STORMWATER HYDROLOGY AND QUALITY - Low Impact Development and Sustainable Stormwater Management - page 36

Figure 2-2 This highly impervious watershed (90%) has even paved the stream bank
(Darby Creek, Pennsylvania).
After decades of destroying natural wetlands, generally in the name of cul-
tivation, we have come to recognize and appreciate their ecologic value. The
statistics on wetland filling and draining processes, some of which are still
being carried out, are depressing from an ecological perspective. Niering and
Littlehales [8] summarize the loss of this resource across North America dur-
ing the past three centuries, and also beautifully illustrate regional systems. The
impact of these losses is much more significant in an individual watershed that
has been “urbanized” during that same period. There are countless examples sur-
rounding each of our urban centers, but the Darby Creek in suburban Philadelphia
is a prime example of the “paving over” of a watershed (Figure 2-2), where
scarcely a wetland can be found except at the lower end of the basin (John
Heinz Wetland Center, USF&WS, Tinicum, PA). Here the ground was sim-
ply too wet to be built upon without massive filling, and while conservation
efforts have preserved some of the original wetland habitats, little remains of the
natural system.
The question of greatest interest here is: How do we develop a site that
contains wetlands, or is it impossible to do so without further loss? The answer to
this question turns on the specific site drainage conditions, since most wetlands
occur in the lower-lying portions of a tract of land, but are usually sustained
by groundwater flows that begin on the surrounding uplands, the very areas that
would be considered “high and dry” prime development land. Where this is
the case, the types of infiltration and groundwater recharge of rainfall systems
discussed here must be situated at or near the built structures, to assure the
continued recharge of groundwater. One critical aspect of most wetland systems
is that they are supported by a shallow water table, and upland recharge must
maintain the water table elevation in the wetland portion of the parcel. Countless
wetlands have been destroyed by a relatively small lowering of the local water