Hydrologists often design for
quantity treatment where
large infrequent design flows
are considered for the purpose
of flood protection and drainage.
As a result, the Rational Method is
adopted for conveyance designs.
Figure 1— Cumulative frequency plot of runoff volume vs. flow rate
The Stormceptor® System is
designed for quality treatment. Historical
rainfall data is examined to determine
the characteristics of storms that
represent the majority of the events
that occur over the long term. These
events tend to be frequently occurring
small storms.
This design philosophy is similar to
that presented in the general guidance
document for Urban Runoff Quality
Management prepared by WEF and
ASCE.1 By analyzing historical rainfall
records, general trends indicate
that frequently occurring storms tend
to be small storms that constitute the
majority of runoff volume over the
long term. To illustrate, runoff from a
2.5 acre (1 ha), 100% impervious site
is plotted using historical rainfall data
for various areas in the state of Ohio
(Figure 1).
Figure 1 also demonstrates that a
large percentage of runoff volume
(almost 90% in this example) is contributed
by small storms that yield low
runoff rates (< 1.5 cfs or 42 L/s). By
designing stormwater quality treatment
systems to treat most frequently
occurring flows, a high level of treatment
and an economically feasible
solution is achieved. Treating higher
runoff rates does not necessarily yield
a significant increase in removal efficiency
and, therefore, does not provide
an optimum cost-effective solution.
The most cost-effective design for
stormwater quality treatment can be
determined using data found at the
“knee of the curve” in Figure 1.
For infrequently occurring higher
flows, the Stormceptor System treats
the “first flush” portion of a large
storm when pollution loading tends to
be higher. When the remainder of the
storm gives way to higher flows, the
patented internal bypass enables the
system to treat up to its maximum flow
rate and to bypass the remainder of the
runoff volume.
By capturing frequently occurring
events and bypassing infrequent excess
flows, a high level of treatment is
achieved without the risk of scouring
previously captured materials. Scouring
generally occurs when flows or
velocities entering a device are high
enough to re-suspend settled sediment.
The Stormceptor System prevents
scouring by controlling flows in the
lower chamber and internally bypassing
excess flows. It is the only proprietary
stormwater Best Management
Practice with an in-line bypass capability.
It effectively allows excess flows to
bypass the lower chamber without the
added cost of installing upstream and
downstream off-line bypass structures.
1 Water Environment Federation and American
Society of Civil Engineers (1998)
Urban Runoff Quality Management.
WEF Manual of Practice No. 23,
ASCE Manual and Report on Engineering Practice No. 87, USA.
Penh Tov is Stormwater Specialist with
Stormceptor Group of Companies.
For further details, e-mail: mmckenzie@monteco.ca.
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