Within this project we worked together with the Ward Officer in implementing a few sections of internal drains, where the existing drains got a complete make-over. We worked together with the Ward Officer to make a design for these new drains, provided technical expertise and supported in financing the materials.
One of the most important factors for a drainage system to function well is the bed slope, basically the bottom of the drain, which should slowly decline in the direction the water should flow.
In addition, we proposed to construct drains with a trapezium design, instead of the standard rectangular cross-section. Given the same space for the drain on the ground, the drain with a trapezium shape can provide higher flow velocity in the dry season compared to the standard rectangular drain. This higher flow velocity will lead to less waste accumulation and less stagnant water, which are favourable conditions for mosquito breeding and bad water quality in the drains.
Finally, it is important that the water will not be blocked by waste and/or sediments, as then the flow will be blocked and influence the drainage capacity.
Choosing the shape of drains for construction
We explained different options for possible drain shapes with pros and cons to YCDC and the Ward Officer. As mentioned above, the drains with a trapezium shape, or another shape with a sloping side (instead of fully vertical, as in a rectangle) can discharge more quickly during both dry and wet seasons. The following images show four shape options from lowest (Option 1) to highest flow velocity(Option 4) assuming every other factor such as materials, bed slope, space on the ground, etc. are the same for every cross section. Although Option 4 (steep V-shaped drain) can provide the highest velocity among different options, it will be challenging in construction to keep the side slope stable without erosion. Therefore, we decided to go with Option 2: A trapezium-shaped drain, which will be easy for the Ward Officer to build. The sketch of the draft design is also shown in the figure.