The Engine Room Vol 2 Issue 1

E d i t o r i a l

Barbados floods too easily. It appears to be quite common in recent times that when the rain falls, it automatically floods, and these floods cause schools to close, the public bus service to be cancelled, roads to be blocked (triggering massive traffic jams), road damage and land slippage and a whole heap of other inconveniences

This disruption to the normal way of life for a few inches of rain is problematic particularly in an island that has increased rainfall during its hurricane season for at least 6 months of the year, very dependent on its dense road network for transportation of goods and people. This seems to be different to 20 or 30 years ago.

In addition to the inconveniences mentioned previously, flooding can cause widespread damage to physical infrastructure, house-hold contents and cause injury and loss of life. Recently, the United Kingdom, where 1 in 5 live in flood risk areas, experienced significant floods in York, Shrewsbury, Somerset and other areas. In January 2023, in Italy, EmiliaRomagna floods left about 15 people dead and forced 13,000 from their homes due to intense rainfall (reportedly, half of their annual rainfall fell in 36 hours) - a true freak event. Closer to home, in our sister island, St. Vincent and the Grenadines, on Christmas Eve 2013, torrential rains caused flooding (and landslides and collapses of bridges), which all resulted in 8 fatalities and several injured.

We simply cannot afford to lose precious time and resources because of a failure to act on potential flood mitigation.

Drainage and flood management should be seen as a crucial cog in the wheel of civil works infrastructure in any country Especially when there are major changes to the built environment to affect the natural flow of surface run-off Flooding is a serious issue and it is an engineering issue which means that it must be met with an engineering solution.

It is apparent that we have long-standing runoff problems in certain areas of this countrye g , Goodland, Christ Church and Woodbourne, St. Philip. We know that it would take some resources to remediate those issues - but it must be done; and sooner rather than later

It is time to take a holistic look at integrated flood management as a means to achieve our goals of mitigating inundation. Consideration should be given to island-wide topographical surveys (done by Lidar drones), watershed modelling and analysis (carried out by professional registered engineers), the collection and distribution of the latest rainfall data (by the relevant collection agencies) and the creation of bespoke drainage policy that would give guidance and instruction to the development of lands in critical areas (by the relevant government departments - i.e., Drainage Division in the Ministry of Transport, Works and Water Resources) The approach must be multidisciplinary, multi-sectoral and a collaboration between the public and private sector.

Despite Barbados facing drought conditions currently, it would not be wise to ignore these matters since we are entering our "wet" season.

We do not have the time to delay this matter

Photo 1 - Andrea Ricci, 26, sat exhausted on a dingy that he used to help deliver goods and water to people in the flooded district of Lugo, Italy

Credit:

https://www.nytimes.com/2023/05/27/ world/europe/italy-floods-emiliaromagna.html

Credit:

https://floodlist.com/america/caribbean -life-as-we-know-it-at-serious-riskexpert

Photo 2 - Flood Damage in Dominica

The Engine Room is the resurgent enewsletter of the Barbados Association of Professional Engineers (BAPE). Its goals include providing interesting and informative articles to its readership, as well as giving a voice to those who wish to contribute. A ship’s engine room is the place where the input is converted to its output, thus giving it power to travel to its destination; and this e-newsletter draws inspiration from this analogy - a driving force for our society. To create a space for engineers and others to debate, to opine, to share ideas and to collaborate for the better of engineering and the wider society, for this generation and for future generations to come.

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President’s Column

The vulnerability of Caribbean countries to natural hazards, and particularly climaterelated hazards, is well known The Region’s population is highly and increasingly urbanized, much of the key and critical infrastructure is coastal, and many settlements and the infrastructure serving them are low-lying or located in mountainous terrain Hydrometeorological hazards, such as extreme rainfall events, floods, tropical storms, hurricanes, and landslides, impact the Caribbean with noticeable frequency. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (AR6) notes that due to their limited size, resource constraints and limited connectivity, the risks from climate change impacts and risks are amplified for small islands, and small islands present the most urgent need for investment in capacity building and adaptation. Strengthening our capacity to manage the effects of high rainfall events is one such urgent need

Understanding rainfall and its likely impact on any location in our physical environment is essential for the safe and efficient provision of infrastructure to serve the social and economic needs of our community This applies to roads, storm drains, housing and any other land development, including for agriculture. Too often, due to a combination of factors, infrastructure is inadequate to manage the volume of rainfall experienced, contributing to flooding and attendant risks to life, health and property.

Updated rainfall intensity-duration-frequency curves (IDF) are not available for most Caribbean countries IDF curves are a basic tool used to provide rainfall parameters appropriate for the design of drains, culverts and other infrastructure, according to applicable building codes or relevant design reference documents IDF curves are prepared from statistical analysis of rainfall data.In their 2011 paper[1] on the challenges experienced in attempting to produce IDF curves for five Caribbean countries, a consulting team comprising specialists from HR Wallingford Ltd. and the Caribbean Institute of Meteorology and Hydrology noted the decreasing number of rainfall stations over past decades, the lack of long records of short-duration rainfall data, and the inability to obtain data from national meteorological and hydrological agencies among the main limitations A recent study[2] of 22 hydromet agencies in the Caribbean found these agencies to have key capacity gaps in the technical production, translation, transfer, and facilitation of the use of climate information, with 17 having low or moderate capacity to perform climate data rescue or data mining

The consequence of the lack of capacity in this particular area can be a lack of confidence and rigour in the design of some of the drainage and flood management systems in the Caribbean because of the use of generic data and assumptions in their design, or the use of IDF curves from other countries or nearby locations for which topography or other critical characteristics might be different. Where IDF curves are not available, engineers engaged on some larger development projects perform the statistical analysis on the data available to determine the design storm parameters for those individual projects, an approach likely to be sub-optimal. With limited financial resources available, it is important to ensure that funds allocated to infrastructure are used efficiently, contributing to increased resilience.

Designs should be based on data and engineering approaches supported by national building and disaster management regulations. In the aftermath of the deaths, damage and losses caused by Hurricane Sandy, the reliance on outdated IDF curves to assess hydrological impacts on infrastructure design in the USA was highlighted as a flaw, as projections in the New York area were for the events then classified as 100-year events to occur once every 35 to 55 years by 2050[3] Similar observations made in Europe have led to adjustments in the engineering design standards to account for climate change, one change being a reduction in the frequency with which flooding from storm drains is permissible in various spatial settings[4]

AR6 did not identify a significant long-term trend in rainfall in the Caribbean over the period 1901 to 2012, but projected the Eastern Caribbean would be slightly wetter over the period to 2100, with more extreme seasonality and significant local variability, with slightly fewer storms continuing to be the main driver of flooding in the Lesser Antilles A number of remote sensing precipitation products are available around the world based on satellite data, atmospheric physics models and other sources

A 2022 study of the performance of five of these datasets when applied to rainfall in the Greater and Lesser Antilles [5] found one dataset, Multi-Source Weighted-Ensemble Precipitation, to perform satisfactorily for hydrological applications on a watershed scale, though when compared to rain gauge data, the correlation was significantly weaker than observed in other parts of the world

The availability of IDF curves for Caribbean countries, and sub-regions of those countries with different rainfall characteristics, is therefore key not only to the efficient performance, by engineers, of routine drainage design, but to strengthening the resilience of communities to the effects of hydrometeorological hazards and climate change Development of IDF curves for countries for which they don’t exist, and updating and refinement of IDF curves in other countries should be seen as a priority in adaptation to the current climate crisis

[1] Lumbroso, D., Boyce, S., Bast, H. and Walmsley, N. (2011) The challenges of developing rainfall intensity-durationfrequency curves and national flood hazard maps for the Caribbean The Journal of Flood Risk Management, 4 (1). pp. 42-52.

[2] Mahon, R., Greene, C., Cox, S.-A., Guido, Z , Gerlak, A K , Petrie, J -A , et al (2019) Fit for purpose? Transforming National Meteorological and Hydrological Services into National Climate Service Centers. Climate Services, 13, 14-23. https://doi org/10 1016/j cliser 2019 01 002

[3] Wagner, M , Chhetri, N , & Sturm, M (2014) Adaptive capacity in light of Hurricane Sandy: The need for policy engagement. Applied Geography 50: 15–23.

[4] Kundzewicza, Z W and Licznar, P (2021) Climate change adjustments in engineering design standards: European Perspective. Water Policy Vol 23 No S1, 85 doi: 10.2166/wp.2021.330

[5] Bathelemy, R , Brigode, P , Boisson, D , Tric , E , (2022) Rainfall in the Greater and Lesser Antilles: Performance of five gridded datasets on a daily timescale. Journal of Hydrology: Regional Studies, doi org/10 1016/j ejrh 2022 101203

In this Issue

In this issue of The Engine Room, we are delighted to bring to you a number of technical articles touching on the broad topic of Stormwater Design & Management

It is our belief that this is an important topic that must be discussed broadly, properly understood, and actioned accordingly

In our Fellow's Corner, Eng Andrew Hutchinson speaks to not only design considerations for drainage, but to analysis and methodology; and does a review of local stormwater drainage practice Eng Justin Jennings-Wray provides timely information that would be useful to our civil engineering design community by delving into three methods for drainage design - rational method, the modified rational method and the TR55 peak flow method (including their advantages and limitations) Importantly, we have crucial input from the Caribbean Institue of Meteorology and Hydrology, through a contribution from their Shawn Boyce and Kayshawn Hall as they discuss the revision of the IDF curves, rainfall data and collection, and limitations of methods used.

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Cover Photo Credit: Floods in St. Vincent https://www iwnsvg com/2016/11/29/housesdestroyed-as-rains-trigger-floods-across-stvincent/

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Fellows' Corner

BACKGROUND

Most Barbadians have experienced some form of stormwater flooding through the years, particularly the events associated with Atlantic tropical cyclones that pass our way annually.

Of note is the extreme flood events of 1819 (Sobers Bottom), 1901 (Jordan Flood), 1949 (Constitution Flood), 1970 (Constitution Flood) and the 1995 (West Flood that took the life of “Carew”), to name a few. These extreme events mainly occur in the period July –November, generally referred to as the “wet” season.

The larger watersheds feed into gully systems that terminate in coastal communities with significant populations; Constitution River (5,506 hectares) into Bridgetown, Bruce Vale River (3,300 hectares) into Belleplaine, Holetown (1,215 hectares) and Speightstown (810 hectares)

The Barbados landscape has always had a network (500km) of gullies that discharge runoff to the sea Since settlement some 400 years ago the initial forested topography has experienced radical transformation to facilitate development, with significant increase in runoff.

Today’s Engineers are therefore challenged to apply appropriate technical solutions to mitigate the impact of stormwater runoff and the consequential damage.

There are currently nineteen (19) Fellows of BAPE, all with significant experience, who have all contributed greatly to the built environment of Barbados, and other countries in the region. They have, without a doubt, been in a position of senior responsibility and/or significant autonomy in their particular field; and influenced policy and strategy-making decisions in either a technical or business environment.

This column is in honor of their sterling service – providing an opportunity for them to contribute articles and be profiled (either their professional career or any interesting projects)

As at the date of this publication, the current Fellows are (in alphabetical order):

Eng Abdul Pandor

Eng Andrew Hutchinson

Eng Andrew Gittens

Eng Bjorn Bjerkham

Eng David Lashley

Eng Errol Clarke

Eng. Frank McConney

Eng. Grenville Phillips II

Eng. Ken Blackman

Eng. Peter Date

Eng Peter Simpson

Eng Peter Williams

Eng Philip Sobers

Eng Philip Tudor

Eng Ralph Adams

Eng Ralph Williams

Eng Lt Col Trevor Browne

Eng. Tony Gibbs

Eng. Vivian-Anne Gittens

DESIGN CONSIDERATIONS

Whether the design of a stormwater system is based on the “Rational Method” or other hydrologic computer aided designs available today, the basic parameters such as watershed area, soil types, land use, hydraulic grades, storage due to topography/vegetation and the Time of Concentration (Tc) must be determined or assessed.

The Tc for a watershed is the time it takes a drop of water to flow from the outer edge of the watershed to the point of interest. The Tc therefore establishes the duration of the rainfall event necessary to generate a peak flow at the point of interest – this is critical in selecting the rainfall duration for the analysis.

Rainfall is an important factor as the design storm used for analysis must have a duration at least equal to Tc and an appropriate return period; 1 in 10 years for urban drainage, 1 in 25 years for significant culverts/bridges or 1 in 100 years for water impoundment structures

Generally, Barbados has good rainfall data, “Rainfall Intensity-Duration-Frequency Maps” first published in 1971 (J.F. Lirios Report, CMI) and since updated by the various consultancy studies on drainage and water resources

The Wallace Evans Drainage Study (1973), the Stantec Water Resources Study (1978) and the Cummings Cockburn Drainage Study (1996) all analyzed Barbados’ hydrology, the latter being the most useful with rainfall ‘hyetographs’ for design engineers.

Unfortunately, published rainfall data for the East Caribbean Island States is very limited, when compared with what is available for Barbados.

Figure 1: 10 Yr. 24 Hr. Hyetograph

Figure 2: 20 Yr. 24 Hr. Hyetograph

ANALYSIS METHODOLOGY

Most civil engineers are taught the “Rational Method” (Qp = CIA, C = runoff coefficient, I = rainfall intensity and A = watershed area) for computing the peak flow for various rainfall return periods However, this method is only appropriate for small (1 to 2 hectares) watersheds.

More appropriate analysis can be done using a computer aided design program such as HydroCAD or other similar, for modeling the hydrology and hydraulics of stormwater runoff. HydroCAD is largely based on hydrology techniques developed by the USA’s Natural Resources Conservation Service (NRCS)

The hydrologic model allows inclusion of many watershed parameters and rainfall data sets thus allowing the Engineer to explore various mitigation measures before arriving at the most cost effective and environmentally appropriate drainage solution.

NRCS used rainfall data published by the USA’s National Weather Service to develop 24- hour rainfall distributions for the generation of hydrographs. The NRCS Type 3 storm represents the Gulf of Mexico and Atlantic coastal areas where tropical storms bring large 24-hour rainfall amounts.

HydroCAD provides the opportunity to explore the most cost-effective watershed mitigation measures through the introduction of attenuation measures for reduction of peak flows which impact the cost of drainage infrastructure.

LOCAL STORMWATER DRAINAGE PRACTICE

Suck Wells

Generally, constructed stormwater drainage systems were based on the local cultural practice, often with little or no engineering design input. This practice has developed over the years using the “Suck Well” to infiltrate runoff into the 100m coral cap that covers most of the island. Suck Wells are often empirically located at low points on a road without any analysis – if one well fails then another can be added On most watersheds the rate of runoff exceeds the capacity of the suck well to infiltrate the water into the rock formation; it is therefore important to provide adequate storage adjacent to the suck well – in the form of an earth ditch or buried ‘synthetic storm chambers’ or by sloping the car park area to a central catch basin that allows for temporary ponding

Hydraulic tests were carried out on suck wells in the Belle area by Stanley Associates in the 1984 Sewerage Study (Technical Memo No 10). It was recommended that suck wells be designed for an average infiltration rate of 0 5 Liter/sec per square meter of wetted area. It is recognized that the rate can vary significantly depending on the presence of fissures in the rock, but the infiltration rate provides a benchmark for the design of suck wells

Figure 3: Suck Well with storage ditch

Figure 4: Storm Chambers for storage

Roadway Drainage

It is observed that most roadway drainage is provided with little or no engineering input. This is evident in numerous locations where small slots are provided in the vertical curb to allow runoff into a box drain under the sidewalk The slots are often unable to capture runoff flowing in a slipper with a grade, especially where the slots are blocked by debris Roadside drainage should be managed with the installation of propriety catch basins with curb inlets that induce runoff into the catch basin with a grade change at the catch basin.

Stormwater Gully Management

The network of gullies across Barbados provides several drainage paths to the coast via coastal communities that are impacted by flooding However, after several drainage studies by various consultants, various Governments have failed to implement the consultant’s recommendations for check dams in the gullies to mitigate the peak flows that impact the coastal communities Holetown and Speightstown are two areas often impacted by flooding.

In addition, the check dams will increase l aquifers a water

Figure 5: Double

#15, 3rd Ave. Wanstead Terrace, St. Michael, Barbados Tel: (246)425-1457

e-mail: ctabds@caribsurf.com

Peak Flow Estimation

Introduction

This article discusses three common methods used for estimating stormflows in drainage design - the Rational Method (RM), the Modified Rational Method and the TR55 peak flow method. The methods are briefly introduced along with the limitations of their use Recommendations for the use of these methods are also provided

The Rational Method

The Rational method is a simple and widely used approach for estimating peak runoff rates from small catchments Three inputs are required: the runoff coefficient, the rainfall intensity, and the drainage area. The method is generally the first choice when conducting preliminary drainage device designs The rational method is based on the following equation:

Q = CiA where:

Q is the peak runoff rate in cubic feet per second (cfs)

C is the runoff coefficient, a dimensionless factor for the portion of rainfall that turns into runoff

I is the rainfall intensity in inches per hour

A is the drainage area in acres

The runoff coefficient is a function of the type of land use, soil type, topography, and other factors that impact runoff Typical values for the runoff coefficient range from 0.1 for pervious areas, such as grassed areas, to 0.9 for impervious areas – hard standings, roadways, parking lots and roofs

The rainfall intensity is obtained from intensity-duration-frequency (IDF) curves for the area determined from local or regional rainfall data

Limitations of the Rational Method include:

The assumption that the runoff coefficient, rainfall intensity, and drainage area are constant over time, which is generally not the case for complex watersheds

No hydrograph production or runoff timing, which are important for the design of flood control/attenuation systems.

The Modified Rational Method

The Modified Rational method is a variation of the Rational Method that considers the time of concentration of the catchment in its calculation. By including a correction factor for the time of concentration, the method provides more accurate estimates of peak flow rates It is a more complex method than the Rational Methods requiring additional inputs as a correction factor and the time of concentration

The modified rational method uses the following equation:

Q = CIA/(t + T) where:

Q is the peak runoff rate in cubic feet per second (cfs)

C is the runoff coefficient, a dimensionless factor for the portion of rainfall that turns into runoff

I is the rainfall intensity in inches per hour

A is the drainage area in acres

t is the time of concentration in minutes

T is the time of concentration correction factor

The time of concentration correction factor is a function of the shape and size of the catchment; it accounts for the lag time between peak runoff rate and the peak rainfall intensity. It is a more accurate method than the original rational method, especially when considering larger catchments or catchments with longer times of concentration.

The Modified Rational Method has its limitations:

It assumes the rainfall intensity and runoff coefficient are constant over time, which is generally not the case for larger more complex watersheds

The method does not provide information on the full hydrograph or runoff time, which are key for flood control/attenuation design

TR55 Peak Flow Method

The TR-55 method is a hydrologic model developed by the United States Department of Agriculture’s (USDA) National Resources Conservation Service (NRCS) for estimating runoff volume, peak flow, and hydrographs from small watersheds. The method is based on the Soil Conservation Service (SCS) Curve Number (CN) method – the SCS Method; this method utilises a dimensionless curve number to characterize the combined effects of soil, land use, and management practices on the runoff process The TR-55 method is more complex than the Rational and Modified Rational methods. It requires additional inputs such as the curve number, antecedent moisture condition, and initial abstraction.

However, it provides additional information on the runoff process, including the full hydrograph and the timing of the runoff making it a preferred method for the design of flood control/attenuation systems The TR55 method is more suitable for assessing complex watersheds as it accounts for runoff coefficient variability and variable rainfall intensity over time

The key steps in the TR-55 method include:

1. Determination of the average CN for the watershed based on the land use and soil type

2. Determination of rainfall depth and duration for the design storm based on the probability of occurrence and rainfall data/IDF curves

3 Calculation of rainfall excess using the SCS dimensionless unit hydrograph method. This method assumes that the runoff is generated uniformly throughout the watershed and that the runoff rate is proportional to the rainfall excess

4. Estimation of runoff volume using the CN method, which converts the rainfall excess to the runoff volume based on the CN and the initial abstraction

5 Estimation of peak flow rate utilising the runoff volume, the drainage area, and the time of concentration.

The TR55 method also has limitations:

It assumes that the runoff is generated uniformly throughout the watershed and that the runoff rate is proportional to the rainfall excess

The method requires more input data and may be more time-consuming and difficult to use than the Rational and Modified Rational methods

Discussion

The Rational Method (RM) is typically used for sizing drainage inlets and pipes, and ditch networks for small catchments up to 25 acres or so; some municipal drainage departments in the United States of America permit its use on catchments up to 200 acres in area. A common rule of thumb is the use of the RM for catchment areas less than 50 acres and the use of the TR55 peak flow method for catchments greater than 200 acres. The TR55 method should be used for hydrograph production and for flood routing to ponds/reservoirs A primary benefit of the TR55 method is its ability to model the response of a drainage system to prolonged rainfall events

The RM considers a storm of a short critical duration to determine peak flows and as such is typically not used for (or recommended for) storage calculations as would be required for sizing or evaluating attenuation ponds The Modified Rational Method (MRM) can be used for sizing detention and retention facilities. For this method a series of trapezoidal shaped hydrographs are created for different storm durations; the greatest difference in volume between the pre and post hydrographs becomes the critical hydrograph for the subject storm.

With respect to rainfall/runoff coefficients, the coefficients for the RM - developed mostly for the design of culverts and ditches – tend to be conservative and when projected to large areas, where longer storms are more appropriate, tend to produce peak flows that are excessive.

Research[1] comparing the results of the RM and MRM with the SCS method revealed the following:

RM/MRM peak flow rates are close to those of the SCS for larger drainage areas

Runoff volumes are significantly different (RM/MRM being 3 to 4 times less).

Detention basin/dry pond volume will be significantly smaller when flood routing is done by the MRM hydrograph method

Conclusion

In conclusion, each method for estimating runoff has its own benefits and disadvantages The Rational method is simple and easy to use but has limitations in accuracy. The Modified Rational method is an improvement over the Rational Method by accounting for the time of concentration but still has limitations with hydrograph production. The TR-55 method is the most sophisticated one however it is a more difficult model to utilize - especially if calculations are to be done manually The method of choice will depend on the size of the watershed, and whether hydrograph production and flood routing calculations are required

References

[1] Paul Schiatriti – Mercer County Soil Conservation District – Basic Hydrology – TR55 vs MRM

The Revision of Intensity-DurationFrequency Curves for Barbados

Introduction

The application of frequency analysis to rainfall data allows for the estimation of rainfall depths and intensities along with their associated return periods. These estimated rainfall depths or intensities and their probabilities of exceedance are important for flood hazard assessments, and the design of drainage infrastructure amongst other water resources management applications.

The Caribbean Institute for Meteorology and Hydrology (CIMH) manages several rainfall stations across the Caribbean inclusive of Barbados with data from these stations being transferred to the CIMH in near real-time to support impact-based forecasting and decision-making - current focus being the disaster management community although other sectoral applications are possible and will be explored in the coming years

This study utilizes the stations on Barbados with supporting data to revise the IntensityDuration-Frequency (IDF) data for Barbados This article (i) outlines the methodology for the development of IDF curves, (ii) demonstrates the application of the methodology at a location of interest, (iii) provides a revision to IDF data for Barbados and (iv) shares limitations with regards to their current application

Data and Methods

Table 1 presents metadata for the rainfall station located at Apes Hill, St James that were used to demonstrate the procedure for IDF data generation. The methodology is outlined below:

Obtain sub-daily rainfall data from target station; Extract annual rainfall maxima corresponding to 5, 10, 15 and 30 minute and 1, 2, 6, 12 and 24 hour durations; Test the extracted rainfall series for outliers, homogeneity and independence and reject unsuitable data series; Rank the data series (lowest to highest) for each duration of interest and calculate the probabilities of exceedance;.

Fit a suitable extreme value distribution (Generalised Extreme Value) distribution to the plotted data points;

Estimate for each duration the annual maximum rainfall depths for the return periods of interest (e.g. 2, 5, 10, 25 and 50 yr);

Fit an appropriate Depth-DurationFrequency (DDF) function to the depthduration data;

Convert the DDF data to Intensity-DurationFrequency (IDF) data if required

In the absence of adequate sub-daily rainfall data, scaling techniques and disaggregation factors may be applied to generate IDF information (Nguyen et al 1998 and Lumbroso et al 2011)

Results

Table 2 presents the annual maxima series for the durations of interest that were extracted from the Apes Hill rainfall data

The maximum 24 hour duration rainfall of 125.5 mm was reported in 2016 while the maximum 1 hour duration rainfall of 56.5 mm was reported in 2017 This highlights the variations of intensity within rainfall events and demonstrates the importance of considering intensities rather than daily rainfall accumulations for design purposes.

Note the 2015 data series shows lower rainfall accumulations for the various durations than is expected due to an incomplete data series As such, the data series failed outlier statistical tests and was removed from the IDF curve development process.

Figure 1 illustrates the fitted GEV distribution for each extreme value rainfall series (see Nguyen et al 1998) on Gumbel plots which are a standard method of representing annual maxima data. The magnitude of the event is plotted on the y-axis and the frequency (return period) plotted on the x-axis using a Gumbel reduced variate scale

The DDF data was extracted from the GEV distribution fits and converted to IDF data.

Table 3 presents the IDF table for the station at Apes Hill The corresponding IDF curves are presented as Figure 2 Each line on the graph represents the likelihood of an event occurring The green line at the top, for example, shows events that have a 1/50 or 2 percent chance of occurring. The red line to the bottom represents events that have a higher percentage chance of occurring , at 1/2 or 50 percent.

Application

IDF curves for Barbados were first published in Lirios (1971) who used nine (9) recording rainfall stations, seven (7) of which had less than ten (10) years of data. Eight (8) of the recording rainfall stations used in Lirios’ study went into disrepair some time ago, hence, updates at these locations are no longer possible.

The procedure showcased above was used by the CIMH to develop revised IDF curves for ten (10) selected locations in Barbados - Halfmoon Fort (St Lucy), Apes Hill (St James), Springvale (St. Thomas), Holetown (St. James), Zhores (St. John), Crab Hill (St. Lucy), Canefield (St. Thomas), Orange Hill (St. Peter), Husbands (St. James) and the Grantley Adams International Airport (Christ Church).

The IDF data produced may be used to generate IDF grid and contour maps for return periods of interest for the varying durations through the application of spatial interpolation techniques The grid demonstrated in Figure 3 was created using a b-spline interpolation technique. Other interpolation strategies such as kriging which preserves values at station locations will be investigated.

Utilizing IDF grids within a GIS environment allows for IDF estimates to be obtained at the locations of interest between station locations to (i) support the estimation of peak flows for engineering design, (ii) provide intensity data for modeling applications and (iii) support flood hazard mapping assessments Notwithstanding, users of the information provided need to be aware of data quality concerns and uncertainties before integrating such data into their applications.

Discussion and Limitations

Ideally, the number of station years required for meaningful frequency analysis should be at least ten (10) years and should be more than or equal to half the return period being estimated. For example, 25 or more years of data would be required to estimate values corresponding to a 50 year return period. Therefore, extrapolating curves beyond their useful range significantly increases uncertainties Currently, the majority of current high-temporal resolution recording rainfall stations across the Caribbean including Barbados have not yet reached this minimum standard With historic stations in a state of disrepair due to a combination of

natural hazards and limited resourcing, suitable data availability is limited Further, spatial interpolation of rainfall data is often challenging given the spatial variability resulting from elevation differences and local climatic profiles. Hence, application of any data presented above should be done with due caution in consideration of such limitations especially considering the potential impact on effectiveness and cost of drainage design works

Over the last 1-2 decades, there has been significant improvements regarding the maintenance of local and regional monitoring networks through the work of the CIMH, local stakeholders and the donor community. Continued investments will support the continuous update of IDF data for Barbados and the Caribbean region The CIMH has developed and implemented a procedure that consults its hydrological database, extracts the required data for IDF curve development, performs the requisite statistical tests (e.g. Grubbs, Mann-Kendell), fits the GEV distribution for frequency analysis, calculates DDF data and publishes IDF curve data.

The procedure will be used in the future to generate design rainfall data for flood hazard assessments to minimise the impact of extreme rainfall events on critical infrastructure provided that the stations continue to be maintained through adequate resourcing.

Research continues on the impact of climate change on rainfall intensities in the Caribbean with no definitive trends identified Notwithstanding, the application of IDF data, which is based on an analysis of historical data, for "climate smart" design raises some interesting questions given the likely nonstationarity.

References

Lumbroso, D M , S A Boyce, H Bast, N Walmsley (2011), The challenges of developing rainfall intensity curves and national flood hazard maps for the Caribbean, Journal of Flood Risk Management, Vol 4 (1), 42-52

Nguyen, V. T. V., T. D. Nguyen, H. Wang (1998), Regional estimation of short duration rainfall extremes, Water Science Technology, Vol. 37 (11) 15-19

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The Association was conceived on 2 October 1964 at the Drill Hall where a Steering Committee held 4 meetings to develop the constitution The first general meeting was held at the Marine on 19 November 1964 where Sir Frank Hutson was elected the first President. Other Founding Members of BAPE include:

F Gill, G Crawford, P Browne, C Evelyn, F McConney, P. Cheeseman, T. Gill, J. Nelson, J. Connell, G. Heywood, M Roachford, Sir Frank Hutson, J Rowe, E Elliot, K Johnson, H Sealy, C Emptage, L Johnson, D Sinson, C Evelyn, B Mahy, A Wason, A Mail & L Williams

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The Executive Commtiee wishes to thank you the reader for engaging with our content We hope you enjoyed the newsletter and continue to be an active participant in other BAPE Activiities

Mr.

Mrs. STACEY

Mr. VINCENT JONES

Eng. WILLIAM ASHBY

Eng. LORETTA WALKER Eng. JOAN BOURNE

Eng. ANDRE'A PALMER Eng. REY MOE

Eng.

Felicia Cox