Dear Editor,
I read with interest the many experts who have shared their ideas on flood relief strategies for Georgetown and the East Coast Demerara. As a professional who has modeled drainage systems, I prefer to use the term attenuate, meaning safely finding a place to hold the excess water and releasing it in a controlled manner so as not to cause flooding further downstream. Guess it is possible but a digital model will determine the next action.
In many of the letters, none of the writers have offered a technical approach and hence the reason for my writing. I shall assume the lead and provide the relevant Ministry on the means to be adopted in understanding the existing problems with drainage in Georgetown. Of course, these few lines in this letter column do not define it all, but they are a good start for the Task Group that will be formed by the new Government. My letter is intended for the City drainage and that includes Greater Georgetown but can be used for other low lying areas.
On April 23, 2009, I wrote a letter to the Editor of Stabroek News titled, “Georgetown Needs a Drainage Plan’. In that letter I opined that the use of the drainage coefficient method (1.5-inches in 24-hours or thereabout) is a simplistic approach to a dynamic drainage problem. Water will naturally flow from a high to a low point. This method may predict the total water generated from a storm but does not help in determining the accumulative amount of water at any point in the storm, or days after the storm. In the absence of an accurately derived drainage coefficient, the amount of water cannot even be estimated.
The relevant Departments, and by that I mean the Hydro-meteorological and the Hydraulic Departments, need to work in tandem. This really should be just one Department headed by a strong professional who has a decent background in both Hydrology and Hydraulics. It is perhaps the case that our Hydraulic Engineers do not have a good understanding of Hydrological principles, and vice versa. To avoid a long lecture on Hydraulics/Hydrology, the rest of my letter will be dedicated to getting started on developing a Drainage Plan for Georgetown.
First, develop a unit hydrograph for the rainfall in the City. Simply explained, it is a graph showing the runoff from a unit storm (say 1-inch rainfall). Second, plot frequency curve for the historic storms and predict the return period for the recent storm say the July 2015 storm). How does it compare to the 100-year return period? Predicting the return period of a storm should not be difficult since the NDIA used a 10,000 year return period to design the famous Hope Canal. This graph should be in the design report of the Hope Canal.
Third, define the catchment area of each village/town and calculate the impervious areas, i.e., roof, concrete area, roads. Come up with a combined hydrological curve number for the village. Fourth, determine how each village is drained to a collector (tertiary) drain and record the dimensions of the outlet structure and the bottom flow level. This structure may be a culvert or bridge. The bottom (invert) level shall be taken as the existing height of the dirt within the structure. Do not cheat and give a theoretical invert value. We will need both inlet and outlet ends.
Fifth, come up with the tidal cycle at each sluice, especially during the May-June months. Tidal cycles should be readily available from the MOA. Sixth, determine the existing state of the outfall drains (the 40-foot canal is an example) within the city and determine their dimensions, length, and existing condition to determine efficiency. Each drain is a conveyance system and the model will pick up on restrictions interfering with flows in the drainage system.
Seventh, determine storage areas in the city where water can stand without overflowing into housing areas, there may not be many places but playgrounds can be flooded in a storm. Also
determine the elevation of the existing ground in each village or town. We want to come up with total volume for storage (typically referred to as an elevation to storage graph). This will help in identifying the areas that will be prone to flooding first as the storm is modeled.
Does this sound like a little work? Of course not but it is my starting point for the Drainage Plan. The next step is for the Ministry to acquire license for a suitable modeling software into which the information is inputted to generate a digital model for each village/town, recognizing that each village/town has its own outlet drain or structure that is limiting its ability to drain.
Here the Unit Hydrograph, as in point one above comes into play. Various storms can be run from this computer model using the Unit Hydrograph. During each storm model exercise, the task group will be looking to identify the time when a particular storm begins to overtop its defined storage area. Print these results for each village/town. Carry out this modeling until each village/town is completed for all catchment areas within the City & Greater Georgetown.
I do not think we need to make the model too complex and an experienced modeler will know when additional inputs are required. My reason for modeling villages is because the modeler will be able to solve modeling problems and determine what information should be put into a bigger model to be explained next.
Now that all villages have been modeled successfully, it is time to input the entire City and Greater Georgetown in one big model since rain does not fall in one village alone. The bigger model must include the sluices and tidal cycles as the boundary structures that controls/restricts the outflow of water to the Ocean/River. Still sounds simple? It is not. It will take a lot of effort and technical personnel who know what needs to be done. Again various storms can be modelled over any interval such as a 25-year storm, the 100-year storm or even a 10,000-year storm.
During the modeling exercise, identify the storm event when the City starts to show signs of flooding based on the ground level. This will be the storm that the existing drainage system is capable of handling without consequential flooding. Remember I used the term attenuate in starting my letter. Here is my explanation. Rain will not run off the land and go straight to the sluice immediately, it takes time. The tidal cycle would have been long gone, the result is that water will move to low lying areas to accumulate until the next cycle of tidal drainage. By modeling, you will be able to consider the storage attenuation and see the villages that will be affected.
In principle, the model will, in a scientific manner, give the amount of additional storage required to improving the City existing drainage system and will also predict the number of hours it will take water to recede. The big question is, where can we get additional storage as we model for larger storms?
The reason, for wanting to model the 100-year storm is that in some countries, the 100-year storm elevation is used as a basic by their Building Department to set the ground floor level for building. It is also used for flood insurance purposes. I do recall, a long time ago, retail shops used to be built with a few flight of stairs. Well, history has a way of repeating itself.
I think, the Housing Authority should also make use of a 100-year flood elevation for new housing developments. Major roadways should also be built above an arbitrary agreed flood plain level as an escape route in disaster preparedness plans.
Coming back to the drainage model, an experienced professional should be tasked with interpreting the print outs of the various model scenarios. The Task Group should consider realistic system retrofit or upgrades that can be put into the model, for example, adding pumps, widening of sluices or widening the main drains or even lining the main drains with concrete, rainwater detention on individual properties is also an option.
In New York, property owners are responsible for detaining water on their own property for a period of time. Actually an engineer friend of mine suggested lining the 40-foot trench which I thought is not a practical retrofit. We need to be practical in our consideration of retrofits. Whatever upgrades are deemed realistic can be thrown into the model and the results studied. The Task Group should bear in mind that with the flatness of the City drainage systems will not help much to increase the flow of water in the earthen drains. By widening of drains, maybe, but we have squatting or other construction hindrances to deal with.
By my letter, I hope the new Government knows what it is getting into in order to come up with a Drainage Plan for the City and the kind of personnel and information that is required.
In conclusion, I do hope that my contribution will help to kick start the Task Force and that the new Government will have a better understanding of the type of resources that will be required to make the Drainage Plan for Georgetown a reality. I cannot foresee a drainage plan without a digital model.
I shall allow other writers to share their opinions on how important weather forecasting is to the drainage plan. That is, to open the koker doors to lower the level of the drains in preparation for collecting water for a storm that is coming.
I wish the New Government and the task force the best of luck.
Yours faithfully,
Ralph V. Seegobin, P.E.
