Stormwater Calculations - Mathematics and Statistics for the Environment - page 789

Manipulation of the site grades and strategic placement of the permanent features such as build-
ings and parking lots can usually provide adequate storage volume within a basin. Sometimes the
site topography and available outfall location will dictate the location of a stormwater facility.
25.7.2 a lloWable r elease r ates
The allowable release rates for a stormwater facility are dependent on the proposed function(s)
of that facility, such as flood control, channel erosion control, or water quality enhancement. For
example, a basin used for water quality enhancement is designed to detain the water quality vol-
ume and slowly release it over a specified amount of time. This water quality volume is the first
flush of runoff, which is considered to contain the largest concentration of pollutants (Schueler
1987). In contrast, a basin used for flood or channel erosion control is designed to detain and
release runoff from a given storm event at a predetermined maximum release rate. This release
rate may vary from one watershed to another based on pre-developed conditions. Localities,
through stormwater management and erosion control ordinances, have traditionally set the allow-
able release rates for given frequency storm events to equal the watershed's pre-developed rates.
This technique has become a convenient and consistent mechanism to establish the design param-
eters for a stormwater management facility, particularly as it relates to flood control or stream
channel erosion control. Depending on location, the allowable release rate for controlling steam
channel erosion or flooding may be established by ordinance using the state's minimum criteria,
or by analyzing specific downstream topographic, geographic, or geologic conditions to select
alternate criteria. Obviously, the engineer should be aware of what the local requirements are
before beginning the design. The design examples and calculations in this text use minimum
requirements for illustrative purposes.
25.7.3 s torage v olume r equirement e stimates
Stormwater management facilities are designed using a trial and error process. The engineer does
many iterative routings to select a minimum facility size with the proper outlet controls. Each itera-
tive routing requires that the facility size (stage-storage relationship) and the outlet configuration
(stage-discharge relationship) be evaluated for performance against the watershed requirements.
A graphical evaluation of the inflow hydrograph vs. an approximation of the outflow-rating curve
provides the engineer with an estimate of the required storage volume. Starting with this assumed
required volume, the number of iterations is reduced. The graphical hydrograph analysis requires
that the evaluation of the watershed's hydrology produce a runoff hydrograph for the appropriate
design storms. Generally, local stormwater management regulations allow the use of SCS methods
or the modified rational method (critical storm duration approach) for analysis. Many techniques are
available to generate the resulting runoff hydrographs based on these methods. It is the engineer's
responsibility to be familiar with the limitations and assumptions of the methods as they apply to
generating hydrographs.
Graphical procedures can be time consuming, especially when dealing with multiple storms, and
are therefore not practical when designing a detention facility for a small site development. Shortcut
procedures have been developed to allow the engineer to approximate the storage volume require-
ments. Such methods described in TR-55 include storage volume for detention basins (Section 5-4.2)
and critical storm duration—modified rational method—direct solution, (Section 5-4.4), which can
be used as planning tools. Final design should be refined using a more accurate hydrograph routing
procedure. Sometimes these shortcut methods may be used for final design, but they must be used
with caution because they only approximate the required storage volume.
It should be noted that the TR-55 tabular hydrograph method does not produce a full hydro-
graph. The tabular method generates only the portion of the hydrograph that contains the peak
discharge and some of the time steps just before and just after the peak. The missing values must be