Hydraulic Design of Highway Culverts - DOT On-Line Publications

volume of the remaining runoff hydrograph is calculated. This is termed the direct runoff volume.
(3) The direct runoff volume is then distributed over the entire watershed (divided by the
watershed area) to determine the equivalent runoff depth. (4) The ordinates of the runoff
hydrograph are divided by this runoff depth to produce the unit hydrograph for the storm
duration.
Figure II-2--Unit hydrograph determination procedure
The unit hydrograph can be used with the concepts of linearity and superposition to predict the
watershed response to a design rainfall with a specified return period. Linearity implies that if
the unit hydrograph represents a basin's response to 1-mm (1-inch) of runoff for a given storm
duration, 2-mm (2-inches) of runoff over the same duration doubles the discharge at each point
in time. Superposition allows for the accumulation of individual runoff responses. For example, if
a storm event which generates 1-mm (1-inch) of runoff over a given duration is followed
immediately by another 1-mm (1-inch) runoff storm event of the same duration, the basin
response will be the accumulation of the individual effects over time (Figure II-3).
A unit hydrograph is derived for a specified storm duration. Since storm durations vary, many
different unit hydrographs exist for any particular watershed. Techniques exist to vary the
duration of a unit hydrograph such as the "S" Curve (Summation Curve) approach (11). These
methods are useful in matching the design unit hydrograph to the duration increment of a design
rainfall. Methods are also available to formulate design rainfalls using U.S. Weather Service
data. (12,13)
b. Synthetic Unit Hydrograph. A synthetic unit hydrograph may be developed in the absence of
stream gage data. The methods used to develop synthetic unit hydrographs are generally
empirical and depend upon various watershed parameters, such as watershed size, slope, land
use, and soil type. Two synthetic procedures which have been widely used are the Snyder
Method and the Soil Conservation Service (SCS) Method. The Snyder Method uses empirically
defined terms and physiographic characteristics of the drainage basin as input for empirical
equations which characterize the timing and shape of the unit hydrograph. The SCS method
utilizes dimensionless hydrograph parameters based on the analysis of a large number of
watersheds to develop a unit hydrograph. The only parameters required by the method are the
peak discharge and the time to peak. A variation of the SCS synthetic unit hydrograph is the
SCS synthetic triangular hydrograph.
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