The following exercise should give you some additional practice
at applying the unit hydrograph concepts of (1) linearity and
(2) superposition. I've kept the numbers simple so that you
can focus on the procedures.
The 2-hour unit hydrograph for a watershed is:
| Time (hours) |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
| UH (cfs) |
0 |
2 |
7 |
8 |
5 |
3 |
1 |
0 |
Preliminaries
- Compute the volume of direct runoff (Vd) for the unit hydrograph
(in cfs-hrs).
- What is the rainfall excess hyetograph (1 hour time step) associated
with this 2-hour unit hydrograph event?
- Compute the S-hydrograph for the watershed. What is the value of the
S-hydrograph at time t = 10 hours?
Prediction (for each problem, you must select the appropriate tr-UH needed to predict the DRH for the given storm)
- Predict the direct runoff hydrograph for the following storm:
| Time (hours) |
0-2 |
2-4 |
| Pe (in) |
1 |
2 |
Verify that the volume of direct runoff is correct.
- Predict the direct runoff hydrograph for the following storm:
| Time (hours) |
0-4 |
4-8 |
| Pe (in) |
1 |
2 |
Verify that the volume of direct runoff is correct.
- Predict the direct runoff hydrograph for the following storm:
| Time (hours) |
0-1 |
1-2 |
2-3 |
| Pe (in) |
1 |
2 |
1 |
Verify that the volume of direct runoff is correct.
- Predict the direct runoff hydrograph for the following storm:
| Time (hours) |
0-3 |
3-6 |
6-9 |
| Pe (in) |
1 |
2 |
1 |
Verify that the volume of direct runoff is correct.
Some things to think about
- Compute the 4-hour unit hydrograph using the two methods. Note the
results must be the same. Verify that the volume of direct runoff is
correct for a unit hydrograph.
- Use the results from question 6 to estimate a unit hydrograph using
the single event method. That is, assume that the rainfall excess
hyetograph and the computed direct runoff hydrograph are given. What is
the unit hydrograph? What is the effective duration tr of the unit
hydrograph? How does this approximate tr-UH compare with the true tr-UH?
The solutions are available.