PROJECT
DESCRIPTION
Charlotte and
Hinesburg are concerned about present and future consequences of
increased percent imperviousness in the towns in the face of
increasing development pressure, and the implications for stormwater
management. While neighboring towns to the north are currently in
“crisis-mode”, addressing stormwater issues in already urbanized
settings, Charlotte and Hinesburg are well positioned to plan for
development that is more compatible with our natural systems.
Based on a recent analysis of percent imperviousness conducted by
Lewis Creek Association (LCA), the portion of Charlotte contained in
the Lewis Creek watershed has impervious cover up to 3.4 %,
while for Hinesburg the estimate is up to 2.8 % (Underwood, 2002).
These estimates were made following methods of Center for Watershed
Protection (CWP, et al, 1999) using a detailed land cover / land use
data set developed specifically for the Lewis Creek watershed (Capen
et al, 2000) based on 1995 to 2000 orthophotographs (see Figure 1).
While these percent impervious estimates are not as high as those for
some urbanized Chittenden County watersheds (3.9 to 25.1 %; Pease,
1997), the percent impervious values calculated for the Lewis Creek
sub-watersheds are already in a range which correlates with geomorphic
impacts to the river, leading to streambank erosion, loss of riparian
lands, damage to infrastructure, and mobilization of sediment and
phosphorus to receiving waters.
Statistical relationships for Vermont watersheds have been
demonstrated between stream channel enlargement / instability and
percent imperviousness (CWP et al, 1999). For eight Vermont
watersheds, varying in area from 0.5 square mile to 24 square miles,
alluvial streams were found to demonstrate evidence of geomorphic
stress at total basin imperviousness of 2 % or more. Active
adjustment of channel form was observed for imperviousness exceeding
9%. Increases in percent imperviousness have also been statistically
correlated to decreases in biological diversity and overall
macro-invertebrate health in receiving waters (CWP, 2000).
Geomorphic assessments conducted to date in the Lewis Creek (2001 and
2002) indicate that most of the reaches in Fair to Poor geomorphic
condition are located in sub-watersheds of greater than 2 %
imperviousness (LCA, 2002). Many river reaches in these In-Transition
sub-watersheds (Figure 1) are exhibiting active lateral and vertical
adjustments in response to development-related stressors including
floodplain encroachment, undersized bridge crossings, reduced riparian
buffers, and conversion of agricultural and forested lands to
residential and commercial use.
At present, neither Charlotte nor Hinesburg have a stormwater
ordinance. Certain industrial projects, and larger commercial or
residential projects exceeding one acre of developed surface will
require stormwater management permits, falling under State and Federal
stormwater regulation to achieve stormwater runoff equivalent to the
pre-developed condition. However, based on past development patterns,
it is expected that the majority of future development in the towns
will be single-family dwellings on smaller acreage lots (1 to 10+
acres). A substantial percentage of these residential development
projects will not fall under State or Federal oversight requiring
stormwater management structures or practices. The Towns expect that
ultimate buildout under current zoning regulations could result in
aggregate percent imperviousness that exceeds geomorphic thresholds
for sub-watersheds located within town boundaries. Charlotte and
Hinesburg (and adjoining towns) will then be faced with increased
expenses associated with damage to infrastructure, degradation of
water quality, and mobilization of sediment and phosphorus to
receiving waters.
Based on a preliminary, ultimate buildout scenario presented to the
town by LCA in the summer of 2002 Charlotte and Hinesburg can expect
to more than double their number of residential structures under
current zoning. With additional funding, the town Planning
Commission can work with consultants to refine buildout scenarios
considering alternate planning and zoning approaches (e.g., changing
minimum acreage in zoning districts, changing minimum lot coverages
per zoning district, adding high-density downtown or village districts
in exchange for reducing density in outlying areas) and considering
valuation of natural resources (e.g., natural communities, wildlife
corridors, stream buffer overlay districts.
The proposed project involves merging GIS-based calculations of
percent imperviousness, on a watershed basis, with the GIS-based
community build-out analysis to predict geomorphic condition resulting
from increasing percent imperviousness. This enhanced buildout
product will serve as a planning tool to improve decision-making,
facilitate consensus-building, and manage development with the goals
of minimizing impacts to water quality, and reducing future costs to
the town associated with stormwater–related consequences.
Merging the percent imperviousness calculations with various build-out
scenarios for the town will help to visualize and quantify the impacts
of expected low-density (5+ and 10+ -acre lot subdivisions) upland and
riparian development on the hydrologic and geomorphic conditions and
the serious potential for exacerbated contributions to non-point
source pollution. These tools can help to estimate at what
approximate level of buildout (under current or proposed zoning
scenarios), percent imperviousness will exceed the statistical,
geomorphic threshold of 9%.
As this project crosses several political boundaries, it will be
coordinated by Lewis Creek Association (Marty Illick, Director). LCA
has conducted initial impervious cover analysis in the Lewis Creek
watershed and has several natural resource data layers to contribute
to the project, as well as excellent working relationships with the
principal and adjoining towns. LCA will be responsible for assembling
the Project Steering Committee for an initial project kick-off meeting
and regular progress meetings during the course of the grant period.
LCA will also facilitate at project meetings.
The Project Steering Committee will consist of a representative from
each of the principal towns, Hinesburg and Charlotte; a representative
from CCRPC (Michelle Maresca, GIS Modeling/ Land Use Analyst); a
representative from ACRPC (Kevin Behm); and a chosen consultant(s) or
consultant team.
