Protecting Strawberry Creek from urban runoff with low impact development This study estimates the current and predevelopment hydrological conditions of the site using ArcGIS, determines the major stormwater-related problems of the study site, and proposes an LID practice to best protect the stream habitat.
Manon von Kaenel
Environmental Sciences & Geography, UC Berkeley 2015
Introduction
Overview of Approach
THE PROBLEM: Urban development often leads to “urban stream syndrome,” symptoms of which include: increased discharges during flood events, reduction in groundwater recharge, higher concentrations of pollutants and nutrients, erosion and channel enlargement, increased temperature, and reduced biodiversity in local creeks. Conventional stormwater infrastructure often fails to address these and other impacts of developed impervious areas and the associated surface runoff on urban stream health and vitality.
ArcGIS Hydrology Model
THE SOLUTION: Low Impact Development (LID) has become a popular alternative stormwater management tool. LID uses simple design features such as vegetation, natural slope gradients, and various forms of pervious surfaces to mimic pre-development hydrological regimes and mitigate the harmful effects of urbanization on local waterways.
FLOW ACCUMULATION: I constructed a base flow accumulation model of my study site using hydrology tools on ArcGIS. I also calculated six parameters to describe the hydrology of my site under current and predevelopment conditions for a 2-year, 24-hour design storm. FLOW-PERMEABILITY SCORES: I mapped the relative effect of surface permeability on runoff infiltration by adding flow accumulation scores to the permeability scores (based off of the Soil Curve Number) of the underlying surface for both current and predeveloment conditions. WATER QUALITY TESTS: I tested runoff and creek samples for color, copper, zinc, conductivity, phosphorus, nitrates, chloramines, and pH to estimate pollutant loads.
LID Selection and Evaluation
Site Description > 30.7 acres, runoff drains into upper North Fork of Strawberry Creek. > History of heavy metal pollution from larger, predominantly residential North Fork watershed. > Major stormwater infrastructure system leads to first major discharge outlet on creek at southwestern corner.
LID PHILOSOPHY AND SELECTION PROCESS: The LID site design process can be broken up into five steps: site assessment, definition of goals, implementation of LID principles, use of LID Best Management Practices (BMPs), and evaluation of design. Using this framework, I selected an appropriate LID solution to the prolems identified by my model and site assessment. SIZING AND LOCATION: I used a weighted suitability analysis on ArcGIS (with constraints of: slope, distance to existing development, and opportunities of: low surface permeability, high flow accumulation, and surface type) to select a location, and the water quality volume of the chosen location’s watershed to size my LID. EVALUATION: I estimated the changes to the study site site caused by the LID installation by reducing the Soil Curve Number of the area pertaining to said installation and re-running the various permutaitons of the ArcGIS Hydrology model. I compared these impacts to the goals outlined in UC Berkeley’s Phase II Small MS4 General Permit and in other case studies.
The Problem: the impacts of urbanization on runoff > An increase in impervious surfaces has led to more voluminous and faster-flowing runoff.
Table 1. Hydrological Parameters
> The heaviest runoff occurs over University Drive along the northern edge of Memorial Glade. > Stormwater here exceeds acidity, zinc, and copper standards for aquatic life protection. > To return at or below the threshold for ecological well-being based on impermeability, the campus would need to remove about 14 acres of impervious surfaces from the site. *The CN for oak forest **Calculated for 24-hour 2-year interval precipitation of 2.3 inches
Figure 2. Flow-Permeability Scores
<< The darkest blue marks areas with both high flow accumulation and low permeability. These are concerning because a storm event here will produce high runoff with little infiltration capacity - leading to fast-flowing, potentially pollutantladden overland flow. The areas with the lowest flowpermeability score, in yellow, indicate both low flow accumulation and high permeability.
Figure 3. Violations of water quality standards in runoff
<< By the time storm runoff reaches the creek (around site A), it will already exceed EPA’s national standards for the protection of freshwater aquatic life for zinc, copper, and acidity. Heavy metals originate from vehicles, roads, and roofs, and can be toxic to aquatic organisms. High acidity can alter water chemistry and affect biologiocal processes of aquatic life.
The Solution: permeable pavement as a Low Impact Development installation > Permeable pavement on University Drive to the north and west of Memorial Glade can: slow the rate of runoff via infiltration, reduce the heavy metal concentration through filtration, and disconnect impervious areas. Unit pavers or brick on sand is the most appropriate kind.
Table 2. Impact of permeable pavement on current conditions
Figure 4: Proposed LID locations
> Impacts to current hydrological conditions from proposed LID installations (Fig. 4) include a minimal reduction in runoff volume and a slight increase in overland flow time (Table 2). > Other potential Low Impact Development practices include: low impact landscaping around the Memorial Glade drain to filter out lawn-related contaimants; permeable parking lots by Haviland Hall and McCone Hall to filter and infiltrate runoff; and screening filters at storm drains to remove pollutants.
Conclusions: Last Thoughts
**Calculated for 24-hour 2-year interval precipitation of 2.3 inches CN of permeable pavement is 45 (BASMAA 1999)
Acknowledgements and Key References
NEXT STEPS: Before installation, an economic cost-and-benefit analysis should be conducted. After installation, careful monitoring should track the performance of the LID. FUTURE DIRECTIONS: This approach is most applicable to small urban subwatersheds with limited structures (such as Faculty Glade on the UC Berkeley campus). Future research should address the behavior of groundwater flow and drainage through existing stormwater infrastructure of the study site, perhaps using one of many alternative stormwater models.
Chancellor’s Advisory Committee on Sustainability (CACS) Green Fund Grant Office of Environmental Health and Safety UC Berkeley Environmental Science, Policy, and Management Department Hans, K., Maranzana, S. 2007. University of California, Berkeley Strawberry Creek Hydrology – 2006 Status Report. Office of Environmental Health and Safety. 2006 Status Report. University of California at Berkeley, Berkeley, CA, USA. BASMAA 1999. Start at the Source: Design Guidance Manual for Stormwater Quality Protection.
manon_vonkaenel@berkeley.edu cacs Chancellor's Advisory Committee on Sustainability