Question

Develop the flood hydrograph that would result from 28 cm of rain falling on the 1450 km2 watershed of problem 2 above during a 72-hour period. The spatially averaged rainfall is 10 cm during the first 24 hours, 12 cm during the second 24 hours, and 6 cm during the third 24 hours, for a total of 28 cm falling over 72 hours. The watershed runoff characteristics are represented by a CN of 78. The stream flow just before the rain storm was 60 m3/s. Assume a constant base flow of 60 m3/s during the flood.

Answer 1

To develop the flood hydrograph resulting from 28 cm of rain falling on the 1450 km2 watershed over 72 hours, we calculate the total runoff volume using the curve number (CN) and the total rainfall. Then, we divide the runoff volume by the total time to find the average streamflow.

To develop the flood hydrograph resulting from 28 cm of rain falling on the 1450 km2 watershed over 72 hours, we need to calculate the total runoff volume and divide it by the total time to find the average streamflow.

First, we calculate the total runoff volume using the curve number (CN) and the total rainfall. The total rainfall is the sum of the rainfall during each 24-hour period (10 cm + 12 cm + 6 cm = 28 cm). So, the total runoff volume (V) can be calculated using the formula V = (P - 0.2S)^2 / (P + 0.8S), where P is the rainfall and S is the potential maximum retention (CN - 10).

Once we have the total runoff volume, we divide it by 72 hours to calculate the average streamflow. The streamflow just before the rainstorm was 60 m3/s, so we add this base flow to the average streamflow of the flood hydrograph.