Technical data

Chapter 3 Performing Hydrologic Studies
34
Overview of Hydrologic Studies
Early in the process of evaluating a site, you must evaluate how your
proposed development will affect watershed runoff. In general, most
urban and rural developments alter the hydrological character of a site
by reducing the pervious surface area, which ultimately decreases
infiltration and travel times.
Since the amount of runoff is directly related to the infiltration
characteristics of the site, any development which decreases the
pervious surface area adversely changes the watershed’s runoff
response to precipitation resulting in higher peak discharges. 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.
For example, it is commonly a requirement of most reviewing
agencies that post-development discharges do not exceed pre-
development discharges for one or more storm frequencies. 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 water-retention structures to store
excess runoff
■
Design and analyze hydraulic conveyance structures such as
channels, culverts, and weirs
For example, if you are building a shopping center with a large
parking lot that covers existing pervious sandy ground, you can use
Autodesk Civil Design to ascertain how the impervious surface area of
the parking lot will affect the water runoff. You may decide that
drainage culverts that lead to a detention pond may be the best way to
prevent flooding problems. You can calculate the type of culverts
needed to convey the excess runoff from the parking lot area to an
appropriately sized detention pond, including the necessary outlet
structures to control discharge to pre-development levels.