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FIGURE 25.5
Rainfall hyetograph and associated runoff hydrograph.
25.3.1.4 Synthetic Storms
The alternative to a given rainfall “distribution” is to input a custom design storm into the model.
This can be compiled from data gathered from a single rainfall event in a particular area, or a syn-
thetic storm created to test the response characteristics of a watershed under specific rainfall condi-
tions. Note, however, that a single historic design storm of known frequency is inadequate for such
design work. It is better to synthesize data from the longest possible grouping of rainfall data and
derive a frequency relationship as described with the IDF curves.
25.3.1.5 Single Event vs. Continuous Simulation Computer Models
The fundamental requirement of a stormwater management plan is a quantitative analysis of the
watershed hydrology, hydraulics, and water quality, with consideration for associated facility costs.
Computers have greatly reduced the time required to complete such an analysis. Computers have
also greatly simplified the statistical analysis of compiled rainfall data. In general, there are two
main categories of hydrologic computer models: single-event computer models and continuous-sim-
ulation models. Single-event computer models require a minimum of one design-storm hyetograph
as input. A hyetograph is a graph of rainfall intensity on the vertical axis vs. time on the horizontal
axis, as shown in Figure 25.5. A hyetograph shows the volume of precipitation at any given time as
the area beneath the curve, and the time-variation of the intensity. The hyetograph can be a synthetic
hyetograph or an historic storm hyetograph . When a frequency or recurrence interval is specified
for the input hyetograph, it is assumed that the resulting output runoff has the same recurrence inter-
val. (This is one of the general assumptions that is made for most single-event models.)
Continuous simulation models, on the other hand, incorporate the entire meteorological record
of a watershed as their input, which may consist of decades of precipitation data. The data are
processed by the computer model, producing a continuous runoff hydrograph. The continuous
hydrograph output can be analyzed using basic statistical analysis techniques to provide discharge-
frequency relationships, volume-frequency relationships, flow-duration relationships, etc. The
extent to which the output hydrograph may be analyzed is dependent upon the input data available.
The principal advantage of the continuous simulation model is that it eliminates the need to choose
a design storm, instead providing long-term response data for a watershed that can then be statisti-
cally analyzed for the desired frequency storm.
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