Fellows' Corner: Storm Drainage - Theory & Practice

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.

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

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

#15, 3rd Ave. Wanstead Terrace, St. Michael, Barbados Tel: (246)425-1457 e-mail: ctabds@caribsurf.com