ENVIRONMENTAL - International Erosion Control Association

PEER
review
Case Study:
Maho Bay Watershed Road Erosion
Reduction Project, St. John, USVI
By Carlos E. Ramos-Scharron, PhD
Increased sediment delivery associated with land development and unpaved roads is a key stressor of nearshore coral reef
systems in the U.S. Virgin Islands. This article briefl y describes eff orts to reduce sediment production rates from an unpaved
road segment in the island of St. John, USVI. Five insloped cemented swales and a paved ditch were built along a 230 m
long road segment to reduce the forces applied by fl owing water on the road surface. The method proved to be eff ective as
sediment production data collected with a sediment trap showed that post-treatment erosion rates were only 30 percent of
pre-treatment levels.
The increased rate of sediment delivery
that typically accompanies
land development is one of the
greatest stressors threatening the coral
reef systems of the U.S. Virgin Islands.
Previous studies conducted on St. John-
USVI have identified the unpaved road
network as the most important source
of sediment on the island [1,2] . These
studies indicate that unpaved roads
erode at rates that may be up to 10,000
times higher than undisturbed hillslopes
[3,4] , and that sediment delivery
into coastal waters from watersheds
containing unpaved roads are 300–900
percent higher than from undisturbed
watersheds [5,6] .
An important goal of erosion studies
conducted between 1992 and 2001 by
Colorado State University (CSU) and
Island Resources Foundation (IRF) in
St. John was to provide guidance for
public and private agencies, as well as
local communities, in the development
and application of sediment control
strategies. Between 1999 and 2003 we
collaborated with the VI-Department
of Planning and Natural Resources
(VI-DPNR), VI National Park, and
homeowner associations in the development
and application of strategies
to reduce sediment loading rates into
several bays on St. John. An evaluation
of these efforts was performed with the
GIS-based STJ-EROS model [7] , and it
concluded that erosion control projects
reduced watershed-scale sediment load
rates by 24, 36, and 5 tons per year into
Lameshur Bay, Fish Bay, and Cinnamon
Bay, respectively. These reductions
represented a 10–45 percent drop in
sediment yields relative to rates preceding
the implementation of the erosion
control projects. Within some of these
watersheds, sediment yields have continued
to increase in spite of these efforts
as the unpaved road network continues
to grow.
The Maho Bay Watershed Erosion
Reduction Project was a derivation of
CSU’s and IRF’s efforts to actively participate
in locally-based erosion control
efforts. The road providing access to the
Maho Bay Camps (hereafter referred to
as Maho-Road) (Figure 1) was selected
as an adequate site for an erosion control
demonstration project for several
reasons. First, for many years managers
and maintenance crews at Maho Bay
Camps puzzled at trying to develop a
cost-effective strategy to reduce erosion
along Maho-Road. Maintaining
a drivable surface on Maho-Road was
very difficult, particularly during the
rainy season, as it tended to suffer severe
rilling damage during intense rain
events (Figure 2). Secondly, previous
field data identified Maho-Road as an
erosion ‘hot-spot,’ because observed
sediment production rates from Maho-
Road were higher than most other road
segments being monitored as part of
those studies.
Maho-Road also was visited by
community members and government
employees representing both territorial
and federal agencies during a 1999 University
of the Virgin Islands Water Resources
Research Institute special seminar
directed by this author. Seminar
participants observed the approximately
27 m 3 of sediment that had accumulated
on a silt-fence used for monitoring
sediment production from Maho-Road
over the previous two years. This visual
corroboration of the volume of
sediment that was being produced by
18 • ENVIRONMENTAL CONNECTION