I have perused letters from Messrs Malcom Ali, Ragwin, C Sohan and R Seegobin. These engineers, particularly Malcom are all expressing their valuable opinion on flood relief for the EDWC. However, I am not sure whether the average Guyanese comprehends whether the Hope Canal is technically feasible or not. I am not entering a technical discourse on risk analysis, etc, but hopefully will bring to the average person basic and common-sense detail that ought to be a factor in the construction of Hope relief canal. In doing so, I would like to draw their attention as to how functional some existing drainage canals are, hoping some lessons can be learnt for the proposed Hope Canal.
Firstly, Canals No 1 and 2 in West Demerara. The Canals Polder is supposed to be drained by two major canals about 7 miles long, which are tidally controlled at the outlet. In spite of this, the Canals Polder floods incessantly. The farming community has never escaped inundation during significant rainfalls. The reason is that input (rainfall) less the natural losses, exceeds the output (sluice discharges), hence there are accumulations (flooding). Assuming that the canals are perfectly maintained, a flood wave emanating from the top of the catchment would have to travel at a speed that would erode the channel to reach the sluice. This does not happen as the canals are designed to flow at a velocity of 3 ft per second. The actual velocity is much lower since the canals are poorly maintained, and the velocity is checked by the weeds and sediment in the canals. Let us be conservative and say that the canals presently flow at the designed velocity. With that rate of movement of water, it will take more than 10 hours for the headwater to be conveyed. Considering that the average time the sluice opens at low tide to be 4 hours, the headwater would not reach the Demerara in time. This results in a backwater effect in the canals, resulting in the ponding and stagnation of water. Because the canals are not delivering the water in sufficient quantity, the sluices operate below capacity. Hence, we can conclude that the flooding in the polders is partly due to the lack of adequate conveyance.
The Naamryck canal is another example of a channel poorly conveying water. Naamryck is also long (4+ miles) and is controlled by spillway/gates that discharge into the Essequibo River. Like the Canals Polder, the Naamryck outfall is likewise discharging below capacity, and flooding ensues. In December 1987, when the Boeraserie Conservancy was overtopping, Naamryck was used as the relief. The government engineer of West Demerara said that the water was not moving fast enough through the canal to the spillway outlet, and there was extensive flooding due to the accumulation. Input did not equal output.
The canal leading to the two pumps at Onverwagt is an interesting case study. The two pumps at Onverwagt are part of the MMA flood relief infrastructure. The pumps are meant to be fed with a fairly wide and deep channel that drains a part of the MMA. I observed that these two pumps have a discharge capacity in excess of the rate at which the channel is capable of delivering the flows. I made those observations when I tested those pumps for hydraulic efficiency in 1982. I realised then that only one of those pumps could have been tested over a 2-hour cyclic period as the pumps were cutting off due to starvation. This means that water was not reaching the pumps fast enough via the canal. It was determined that about 2 hours was required to allow the flows to re-accumulate in the reach of the canal immediately upstream of the pumping station, hence the 2 hour cycle was chosen. I concluded in my report to the then Hydraulics Division that the canal could not deliver the water required for even a single pump to operate on a continuous basis. (A major problem that had to be considered in the conveyance of the canal is the rapid drawdown of the water level in the channel, which caused the banks to fail.)
Another example is the failure of the conveyance channel to provide flood relief to my native village. I grew up in Meten Meer Zorg, and frequently woke up to the villages flooded with overnight rain. The flood relief was again a sluice discharging into the Atlantic. I vividly remember that the discharge at the sluice never appeared to be at capacity in spite of the fact that the village is knee-deep in water. I later understood the reason. Simply, the floodwater was never conveyed efficiently to the sluice to be discharged during low tide periods as there is a travel time required to move floods to the sluice. Obviously, MMZ flooded because the required travel time for the flood waves exceeded the hours that the sluice could remain open, so like all other villagers, I waited on the next low tide. In the meantime, there was a build-up of water, backwater effects were created in the trench and increases in accumulations occurred.
What lesson can we learn from the above mentioned examples of drainage canals failure to convey the floods to the outfalls? Liliendaal and Anna Regina are other examples. Knowing that the discharges for the Hope Canal could take 2 to 3 days to travel the length of the proposed canal to the Atlantic, the outflows/discharges would be tidally influenced, and given the above four examples of flooding attributed to travel time (conveyance) failures, do Guyanese have hope that the Hope Canal would function effectively? The Guyanese public is eager to know that the government engineer/consultant knows the answer before they begin to spend $3B.
I acknowledge Mr C Sohan for recognising the conveyance problem. You may recall that he said, “It is estimated that floodwater from critical parts of the EDWC could take from 24-48 hours to reach any one of the relief sluices” (SN February 9), in his review of the Hope Canal proposition. For Hope to function, a steady state condition has to be created, where output from the conservancy can be equal to the output at the sluice gates. But for this idealized situation to happen, the coordination between the release at the conservancy and the discharge has to be continuous beyond the time the tide allows the sluices to open. The proliferation of weeds, bank failures and sediment deposition would create imbalances that would affect the desired steady-state conditions. Let us not build another Canals Polder, Naamryck, or Onverwagt canal.