National Urban Stormwater Modelling Guideline

September 2023

National Urban Stormwater Modelling Guideline

Phase 1 – Literature Review and Gap Analysis

Water New Zealand, Metis Consultants Limited, Awa Environmental and individual contributors make no representations and give no warranties of any kind, whether expressed or implied, concerning the information contained in this guideline. The guideline and all information contained in it are provided “as is” and are subject to change without notice. Any risk arising out of its use remains with the recipient. Water New Zealand, Metis Consultants Limited, Awa Environmental and individual contributors are not responsible for the results of any actions taken on the basis of information in this guideline or any loss, damage, costs or expenses of any kind which may arise in any way out of, or result from, any use of material or information in this guideline.

Acknowledgements

The development of this document was funded by the Department of Internal Affairs. The project was facilitated by Water New Zealand (Nicci Wood – Technical Advisor) and delivered by a technical team from Awa Environmental Limited (led by Rathika Jebamony) and Metis Consultants Limited (led by Michael Arthur). This work builds on a volunteer initiative to develop stormwater modelling guidance led by Nadia Nitsche (Wellington Water), Fiona Macdonald (Auckland Council) and other Water New Zealand Modelling Special Interest Group members between 2019 and 2022.

An Advisory Group drawn from the Water New Zealand Stormwater and Modelling Special Interest Groups along with other volunteers from the wider industry provided guidance and review input to inform this document. We give our thanks to the Advisory Group members as listed below for freely contributing their time, knowledge, experience and opinions:

• Alistair Osbourne - Wellington Water

• Cheryl Bai - Auckland Council

• Emily Lane - NIWA

• Kelly LaValley - Waimakariri District Council

• Matthew Wilson - Canterbury University

• Nadia Nitsche - Wellington Water

• Sue-Ellen Fenelon - Ministry for the Environment

• Suzana Shipton - DHI

• Thomas Nikkel – WSP

• Tyler McMillan - VirtuGlobal

• Pranil Wadan - Woods

• Victor Su - Tauranga City Council

The content of this document was also informed through industry surveys, interviews and workshops as listed below. We thank those who took the time to contribute to the surveys, discussions with our technical team and / or attend the workshops to share their experiences, knowledge and opinions.

• Industry Survey – July / August 2020

• Water New Zealand – National Conference Workshop – November 2020

• Water New Zealand – Modelling Symposium Workshop – June 2021

• Water New Zealand - Modelling Symposium Workshop – March 2023

• Water New Zealand – Online Workshop – April 2023

• Industry Survey – April / May 2023

• Water New Zealand – Stormwater Conference Workshop – May 2023

• Interviews with council staff and consultants in the industry – April to May 2023

Revision History

Glossary

Term / Abbreviation

Definition

AC Auckland Council

ADRH Australian Disaster Resilience Handbook

Advisory Group / AG Group of volunteers who supported the development of this report and associated guidance document

AMC Antecedent Moisture Content

ARI Average Recurrence Interval

AEP Annual Exceedance Probability

ARR Australian Rainfall Runoff Guidelines

BOPRC Bay of Plenty Regional Council

CCC Christchurch City Council

CIWEM Chartered Institution of Water and Environmental Management

CoP Code of Practice

DAPP Dynamic Adaptive Policy Pathways

DCC Dunedin City Council

Defra Department for Environment, Food and Rural Affairs (United Kingdom)

DiA Department of Internal Affairs

ES Engineering Standards

FEH Flood Estimation Handbook

FEMA Federal Emergency Management Agency (US)

GWRC Greater Wellington Regional Council

HEC-RAS Hydrologic Engineering Center's River Analysis System (modelling package developed by US Army Corps of Engineers)

IPWEA Institute of Public Works Engineering Australasia

KCDC Kapiti Coast District Council

LoS Level of Service

Metadata

Data that describes other data – structured reference data that helps sort and identify attributes of the information it describes. It makes data easier to find, use and re-use.

MfE Ministry for the Environment

Term / Abbreviation Definition

MSM Minnesota Stormwater Manual

NAM Nedbor Afstromnings Model

NOAA National Oceanic and Atmospheric Administration (US)

NIWA National Institute of Water and Atmospheric Research

NTU National Transition Unit (part of DiA)

QH Flow (Q) – Height (H) relationship

QUDM Queensland Urban Drainage Manual

RCP Representative Concentration Pathway

RFHA Rapid Flood Hazard Assessment

RMA Resource Management Act (1991)

RoFSW Risk of Flooding from Surface Water (note that Stormwater = Surface Water in the UK)

RORB

The 'ROR' of 'RORB' stands for 'runoff routing'. The 'B' no longer has significance but at one time indicated that the program was developed and maintained on a Burroughs B6700 computer.

SCS

United States Soil Conservation Service (recently re-named as the Natural Resources Conservation Service – NRCS)

SEPA Scottish Environmental Protection Agency

SIG Special Interest Group

SPP Shared Socio-economic Pathways

Surface Water

Stormwater (surface water is the term used for stormwater in the United Kingdom)

SWMM Stormwater Management Model

TCC Tauranga City Council

URBS Unified River Basin Simulator

USDCM Urban Storm Drainage Criteria Manual

WaterNZ Water New Zealand

WBNM Watershed Bounded Network Model

WRC Waikato Regional Council

WWL Wellington Water Limited

Executive Summary

Floods are New Zealand’s most frequent and most significant natural hazard, estimated to cost the country $160 million per year. Urban stormwater networks (and flood protection schemes) underpin the integrity of public and private assets, provide resilience and security to communities and their investments, and are recognised as nationally important infrastructure.

Flood hazard modelling involves the use of hydrological and hydraulic models to estimate the range of possible floods that could occur in a catchment and the hazard associated with these events. The output produced from flood hazard models is generally a series of flood hazard maps and tabulated data for each scenario modelled. Model results are used to predict flood extents, define overland flow paths and set minimum freeboard levels, whilst understanding the risk.

Having a holistic and consistent understanding of the flood hazard in different areas enables informed decisions to be made about the best ways to manage risk. This may be through engineered approaches to managing or reducing the risk to existing development, and future planning decisions such as excluding development in higher hazard areas.

The overall objective of developing a national urban stormwater modelling guideline is to provide a consistent and robust urban flood hazard modelling process to aid understanding of risk and our management of it. A consistent approach will enable better decision making by communities, planners, and engineers. The guideline will give greater certainty for councils, future Water Service Entities, developers and the public that modelled urban flood risk data can be used to enable common understanding and clear communication of urban flood risks and associated impacts. The objectives of the guideline document are to provide:

• A clear and consistent approach to urban stormwater modelling at a range of scales

• A framework for assessing confidence in models and their outputs

• A minimum standard for planning, building, reviewing / quality assuring, using, maintaining and sharing urban stormwater models

• Improved stormwater management planning to enable existing networks and future developments to be prepared for the predicted impacts of climate change

This report is the main deliverable from Phase 1 of guideline development. Phase 1 consisted of the following tasks:

• A literature review of existing guidance documents, engineering standards and codes of practice within NZ plus a review of best practice guidance documents from Australia, USA and the UK

• Review of legislative drivers for stormwater modelling in local government and the private sectors

• Proactive engagement with the NZ industry through a range of in person and online workshops supported by online surveys

• Targeted interviews with authors of other WaterNZ modelling guidance documents and other leading industry practitioners

These tasks were used to identify gaps and inconsistencies needed to be overcome to arrive at a consistent national approach for stormwater modelling Key gaps / inconsistencies identified are

summarised in Table ES1 below. Note that item numbering is for reference purposes only and does not imply any priority or relative importance

Table ES1: Gap Analysis Summary

The engagement and research completed for Phase 1 of this study has also shown there is a clear mandate from the wider industry for national stormwater modelling guidance. Both industry surveys showed more than 95% of respondents supported the development of national guidance. Similarly, the majority of attendees across all workshops run for this work have supported the need for national guidance. Phase 1 has also considered the format of the guidance document.

We recommend that this study progresses to Phase 2 to develop a national stormwater guidance document in the following general form:

• Basic structured website

• Functionality must include print to PDF options (or a parallel master document maintained with the same content that can be downloaded)

• Dedicated URL (not a sub-domain of the WaterNZ website)

• Hosted & maintained by WaterNZ (funding model will be required)

• Version tracking is actively managed using a combination of proprietary website tools and / or PDF records

The recommended work scope for Phase 2 is summarised below.

• Technical content is developed generally in line with the structure detailed in Appendix B of this document (noting that the structure remains subject to further discussion and development within the agreed programme and budgetary constraints of the project)

• Content development will be prioritised based on the gaps identified. All sections within the proposed structure will have basic content as a minimum, but only priority areas identified by the gap analysis will be developed in detail.

• A range of approaches will be used to deliver the content including:

o Flow diagrams

o Tables & figures (either generated for this document or adapted from other best practice documents)

o Signposting to other key documents (to avoid repetition and need for future updates when external documents are superseded)

o Case studies demonstrating best practice approaches

• The themes guiding content will be:

o Conciseness

o Accessibility for a range of end users

o Ease of maintenance / updates / addition of future content

It is recognised that not all content will be fully developed within Phase 2. This aligns with the purpose of Phases 1 and 2 of guideline development – to provide an overall framework, but not necessarily all the detailed components due to programme and budgetary constraints. We recommend the tasks listed below (in order of priority) are added to the Phase 2 work scope to deliver efficiencies and value for money for the project sponsor (DiA) and end users.

1. Collate current approaches used within NZ for rainfall-runoff estimation to inform future work in this area

2. Develop a basic metadata standard for flood models and flood model outputs (including identifying and working with relevant partner organisations)

3. Work with the MfE to address issues around inconsistent language and poor understanding of common technical terms associated with extreme rainfall, flooding and flood mapping

We recommend that the prioritised list of work packages below are delivered in the next 3-5yrs to support and update the national urban stormwater modelling guidance document.

1. Develop a NZ standard methodology for rainfall-runoff estimation

2. Support MfE to address issues around inconsistent language and poor understanding of common technical terms associated with extreme rainfall, flooding and flood mapping – including updating the guidance to align with agreed terminology as required

3. Determine the need, scope and support for developing national water quality modelling guidance

1.1 Background

Floods are New Zealand’s most frequent and most significant natural hazard, estimated to cost the country $160 million per year. Urban stormwater networks (and flood protection schemes) underpin the integrity of public and private assets, provide resilience and security to communities and their investments, and are recognised as nationally important infrastructure.

Flood hazard modelling involves the use of hydrological and hydraulic models to estimate the range of possible floods that could occur in a catchment and the hazard associated with these events. The output produced from flood hazard models is generally a series of flood hazard maps and tabulated data for each scenario modelled. Model results are used to predict flood extents, define overland flow paths and set minimum freeboard levels, whilst understanding the risk.

Climate change is increasing frequency and intensity of storm events, along with growth and intensification of our urban environment, both increase the risk of flooding. There is need for more specific, targeted, consistent approach for stormwater modelling to help address these risks. Currently there are significant gaps in flood risk information, absence of nationally consistent flood hazard maps and variations between councils' approach, design standards and policies related to flooding.

Having a holistic and consistent understanding of the flood hazard in different areas enables informed decisions to be made about the best ways to manage risk. This may be through engineered approaches to managing or reducing the risk to existing development, and future planning decisions such as excluding development in higher hazard areas.

1.2 Objectives

The overall objective of developing a national urban stormwater modelling guideline is to provide a consistent and robust urban flood hazard modelling process to aid understanding of risk and our management of it. A consistent approach will enable better decision making by communities, planners, and engineers. The guideline will give greater certainty for councils, future Water Service Entities, developers and the public that modelled urban flood risk data can be used to enable common understanding and clear communication of urban flood risks and associated impacts. The objectives of the guideline document are to provide:

- A clear and consistent approach to urban stormwater modelling at a range of scales

- A framework for assessing confidence in models and their outputs

- A minimum standard for planning, building, reviewing / quality assuring, using, maintaining and sharing urban stormwater models

- Improved stormwater management planning to enable existing networks and future developments to be prepared for the predicted impacts of climate change

1.3 Report Purpose

This report is the main deliverable from Phase 1 of guideline development. Phase 1 consists of the following activities:

- Collating national, regional and district plan documentation that define the regulatory context for modelling schematisation decisions.

- Engagement with the Advisory Group, Modelling Special Interest Group (SIG) and Stormwater SIG to understand work already undertaken,

- Collate and review known guidelines in NZ and relevant international guidelines from Australia, the United Kingdom and the United States

- Engage with practitioners to locate guidelines and specifications, including stormwater codes of practice / design codes.

- Complete targeted interviews to understand lessons learnt from practical application of existing guidance

- Review documentation and interview materials to identify:

o Relevant trends that influence stormwater modelling objectives and outcomes

o Common approaches

o Discrepancies / inconsistencies

o Strengths and weaknesses

- Highlight key issues and provide prioritised recommendations on how to achieve a consistent national approach for stormwater modelling

- Prepare a report (this document) that:

o Summarises the outcomes from the above activities including:

 Overview of literature review completed

 Gaps that need to be overcome to achieve a consistent national approach

o Makes a clear case for national guidance

o Defined the work scope for Phase 2 including structure and delivery format

Phase 2 of guideline development process will deliver the first version of a National Urban Stormwater Modelling Guideline. It is acknowledged that this first version will provide an overall framework, but not necessarily all the detailed components due to programme and budgetary constraints. The guideline will be a live document and it is envisaged that further development / update will be undertaken in future work phases as funding becomes available.

2 REVIEW OF MODELLING APPROACHES

2.1 Overview

The following sections summarise the review of current modelling guidance documentation in New Zealand (NZ). Relevant documents were identified through the following methods:

- Industry knowledge and experience of the report authors and their supporting teams

- Engagement workshops with industry members (held in person at the WaterNZ Modelling Symposium in March 2023, online in April 2023 and in person at the WaterNZ Stormwater Conference in May 2023)

- Industry surveys circulated to WaterNZ Members (July 2020 and April-May 2023)

- Feedback from the Advisory Group

- Interviews with NZ industry experts – council staff involved with creating documents, consultants and authors of other WaterNZ modelling guidance documents (Water Supply and Wastewater)

Relevant documents were then prioritised, and documentation that is used frequently by a range of parties were given precedence for review It should be noted that review effort was focussed on modelling guidance – not detailed modelling specifications which are generally software specific and / or specific to local conditions.

Engineering standards also inform a significant portion of the model schematisation. A national stormwater code of practice document (recently renamed as the National Engineering Design Standards - NEDS) is currently being developed and is due to be complete in early 2024. The consultant team developing the NEDS are coordinating inputs from council representatives on specific topics to incorporate into this document. At the time of this literature review (May / June 2023), the modelling guidelines team have attended two of their technical working group meetings to coordinate overlaps and identify gaps.

2.2 New Zealand

Several established modelling guidance documents in NZ were reviewed in detail. These are summarised in the table below. Further details of these documents that were reviewed are available in Appendix B with summaries on methods used by specific local authorities.

Table 2-1: New Zealand Modelling Guidance Documents

Documents Reviewed Document Owner Location of Document

NRC Flood Hazard Modelling Guidelines, July 2022

Stormwater Flood Modelling Specifications, 2011

Flood Hazard Modelling Standard, May 2021

Northland Regional Council

Auckland Council

Greater Wellington Regional Council https://www.gw.govt.nz/assets/Documents /2021/12/GWRC-Flood-Hazard-ModellingStandard-R1-May-2021.pdf

Documents Reviewed Document Owner Location of Document

Hydraulic Modelling Guidelines, May 2021

Guidelines for Stormwater Modelling, April 2022

Regional Stormwater Hydraulic Modelling Specifications, V6 –January 2023

Stormwater Modelling Specification for Flood Studies, July 2012

Waterways, Wetlands and Drainage Guide (WWDG), February 2003

Bay of Plenty Regional Council

Unpublished, supplied by Council.

Tauranga City Council Supplied by Council.

Wellington Water

Unpublished, supplied by Wellington Water.

Christchurch City Council Supplied by Council.

https://ccc.govt.nz/environment/water/wat er-policy-and-strategy/waterwayswetlands-and-drainage-guide/

Several other modelling specifications and methodology documents used by councils and consultants were reviewed at a high level using online documentation, unpublished documentation and/or interviews. These include methodologies for councils who own and maintain stormwater models such as Hamilton City Council, Rotorua Lakes Council, Kapiti Coast District Council, Kaipara District Council, Dunedin City Council, Waikato Regional Council. General structures for other Water NZ modelling guidance documents were also reviewed, including the Water NZ Good Practice Guide for Addressing wet weather wastewater network overflow performance (November 2022), Water NZ Guidelines for Modelling Water Distribution Systems (September 2021), Water NZ WastewaterModellingGuidelines(October 2017).

The following sections comment on how the modelling guidance documents in NZ address specific topics. Examples of councils where certain methodologies are applied are shown within brackets. Auckland Council (AC), Wellington Water Ltd (WWL), Christchurch City Council (CCC), Bay of Plenty Regional Council (BOPRC), Northland Regional Council (NRC), Kapiti Coast District Council (KCDC), Tauranga City Council (TCC), Greater Wellington Regional Council (GWRC).

2.2.1 Document Structure and Format

Most guidance documents are developed by regional, city or district councils who own and maintain hydraulic models that are built in-house or through consultants (often an established panel of consultants). The documents have a level of detail specific to each council’s needs.

Documents are generally in .pdf format and are distributed to interested parties as required. Very few are available as online links to the pdf document via council websites.

As guidance documents often need to target different audiences, documentation that separates narrative from implementation and detail are easier to understand and use.

Guidelines / specification documents with easy to use and informative structures include those developed by:

o Greater Wellington Regional Council

o Bay of Plenty Regional Council

o Northland Regional Council

o Tauranga City Council

o Wellington Water

2.2.2 Software

Modelling software packages “Infoworks ICM”, “TUFLOW” or “MIKE by DHI (“MIKE FLOOD”) are generally used for large, detailed 1D-2D coupled catchment models and by councils / larger consulting companies. Many of the guidelines specify the use of only one of these packages when delivering work on their behalf, with guidance documentation specifying implementation details using this software. MIKE URBAN, HECRAS, SWMM and DRAINS are used frequently by smaller councils or consultants, particularly for development / engineering options impact assessments.

2.2.3 Model Methodologies

The following tables summarise the types of methodologies applied across the country for each model component as described in the reviewed guidelines.

Spatial Distribution within modelled area

Dynamic

Historic events

Uniform

TP108 Centred Hyetograph (AC), Centred Nested Hyetograph (TCC), 67% offset nested hyetograph (WWL), Triangular (CCC), ¾ centred nested (BOPRC), Centred Nested Hyetograph (KCDC)

Moving centred nested storm for upper catchments (BOPRC)

Thiessen polygons or closest rainfall gauge (TCC, WWL, etc).

Rainfall radar (BOPRC)

No spatial distribution (commonly used)

Thiessen polygons Used for specific projects, minimal guidance

Interpolated between gauges

Distributed – radar

Areal Reduction

Factors for nondistributed rainfall

Historic events

Isohyets

Used for specific projects, minimal guidance

Used for specific projects, minimal guidance

Used for specific projects, minimal guidance

Thiessen polygons or closest rainfall gauge (TCC, WWL, etc).

Rainfall radar (BOPRC)

TP108 (AC)

Depth Scaling

Single rainfall based on local assessment and moving storm

HIRDS v3 or v4 depths (TCC), HIRDS v4 grids for gridded rainfall (KCDC)

Table 2-2: Rainfall

Parameter / Variable Sub-type Examples

Duration for design N/A

18hr (CCC), 24hr (AC, KCDC), 6hr – 72hr nested storm (TCC), 12hr (WWL)

Note,rainfallvariesspatiallywithlocalisedeffects,sowhatisappropriateforonecouncilisnotnecessarily appropriateinanotherlocation.Thereviewhighlightsthisthroughthevarietyofapproachessummarised above

Parameter / Variable Sub-type Examples

Constant N/A

Mean High Water Springs (AC)

Sample from a gauge Calibration/Validation (TCC, KCDC, WWL) Generated from harbour modelsNIWA forecaster, hydrodynamic models (TCC, KCDC)

Dynamic

Timing of peaks synchronised with hydrodynamic model peak discharge (TCC, KCDC, WWL) Inclusion of Joint probability of storm tide and wave setup

Coastal calculator by NIWA - (BOPRC) Inclusion of wave setup (KCDC, TCC)

Type

Antecedent conditions

Sub-type Examples

Antecedent Moisture

Content (AMC) factors

Initial losses

Groundwater table N/A

Incorporated into losses based on soil

drainage classification N/A

TP108 (AC)

Set to zero for design (common practice)

Modified to account for evapotranspiration in rootzone (CCC), groundwater table based on groundwater model (KCDC)

Frequently applied

Table 2-3: Tide / Sea Level

Table 2-4: Subsurface

Parameter / Variable Sub-type

Fixed increase (e.g. 20%) or set for 2090/2100.

RCP based approach for temperature estimates (climate change scenario and horizon are required)

Comments

Fixed increase is most common approach as it is easy to apply.

HIRDS guidelines (nation-wide temperature increase for different RCPs), MfE guidelines (estimates for each region)

Rainfall

Augmentation factors

Shared Socio-Economic Pathways (SSP)supersedes RCP approach. Climate change pathways and horizons.

Localised studies

Sea level rise

HIRDS 2018, MFE – older reports

No guidance on how it applies to rainfall yet. Used for specific projects, but no general guidance provided.

Commissioned by regional councils.

Tide / Sea Level

Localised studies

Takiwā platform by NZSeaRise (latest). Previously, regional studies have been undertaken that relate local effects to nationwide sea level rise.

Commissioned by regional councils

Included in Takiwā platform. Based on recent measurements and does not consider other unpredictable land movement (e.g. earthquakes) Dynamic Adaptive Policy Pathway (DAPP) methods

There are range of approaches that use varying combinations of future scenarios applied to stormwater models. None of these are standardised or coordinated.

Parameter / Variable Comments

Data based correlation

Lack of evidence and lack of data to show nationally consistent correlation between rainfall and storm surge. Some recent studies undertaken by WWL and KCDC using gauged data.

Guidance from Australia is often referenced.

Conservative estimates

A few regional councils have defined conservative estimates for joint probability. For example, 100yr-ARI rainfall with 20-year ARI storm surge impacted tide (BOPRC, TCC). Others have assumed no correlation (AC, WWL).

Table 2-5: Climate Change

Table 2-6: Joint Probability

Parameter / Variable Sub-type

Comments

SCS Curve number calibrated for local conditions (AC, WWL)

Time of concentration specific to the region (AC), Three-part estimate based on three flow types (WWL)

Lumped / Subcatchment based

Loss Models

Initial and Continuing loss – (TCC lower catchments)

Hortons infiltration coefficients (DCC)

Proportional Loss - (TCC Upper Catchments, DCC impervious area)

Variant of the SCS Method - Upper catchments (KCDC)

SCS Dimensionless curve “standard peak rate factor 484” (AC, WWL, TCC)

Clark Unit Hydrograph (Upper Catchments KCDC, TCC)

Time-Area (TCC)

Routing Models N/A

Kinematic Wave (TCC – older models)

Nedbor Afstromnings Model (NAM) - used for specific purposes

Runoff Routing (RORB) - initial and fixed continuing loss model (CCC upper catchments, GWRC joint probability, KCDC joint probability)

Rain-on-grid

Loss Model

Initial and Continuing at a constant rate. (TCC, KCDC, CCC)

Includes soil capacity limit (CCC, KCDC)

Hydraulic Routing 2m resolution for urban areas (TCC, KCDC, KDC)

Variable Mesh (CCC, WWL, AC)

Table 2-8: Primary System

Parameter / Variable

Sub-type

Comments

Drainage Networkpipes, channels, structures, pond outtakes, stormwater inlets, service leads to structures and private properties, soakage, outlets, etc.

Usually represented in 1-dimension (detailed models) or assumed to be blocked in high level assessments (not modelled).

Channels sometimes represented in 2dimensions.

Some high level models represent the drainage system using constant losses in model hydrological component

Some specifications exist (AC, TCC, WWL),

Varying level of detail on the schematisation of the primary system (from all pipes to only pipes exceeding a certain diameter)

Minimal guidance on level of detail required to align with the purpose of the model

Minimal guidance on level of detail required to align with level of service requirements

Table 2-7: Hydrology

Parameter / Variable

Comments

Overland flow outside of the primary system.

Usually represented in 2-dimensions

Representation of infrastructure that is outside of the council’s control and is often not designed to convey stormwater. E.g. buildings, properties, roads, subsurface system, etc.

Inconsistent approaches to representation of buildings – such as high roughness or blocking them out of the 2-dimensional space entirely.

Minimal guidance on level of detail required to align with the purpose of the model.

Minimal guidance on level of detail required to align with level of service requirements.

Sensitivity

Freeboard

Limited guidance available. Though some specification exists (WWL, Waikato Regional Council, TCC)

Some general guidance exists in all documents, with a few definitions on an acceptable calibration (AC, WWL). However, a general lack of gauged data means that stormwater models are not calibrated.

Where possible, flood incident information (photos, media reports, fire service call outs, etc) are utilised to validate / verify a model.

Project specific sensitivity is often recommended.

Minimal guidance on the process and criteria for success.

A few councils specify a comprehensive list of sensitivity scenarios to be simulated as a standard (CCC, WWL).

Constant value applied to flood levels.

Scenario tests undertaken to inform a localised freeboard (WWL).

Table 2-9: Secondary System

Table 2-10: Boundary Conditions

Table 2-11: Model Confidence

Table 2-12: Model Maintenance

Parameter / Variable Comments

Folder and file naming conventions

Metadata guidance (AC, TCC, WWL)

Model update / maintenance plan / process

Some guidance available (AC, WWL, TCC, CCC)

Some guidance available (AC, WWL, TCC)

Limited - though some good examples exist (TCC)

2.3 Australia

The table below lists the Australian guidance documents that were reviewed. Document commonalities and differences are discussed in the following sections. Further detail can be found in Appendix B. This section provides less detail than the NZ review section as the majority of regional documents use the Australian Rainfall and Runoff Guidebook as their main reference source.

Table 2-13: Australian modelling guidance documents

Documents Reviewed Document Owner

Australian Rainfall and Runoff Guidebook (ARR)

Australian Disaster Resilience Handbook

7: Managing the Floodplain (ADRH)

Technical Guideline on Hydrologic and Hydraulic Modelling

AM STA 6100 Infrastructure Projects in Flood-Prone Areas

AM STA 6200 Flood Mapping Project Specifications

Queensland Urban Drainage Manual (QUDM)

Location of Document

Commonwealth of Australia https://www.arr-software.org/arrdocs.html

Australian Institute for Disaster Resilience

https://knowledge.aidr.org.au/media/3521/adrhandbook-7.pdf

Queensland Department of Transport and Main Roads

Melbourne Water

https://www.tmr.qld.gov.au/businessindustry/Technical-standardspublications/Hydrologic-and-HydraulicModelling

Unpublished, supplied by Melbourne Water.

Queensland Government

Available for purchase from the Institute of Public Works Engineering Australasia (https://www.ipwea-qnt.com/qudm)

Parameter / Variable Sub-type

Comments / Examples

• Three regional and two national documents reviewed. ARR is considered the master document to which most other guidance refers.

Document Structure and Format

N/A

• Most documents are freely available online in pdf format accessed via organisation websites. The Melbourne Water document is supplied upon request, and QUDM needs to be purchased (revenue earned through sales is used to maintain the guidance document)

• TUFLOW is the most specified software package (Melbourne Water, Queensland Transport)

Software N/A

• ARR provides a range of options and summarises the capabilities of the main software packages

Hydrology

Rainfall

Most documents refer to ARR approaches ARR provides multiple rainfall products, including spatial intensity-frequency-duration, design temporal hydrographs, areal reduction factors, and pre-burst rainfall

Losses

ARR provides ranges for initial and continuing losses for pervious and impervious areas. Recommended to use GIS investigations combined with local knowledge to estimate these

Runoff routing

Software solutions often recommended. Preferred packages include RORB (Melbourne Water) and XP-RAFTS (Queensland Transport). Unified River Basin Simulator (URBS), RORB, and Watershed Bounded Network Model (WBNM) approaches also used.

Primary System

Rain on grid vs. lumped hydrology

Rain on grid appears less commonly used. It is discouraged or not supported by Queensland Transport and Melbourne Water; little detail in ARR.

Piped network

• Regional documents mostly require explicit representation of 1D network (Queensland Transport, Melbourne Water). Some provide details on how to assess blockage and drainage structure failure.

• ADRH recommends validating GIS pipe network information using surveys and aerial imagery

Open channels

Secondary Systems Overland flow

• Represented in 1D in QUDM and Melbourne Water

• ARR provides option to represent in 2D

• Represented in 2D (ARR, QUDM)

Table 2-14: Review summary of Australian modelling guidance documents

Parameter / Variable Sub-type Comments / Examples

• No consensus on representing buildings (Melbourne Water and ARR provide some options but no guidance on choosing between them).

Boundary Conditions

Upstream

• Most documents recommend using inputs from a lumped hydrological model as part of the upstream boundary condition

• Assumptions are required on flow distribution along the boundary

• Tailwater or tidal levels recommended for use as constant, time series, or rating curve

Downstream

• ARR recommends a first pass assumption of horizontal water surface elevation across the downstream boundary

• Fixed percentage increase of 5% per °C of local warming

Climate Change

Rainfall

• ARR recommends an online tool for projecting future heating

• QUDM provides projected future heating (2°C by 2050, 3°C by 2070 and 4°C by 2100)

• Fixed: Melbourne Water recommends 0.8m increase, QUDM recommends a minimum of 0.3m

Sea level rise

• Other documents do not provide specific guidance

• None of the reviewed documents provide guidance on vertical land movement

Joint Probability N/A

• Most guidance documents refer to ARR, which recommends assessing joint probability only for factors which are highly variable and significantly affect flooding. In these cases, the design variable method is recommended.

• A probability flood model can be fitted to observed data to avoid separately assessing joint probability (ARR).

• All documents mentioned the importance of validation against independent real-world data, e.g. observed flooding, gauge data

Calibration / Validation

/ Verification

• Queensland Transport also mentions verifying head loss for bridges and structures using numerical methods

Model Confidence

Sensitivity Testing

• ARR notes the difficulty obtaining validation data

• Guidance provided only in ARR, Melbourne Water, and QUDM. Others either omit sensitivity testing or mention it in passing only.

Parameter / Variable Sub-type Comments / Examples

• ARR suggests a list of parameters for sensitivity testing, advising a ±20% variation.

• Common parameters highlighted are roughness coefficients, tailwater levels, pipe blockages, and climate change

• Most documents recommend a minimum of 300mm, up to 600mm

Freeboard

• Increase in freeboard often suggested for areas of high model uncertainty

• Melbourne Water recommends applying climate change to freeboard allowance for structures with long design life

• ARR recommends defining output types at start of project to guide modelling

Outputs and Presentation N/A

• Most documents recommend documenting model assumptions and data use in a report. Some provide templates (Melbourne Water, Queensland Transport)

• Models are also usually provided back to the Council in digital format

• Exact mapping requirements vary by organisation but generally include flood depth, extent, and flooded buildings

Model Maintenance N/A

• None of the reviewed documents provide detailed guidance on this parameter

2.4 United Kingdom

Three UK modelling documents were reviewed, as shown in the table below. Document commonalities and differences are highlighted in the following sections. Further detail can be found in Appendix B.

Documents Reviewed

Flood Modelling Guidance for Responsible Authorities

Integrated Urban Drainage Modelling Guide

Risk of Flooding from Surface Water (RoFSW) Technical Specification

Document Owner

Scottish Environmental Protection Agency (SEPA)

Chartered Institution of Water and Environmental Management (CIWEM)

Department for Environment, Food and Rural Affairs (Defra – England and Wales)

Location of Document

https://www.sepa.org.uk/media/219653/f lood_model_guidance_v2.pdf

https://www.ciwem.org/assets/uploads/I UD_1.pdf

Unpublished, supplied by Defra

• Documents are regional or national. CIWEM provides generalised guidance applicable to a wide range of scenarios, while the RoFSW document is for a specific high-level countrywide stormwater (surface water) flood map. The SEPA document provides more detail on local-level models.

• Documents are available in pdf format. Two can be accessed online (SEPA, CIWEM), while one was supplied by the relevant authority for England and Wales (Defra)

• CIWEM provides a useful a process diagram, presented at the beginning of each section to signify the corresponding stage in the modelling process.

• CIWEM also classifies models into four types based on level of detail

• CIWEM is the only document to explicitly mention available software packages. It provides typical use cases for each package.

• Both CIWEM and SEPA provide considerations in software selection (ability to meet project objectives, training availability, industry standards)

Table 2-15: UK modelling guidance documents

Table 2-16: Review summary of UK modelling guidance documents

Parameter / Variable Sub-type Comments / Examples

• RoFSW requires, as a minimum, software that uses shallow water equations to produce reliable depth and velocity data.

Hydrology

Rainfall

CIWEM presents four options for urban drainage

All three documents recommend using the Flood Estimation Handbook (FEH) 2013, which provides spatial depths and temporal variation. Runoff routing

– fixed percentage runoff, New UK Percent Runoff method (NewUK PR), UK Water Industry Research (UKWIR) method and Wallingford Procedure, which include a consideration of losses. RoFSW recommends the fixed percentage runoff method with a time-varying loss based on soil profiles. SEPA recommends the lumped Revitalised Flood Hydrograph (ReFH 2) procedure.

• SEPA and RoFSW offer options for implicit modelling of pipe network using fixed drainage loss

Primary System Piped network

• CIWEM provides guidance for explicit 1D modelling. Gullies / catchpits are not typically recommended for modelling.

• Defined in CIWEM as above-ground drainage (channelised flows, overland flow paths).

Secondary Systems Overland flow

• CIWEM recommends modelling in 2D and including small-scale linear features for accurate flow routing. These include kerbs, walls, and hedges.

• CIWEM provides some approaches to modelling buildings.

• SEPA and RoFSW do not provide guidance on secondary systems.

• Outfall levels can be set using downstream hydrographs or tide levels

Boundary Conditions

N/A

• RoFSW considers a 'baseline' downstream boundary, and does not account for flood barriers / defences, high tide, or high river levels at outfalls

• There is no single standard approach across the documents reviewed

• CIWEM does not provide a specific percentage uplift for climate change as the recommendations frequently change

Climate Change

N/A

• Climate change uplifts to rainfall / river flows are specified in separate guidance documents by the Environment Agency (England) and Natural Resources Wales (Wales).

Parameter / Variable Sub-type Comments / Examples

• SEPA recommends an uplift based on a previous scientific report, allowing 50% increase for 2080s

• Defra does not consider CC in the RoFSW mapping

Joint Probability N/A

• CIWEM and SEPA both highlight the importance of joint probability. Both link to an external technical report by HR Wallingford ('Joint Probability: Dependence Mapping and Best Practice'), which provides further detail, considering combinations of wave height, sea level, surge, tide, river flow, and precipitation.

• RoFSW only considers the joint probability of antecedent soil moisture conditions, but provides no guidance on how to include this

• CIWEM suggests running very low AEP events as a proxy for considering joint probability.

• CIWEM and SEPA both differentiate calibration requirements based on model purpose.

Model Confidence

Calibration / Validation / Verification

• SEPA recommends only sensitivity testing for strategic models, and calibration for catchment and local models where gauging is available. A minimum of three calibration events and one validation event is recommended.

• CIWEM: Possible calibration parameters include inflows to model and boundary conditions, using gauges where available. Detailed verification guidance is given for different types of data.

• RoFSW only mentions calibration for rainfall. It does not require validation but gives models a confidence score based on level of validation undertaken.

• CIWEM provides detailed guidance including suggested sensitivity tests and ways to assess their results.

Sensitivity Testing

• SEPA states that sensitivity tests should be undertaken for all modelling studies, with required tests specified in the initial scoping phase.

• RoFSW does not mandate sensitivity testing

• SEPA suggests quantitative uncertainty analyses to calculate freeboard requirements.

Freeboard

• Neither CIWEM nor RoFSW explicitly mention freeboard.

Parameter / Variable Sub-type Comments / Examples

• All documents require a model build report and provide suggested content.

• Output file size considerations are included in CIWEM.

Outputs and Presentation N/A

• SEPA specifies hazard calculation formula and values to use for a 'nodata' field while post-processing.

• RoFSW specifies a rectangular grid resolution of between 2m and 5m in outputs.

Model Maintenance N/A

• CIWEM dedicates a chapter to model maintenance, including ownership, filing considerations, and possible maintenance approaches.

• SEPA and RoFSW do not provide model maintenance guidance

2.5 United States

Three US modelling documents were reviewed in detail, as shown in the table below.

Table 2-17: US modelling guidance documents

Documents Reviewed Document Owner

Stormwater Management Model (SWMM) Reference Manual – Volumes 1 and 2

Urban Storm Drainage Criteria Manual (USDCM)

Minnesota Stormwater Manual (MSM)

United States Environmental Protection Agency

Location of Document

https://nepis.epa.gov/Exe/ZyPDF.cgi?Do ckey=P100NYRA.txt

https://nepis.epa.gov/Exe/ZyPDF.cgi/P1 00S9AS.PDF?Dockey=P100S9AS.PDF

Mile High Flood District (Denver, CO)

Minnesota Pollution Control Agency

https://mhfd.org/resources/criteriamanual/

https://stormwater.pca.state.mn.us/inde x.php?title=Stormwater_models,_calcula tors_and_modeling

In general, it proved challenging to find relevant urban stormwater modelling guidance documents from the US. The documents in Table 2-17 contain some stormwater modelling information but are not comprehensive modelling guides. Several other documents, recommended from the Stormwater Conference workshop and the WaterNZ survey, were briefly reviewed but disregarded due to their lack of information on urban stormwater modelling processes These include the HEC-RAS manual (focused on open channels and specific approaches used within the HEC-RAS software), Georgia Stormwater Management Manual and Stormwater Management Manual for Western Washington (limited information on hydraulic modelling), and FEMA Guidance for Flood Risk Analysis (very specific to US hazard calculations). Document commonalities and differences are highlighted below. More details can be found in Appendix B.

/ Examples

• Two of the documents (SWMM and USDCM) were pdf documents, while one (MSM) was set up as a wiki page online

• USDCM and SWMM are split into two volumes

• MSM contains a comprehensive list of different software and provides a high-level summary of their capabilities

• SWMM is a software-specific guide

• USDCM also recommends EPA SWMM

SWMM had a strong focus on using real world data for all model parameters (rain gauges for rainfall, measured evaporation and infiltration, etc.)

Table 2-18: Review summary of US modelling guidance documents

Parameter / Variable Sub-type Comments / Examples

Rainfall

NOAA Atlas 14 was recommended as a rainfall depth tool by both MSM and USDCM

Spatial distribution

SWMM recommended using multiple rain gauges throughout the catchment, or spatial averages such as Thiessen weighted as a last resort. MSM recommended SCS Type II, NOAA Atlas 14 Nested or Huff probabilistic distributions.

Temporal distribution:

Losses

Runoff routing

USDCM provides design storm distributions to convert NOAA 1hr intervals to 5min intervals

MSM recommends using US SCS Curve Numbers. USDCM provides indicative loss values to use, while SWMM recommends ground measurement.

Lumped subcatchment methodology was common to both USDCM and SWMM

Primary System

Piped network

• Explicit modelling of the pipe network is required in SWMM. USDCM recommends modelling only complex networks, while MSM does not make recommendations on network representation.

• SWMM provides a variety of node and link types for accurate representation.

Open channels

USDCM recommends the use of HEC-RAS for modelling channels in 1D

Secondary System

Overland flow

SWMM and USDCM have overland flows modelled in 2D. MSM does not make recommendations on overland flow representation.

Upstream

SWMM has options for applying external inflow to specific nodes as either subcatchment runoff, groundwater discharge, or infiltration inflow

Boundary Conditions

Downstream

Climate Change N/A

• Represented in SWMM as the hydraulic head at outfalls

• MSM and USDCM do not mention boundary conditions

• None of the three documents reviewed provided explicit guidance on accounting for climate change, although USDCM briefly mentioned it as a factor to consider while selecting design storms

Joint Probability N/A

• For the joint probability of high receiving stream levels, USDCM recommends applying the 10yr ARI fluvial flood level to outfalls while running the 100yr ARI pluvial storm

• Neither MSM nor SWMM mention joint probability

Parameter / Variable Sub-type Comments / Examples

• USDCM mentions calibration exercises but does not provide specific guidance.

Calibration / Validation / Verification

• SWMM recommends no calibration or validation as its models focus on real-world data.

• MSM does not mention calibration or validation.

Model Confidence

Sensitivity Testing

• SWMM lists some parameters that affect runoff volume calculation and the degree of sensitivity.

• MSM and USDCM do not mention sensitivity.

Freeboard

Outputs and Presentation N/A

• USDCM specifies exact required freeboard above 100yr flood level depending on type of development

• MSM and SWMM do not provide freeboard guidance.

• No guidance provided on mapping or reporting in MSM or SWMM

• USDCM provides some details on the process to update Federal Emergency Management Agency (FEMA) flood hazard maps

Model Maintenance N/A

• None of the reviewed documents provide detailed guidance on this parameter

3 REVIEW OF LEGISLATIVE REQUIREMENTS FOR STORMWATER MODELLING

3.1 Overview

In New Zealand, the level of detail required in flood models is most often driven by relevant statutory and regulatory drivers. Some examples of this include:

• Regional and Territorial Authorities may use hydraulic modelling to inform the development, implementation and administration of relevant Plans and Policy Statements as required under the Resource Management Act 1991 (RMA).

• Civil Defence Groups may employ hydraulic models to inform planning and preparation for emergencies including response and recovery plans.

• Land developers and their designers may use hydraulic modelling to support and inform their designs and submissions for resource consent, building consent and / or engineering plan approval.

• Asset owners may use hydraulic modelling to monitor and track compliance with relevant levels of service, consent conditions and / or Codes of Practices. Asset owners may also employ hydraulic models for ongoing operational and asset management purposes.

While there is no single flood management statute in New Zealand, the key pieces of relevant legislation are the:

• RMA

• Civil Defence Emergency Management Act 2002 (CDEM)

• Building Act 2004 (BA)

• Local Government Act 2002 (LGA)

• Soil Conservation and Rivers Control Act 1941 (SCRCA)

A brief overview of each of these statutes is provided in the following sections along with how they may play a role in the scoping and development of hydraulic models.

3.2 The Resource Management Act (RMA)

The RMA requires Regional and Territorial Authorities to control the use of land for the avoidance or mitigation of natural hazards and any adverse effects on the environment (Sections 30 and 31) As a result, both Regional and Territorial Authorities may assess and quantify stormwater or river flood risk and associated hazards within their jurisdictions. Hydraulic modelling can be a key tool in helping the Councils establish at risk areas, quantify hazards and inform risk management, policy or management approaches. Modelling outputs may also be used to support any resource consent applications.

The Government has announced its intention to repeal the RMA and replace it with three new pieces of legislation. A high level summary of these proposed Acts and their objectives are provided in the following sections.

3.2.1 Natural and Built Environment Act (NBA)

This Act has the purpose to enable the use, development, and protection of the environment in a certain way and to recognise and uphold te Oranga o te Taiao. The Act will require the risks arising from natural hazards and the effects of climate change are reduced and other measures are taken to achieve an environment that is more resilient to those risks. Modelling will be the method for identifying the extent of the natural hazard of flooding.

Under the Act, a new National Planning Framework (NPF) is being drafted and will be delivered in stages. The NPF must provide national direction on the system outcomes - there will be mandatory national direction for risks arising from natural hazards and the effects of climate change. The scope for the first NPF is direction to Regional Planning Committees on development of Regional Spatial Strategies (RSS) These are intended to incorporate exiting national direction (such as National Policy Statements and National Environmental Standards) and new content (such infrastructure planning).

3.2.2 Strategic Planning Act (SPA)

This Act will require the development of long-term Regional Spatial Strategies. RSSs would have to promote integration in the performance of functions under the NBA bill, the Land Transport Management Act 2003, the Local Government Act, and the Water Services Entities Act 2022. Under this act, modelling at a range of scales will be needed to inform relevant spatial planning requirements and associated outcomes.

3.2.3 Climate Adaptation Act (CAA)

This Act aims to address legal issues associated with managed retreat, including funding and financing of adaptation measures. This Bill is not expected to be introduced this Parliamentary season. Again, hydraulic modelling is expected to play a key role in helping determine areas in need of retreat and inform associated processes. Hydraulic modelling will be a key tool to inform policy and rules, taking climate change and flood hazards risk into account to inform RSS and in the scoping / development of adaptation measures.

3.2.4 Application to stormwater modelling

The provision of a stormwater modelling guideline would support a nationally consistent approach

Modelling, at an appropriate scale, will inform land use planning and decision making across both NPF and RSS activities. Thes will in turn identify where more detailed modelling is required, perhaps in cases where communities need to consider retreat under the CCAA.

3.3 Civil Defence Emergency Management Act 2002 (CDEM)

The CDEM aims to improve and promote the sustainable management of hazards and enable communities to achieve acceptable levels of risk via planning and preparation for emergencies and response and recovery. The CDEM is about planning to respond to a natural hazard. It recognises that even where good land use planning is applied, there will still be natural hazard events that cause the need to evacuate, respond and recover to the event.

The Emergency Management Bill has been introduced to Parliament with its first reading in June 2023 and is at the Select Committee stage until December 2023 The Bill will replace the Civil Defence Emergency Management Act (2002). The Bill introduces critical infrastructure entities (replaces lifelines) and includes Water Service Entities (WSE) as critical infrastructure entities. It represents a more coordinated approach to planning and responding to emergency event, especially as the water sector (stormwater and river flood control) are recognised.

3.4 Building Act (BA)

The BA primarily aims to ensure any proposed new development is compatible with any existing flood hazards and will not accelerate, worsen, or result in a natural hazard on that land or any other property. The act also requires any flood hazards identified with a new development to be registered on the Certificate of Title by the relevant Building Consenting Authority. The BA also gives effect to the Building Code which provides minimum floor level requirements relative to the 2% AEP flood level and acceptable design standards for drainage infrastructure.

3.5 Local Government Act (LGA)

The LGA provides the general framework and powers under which New Zealand's local authorities operate. It also sets out a range of obligations, restrictions and powers that affect land development, and public infrastructure. This includes infrastructure assets that become public assets through a vesting process governed by provisions in the RMA for land use consents

A challenge in this area is that Territorial Authorities administer land use consents for developments, which are different from the stormwater discharge consent purposes set by Regional Councils at a catchment or development scale framing for flood management. Both can have a stormwater management and approval component. There is often a gap between land use consents and codes of practice that specify the design of the stormwater system, which might be a mix of BC and RMA requirements and not clearly from one or the other. Within this context, hydraulic modelling can be used to inform a broad range of public infrastructure related requirements ranging from system performance analysis to long term planning, design, and quality control during vesting and / or Engineering Approval processes. Use of consistent stormwater modelling approach would enable information sharing and coordination of outcomes where stormwater requirements cross over RMA, BC and LGA responsibilities delivered by Regional and Territorial Authorities.

3.6 Soil Conservation and Rivers Control Act (SCRCA)

The purpose of this Act is to allow for the conservation of soil resources, the prevention of damage by erosion and protect property from flood damage. To achieve the Act’s purpose and objects, Catchment Boards are allowed to be established under the Act. These Boards can be given wide-ranging powers to achieve the purpose and objects of the Act. Where rural rivers flow through towns or cities, then there is an interface with urban stormwater.

3.7 Application of Stormwater Modelling under current Regulatory Context

It should be noted that ownership and management of urban stormwater management differs from flood protection. Stormwater management is not primarily driven by RMA planning needs, it has more significant interactions with Territorial Authority responsibilities under the LGA and associated asset management planning.

3.7.1 Regional Councils

Under the RMA, Regional Councils are typically responsible for the management of effects associated with the use of natural resources including air, freshwater, land and coastal waters. Within the context of stormwater, Regional Councils are responsible for:

• The control of the use of land for the purpose of:

o the maintenance of the quantity of water

o the avoidance or mitigation of natural hazards

• Discharges of contaminants into or onto land, air, or water and discharges of water into water

Under the LGA, Regional Councils are responsible for managing assets associated with flood control The risk from river flooding is managed via regional council’s flood protection schemes. Each region manages their own flood protection schemes based on available resources and priorities. Regional Councils also administer stormwater discharge consents - one of the key tools for managing flooding and runoff quality. Due to the cumulative effects problem of dealing with consents one by one, then modelling of a catchment, with allowance for different land use scenarios, becomes a key tool for understanding the cumulative effects of many stormwater discharge consents

To support the above functions in a policy and asset management context, Regional Councils may assess and quantify flood risk and hazards within their regions along with runoff quality. A Regional Council may develop and maintain a range of models with varying levels of detail and accuracy to suit their specific needs and responsibilities.

The level of detail required in Regional Council flood models could vary significantly from Rapid Flood Hazard Assessments (RFHAs) based on low resolution topographical data to more detailed 1D/2D integrated models developed using higher resolution topographical and asset data. Typically, a RFHA would suffice if the objective of the Council was to establish and manage development and associated risks within flood plains. However, where the Regional Council may be responsible for flood mitigation and / or environmental management schemes, more detailed modelling of such systems may be desired.

3.7.2 Territorial Authorities

Urban stormwater piped networks are managed by city and district councils to avoid nuisance flooding and consequential damage to public and private property (the avoidance or mitigation of natural hazards and the prevention or mitigation of adverse effects of land development). Councils build hydraulic models to understand the capacity of a stormwater networks, the associated potential flood risk and to inform forward capital works programmes. Flood hazard maps are used to inform development decisions, including floor levels and overland flow paths.

3.7.3 Private Sector

A broad range of private sector users ranging from landowners and developers to asset owners may choose to develop and maintain hydraulic models to inform and support a variety of needs. These may include supporting applications for resource consent, design of infrastructure or land development solutions to comply with relevant statutory and regulatory requirements such as resource consent, building consent and / or Engineering Plan Approval. Hydraulic models that are built to demonstrate regulatory compliance will be undertaken in consultation with relevant Regulatory Authorities and their hydraulic modelling specialists. Typically, this would involve improving the level of detail and / or accuracy of an existing Council model to incorporate more accurate site-specific data or characteristics of a proposed future development scenario.

3.7.4 Summary

The RMA is the main legislation responsible for the management of natural hazards and the functions fall to regional and territorial authorities. There is no flood management statute, national guideline or standard in New Zealand. While central government (i.e. the Ministry for the Environment) has a duty to provide advice on flood management issues, the primary responsibility of managing flood risk on people and property falls on regional and territorial authorities. In the absence of strong national direction on natural hazards, regional and territorial authorities have been left to develop their own risk assessment methodologies, and risk management policies and rules. This is complicated by the BA and SCRCA requirements which act across the RMA requirements. As a result, flood risk management policy and processes are inconsistent across the country, determined by an individual authorities capability, resources and competing priorities.

Mā te haumaru ō nga puna wai ō Rākaihautū ka ora mo ake tonu (Increasing flood resilience across Aotearoa) programme is providing a national high level flood map. When completed, this will be able to provide flood hazard information where it does not currently exist and a rapid screening tool for locations where further modelling is required

It is also necessary to recognise that different regions, cities and districts will require different approaches to suit local needs and natural characteristics. If a national guideline or standard was to be developed, it will need to provide the necessary local flexibility whilst adhering to a nationally consistent framework.

3.8 Application of Stormwater Modelling under possible future Regulatory Context

At the time of writing this report, significant national reform initiatives were ongoing for three waters management and the Resource Management Act (1991) within NZ. Documents and legislation reviewed for the purposes of this project were limited to those currently in use within the industry The project delivery team and Advisory Group members are aware of the reforms and note the potential impact they may have on a future regulatory context. Due to the ongoing nature and unknown ultimate outcomes of the reforms, it was decided that the potential future stormwater modelling guidance document should:

• Have a technical focus and minimise reference to specific legislation

• Be delivered in a format that was easy to update to accommodate future changes

• Be applicable for a range of scales and contexts to ensure it is a practical tool for use by the land development community, territorial authorities and potential future Water Service Entities

• Provide a robust and common basis for development of models to inform a range of applications

4 GAP ANALYSIS

The purpose of this section is to summarise the key gaps and inconsistencies that need to be overcome to arrive at a consistent national approach for stormwater modelling. These have been identified through:

- Industry knowledge and experience of the report authors and their supporting teams

- Engagement workshops with industry members (held in person at the WaterNZ Modelling Symposium in March 2023, online in April 2023 and in person at the WaterNZ Stormwater Conference in May 2023)

- Industry surveys circulated to WaterNZ Members (July 2020 and April-May 2023)

- Feedback from the Advisory Group

- Literature review of relevant guidance and supporting documentation within New Zealand and internationally (including documents from Australia, United Kingdom and the United States) – as presented in Sections 2 and 3

- Interviews with industry experts – including authors of other WaterNZ modelling guidance documents (Water Supply and Wastewater)

The gap analysis has been completed in tabular format and is presented in Table 4-1 Note that item numbering is for reference purposes only and does not imply any priority or relative importance.

4.1 Achieving a Consistent National Approach

Details of gaps and inconsistencies identified that need to be overcome to achieve a consistent national approach are provided in Table 4-1. For ease of reference, both gaps (missing guidance) and inconsistencies are referred to as ‘gaps’. Table headings relevant to this section are described below:

- Gap – Description of gap identified

- Impact – How the gap impacts modelling outcomes

4.2 Addressing inconsistencies

To avoid repetition of gaps, Table 4-1 also identifies the actions required to address the gaps. Table headings relevant to this section are described below:

- Action(s) – What action is required to resolve the gap

- Timing – Which project phase will address the gap:

o Phase 2 (in scope) = Addressed within current Phase 2 work scope (2023 delivery)

o Phase 2 (out of scope) = Not currently in scope for Phase 2 but recommended for consideration by Advisory Group / WaterNZ / DiA for inclusion and delivery within Phase 2 scope / timeframes.

o Phase 3+ = Future work required beyond Phase 2, but within the next 3-5years

- Ownership – Recommendation on who should own the action(s) to ensure delivery including lead (L) and delivery support (D) parties.

Note that Section 4.3 and Table 4-2 provide a prioritised task list for addressing gaps that are not within the current work scope for Phase 2.

Table 4-1: Gap Analysis

No. Gap Impact Action(s) Timing Ownership

1 Modelconfidencerating – There is no consistent method within NZ for defining confidence in model outputs or what confidence level is needed for certain applications (e.g. what confidence is needed for setting floor levels relative to flooding within a development application)

2 Rainfall-runoffmethodology – There is no nationally consistent method available. Methods vary significantly across NZ. Methods applied are often out of date and / or based on historic data that is more than 30yrs old.

Poor level of industry understanding of model output confidence and application results in poor / inconsistent decision making

Include confidence rating system within Phase 2 guidance development work scope.

Phase 2 (in scope)

Phase 1/2

Consultant Team (L+D)

3 Lackoflinksbetweencouncilledanddeveloper ledstormwatermodelling – Modelling exercises are undertaken by these organisations for differing purposes often using substantially different underlying assumptions and resolution levels.

Some regional methods (such as Technical Publication 108 / TP108) are applied inappropriately in other regions. International methods are often applied with no consideration of suitability for NZ conditions

Consider investing in development of a national rainfall-runoff methodology.

An initial step could be a summary of pros and cons / applicability of common methods. Different regions and applications may need to use different methods.

Phase 2 (out of scope) –Initial research

Phase 1/2

Consultant

Team (L+D)

Phase 3+full methodology

MfE (L)

WaterNZ (D)

Water Service

Entities (D)

Models are delivered for differing purposes, to vastly varying resolutions and it is often difficult for end users to interpret, understand and compare outputs

Define standard approaches for a range of applications and clearly link with the confidence rating system noted above to ensure comparability of outputs

Phase 2 (in scope)

Phase 1/2

Consultant Team (L+D)

Table 4-1: Gap Analysis

4 Modelvalidation/ calibration – There is a lack of guidance on how to validate models against flood incident or operational data. Where calibration data is available, there is often minimal guidance on target calibration thresholds (e.g. peak flow / volume degree of match)

5 Data collection – there are limited amounts of level and flow data measured and flood incidents consistently recorded during or immediately after a flooding event.

6 Jointorcoincidentprobability – Limited guidance on how to select and apply events in this context (particularly river and / or sea levels in combination with urban stormwater models plus application of climate change in this context)

7 Understandingwhyamodelisneeded and how it will be used – setting objectives and understanding end use.

Model outputs are often uncalibrated and / or unvalidated with limited links back to the reality they are aiming to represent. This can lead to misplaced confidence or unjustified reliance on outputs

There is not enough data to calibrate/validate/verify models.

Define standard workflows for validation and calibration along with guidance on appropriate thresholds

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

8 Scenariotesting – Inconsistent guidance on how & when to model a range of scenarios including blockages, rainfall spatial variability & duration and future landuse / development conditions.

Flooding predictions are over or underestimated resulting in a misunderstanding of risk because of poorly selected boundary conditions (also refer sensitivity analysis).

Model outputs do not meet the needs of end users. Methods or assumptions applied are not appropriate for end use.

Urban stormwater model schematisation needs to be tested for a range of existing operational scenarios and potential future conditions. This is often missed or inconsistently applied.

Highlight the importance of data collection and provide guidance

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

Select an appropriate good practice approach from available literature and replicate (or reference) in guidance document

Include content on model planning and schematisation in guidance document

Coordinate with ongoing Three Waters Code of Practice development.

Phase 2 (in scope) Phase 1/2

Phase 2 (in scope)

Consultant

Team (L+D)

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

DiA (L)

Phase 1/2

Consultant

Team (D)

Table 4-1: Gap Analysis

No. Gap Impact Action(s) Timing Ownership

9 Designstormselection – Within NZ there are inconsistent requirements, standards and levels of service (LoS) for primary and secondary stormwater systems.

Stormwater system LoS and flood mapping is often inconsistent between districts and regions. This creates confusion particularly in the land development sector where requirements are often incorrectly applied.

Coordinate with ongoing Three Waters Code of Practice development.

Phase 2 (in scope) DiA (L)

Phase 1/2

Consultant Team (D)

10 Existingspecificationsaretoo detailed – While is it noted that detail is required to create consistency, the lack flexibility to apply new knowledge or techniques limits innovation in the sector.

11 Floodplainmappingcut-off levels(theflooddepth atwhichnofloodingismapped)andapproaches for‘simplifying’orcleaningfloodmapoutputs –This varies substantially across regions within NZ.

End users do not benefit from application of innovative approaches in stormwater / flood modelling.

Guidance must not be too detailed and allow for innovation by focusing on confidence and outcomes rather than specified methods.

Phase 2 (in scope) Phase 1/2

Consultant Team (L+D)

Flood maps may appear more ‘detailed’ or flooding more extensive when comparing district or regional level information. Inconsistent flood extents are used for planning purposes between districts.

Guidance should clearly link model methodology / confidence with appropriate floodplain mapping cut-off levels

12 Designstormsvs.actualevents – There is limited guidance on how design storm and actual event modelling should be used. Also noted in this context that fundamentally different hydrological modelling approaches are needed when considering design and actual events.

Models are often built using design storms, then used to replicate actual rainfall events or actual events are used as design standards. This results in inconsistencies and potential inaccuracies in outputs

Guidance should clearly define appropriate uses for design / actual rainfall events and associated limitations.

Phase 2 (in scope) Phase 1/2

Consultant

Team (L+D)

Phase 2 (in scope) Phase 1/2

Consultant Team (L+D)

Table 4-1: Gap Analysis

No. Gap Impact Action(s) Timing Ownership

13 Levelsofserviceforhabitablefloorflooding - This varies substantially across regions within NZ.

Inconsistent drivers for investment in stormwater / urban flood management infrastructure between regions.

Coordinate with ongoing Three Waters Code of Practice development.

Phase 2 (in scope) DiA (L)

Phase 1/2

Consultant Team (D)

14 Freeboard – Allowances vary substantially across regions within NZ. Also noted that the concept of freeboard is often misunderstood and used inappropriately.

15 Generationanduseoffloodhazardrating - The definition and application of flood hazard varies locally and internationally.

Inconsistent requirements for development between regions.

Coordinate with ongoing Three Waters Code of Practice development.

Phase 2 (in scope) DiA (L)

Phase 1/2

Consultant Team (D)

16 Boundaryconditions – There is varying levels of detail within current guidance documents on consideration of and setting of key boundary conditions such as river levels, groundwater levels, sea level & sea level rise.

Flood hazard mapping and use of flood hazard data to assess potential flood impacts is inconsistent across NZ.

Model boundary conditions (initial and dynamic) are often poorly selected or set to default values without considering the impact of external influences. Also refer sensitivity analysis.

Guidance should clearly define flood hazard and hazard rating bandings based on current best practice.

Guidance should provide best practice workflow and checklists for setting boundary conditions.

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

Phase 2 (in scope)

Phase 1/2

Consultant Team (L+D)

17 Applicationofclimatechangetosealevelrise, riverlevels,groundwaterandrainfallprofiles –Most guidance documents provide content on these issues, but there is substantial variation in assumptions made. Many documents are out of date and apply superseded climate change guidance.

Inconsistent assumptions are made around climate change which result in outputs that cannot be compared to other studies or are inconsistent between regions.

Guidance should signpost / reference current best available guidance for climate change. The document could also include a flow chart for guiding appropriate selection of climate change scenarios based on purpose of model outputs.

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

MfE (D)

Table 4-1: Gap Analysis

No. Gap Impact Action(s) Timing Ownership

18 Modelandoutputs metadata – There is currently no standard for recording model or output metadata. Consistently recording metadata is essential for inter-organisational sharing and use of flood data.

Model and their outputs can be used inappropriately as the end user does not have easily accessible knowledge on key modelling assumptions or quality of input data. Data sharing is difficult as there is no consistent format for recording assumptions and limitations associated with the data.

Guidance should provide a minimum standard for metadata on models and outputs

It is noted that KāingaOrais currentlydeveloping a metadata standard for their own use. Collaboration with them and others could turn this into a national standard.

Phase 2 (out of scope)

Phase 1/2

Consultant

Team (L)

NIWA (D)

DiA (D)

19 Modelbuildlog/modelbuildreporting – There is no minimum standard for recording or reporting on key model build assumptions or methods.

Models cannot be re-used if key assumptions or methods have not been recorded and shared with the model.

Guidance should provide minimum standard for model build log components and build reporting (this should not be a standard template or table of contents – but a checklist of minimum requirements)

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

20 Sensitivityanalysis – This is well understood and applied for medium / large catchment scale models. It is poorly understood and often not applied for smaller scale models built to inform land development and / or infrastructure design.

Fundamental assumptions on key input parameters are not appropriately tested which results in significant under / over sizing of infrastructure or inappropriate setting of minimum floor levels.

Guidance should provide a standard workflow for sensitivity testing of key parameters that is scalable to the size / complexity / application of the model.

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

Table 4-1: Gap Analysis

No. Gap Impact Action(s) Timing Ownership

21 Groundwater/infiltration – The impact of infiltration and groundwater is difficult to model. There are no consistent practices or guidance available on how to schematise this in a stormwater model

Soakage and groundwater levels can have significant impacts of stormwater flows and flooding. This is often not well represented in modelling and can have significant impacts on results

Guidance should provide clear prompts and simple approaches to ensure this gap is recognised and addressed where appropriate

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

22 Modellingofstormwaterqualityandquantity managementdevices – There is no consistent guidance on how to approach modelling of stormwater management devices

Stormwater management devices are poorly represented in schematisation and models do not provide an accurate representation of their potential performance.

Guidance should provide clear prompts and simple approaches to ensure this gap is recognised and addressed where appropriate. Note this will be focussed on quantitymanagementdevicesas stormwaterqualitymodellingis currentlyoutsidetheworkscope.

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

23 Inconsistentlanguageandpoorunderstandingof commontechnicalterms(ARI,AEPetc) – Both practitioners and the public often confuse or misinterpret these terms.

Model results are poorly communicated and / or misinterpreted by end users including local government, central government and the public

Work with MfE to agree improved terminology and use throughout guidance document (if available within the timeframe of the project)

Phase 2 (out of scope) /

Phase 3+

MfE (L)

Phase 1/2

Consultant

Team (D)

Table 4-1: Gap Analysis

No. Gap Impact Action(s) Timing Ownership

24 Residual risk – Current guidance and specification documents have requirements for primary and secondary system performance, but not many explicitly consider residual risks (those events which exceed the design LoS for secondary systems). This also links with use of probable maximum precipitation or very rare extreme event (>250yr ARI) analysis for understanding impacts when events exceed secondary system LoS.

25 Minimumqualityrequirementsforinternal/ external(peer)reviewofstormwatermodels:

There are no consistent minimum quality assurance requirements for build of stormwater models.

26 Lack of national direction and coordination for floodriskmanagement: A complex mix of legislative duties and no clear direction from central government means flood risk management responsibilities are often poorly understood and variably implemented.

Residual risk is often not considered in planning or hydraulic modelling processes for stormwater. This could result in poor design of stormwater management structures that do not consider risk or impact of complete asset failure or exceedance.

Guidance should provide clear prompts and simple approaches to ensure this gap is recognised and addressed where appropriate.

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

The lack of quality assurance methods or requirements leads to significant variability in quality and reliability of stormwater models.

Guidance should provide minimum quality standards in the form of checklists, thresholds for peer review and / or review guidance.

Phase 2 (in scope)

Phase 1/2

Consultant

Team (L+D)

This gap leads to an inconsistent approach to flood risk management and associated activities across territorial authorities, central government departments, crown agencies / entities and land development organisations.

The MfE aims to address this gap through the Natural and Built Environment Act (2023) and associated National Planning Framework.

Out of scope for this project – but in progress through other initiatives

MfE (L)

4.3 Future work required

Table 4-2 provides a prioritised task list for future work packages. The list consists of Phase 2 (out of scope) and Phase 3+ items from Table 4-1. Table 4-2 expands on these gaps with a justification for further work, next steps, suggested timing and recommended leadership (L) / delivery (D) support organisations.

Table 4-2: Future work required

Priority Gap Justification & Next Steps

1 (2) Rainfallrunoff methodology

Justification

There is no nationally consistent method available. Some regional methods (such as Technical Publication 108 / TP108) are applied inappropriately in other regions. International methods are often applied with no consideration of suitability for NZ conditions. Feedback suggests this should include:

- Design storm generation

- Standardised runoff parameters for common soil types (noting there will be a need for some regional variation)

- Standardised runoff generation methodologies for a range of scales and applications

Next Steps

Phase 1 work to date has already identified most rainfall-runoff methodologies commonly used in NZ. It would provide good value for money to the project sponsors to add this to the current work scope and deliver an initial research summary. The work could then be easily progressed at a later point by MfE and WaterNZ with funding provided by the Water Service Entities and / or the DiA.

Timing Ownership

Phase 2 (out of scope) –Initial research

Phase 3+full methodology

Phase 1/2

Consultant Team (L+D)

MfE (L)

WaterNZ (D)

Water Service Entities (D)

DiA (D)

NIWA (D)

Priority Gap Justification & Next Steps

2 (18) Model andoutputs metadata

Justification

There is currently no standard for recording model or output metadata. Consistently recording metadata is essential for interorganisational sharing and use of flood data. Through the consultation work completed in Phase 1, it was noted that Kāinga Ora and their consultants Tonkin + Taylor are producing a stormwater modelling guidance document and considering developing metadata standards to support it.

Next Steps

Work with Kāinga Ora to determine which other local, regional or central government bodies are considering a similar standard. Work with these parties to deliver a basic standard metadata format for use nationally.

3 (23) Inconsistent languageand poor understanding of common technical terms(ARI, AEPetc)

Justification

Both practitioners and the public often confuse or misinterpret these terms. MfE have recognised this requires addressing, but this may not be within the timeframe of Phase 2 of this project.

Next Steps

The research and engagement completed to date forms a useful input to this discussion. The Phase 1/2 consultant team and the Advisory Group have a diverse knowledge base that could proactively support this initiative being led by MfE.

Timing Ownership

Phase 2 (out of scope) Phase 1/2 Consultant Team (L) NIWA (D) DiA (D)

Phase 2 (out of scope) / Phase 3+

MfE (L) Phase 1/2 Consultant Team (D) Advisory Group (D)

Table 4-2: Future work required

Table 4-2: Future work required

Priority Gap

4 (22)Modelling of stormwater qualityand quantity management devices

Justification & Next Steps Timing Ownership

Justification

While this study can address some of the gaps around modelling hydraulic behaviour of these devices, the modelling of quality performance is not within the scope. Water quality performance modelling is generally completed by a similar, but separate suite of software tools compared to urban stormwater models. However, there is no consistent guidance locally, regionally or nationally within NZ for undertaking water quality modelling.

Next Steps

We recommend that WaterNZ survey it’s member base to determine the potential need and scope for this type of guidance. Assuming sufficient interest exists, a subSpecial Interest Group to the Stormwater SIG could be formed to move the work forwards.

5 RECOMMENDATIONS

5.1 Need for national guidance

The engagement and research completed of Phase 1 of this study has shown there is a clear mandate from the wider industry for national stormwater modelling guidance. Both industry surveys (July 2020 and April / May 2023) showed more than 95% of respondents (approximately 110 people across both surveys) supported the development of national guidance. Similarly, the majority of attendees across all workshops run for this work have supported the need for national guidance. The key themes for guidance content that have emerged across these engagement activities include:

• Consistent rainfall / runoff methodologies

• Model confidence benchmarking with links to how certain methods should be applied

• Deciding when a model is needed and what type is required

• Consistent terminology for rainfall event description and flood hazard description

Phase 1 has also considered the format of the guidance document and included this for discussion in the more recent workshops. Opinions are generally split between a static digital document (PDF) and website format. The key issues that need to be addressed by the formatting are version tracking and usability.

We recommend that this study progresses to Phase 2 to develop a national stormwater guidance document in the following general form:

• Basic structured website

• Functionality must include print to PDF options (or a parallel master document maintained with the same content that can be downloaded)

• Dedicated URL (not a sub-domain of the WaterNZ website)

• Hosted & maintained by WaterNZ (funding model will be required)

• Version tracking is actively managed using a combination of proprietary website tools and / or PDF records (similar approach to the International Infrastructure Management Manual platform managed by IPWEA)

5.2 Phase 2 Work Scope

The recommended work scope for Phase 2 is summarised below. This is supported by an unpublished (due to commercial sensitivity) document that details task breakdown, fees and programme for Phase 2

• Technical content is developed generally in line with the structure detailed in Appendix A of this document (noting that the structure remains subject to further discussion and development) within the agreed programme and budgetary constraints of the project.

• Content development will be prioritised based on the key gaps identified in Section 4.1 of this document. All sections within the proposed structure will have basic content as a minimum, but only priority areas identified by the gap analysis will be developed in detail.

• A range of approaches will be used to deliver the content including:

o Flow diagrams

o Tables & figures (either generated for this document or adapted from other best practice documents)

o Signposting to other key documents (to avoid repetition and need for future updates when external documents are superseded)

o Case studies demonstrating best practice approaches

• The themes guiding content will be:

o Conciseness

o Accessibility for a range of end users

o Ease of maintenance / updates / addition of future content

• Select and purchase appropriate URL, then develop website themes, wire frames and key functionality (navigation, version tracking and PDF support)

• Populate website with developed content, complete technical reviews & functionality checks and publish draft for targeted industry feedback

• Receive and collate feedback, make updates as required and publish version 1.0 of guidance

• Develop and agree a cycle for regular maintenance and triggers for update of the guidance document (potentially supported by the WaterNZ Modelling and Stormwater SIGs)

It is recognised that not all content will be fully developed within Phase 2. Section 5.3 summarises where future work packages will be required to complete the guidance document in the future. This aligns with the purpose of Phases 1 and 2 of guideline development – to provide an overall framework, but not necessarily all the detailed components due to programme and budgetary constraints.

We recommend the tasks listed below (in order of priority) are added to the Phase 2 work scope to deliver efficiencies and value for money for the project sponsor (DiA) and end users. Further details on these items are provided in Section 4.3.

1. Collate current approaches used within NZ for rainfall-runoff estimation to inform future work in this area

2. Develop a basic metadata standard for flood models and flood model outputs (including identifying and working with relevant partner organisations)

3. Work with the MfE to address issues around inconsistent language and poor understanding of common technical terms associated with extreme rainfall, flooding and flood mapping

5.3 Future Work Packages

We recommend that the prioritised list of work packages below are delivered in the next 3-5yrs to support and update the national urban stormwater modelling guidance document. Further details on these work packages are provided in Section 4.3.

1. Develop a NZ standard methodology for rainfall-runoff estimation

2. Support MfE to address issues around inconsistent language and poor understanding of common technical terms associated with extreme rainfall, flooding and flood mapping – including updating the guidance to align with agreed terminology as required

3. Determine the need, scope and support for developing national water quality modelling guidance

REFERENCES

Meeting the challenges of future flooding in New Zealand, MfE, August 2008 Meeting the challenges of future flooding in NZ_FINAL (environment.govt.nz)

Preparing for future flooding, A guide for local government in New Zealand, MfE, May 2010 flooding book_300410.indd (environment.govt.nz)

Resource management system reform | Ministry for the Environment

Resource Management Act 1991 No 69 (as at 13 April 2023), Public Act Contents – New Zealand Legislation

A guide to preparing a basic Assessment of Environmental Effects, MfE, A Guide to Preparing a Basic Assessment of Environmental Effects [Ministry for the Environment]

Overview of the resource management reforms, Ministry for the Environment, https://environment.govt.nz/what-government-is-doing/key-initiatives/resource-management-systemreform/overview/

Soil Conservation and Rivers Control Act 1941 No 12 (as at 28 October 2021), Public Act Contents – New Zealand Legislation

APPENDICES

Appendix A – National Stormwater Modelling Guidance Structure

The current working structure of the guidance document is presented on the following pages. It should be noted that the final delivery format is proposed to be a website and structure / content may vary to accommodate this.

Appendix B – Literature Review Details

specific hydrological method specified, but needs to generate a full-flow hydrograph rather than just estimate peak flows and be agreed with NRC prior to modelling studies. Rain on grid models to have infiltration applied.

and cover different topics for different audiences.

High level document that does not include specific ecommendations.

Model design appears to focus on catchment scale planning, not for property level flood hazard mapping and optioneering.

Description of and justification for hydrological loss approach is provided.

Overall structure provides for technical and non-technical audiences. Generalised guidance with specific implementation guidance for MIKE FLOOD kept separate. Standards and recommendation section at the end of the document as a reference for specific guidance.

standards sectioncomprehensive No urban guidance. No urban guidance. Focus on stop banks and rural infrastructure. Blockage and freeboard section. Based on ARR 2016.

Based on BOPRC guidelines and local studies by NIWA. HIRDS rainfall augmentation is applied. Specific to TCC's online flood mapping needs.

100yr Historic rainfall (HIRDS) with 20% increase for Climate Change. (Analysis in line with RCP 6.0). Currently considering approach to update this method. No formal joint probability. (though some analysis currently being done for Hutt River). 1m sea level rise for climate change at 2115 horizon.

System performance (data quality, primary system performance and quantification of buildings flooded for different events) and flood depth maps is standard output. Maps required for each study need to be confirmed with WWL.

Section describing use cases for models. Detailed models with all public pipes and sumps. Some significant private network included. Detailed specifications on how to represent each component.

Detailed models with all public pipes and sumps. Some significant private network included. Detailed specifications on how to represent each component. 10yr ARI rainfall minimum (curve numbers not representative below this) to design primary drainage. One council requires 25yr ARI design standard.

Upper catchment hydrological method not specified. Detailed specifications for urban areas.

Buildings with direct connections represented in hydrology (sub catchments) and assigned to nearest network. Lumped sub-catchment approach using SCS Curve number calibrated for local conditions. Simple rain-on-grid for early model planning with infiltration losses aligned with SCS method loses. (but not practically implemented often).

Guidance focuses on public system from stormwater mains to catchpits and soak pits. Gives limited guidance on connection of private systems. A number of parameter sets are pregenerated for use across Tauranga for model consistency.

Large buildings voided out of the mesh, roads have reduced roughness, each sump has a mesh zone to match 1D and 2D. 50yr ARI and 100yr ARI rainfall for secondary system.

Specific guidance for Tauranga based on local rainfall studies.

Timeseries for Rainfall and Tide provided as a package in software specific format. Guidelines discuss intent of representing catchment interactions and some of these are provided and updated as models are updated.

MHWS as BC (oscillating tide that peaks at 1.1mRL, small allowance for reduced barometric pressure. Most catchments fully defined to the ridge lines. Rivers and streams hydrographs provided by regional council models. 2D tidal boundary. Neighbouring catchment overland flows accounted for via a 2D surface boundary condition.

General guidance provided with definitions for calibration/validation/verification. No specific measures for quality of calibration. No guidance on sensitivity assessments.(done as required).

Guidance on folder structure, file naming, model archiving, metadata. Brief guidance on model build report and project model reports. Simulation scenario requirements and guidance on outputs based on TCC's foreseen requirements. Hazard defined as DxV.

Some guidance does exist if gauges available with target variance, but most models are validated using flood incident data. Flood maps used to engage with public, which enhances model confidence.

Section on model maintenance and model type categorisation.

section has useful sections to consider before embarking on a modelling project. Process for updating existing model with new information. Guidelines for Stormwater Modelling, April

Required scenarios are specified. Comprehensive guidance on Freeboard and Sensitivity. Sensitivity targeted to the catchment. E.g. 1.5 100yr ARI + CC and targeted blockage scenario. For tidal catchments - no tide scenario. Freeboard outputs from sensitivity tests to select freeboard, then apply to model (dynamic freeboard). Outline of report structure in guidelines.

None

Document version control table. Stormwater modelling process diagram (Figure 1).

Joint probability: rainfall to tide AEP correlation should be 0.1 (Section 5.3.2).

MfE (2008) guidance is recommended (2100 horizon with 2 degrees temperature increase for a 16% increase in rainfall depths).

Some mapping specifications relating to depths and flood hazard (NSW Floodplain Development Manual, 2005).

List of model uses provided to define purpose of stormwater modelling provided at high level without explicit specification of thresholds/levels of service/etc.

WWDG: Part B Chapter 21 provides specifics of the hydrology approach. Only a brief summary is added. Attachments include hydrological and tidal analysis from other sources.

Results of infiltration conceptualisation and parameter test presented with recommendations for model parameters provided.

Very brief without specifying particular modelling approaches. Blockage is not to be represented.

Detailed summary of modelling approaches for urban drainage structures with parameter values used (for example sump inlets).

MIKE FLOOD specific parameters and implementation details.

Very brief summary of boundary conditions to apply. Description of boundary conditions included in attachments.

Sensitivity testing for only roughness and boundary conditions is suggested. Sensitivity should be investigated if calibration is not possible.

Calibration mentioned, but it appears that more of a validation was carried out and there does not appear to be a comprehensive quantitative analysis.

Calibration scenarios with sensitivity of infiltration parameters are briefly described.

High level requirements for model delivery.

Very brief section on ownership of models. Flood damage approach described in brief with datasets available from CCC. Provides a comprehensive example of model build with details of fixes applied to workaround issues. Flow diagram in attachment used for scenario definition. Model component naming conventions and flagging are defined. Includes a "Compliance with Specification" section.

Appendix A3 - United Kingdom Literature Review

Flood Modelling Guidance for Responsible Authorities

Scottish Environmental Protection Agency (SEPA)

https://www.sepa.org. uk/media/219653/floo d_model_guidance_v 2.pdf

Guidance for modelling accompanying flood risk studies conducted in Scotland. Provides Scotland-specific examples.

Figure 2-1: Document layout (pg. 9); Each section starts with a 'key points' summary box;

Figure 8-1: Sources of uncertainty (pg. 91)

Fig 14-1: QA activities by responsible party (pg. 117)

CC uplift based on UKWIR study (Rainfall Intensity for Sewer Design, 2015). ~50% for 2080s

Joint probability analysis according to DEFRA EA Project FD2308 Joint probability: dependence mapping and best practice

Use 0.1m depth threshold on flood rasters. Also postprocess to remove dry islands and puddles (area threshold is judgement based, SEPA maps use 200m2)

- Need watercourse cross-sections at key points (commission surveys if needed)

- Also all hydraulic structures e.g. weirs can be surveyed along with watercourse

Data: use rain gauges and radar for calibration. Identify calibration events based on data availability across gauges. Rainfall model recommended: FEH2013, use summer storm for urban

Runoff rates:

• 100% of the urban area is impermeable

• Urban runoff rate of 70% Drainage loss allowance: 12mm/h

Downstream boundary condition for river reaches: stage hydrograph, flow hydrograph, single valued rating curve, normal depth or critical depth boundary

- Sensitivity testing for strategic models, calibration for catchment and local models

- Minimum of 3 calibration events and one validation event is recommended.

Required scenarios: 10, 30, 50, 100, 200yr; 200yr+CC

Four documents required with each model: technical, non-technical, model handover, and model audit reports Map outputs required: extent and depth, hazard, velocity and flow direction. Preferably as shapefiles/rasters (no geodatabases)

Hazard formula: d(v+1.5)+DF (from Defra report FD2321/TR1 Flood Risks to People, HR Wallingford 2006)

For GeoTIFF, use -9999 as nodata value

For 1D models, tables of water elevation and flow at each 1D node should be requested.

- Intellectual property rights and licensing - Available datasets (need to consider document update implications of including this) - Common modelling problems - NFM

Integrated Urban Drainage Modelling Guide

Chartered Institution of Water and Environmental Management (CIWEM)

https://www.ciwem. org/assets/uploads/I UD_1.pdf

Detailed document covering model purpose, build, use, and maintenance.

- Each section begins with a block diagram indicating which stage of the modelling process the section covers (e.g. pg. 20)

- Table B1 - 3 (pg. 122): data collection needs by model complexity level

- Table B3-1 (pg. 133), summarising model concept types

- IUD models are split into four types, and each section gives guidance on modelling each of the four types

Joint prob: Agree on need for it during scoping - otherwise just run really low AEP events. Refer to HR Wallingford guidance paper (linked in cell B46). May be necessary to create inflow hydrographs for a range of storm durations rather than just the critical.

CC: Follow govt guidance, uplift not provided within modelling guide as numbers frequently updated

- Post-process out shallow depth areas (agree threshold during scoping)

- For hazard mapping, agree upon formula during scoping

Survey culvert inlets. Do not model gullies unless there is significant overland flow or if inlet capacity assessment is needed. How to represent buildings: stubby building, removal from model, thin wall, or void. Whether and how to model small linear features affecting flow (e.g. fences, hedges). Include any openings through embankments (e.g. culverts).

- First understand catchment - delineate boundary, understand parameters influencing runoff generation (land use, soil type, etc.). Avoid double counting inflows.

- Provides overview of runoff routing methods (fixed, NewUK, Wallingford, UKWIR for urban; additionally SPR, ReFH, Horton, Green-Ampt, SCS and PDM for green spaces).

- 1D-2D coupling: for manholes based on weir equation usually but if using head-discharge then ensure suitable parameters used

- Emphasizes the importance of accurately representing culvert inlets and provides guidance on inlet loss parameters (based on culvert type and material)

- If gullies need to be modelled, apply headdischarge relationship. This depends on gully characteristics guidance is given on common gully types.

- Check outfall locations against fluvial flood maps and tide levels (might be a level hydrograph)

- For any watercourses, need inflow hydrographs for upstream

- Watercourse response to rainfall - if time of concentration similar then integrate, otherwise model separately

- Sensitivity tests suggested: roughness, downstream boundary conditions, inflow hydrographs, infiltration losses, runoff coefficients, structure representation and coefficients, pipe blockage, storm duration, joint probability

- Calibrate model using observed catchment conditions and gauges where available

- Need Model Build and Verification

Report (suggested content: purpose, catchment description, issues, previous studies, naming convention, modelling, data collection, model development, hydrology, verification, sensitivity, conclusions, scenarios, QA)

- Model logs also recommended

- Simulation output requirements may influence software selection

- Consider output file sizes if needed

- Guidance on level of detail required and when to use 1D, 2D, or 1D-2D

No consideration of CC. If model outputs are sensitive to antecedent soil moisture conditions, then the joint probability of combinations of rainfall event and antecedent conditions must have been assessed (no guidance provided on how this must be done).

Model grid with minimum 5m resolution for rural and 2m resolution for urban areas

Must account for effect of subsurface drainage by including, at minimum, an implicit allowance for drainage loss. Recommended approach is explicitly modelling pipe network.

Run model for range of rainfall durations and combine to produce map of worst-case flooding outcome for each model cell. Document provides minimum and recommended hydrology parameters. Key ones are below:

Rainfall depth Use FEH13, or derive depth from very long synthetic rainfall series produced by stochastic rainfall generator, calibrated to local observations.

Temporal profile Test with both FEH 50% summer and 75% winter profiles

Urban hydrology: Fixed rainfall losses based on land use and divided according to specific ground surface and the condition of each surface.

Losses Time-varying based on soil types

No guidance on hydraulics parameters or representation

Do not account for flood barriers defences, high tide, or high river levels at outfall.

Confidence scoring criteria provided based on data input and resolution. These are translated into a 'Suitability' GIS layer.

2-5m resolution grid of maximum depth, velocity and hazard with corresponding attribute table. Produced for 30yr, 100yr, 1000yr events. Also need flow direction at time of max velocity.

N/A

on real world data for all hydrology parameters (rainfall, evaporation, infiltration, etc)

Use of rain gauge data strongly preferred.

storms not automatically supplied but can be inputted. Spatial distribution: Use multiple gauges or radar.

averaged (e.g. Thiessen weighted) only as last resort Runoff routing: Subcatchments, with percentage pervious and impervious user defined

Provides details on different types of nodes and links available within the software package

External inflow to specific nodes of the network (subcatchment runoff, groundwater discharge, infiltration inflow)