Dear Editor,
I think the problem of flooding in the City of Georgetown needs to be understood first before a plan of action can be formulated. In my writings the following conclusions can be drawn: an insufficient application of drainage engineering and a lack of understanding of urbanization compounded with a lack of code enforcement may have contributed immensely to the current flood.
First of all, I am convinced that the flood monster was created by the Mayor and City Council (M&CC) and the Government of Guyana (GoG) through landfill sites. GoG must be partly blamed because at that time the Hydraulics Division had the technical capability to intervene, but did not possess the foresight to predict the outcome. There were floods at the time, but less pronounced given the average rainfall. However, lessons can be learnt for the future development of townships and legalizing of squatting areas.
I grew up in the Kitty/Campbellville/Prashad Nagar area and can attest to the changes in land use from 1967 until my recent visit to Guyana. Prashad Nagar had fewer than 10 houses in 1967, Bel Air was more developed and there was the Bel Air Dairy aback of Bel Air residences. Lamaha Gardens and Sophia were undeveloped. In other words there were vast amount of open pasture lands and dry shallow depression areas – called green areas for obvious reasons. To a lesser extent there were impervious areas such as roofs of buildings, drains/trenches, roadways, pavements and concrete areas (ie driveways, sidewalks).
In Guyana, engineers have used a drainage coefficient (say 1.5″ in 24 hrs) for calculating drainage for the city. In order to derive a drainage coefficient, many assumptions have to be made, including the percentage of the rainfall that will run off the land and the actual rainfall amount. This method in my opinion is unsuited for urban drainage calculation and is more applicable to agricultural areas. Recognizing that the limits of Georgetown have increased considerably and the imperviousness has disproportionately increased leaves no doubt in my mind that this coefficient is grossly under-estimating the discharge. The result is that all ancillary structures will be undersized, eg culverts, drains. A method that takes into account the travel time of the rainwater, a run-off coefficient that is weighted to the amount of impervious and pervious areas and a design rainfall based on economic and risk evaluations should be used. In Florida (where I currently practise) the design rain is provided by the permitting agency. Design rainfalls are greater for calculating storage (say 25 year storm) while a lesser storm event is used to design the conveyance canals (say 10 year storm). Building pads are placed above the 100 year rainfall elevation.
Simply put, urbanization creates more impervious areas at the expense of pervious areas. Basic engineering dictates that with an increase in impervious areas, the travel time of water will decrease and the peak volume generated by the storm will be reached more quickly. Hence the city will feel the effect of the storm faster. Are there building codes that limit the amount of impervious area in a yard and if so were the applicable codes enforced?
As rain falls it fills up the shallow depressions before flowing further. If there is insufficient storage in the canals/drains the low areas (ie low-level streets and house lots) will be filled in. It is an ongoing practice for property owners to fill their lots to an elevation to avoid being flooded. The consequence of such action is that the flood water will be displaced elsewhere, ie the neighbour’s yard for instance.
Some time after 1967, the M&CC converted many of those low areas into landfill sites. The name of a few garbage dump sites are Redeemer Lutheran School at Stone Ave (Campbellville), Farnum Playground at Sheriff Street, Campbellville Government School at Sheriff Street, two drainage trenches between Prashad Nagar and Campbellville Housing Scheme, Kitty railway line, playgrounds along Woolford Ave, Ruimveldt Multilateral School, etc. These dumpsites were overbuilt above a presumed base flood level. The result is that many residential areas experienced a greater depth of flood water on their property and surroundings. Where did the M&CC expect the rainwater to go after taking away floodplain areas? For obvious reasons, areas farthest from the sluice will remain under water for a longer period due to a longer travel time for the water to reach the sluices. The usual excuse is garbage in the canals/drains. Why can’t the M&CC code enforcement help control that hazard? There will still be the need for maintenance.
It was observed that the downsizing of drainage trenches has taken place in Georgetown. For example on Sheriff Street, a comparison of the bridge culverts across the main avenues and the concrete ‘covered’ driveways into business places will show that the canal’s capacity has been compromised. In other areas, bridges servicing private properties are below the normal depth of flow of the trench, restricting flows at peak periods.
The M&CC never kept abreast of urbanization. The GoG and local councils may learn from the approach of the M&CC. Drainage is an integral part of planning a township or housing schemes. It is time the GoG initiate guidelines for the temporary storage of rainwater. The establishment of a base flood elevation is important for the survival of our city and communities. Each new housing scheme built within the base floodplain area is taking away storage.
In my years of practice in NY and Florida, there are limits to which property owners can discharge run-off into the city sewers/drains. Drainage design and calculations are part of the permit package. One criterion for a new development allows for a discharge of no greater than the pre-developed discharge (that is, before the site was developed). Post-flow generated above the pre-flow has to be stored on site. Storage may take the form of underground tanks, drywells or open ponds.
In conclusion, I feel the drainage problem in the city is beyond the capability of the M&CC, and the GoG should engage technically competent firms to develop a drainage plan, establish a base flood elevation and provide guidelines for new and established housing developments.
Yours faithfully,
Ralph V Seegobin
