Science-Based Landscape Planning in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 501

Vegetation canopy measurements (aerial photos can help here) to determine where vegeta-
tion removal will create the most slope instability, and where plantings can have the most
stabilizing impact.
This short example demonstrates there is a significant amount of data to collect and sci-
ence to know—even without considering the proposed activity. Besides sloped terrain—
which we might expect to become unstable with development—other seemingly stable
landscapes can be destabilized by human activity if the human activity causes a threshold
present to be exceeded. For instance, wetlands can only withstand a certain amount of
inundation before they become damaged. Seed banks found in freshwater wetlands play
a critical role in the maintenance of many of their plant communities (Leck 1989), and
their disturbance by floods can decrease seedling emergence (Peterson and Baldwin 2004).
Therefore, the possibility of damaging seed banks in the wetlands of urban wetlands must
be considered when routing stormwater flows.
15.4 Watershed Management
As demonstrated, significant reduction of environmental impacts at the parcel scale can
be achieved with science-based landscape planning. Individual parcels, however, are situ-
ated within watersheds of larger geographic extent, and there are also many opportunities
at this broader scale to apply science-based landscape planning methods. Before these
efforts are attempted, there are fundamental differences between parcels and watersheds
that must be considered when applying a science-based planning approach to watersheds.
Some of the major differences include
• Geographic scale . Landscape processes operating a certain way at one scale may
not mimic their behavior at another. Runoff provides an example. Since most indi-
vidual parcels do not contain a stream segment, overland flow is the dominant
form of runoff on the parcel. Across watersheds, however, streamflow transports
the most surface water in a drainage network.
• Landforms . Watersheds contain entire landforms, and their distribution results in
a highly varied terrain over large distances. For example, urbanized watersheds
in sedimentary environments exhibit a wide range of landforms and land cover
types, e.g., kames, eskers, moraines, valleys, lakes, deltas, wetlands, and forests.
Parcels may exhibit significant topographic variations over a small distance, but
typically do not contain a diverse set of landforms and land cover types. It is safe
to assume most homeowners and factories do not have river deltas and valleys on
their property.
• Land use . Urban watersheds contain a wide range of land uses, including agricul-
tural, industrial, commercial, and residential. Due to zoning regulations, a singu-
lar land use almost always exists at the parcel level, and where multiple uses do
exist (e.g., operating a business out of one's home) a zoning variance is required.
Some urban planning strategies, such as “New Urbanism” (Duany et al. 2000) and
“Smart Growth” (ICMA/USEPA 2010), advocate mixing land uses within neigh-
borhoods to preserve the character of older cities—but the mixing occurs at scales
larger than a single parcel.