Hydraulic Design of Highway Culverts - DOT On-Line Publications

Table 2--Peak Determination Methods.
Gaged Sites Ungaged Sites
1) Normal Distribution
2) Log-Normal Distribution
3) Gumbel Extreme Value Distribution
4) Log-Pearson Type III Distribution
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1) USGS Regression Equations
2) FHWA Regression Equations
3) Regional Peak Flow Methods
4) SCS Peak Discharge Method
5) Rational Method
3. Check Flows. Culvert operation should be evaluated for flows other than the peak design
flow because: (1) It is good design practice to check culvert performance through a range of
discharges to determine acceptable operating conditions, (2) flood plain regulations may require
the delineation of the 100-year flood plain, (3) in performing flood risk analyses, estimates of the
damages caused by headwater levels due to floods of various frequencies are required.
Check flows are determined in the same manner as the peak design flow. The hydrologic
procedures used should be consistent unless unusual circumstances dictate otherwise. For
example, a stream gage record may be long enough to estimate a 10-year peak design flow but
too short to accurately generate a 100-year check flow. Under these circumstances the check
flow should be evaluated by another method.
4. Hydrographs. The entire flood hydrograph at the culvert site must be defined if upstream
storage is to be considered in culvert design. Passing the peak design flow through a culvert
neglects the attenuating effects of upstream storage. If this storage is taken into account, the
required culvert size may be substantially reduced. Since volume considerations are now
involved, the flood hydrograph becomes an integral part of the design process.
A flood hydrograph is a plot of discharge versus time. Figure II-1 depicts a typical flood
hydrograph showing the rise and fall of stream flow over time as the flood passes. Actual flood
hydrographs can be obtained using stream gage records. These measured storm events can
then be used to develop design flood hydrographs. In the absence of stream gage data,
empirical or mathematical methods, such as the Snyder and SCS synthetic hydrograph
methods, are used to generate a design flood hydrograph.
The unit hydrograph technique is a popular procedure for determining the response of a
watershed to a specified design rainfall. A unit hydrograph represents the runoff response of a
watershed to a uniform 1-inch rainfall of a given duration. A unit hydrograph may be generated
from data for a gaged watershed or synthesized from rainfall and watershed parameters for an
ungaged watershed. Both methods are briefly described below.
a. Unit Hydrograph Formulation - Gaged Watershed. To develop a unit hydrograph for a gaged
watershed, the designer should have streamflow and rainfall records for a number of storm
events. The rainfall data must be representative of the rainfall over the watershed during each
storm event. In addition, the rainfall events should have relatively constant intensities over the
duration of the storm.
Unit hydrograph generation involves four steps which are illustrated in Figure II-2. (1) The
groundwater or low flow contribution of the gaged flood hydrograph is estimated and removed
from volume consideration. This groundwater or low flow contribution is generally regarded as
constant and estimated to be the amount of stream flow prior to the storm event. (2) The