STORMWATER HYDROLOGY AND QUALITY - Low Impact Development and Sustainable Stormwater Management - page 39

While any volume control guideline must be quite specific concerning the
volume of runoff to be controlled from a development site, it should not limit
the methods by which this can be accomplished. The selection of a BMP, or
combination of BMPs, is left to the design process. In all instances, minimizing
the volume increase from existing and future development is the goal. The LID
measures described in this topic place emphasis on infiltration of precipitation
as an important solution, but this is only one of the three basic methods that
reduce the volume of runoff from land development. These three methods can
be summarized as follows:
1. Infiltration
2. Vegetation systems that provide evapotranspiration
3. Capture and reuse
All of the stormwater management systems described in this topic include one
or more of these methods, depending on specific site conditions that constrain
stormwater management opportunities.
2.4 THE WATER QUALITY IMPACTS OF LAND DEVELOPMENT
The health of our rivers and streams reflects that of the entire ecosystem in which
we live, so their importance to our species goes well beyond the immediate needs
of potable supply. Every stream is a living system comprised of many parts, with
the web of life critical to our own survival, even though in the United States
we may no longer go to the river each day to fish, drink, or bathe. This aquatic
system is delicately balanced and supported by the natural flow in the stream,
with the largest portion of that supply coming from groundwater discharge, as
base flow. Thus, any major reduction in base flow and corresponding increase in
polluted surface runoff degrades that surface system not only by runoff discharges
but also by reduced base flow (Figure 2-5).
The relationship between rainfall infiltration and subsequent base flow dis-
charge from the subsurface to surface streams is usually not recognized as an
important factor in surface water quality, but in fact it is the most important
source of water for the aquatic environment. Some research effort has been made
to determine exactly how depleted a stream flow can become without affecting
the habitat of finfish species. Such models suggest that when the Q7-10 flow
(the lowest 7-consecutive day flow with a 10-year frequency) is not sustained,
trout are unable to move from pool to pool, and spawning is prevented. Each
and every stream has its own set of low-flow impacts, but at some reduction in
long-term base flow, the system cannot be sustained and the aquatic community
collapses.
As a watershed undergoes urbanization, we can recognize degraded environ-
mental quality beyond sediment-laden streams and polluted lakes. Long-term