Technical information

140 | Chapter 10 Hydrology and Hydraulics
Hydrology and Hydraulics
Early in the process of evaluating a site, you must evaluate how the proposed
development affects watershed runoff. In general, most urban and rural
developments alter the runoff characteristics of a site by reducing the pervi-
ous surface area, which ultimately decreases infiltration and travel times.
Since the amount of runoff is directly related to the infiltration characteris-
tics of the site, any development that decreases the pervious surface area
generally results in higher peak discharges and higher runoff volumes. In
addition, decreasing travel times causes the peak discharge to occur earlier in
the storm water event. To evaluate the impact on the watershed runoff, you
can establish pre-development and post-development runoff models, and
then compare the results.
To control post-development peak discharges, you can calculate the required
storage volume for one or more selected storm frequencies, and then design
a detention pond to accommodate increases in storm water runoff for the
selected storm events.
You can use the Hydrology commands to:
■ Calculate runoff from watershed areas using the Rational, the TR-55
Graphical Peak Discharge and Tabular Hydrograph Methods, and the
TR-20 method.
■ Develop pre- and post-development runoff models.
■ Design various types of retention/detention facilities to store excess
runoff.
■ Design and analyze hydraulic conveyance structures such as channels,
culverts, and weirs.