Piedmont Greenway Master Plan - Town of Kernersville

THE WATERSHEDS OF FORSYTH
AND GUILFORD COUNTIES
Effects of Land Use on Watersheds
Land use dramatically effects creeks, rivers and estuaries. Understanding the impacts of land use is
essential to effective watershed management planning. To assess the effects in a meaningful way requires
careful analysis at the watershed level. Rural watershed management plans generally consider spatial
relationships -- how close are grazing lands or manure storage to streams These plans also consider social
and demographic aspects -- who owns the land and who takes care of it By contrast, federal mandates for
urban areas generally require use of “best management practices”. The impacts of urbanization are closely
linked to the spatial pattern of development. Pattern matters more than the proportion of the entire watershed
that is urbanized, and more than the relative proportions of urban land uses. Urbanization causes by far
the most severe impacts on the basin’s creeks, rivers and lakes.
Non-urban land uses are factors, but the biggest challenge is to preserve and enhance streams in urban
areas. Nor can the effects of urbanization be reduced to questions of pollutants per acre. A host of interrelated
changes must be considered such as changes that accelerate runoff, alter patterns of erosion and deposition
and alter the flow of water, sediment and nutrients between riparian areas and streams.
Most municipalities have not made preservation of aquatic resources a goal of their comprehensive
plans. They have the authority to undertake a variety of initiatives, but usually do not have a methodology
for developing and carrying out measures on an appropriate watershed-wide scale.
Effects of Urbanization on Forsyth
and Guilford County Basin Watersheds
Land Uses
Changes in the land use characteristics of a watershed, individually or in combination, can alter its
structure or impair key ecological functions. The interrelated components are connected and can be described
as follows: changes in land use lead to changes in the shapes and contours of streams, thence to
changes in the way water flows through them and how they carry and deposit sediment, with the result of
changing the way stream habitats function, both physically and ecologically.
Suburban Sprawl
Sprawl is characterized by segregation of land uses, overall low density and dependency on automobiles
for transportation. These characteristics have a direct effect on the water quality and natural resources
in the watershed. Vast areas covered by houses and workplaces affect watersheds principally by altering the
way they drain. Particularly significant is the narrowing of streamside corridors and their division into non
contiguous segments by roads and other development.
Urbanization and Imperviousness
A major effect of urbanization is imperviousness. This term is often applied to surfaces -- roads, sidewalks,
rooftops, parking lots -- that prevent or inhibit rainfall from sinking into groundcover and groundwater.
Urbanization and increased imperviousness can produce smaller, more frequent floods. During
extreme events, increased imperviousness has little effect on flows. The reason is that at such times rainfall
saturates the natural soils and renders them effectively impervious. Nevertheless, urbanization and increased
imperviousness can increase smaller, more frequent flooding -- 1.5- to 2-year floods -- by up to 10 times,
particularly during smaller storms and in smaller streams. Greater imperviousness is also correlated to reduced
habitat quality as measured by biological indices. To understand how this process happens it is necessary
to examine the relationship
Changes to stream structure and disconnection from floodplains are the most significant and characteristic
impacts of land use on streams in the Piedmont Greenway Corridor. They include:
• Destabilization of streambeds and banks (caused by the change in amount of timing and
flows as the result of increased impervious cover), increased drainage density, and changes to
sediment inputs.
• Agricultural and urban encroachment on riparian corridors (especially along Reedy Fork
Creek and Beaver Creek Corridors).
• Increased subdivision building (including the Moore’s Creek corridor and areas of Reedy
Fork, Beaver Creek, and Kerners Mill Creek outside of the critical watershed boundary).
• Disconnection of streams from floodplains, caused by erosive down cutting of streambeds
and by construction of channels (stormwater uses from roads, housing etc).
Imperviousness is most significant during the smaller but more frequent storm events because these
bankfull flows most strongly influence stream characteristics. They do the major “work” of a perennial
stream in moving sediment and thereby determining the stream’s form. The configuration of the drainage
patterns of the study corridor suggest a gently sloping plain underlain by alluvial material interspersed with
clays, created by this work of streams, carrying sediment down from hillsides (see figures 7, 14, and 15).
The relatively flat alluvial plain (Reedy Fork as an example) was created by streams moving back and forth
over the valley floor. Periodic flooding is essential to some riparian plants such as willow, and helps to
PIEDMONT GREENWAY
Final Master Plan Report 87