Environmental Engineering Reference
In-Depth Information
Mar-93
Main Branch of Perkiomen Creek at East Greenville, PA
hydeography of stream flow, phosphorus concentration and suspended solids concentration
1200
3500
3000
1000
Hydrograph
2500
800
Phosphorus
Suspended solids
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600
1500
400
1000
200
2500
0
0
3/4/93
0.00
3/4/93
6.00
3/4/93
12.00
3/4/93
18.00
3/5/93
0.00
3/5/93
8.00
3/5/93
12.00
3/5/93
18.00
3/6/93
0.00
3/6/93
8.00
3/6/93
12.00
3/6/93
18.00
Figure 2-10 Chemograph precedes hydrograph during runoff, requiring double integration
to calculate the mass transport.
fairly constant discharge, both in terms of quantity and quality. With stormwater
runoff, however, the pollutant concentration for certain pollutants varies by two
orders of magnitude during a runoff event (Figure 2-10), and the total volume of
runoff generated from a developed site can increase sixfold over predevelopment
conditions. When the variability of rainfall is added to this setting, the pollutant
removal process requires a different set of technical solutions.
Many of the measurements of pollutant transport in stormwater have docu-
mented the fact that a concurrent measurement of chemical concentration over a
hydrograph shows a distinct increase that “peaks” before the flow hydrograph,
leading to the concept of “first flush” transport. In fact, the particulate-associated
pollutants that are initially scoured from the land surface and suspended in the
runoff are transported by the kinetic energy of the moving fluid flow and observed
in a stream or river before the fluid wave peak that occurs. These pollutants
include sediment, phosphorus that is moving with colloids (clay particles), met-
als, and organic particles or natural detritus. Those pollutants that are dissolved
in the runoff (solutes), however, may actually decrease in concentration during
heavy runoff as the result of dilution. These include nitrate, salts, and some
synthetic organic compounds applied to the land for a variety of purposes.
Managing stormwater to reduce these pollutants includes reducing not only
the volume of runoff but also the sources of these pollutants, as well as restoring
and protecting the natural systems that are able to remove them from the runoff.
These include stream buffers, vegetated systems, and the natural soil mantle, all
of which can be used to reduce the discharge of pollutants to waterways.
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