Environmental Engineering Reference
In-Depth Information
Where watershed stormwater planning has been undertaken, hydrologic model-
ing may have been performed to suggest more stringent release-rate criteria on
subbasins within the watershed. As volume-reduction BMPs are incorporated into
stormwater management on a watershed basis, release-rate values will warrant
reevaluation. Volume-control guidelines will reduce or perhaps even eliminate
the increase in peak rate for many storm events.
Water Quality Protection Guideline
The volume control required with the control guideline may result in removal of
the major fraction of particulate-associated pollutants from impervious surfaces
during most storms. Semipervious surfaces such as
lawnscapes
, subject to con-
tinuing fertilization, may generate NPS pollutants throughout a major storm, as
may stream banks subjected to severe flows. While infiltration and landscape LID
measures are very effective in NPS reduction, if the volume control measures sim-
ply overflow during severe storms, they will not achieve the control anticipated.
Solutes (such as NO
3
) will continue to be transported in runoff throughout the
storm, regardless of magnitude, so their control will rely on application reduction
and vegetation management programs.
The control guideline will provide water quality control and stream channel
protection as well as flood control protection for most storms if the measures
drain reasonably well and are sized and distributed adequately. A less stringent
volume guideline will not fully mitigate the peak rate for larger storms, and
will require the addition of secondary measures for peak-rate control. These
secondary measures could provide additional water quality protection. In the
event that this secondary measure is added to assure rate mitigation during
severe storms, the incorporation of vegetative elements could provide effective
water quality controls.
The control guideline recommended for total water quality protection is
Achieve an 85% reduction in postdevelopment particulate-associated pol-
lutant load (as represented by total suspended solids), an 85% reduction in
postdevelopment total phosphorus loads, and a 50% reduction in postde-
velopment solute loads (as represented by NO
3
-N), all based on postdevel-
opment land use.
These reductions may be estimated based on the pollutant load for each land
use type and the pollutant removal effectiveness of the proposed measures, as
shown in other chapters. The inclusion of total phosphorus as a parameter is in
recognition of the fact that much of the phosphorus in transit with stormwater is
attached to the small (colloidal) soil particles, which are not subject to gravity
settlement in conventional detention structures, except over extended periods.
With infiltration or vegetative treatment, however, the removal of both suspended
solids and total phosphorus should be very high, approaching 100%.
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