Geoscience Reference
In-Depth Information
from a uniform steady excess rainfall of 1 cm h
−
1
lasting indefinitely. (b) Calculate the runoff rate
y
(specify the units) after
t
=
5 h resulting from a uniform excess rainfall rate of 2 cm h
−
1
that
lasts for 3 h (i.e.
x
=
2 for0<
t
≤
3 h). (c) Calculate the runoff rate (specify the units) after
t
=
5 h resulting from a uniform excess rainfall rate of 2 cm h
−
1
lasting for 3 h, that is followed by a
rainfall rate of 1.8 cm h
−
1
lasting for 7 h (i.e.
x
=
2for0<
t
≤
3 h) and
x
=
1
.
8for3<
t
≤
10 h.
Note: use the convolution integral and integrate to obtain the answers.
12.8
Multiple choice. Indicate which of the following statements are correct. The unit hydrograph
method:
(a)
is used mainly to calculate the return period of relatively rare events;
(b)
results from the detailed physical analysis of watershed runoff phenomena;
(c)
tends to yield more correct answers for very rare events, than it does for common events;
(d)
in its usual form produces the total runoff, that is storm runoff together with long-term
groundwater runoff;
(e)
makes use of typical storm runoff hydrographs produced by uniform excess rainfall and
reduced to a unit volume under the assumption of superposition and invariance;
(f)
derives its practical appeal from the underlying principle of linearity;
(g)
could, under certain conditions, also be applied to calculate groundwater outflow from a
catchment, as explained in Chapter 10.
Consider the rainfall event of Example 12.3, in which
x
i
=
2
.
0
,
4
.
0
,
1
.
0cmh
−
1
; assume that the
measured runoff is
y
i
=
0
.
63
,
1
.
97
,
2
.
35
,
1
.
38
,
0
.
6
,
0
.
12
.
Calculate the unit hydrograph ordinates
u
i
by the method of least squares, with Equations (12.17) and their analogs for
i
=
3 and 4, and
compare with the “exact” values given in Example 12.3.
12.9
12.10
Multiple choice. Indicate which of the following statements are correct. For a given location and
a given time of year, maximum rainfall:
(a)
intensity of a certain duration decreases with increasing exceedance interval (i.e. return
period,
T
r
);
(b)
intensity for a given return period decreases with increasing duration of the rain;
(c)
volume for a given return period increases with increasing duration of the rain;
(d)
intensity needed in the rational method is determined on the basis of a knowledge of a
design return period and a duration equal to the time of concentration;
(e)
can be determined for an ungauged site by areal interpolation of available data.
12.11
Multiple choice. Indicate which of the following statements are correct. The rational method in
its classical form,
Q
p
=
CIA
:
(a)
is normally used to calculate runoff resulting from infiltrated rainfall;
will work better to calculate surface runoff from catchments larger than 100 km
2
;
(b)
(c)
is normally used to obtain a design peak runoff but not a design hydrograph;
(d)
is based on the assumption that the infiltration rate is a constant loss rate, independent of
rainfall intensity;
(e)
is based on the simplest type of linear runoff model, namely a model consisting of pure
storage.
12.12
Multiple choice. Indicate which of the following statements are correct. The rational method gives
the flow rate in terms of the rainfall intensity.
