Environmental Engineering Reference
In-Depth Information
the disturbed area to its discharge from the disturbed area. Time of concentration
can be affected by adjusting the length or roughness of natural flow paths and
routing through BMPs. If time of concentration is kept constant for pre- and
postdevelopment conditions, the peak rate is completely dependent on the volume
of surface runoff and can be completely controlled by implementing additional
volume control. Repeat steps 5 and 6 for the larger storms and determine if
additional volume control can be implemented to control the peak rate.
The use of widely distributed, volume-reducing BMPs can significantly
increase the postdevelopment runoff travel time to the point of ultimate site
discharge and therefore decrease the peak rate of discharge. The Delaware
urban runoff management model calculates the extended travel time through
storage elements, even when completely full, to adjust peak flow rates [24]. The
extended travel time is essentially the residence time of the storage elements,
found by dividing the total storage by the 10-year peak flow rate. This increased
travel time can be added to the time of concentration of the area to account for
the slowing effect of the volume-reducing BMPs. This can significantly reduce
or even eliminate the detention storage required for peak rate control. This
method can and should be used with simple volume diversion explained above
and does not entail routing through the volume-reducing BMP(s). However, it
should not be used with the routing methods described below.
Methods Involving Routing
Composite BMPs with Routing
For optimal stormwater management, this
manual suggests widely distributed BMPs for volume, rate, and quality control.
This approach, however, can be very cumbersome to evaluate in detail with
common computer models. To facilitate modeling, similar types of BMPs with
similar outlet configurations (weirs, low-flow orifices, etc.) can be combined
within the model. For modeling purposes, storage of the combined BMP is simply
the sum of the BMP capacities that it represents. A stage-storage-discharge
relationship (including ongoing infiltration) can be developed for the combined
BMP based on the configuration of the individual systems. The combined BMP(s)
can then be routed normally and the results submitted. BMPs that are grouped
together in this manner should have similar drainage area/storage volume ratios
to ensure that the individual BMPs function properly. This method should not be
used in conjunction with the travel time/time of concentration adjustment method
described above.
Full BMP Routing, Including Ongoing Infiltration
For situations where the
methods described above do not apply or for projects with a very few BMPs,
these BMPs can be modeled using traditional routing methodologies that take into
account ongoing infiltration. Designers are encouraged to select and design BMPs
that detain and infiltrate stormwater runoff and release at the predevelopment
volume and rate, or as required. See Chapter 7 for design guidance on infiltration
and detention BMPs.
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