Helical Autotechnology, Mulshi

Helical Autotechnology is adding a factory building at Mulshi, in the hills west of Pune, for the assembly of automotive parts - actuators, valve rotators, pressed parts and assemblies, waste-gate valves and relief valves. The new pre-engineered building joins existing buildings and earlier phases on a plot of 15,250 sq m, of which about 4,340 sq m is metal-sheet roof, 9,650 sq m is roads, truck docks and paving, and 1,250 sq m is landscape. Mulshi takes heavy monsoon rain, and on a site that is almost all roof and paving very little of it soaks in: nearly all of it has to be collected and carried off the plot without flooding the docks and yards. The architecture is the architect's; our scope is the storm water drainage design and the report the corporation approves it on.

We prepared the storm water design report, issued as R0 on 31 August and R1 on 2 September 2026. Rainfall: a design intensity of 75 mm an hour. Runoff: the Rational Method, Q = C x I x A / 3600, with runoff coefficients from the CPHEEO Manual on Sewerage and Sewage Treatment Systems - 0.95 for the metal-sheet roof, 0.80 for the paved areas and 0.10 for the landscape. That gives 85.89 litres a second from the 4,340 sq m of roof, 160.83 from the 9,650 sq m of paving and 2.60 from the 1,250 sq m of landscape: about 250 litres a second, 15,000 litres a minute or 9 lakh litres an hour from the whole plot. Drains: each line sized with Manning's equation and checked that its carrying capacity is more than the flow it takes - a 450 mm pipe for the roof catchment (1.10 m a second, 157 litres a second of capacity against 81), a 600 mm pipe for the paved areas (1.33 m a second, 339 litres a second against 161) and a 250 mm pipe for the landscape (0.74 m a second, 33 against 3), all laid at 1 in 200 to the combined outfall. The report sets out the basis, the coefficients, the runoff and the node-by-node sizing for the approval of the corporation.