Agriculture Reference
In-Depth Information
here is to address the majority of annual low volume and provide adequate
time within an SCM for pollutant removal. The exact time or allowable release 
rate may not be speciied. The WQV is often determined as the runoff volume 
produced by the 75th-95th percentile storm event ( Section 2.2 ), or is
sometimes simply given as ~12-40 mm of runoff.
Some jurisdictions include provisions for groundwater recharge to maintain
aquifers consistent with a design storm approach. Living roofs cannot be used to
satisfy groundwater recharge requirements as there is no hydraulic connection to
the subsurface.
  Inherent in the design storm approach is that the performance of the SCM 
during the speciied return frequency event(s) is considered in isolation - e.g., the
2-year, 24-hour ARI or the water quality design storm. The time of year, implying
seasonal variation of climate and/or performance, is not considered. It is assumed 
that the living roof is dry when rainfall begins, or a retention basin is empty and
the full storage capacity is available. In reality, this may be an optimistic approach
that overestimates actual retention performance on a day-to-day basis, but it is
nonetheless common practice. Conversely, living roof champions may face unfair
criticism claiming that this particular technology is less effective during periods of
frequent wet weather. Further complicating the discussion, it has been shown
that water storage predicted by measures of PAW estimate the maximum amount
of stormwater retention on average for storms that exceed the PAW (Fassman
and Simcock 2012).
The characteristics of each design storm are derived from historic precipitation
patterns, but the end result is a statistical agglomeration into a unique
combination of rainfall depth and duration that is rarely observed, due in part to
the enormous variability of actual event conditions. Demonstration of compliance
with objectives in many cases requires only a total depth of runoff or the peak
low, rather than a temporal distribution of the result (i.e., a runoff hydrograph).
It is recognized that design storms have been applied successfully in drainage
design for decades, using, for example, curve numbers ( CN s) or runoff
coeficients to determine low characteristics ( Section 3.5.2 and 3.5.3 ).
While not common in current practice, there is an increasing shift towards
and endorsement of continuous simulation for stormwater design. A continuous
simulation models a long-term time series of alternating periods of dry and wet
weather. It accounts for ET, and thus how much water storage capacity in an
SCM is actually available for any given event. Continuous simulations may be 
conigured on a variety of timescales, considering anywhere from one-minute to
daily time steps, with subsequent implications for input data requirements,
accuracy and interpretation. Continuous simulation is helpful for evaluating
quantitative design objectives such as:
•  Match the runoff hydrograph or low duration curve for a design storm or 
series of storms. This approach is quite progressive, requiring consideration of
peak low, volume, duration and timing of discharges. To achieve these
 
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