Stormwater drainage design for a flood-prone city ward

Ward 9 of Sabarkantha Nagar Palika covers 1.6 km² and has about 38,000 residents. Most streets have shallow open drains built 25 years ago, when the ward was half farmland. Now it is 70% built up. In the last three monsoons, the main road and two low-lying housing colonies flooded to knee height for four to six hours after heavy rain, and the bus depot was closed for a day.

The municipal engineer has a state grant to rebuild the ward's primary and secondary drains, but must send a design and cost estimate to the state urban development department in eight weeks. She wants a design she can defend: how much water each drain must carry, how big it needs to be, and what it costs.

You are given: - a ward map (you will trace it from OpenStreetMap or a satellite image and simplify it to 8 to 12 sub-catchments); - an IDF relationship for the town in the form i = K T^a / (t + b)^n, with K = 5.9, a = 0.16, b = 0.5 and n = 0.82 (i in cm/h, t in hours), which the engineer derived from IMD records; - a design return period of 5 years for secondary drains and 10 years for the primary drain; - the outfall: a nallah at the east edge, with invert level 1.8 m below the lowest road.

What you hand over: - a catchment map with areas, land use and runoff coefficients; - peak flow for every drain reach; - drain sizes, slopes and velocities; - a check of the design under 20% more intense rain; - a bill of quantities and cost estimate; - a short design report.

Done looks like a set of drains that carry the design storm without surcharging, keep velocities between self-cleansing and scour limits, and come in within the grant of ₹6.5 crore, or a clear explanation of why they cannot.