25 October 2004
Conventional Storage Concepts. Conventional stormwater strategies often
include the storage of water in large centralized end-of-pipe facilities. Site designs
direct and convey most runoff as quickly as possible to these facilities and then
discharge through an outlet structure at a limited release rate (e.g., 2-year 24-hour pre-
development runoff rate). Conventional runoff management techniques can
dramatically reduce the flow of runoff into natural storage areas such as wetlands,
depriving a variety of organisms of the level of moisture they need.
Conventional approaches can have other negative impacts. By removing
opportunities for storage onsite, rates of ground water recharge will be reduced. In
addition, the concentrated flow conveyed to large-scale facilities accumulates pollutants
and increases the erosive force of the water, which must be slowed down and treated to
maintain the natural energy and chemical balance of the ecosystem. An increase in
temperature as the water is pooled may also be detrimental to the ecological integrity of
the receiving water.
INFILTRATION. Water stored in depressions will infiltrate into the soil at
different rates, depending on the soil type and the amount of moisture already in the
soil. Some of the water that infiltrates into the ground may then percolate further
downward into an aquifer, or travel horizontally and reappear as surface flow in a
stream. A portion of the water will be held in the soil and extracted by vegetation.
The capacity of the soil to absorb and infiltrate water is dependent on a
variety of factors such as soil structure (e.g., pore spaces and particle size),
classification (percentage of sand, silt, and clay) and biological activity (e.g., roots,
worms). Water is filtered by the soil system by various mechanisms such as adsorption
and chemical and biological reactions. Under natural conditions, a significant portion of
the annual precipitation may infiltrate into the ground. As land is developed, however,
many natural depressions that would otherwise collect water are eliminated, the soil is
compacted, and impervious area is added in the form of buildings and pavement.
Consequently, levels of infiltration typically decrease when a site is developed. The
additional runoff generated often results in degradation of the watercourse because of
bank erosion, increased flooding, and alteration of habitat characteristics.7
The infiltration flow patterns and processes are extremely important to
maintain the water balance in wetlands and the base flow in stream channels. Figure
4-5 illustrates how groundwater feeds an aquatic system.
Gordon et al., 1992.