Question

1. ] Rational Method for Runoff

Using the same example from the in-class assignment, determine the new peak flow for each change. The changes are not additive – always refer back to the initial conditions in the problem and then perform the change below.

1. All of the land types have 8% grades (i.e. rolling), and the storm duration is 20 minutes (because the slope is steeper).
2. Some of the land types are different (replace forest with turf; replace pasture with normal residential: 3 to 6 units/acre)
3. The drainage area is larger (forest = 4.5 acre, light residential = 3.6 acre, pasture = 5.5 acre) but the land types and slope remain the same.
4. Summarize how slope, land type and drainage area affect peak flow.

Answer 1

1. Rational method is the most elementary method for computing runoff

2. Simply it is given by the equation Runoff = CAP where R is the peak runoff; C is the runoff coefficient; A is the area and P is the precipitation or rainfall intensity. The units to be consistent so as to obtain peak runoff as rate/ discharge.

3. The coefficient C includes land-use and slope etc. Of late several imporvements have been made in rational method to account for landuse, slope , soil type and initial moisture conditions. Peak runoff is infulenced directly by rainfall intensity, shape of catchment, average slope and landuse.

4. This approch is explained in most hydrology text books

5. Actual computation is simple as per step 1 and not shown here