Africa-Press – Mauritius. In this Qs & As exploration, Virendra Proag, a former Associate Professor in Civil Engineering at the University of Mauritius, elucidates the causes of floods, factors influencing their occurrences, and the vital role played by catchment areas and runoff coefficients.
Additionally, he scrutinizes the significance of rainfall intensity, return periods, and the implications of climate change on drainage systems.
Viren Proag’s perspective challenges us to ponder the effectiveness of existing infrastructure and the imperative of innovation to mitigate the impact of floods, urging society to grapple with the question: Is it acceptable for one’s house to get flooded every 10 years, or never at all during one’s lifetime? Through historical analysis and engineering acumen, Proag prompts a critical reflection on the need for resilient drainage systems in the face of evolving climatic challenges.
* Why do we have floods?
Being flooded or submerged by water occurs because the soil cannot absorb the incoming rainfall, or because water on the ground is not flowing away fast enough. A sudden heavy rainfall will cause flooding if there is no drain to carry the water away.
A low-lying area will certainly be flooded because all water will eventually accumulate there, and it is usually difficult to make drains at a still lower level.
* What are the factors influencing flood occurrences?
When rain is falling, the amount of water that is going to collect and flow into a river or drain is influenced by (1) the catchment area concerned, (2) the rainfall intensity, and (3) the runoff coefficient.
* Can you elaborate?
Figure 1 shows the process of rainfall, wherein rainfall is the sum of the ensuing evaporation, infiltration, and runoff.
Rainfall = Evaporation + Runoff + Infiltration. The land’s surface always has a slope, however small it might be, which determines the direction of flow (here, the runoff).
Figure 2 indicates how the ridge at the top of a valley slope will divide rainfall, which will run along slopes on either side of the ridge.
The area enclosed by a given ridge determines a catchment area. Depending on the point of interest, the catchment area will vary. Point X determines a smaller catchment area than point Y, and in turn, the area at point Y is smaller than that governed by point Z.
Eventually, the estuary governs an even bigger area.
Figure 3 gives an overall picture of a valley (with smaller valleys inside) and indicates how everything discharges into the lowest point, which happens to be the estuary.
In this connection, there is a parallel with traffic flow. Unless the conveying capacity QOUT is greater than the incoming flow of traffic QIN, there is going to be a traffic jam.
While this results in a halt or lower speed in the case of vehicles, unfortunately, with water, this higher inflow leads to non-stopping flow overtopping the drain and flooding the sides.
* What is the significance of the runoff coefficient?
The runoff coefficient C represents the percentage of rainfall that flows over the ground, in the equation, Rainfall = Evaporation + Infiltration + Runoff.
Neither this runoff nor any of the other parameters is constant, though an annual average may be taken. For example, during a hot sunny day, imagine that some rain falls.
As the raindrops touch the ground (soil or road surface), water vapour can be seen rising in the air. Evaporation is actually occurring, live and direct! If it is light rain, the ground surface will dry up quickly. Either all rainwater evaporates on the road or some of the rain is absorbed into the earth: infiltration is taking place.
Source: Mauritius Times
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