Stormwater Calculations - Mathematics and Statistics for the Environment - page 773

For larger watersheds, attenuation of peak flows through the drainage network begins to be a fac-
tor in determining peak discharge. While there are ways to adjust runoff coefficients ( C ′ factors) to
account for the attenuation, or routing effects, it is better to use a hydrograph method or computer
simulation for these more complex situations. Similarly, the presence of bridges, culverts, or storm
sewers may act as restrictions that ultimately impact the peak rate of discharge from the watershed.
The peak discharge upstream of the restriction can be calculated using a simple calculation pro-
cedure, such as the rational method; however, a detailed storage routing procedure which consid-
ers the storage volume above the restriction should be used to accurately determine the discharge
downstream of the restriction.
25.6.1.3 Design Parameters
The following is a brief summary of the design parameters used in the rational method.
25.6.1.3.1 Time of Concentration (t c )
The most consistent source of error in the use of the rational method is the oversimplification of the
time of concentration calculation procedure. Because the origin of the rational method is rooted in the
design of culverts and conveyance systems, the main components of the time of concentration are inlet
time (or overland flow) and pipe or channel flow time . The inlet overland flow time is defined as the
time required for runoff to flow overland from the furthest point in the drainage area over the surface
to the inlet or culvert. The pipe or channel flow time is defined as the time required for the runoff to
flow through the conveyance system to the design point. In addition, when an inlet time of less than 5
minutes is encountered, the time is rounded up to 5 minutes, which is then used to determine the rain-
fall intensity ( I ) for that inlet. Variations in the time of concentration can impact the calculated peak
discharge. When the procedure for calculating the time of concentration is oversimplified, as men-
tioned above, the accuracy of the rational method is greatly compromised. To prevent this oversimpli-
fication, it is recommended that a more rigorous procedure for determining the time of concentration
be used, such as the one presented in Chapter 15, Section 4, of the National Engineering Handbook .
Many procedures are available for estimating the time of concentration. Some were developed
with a specific type or size watershed in mind, while others were based on studies of a specific
watershed. The selection of any given procedure should include a comparison of the hydrologic and
hydraulic characteristics used in the formation of the procedure, vs. the characteristics of the water-
shed under study. The engineer should be aware that if two of more methods of determining time of
concentration were applied to a given watershed, there would likely be a wide range in results. The
SCS method is recommended because it provides a means of estimating overland sheet flow time
and shallow concentrated flow time as a function of readily available parameters such as land slope
and land surface conditions. Regardless of which method is used, the result should be reasonable
when compared to an average flow time over the total length of the watershed.
25.6.1.3.2 Rainfall Intensity (I)
The rainfall intensity ( I ) is the average rainfall rate, in inches per hour, for a storm duration equal
to the time of concentration for a selected return period (e.g., 1-year, 2-year, 10-year, 25-year). Once
a particular return period has been selected, and the time of concentration has been determined
for the drainage area, the rainfall intensity can be read from the appropriate rainfall IDF curve for
the geographic area in which the drainage area is located. These charts were developed from data
furnished by the National Weather Service for regions of the U.S.
25.6.1.3.3 Runoff Coefficients (C)
The runoff coefficients for different land uses within a watershed are used to generate a single,
weighted coefficient that will represent the relationship between rainfall and runoff for that water-
shed. Recommended values can be found in Table 25.3. In an attempt to make the rational method