Determine the actual diameter of the first three pipes, in series, for a storm sewer system for a storm with 20 year return period. distance from the most remote part of the watershed (in catchment a) to the first inlet is 169 m. assume that min velocity in pipes is 0.75 m/s and the min pipe diameter is 300 mm. the roughness of the pipes is 0.013. available pipe sizes (mm): 300, 375, 450, 525, 600, 750.