Environmental Engineering Reference
In-Depth Information
25, 50, or 100 years. The prediction provides the basis for the selection of protective meas-
ures. The design flood is selected with regard to cost of control and to the degree of dan-
ger to the public from failure of the proposed flood-control system. If failure will result in
the loss of life and substantial property damage, design is based on floods of lower prob-
abilities, such as a 1000-year flood, because this flood level is higher than those of floods
of higher probabilities.
Analytical Forecasting
Factors to consider in analytical flood-level forecasting are:
1.
Topography of the total basin contributing runoff to the study area
2.
Ground cover including soil and rock type and vegetation (to evaluate runoff vs.
infiltration and evaporation)
3.
Maximum probable storm in terms of intensity and duration (based on records)
4.
Season of the year (affects conditions such as frozen ground, snow cover, and
ground saturation, all of which influence runoff)
5.
Storage capacity of the river channel and floodplain (possible future downstream
changes must be considered)
Maximum probable flood computational procedures as described in USBR (1973a) require
estimates of storm potential and the amount and distribution of runoff within the drainage
basin. The general procedure is as follows:
1.
A 6-h point rainfall is selected from an appropriate chart for the geographic loca-
tion. From graphs, the point rainfall value is adjusted to represent a 6-h average
precipitation over the drainage basin, and is also adjusted to give the accumu-
lated rainfall for longer durations, such as 48 h, for example.
2.
Runoff is determined from an evaluation of the soil and vegetation conditions,
and the runoff volume for the drainage area is computed for various time incre-
ments. From the data and simple mathematical relationships, runoff hydro-
graphs are prepared.
Hydrographs
are graphic plots of changes in the flow of water (discharge) or of the water-
level elevations (stages) against time as shown in
Figure 8.l3.
They are often presented in
the form of a unit hydrograph which is a hydrograph for 1in. of direct runoff from a storm
of a specific duration as shown in
Figure 8.l4.
Discharge volumes
for a given period of time are computed from the unit hydrographs.
For flood-forecasting purposes the computed flows are converted into stages (water lev-
els) by the application of stage-discharge relationships for a given location as illustrated
in Figure 8.14. The curves are prepared from data obtained from field measurements.
Computer programs
include the HEC series, which allows rapid computations for the var-
ious elements of a hydrological study once the basic data are collected.
Site planning
for a location where development is anticipated involves computing peak
flows for various storm frequencies by alternative methods to arrive at unit hydrographs
for several conditions including those before, during, and after development as shown in
Figure 8.14. The computed flood levels are checked against the flood levels estimated by
geologic techniques (see the following section). Channel capacities are computed and esti-
mated flood levels are derived and plotted in cross section. If the “after development” unit
hydrograph results in dangerously high flood levels, then flood-control measures are
required, and a storm-water management program ensues to control runoff as also shown
in Figure 8.14.
