Geoscience Reference
In-Depth Information
Table 12.1Values of the 2 h unit hydrograph used in Example 12.1
Flow rate (cm h
−
1
)
Flow rate (cm h
−
1
)
Time (h)
Time (h)
0
0.0000
6.5
0.0654
0.5
0.0080
7
0.0477
1
0.0414
7.5
0.0342
1.5
0.0963
8
0.0242
2
0.1610
8.5
0.0169
2.5
0.2195
9
0.0118
3
0.2471
9.5
0.0081
3.5
0.2433
10
0.0055
4
0.2190
10.5
0.0037
4.5
0.1851
11
0.0025
5
0.1494
11.5
0.0017
5.5
0.1164
12
0.0011
6
0.0883
12.5
0.0008
0
P
a
1
c
(cm h
−
1
)
b
2
a
+
b
+
c
b
1
c
y
a
0
0
5
10
15
t (h)
Fig. 12.2 Example of the storm runoff (heavy line) calculated by means of the 2 h unit hydrograph shown in
Figure 12.1, resulting from a storm with excess precipitation rates of 1 cm h
−
1
for 0
<
t
≤
2h,2cm
h
−
1
for 2
<
t
≤
4 h, and 1.5 cm h
−
1
for 4
<
t
≤
6 h; these successive pulses and their responses are
indicated by a, b and c. The respective volumes are 2, 4 and 3 cm over the catchment area.
time scales of runoff remain independent of the magnitude of the input. This assumption
is acceptable as long as the flow rates do not deviate too much from some average or
characteristic values. However, nonlinearities can be expected to show up when the flow
rate magnitudes of interest cover a wide range; this holds especially true over smaller
catchments. For instance, in the case of free surface flow, the Chezy and GM equations,
(5.39) and (5.41), indicate that the velocity depends on the water depth. This means
that the more water is flowing in the rills, gutters and creek channels of the basin, the
