Environmental Engineering Reference
In-Depth Information
Statistical dry year flows (1970-1994)
10,000
10-year low daily base
flow: 450 gpd/acre
1,000
10-year, 7-day low flow: 192 gpd/acre
100
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 1-21
Base flow analysis of minimum low flow in a stream.
impervious surfaces pours into the stream. The reduction in base flow of any
surface stream usually represents a significant loss of water quality and aquatic
habitat, as countless studies have documented. In effect, when we reduce the base
flow below the minimum required to sustain the aquatic habitat (Figure 1-21),
the system dies or is transformed into something very different.
Altered Stream Channel Morphology
While the increased runoff volume produced and the pollutant load transported
with stormwater are important, the impact of this flow in the network of natural
stream channels through which it passes is of equal concern. Even where a
detention system has been constructed in one or many development sites, the
total increase in runoff volume results in a greater flow for longer periods, which
affects the natural stream channel by altering the morphology of these channels.
Since most surface streams have reached equilibrium with the flow variability
under natural conditions, the shape, size, depth, and bank condition of the riparian
section is long established prior to land development, although larger river sys-
tems that have formed a broad floodplain over millennia will move the channel
within this floodplain as discharges vary [18]. On smaller streams that have no
tributaries, referred to as
first-order streams
, the impact of the increased volume
of runoff is significant, and erodes the channel banks to increase the non-point
source pollutant load as the channel readjusts to convey this greater flow.
Water Supply Impacts
A modern water treatment plant is capable of removing virtually all of the sed-
iment and associated pollutants that are attached to the soil particles from a
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