HomeMy WebLinkAboutSUB201000156 Assessment - Environmental 2011-03-30 Expected Impacts of Hillslope
Development on Willow Lake
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Joanna Crowe Curran, Ph.D.
Assistant Professor
Civil and Environmental Engineering
University of Virginia
This document details the expected sedimentation and nutrient influx to Willow Lake that will occur
upon disturbance of the adjacent hillside. It is well known that development will disturb a hillside and
that development will increase the sediment erosion from that same hillside. Below I provide a brief
summary of how the hillside is known to change,case studies for comparison, and the results of
applying common engineering models to estimate erosion and nutrient flux from the proposed
development around Willow Lake.
Development and impervious cover:
The volume of runoff from a site increases linearly with watershed size. However, when urbanization
occurs in a watershed this natural trend is altered.As the percent of impervious cover increases past
20%of the watershed area,the relation between runoff volume and impervious cover is no longer liner.
The volume of runoff increases at a greater rate with the increase in impervious cover over 20%,
regardless of any steep slopes or other watershed topography.
No matter the watershed,over 20%impervious cover dramatically increases the runoff from a storm.
Physics of steep slopes:
Development on a steep slope is more detrimental than on a milder slope, and every degree of slope
matters as the slope steepens.The primary force driving the erosion down the hill, including the creep
of soils and the slope failures, is gravity. When a slope becomes steep the critical values for initiating soil
movement are adjusted because the gravitational force becomes much more significant than at lower
slopes.'
Engineered/Developed slopes:
A comparative study in Singapore examined a forested hillslope, a hillslope under construction, and a
hillslope post-construction and re-vegetated over a full year.2 Local slopes of these hillsides reached
20°.All slope processes were measured and monitored, including infiltration rates, pore water suction,
runoff, rill formation, rain splash,ground lowering, and slope instability. As expected,the bare
construction site experienced high rates of instability and erosion. More surprising is that the
engineered and re-vegetated slopes showed fundamental changes in infiltration paths and rates,
illustrating a fundamental change in the way the hillslope responds to rainfall.There was not significant
rain splash or rill formation, meaning that the engineered slope appeared to function well on its surface
while the subsurface processes were altered. Differences were quantified in the suspended sediment
concentration coming off the engineered and re-vegetated slopes as well as in the slope stability.
Suspended sediment concentrations from the engineered slopes were 4-8 times higher than from the
forested slopes.Another significant finding was that the failures on the engineered slope were larger in
volume than for the forested land.The number of local slope failures was not greater but the types of
failures were much larger and deeper on the engineered slope.These failures were large and rotational,
indicating a deep failure plane within the hillside.
New York City Local Law 7 recommends avoiding any development on steep slopes, defined as slopes of
15%, and banning all land-disturbing activities on severe slopes defined as over 25%. The reasons for
this are the known detrimental and costly effects of development on steep slopes.There was an
increase in landslides on slopes subjected to cut and fill activities. Slope stability with development was
decreased by excavation, mechanical overloading by fill placement, the introduction of additional water
with sewer and water pipes, concentration of stormwater runoff, removal or conversion of vegetation,
or a combination of items listed. Where the slope appears stable after development, soil creep was
found to be accelerated and caused damage to residences and public services. Re-vegetation of slopes is
encouraged but with the caveat that it only reduces surface erosion and does not repair damage to the
subsurface. The report recommends that on slopes over 45°strong rooted woody shrubs be planted
instead of trees, as trees add weight and transmit vibrations to the soil when subject to heavy winds.3
Although the surface of an engineered hill appears stable, the underlying physical processes have been
altered. Re-vegetation of a slope only reduces surface erosion and hides underlying slope instabilities.
Suspended sediment concentrations are increased after slope disturbance. Slope failures are larger and
deeper. Hidden instabilities in the disturbed and re-vegetated hillslope will appear over time.
Lessons from Albemarle County:
There are already well-known issues with sedimentation of Lake Hollymead. Sedimentation of the lake
was a result of impervious cover and development in the drainage area. To avoid repeating the same
sedimentation problems as Willow Lake, below are a few of the statements in response to the situation
at Hollymead and what was found about the problem.
Erosion and sediment control measures that conform to minimum state law requirements control at
most 60%of the sediment under normal (not storm) conditions. Reaching minimum standards is
comparable to a C grade. When developers do more and install numerous stormwater BMPs, the
capture rate increases to only 80%of the sediment in runoff.4'5 Unfortunately it is well known that many
stormwater BMPs are improperly installed and violations of the rules is common.'
The harm in the case of lake sedimentation is to the immediate property owners. Albemarle County staff
for the Board of Supervisors did do some preliminary analysis on how Hollymead Lake could be
maintained and the sediment removed. "None of the alternatives are considered ideal and all appear
to require initial expenditures upwards of$500,000 for some base level of service. There would also be a
need for ongoing maintenance after the initial work.i6
Deer Lake Estates in Ruckersville,VA:
The sediment detention pond at Deer Lake Estates in Ruckersville is approximately 8 acres in size and
has developed a significant algae problem since it was created in the 1970s. Algae grow where there is a
large supply of nutrients and a shallow water depth, making the presence of algae an excellent indicator
of detention pond health. The pond was 15ft when constructed in the 1970s and has filled such that in
2010 it averaged around Oft deep.There has been a large amount of development in the area of the lake
as housing in the drainage area has been built in recent decades. The resulting influx of sediment along
with nutrients from lawns (which have replaced fields) has generated the algae blooms.
To reduce the algae and attempt to return Deer Lake to a cleaner state, the HOA has choices of
dredging, introducing alum into the lake, building a fountain and aeration system in the lake,
biomanipulation of the aquatic system by introducing carp, or altering the land use and practices of the
homeowners in the drainage area. None of these fixes is easy or inexpensive. The area around the lake
has lost much of its riparian buffer and many of the lawns are manicured and fertilized. With more
development planned in the drainage area, it is not clear how much the HOA can do.'
Willow Lake:
Willow Lake was created in 1939, and is roughly 2.74 acres in size. It has served as a sediment detention
pond for the Willow Creek Community. It is a small lake but has remained clear (low turbidity) and free
of algae over the years. This is unusual for a detention basin that has no algae or sediment reduction
mechanisms (no fountains or dredging). The clean state of Willow Lake can be attributed to the lack of
development along the slopes immediately surrounding the lake. These slopes are steep and have been
left to vegetate on their own without mowing or landscaping. They act as a natural buffer to stormwater
coming from houses and condos in the upper portions of the Willow Lake development. If they are
developed, there will be an unavoidable increase in the sediment delivery rate to Willow Lake and it will
fill with sediment, shallow, and develop water quality problems.
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Photo shows the current state of the hillslope development area adjacent to Willow Lake.
Soils around Willow Lake:
The USDA Natural Resources Conservation Service (NRCS) provides custom soil reports.8 Soils along the
slope of interest are predominantly clay and do not drain well. The NRCS provides ratings of the
suitability of soils to different types of use and development.The soils on the slope around Willow Lake
are listed as Very Limited and Unsuitable in categories that include dwellings, local roads and streets,
shallow excavations, and lawns and landscaping. The soils are limited and unsuitable due to high clay
•
content (limited drainage and high shrink-swell ability) and slope. The soils will also preclude the use of
permeable pavement or pavers in the proposed development.
Sediment Erosion:
The Revised Universal Soil Loss Equation model Version 2 (RUSLE2) is a standard engineering model used
to estimate the soil erosion from sites under different management conditions.9 Slope length, slope
steepness, soil type, climate, and land management practices are input by the user. In all scenarios of
Willow Lake modeled, there was no contouring of the slope, no strips or barriers, and no diversions from
the slope. Three management scenarios, or vegetation covers were modeled: dense grass to simulate an
undisturbed state; rough bare land to simulate the ground during construction but with some erosion
protection due to roughness; 4 year old grass cover to represent a re-vegetated state.
The development of sites C8 through C3 is of the most interest as they are planned for the slope
adjacent to Willow Lake. All of these sites are dominated by Rabun Clay soils and contain critical and
non-critical slope segments. Erosion from the critical and non-critical slope segments is calculated
separately and summed. The development area encompassed by these sites is 2.19 acres, which is
applied to provide an estimate of sediment loss and delivery from sites C8-C3 each year. The conversion
from tons/acre to tons/ft3 of sediment was done to provide an estimate of the volume of soil erosion
per year.The bulk soil density as reported by NRCS was used.
Results of RUSLE2 Modeling:
Soil Type Length Slope Management Sediment Sediment Sediment
delivery, delivery, delivery,
tons/acre/year tons/year ft3/year
Rabun clay 75 32 dense grass 0.17 0.15 2.1
Rabun clay 195 20 dense grass 0.12 0.16 2.2
sum for dense grass scenario 0.29 0.31 4.3
Rabun clay 75 32 bare, rough 120 105.12 1460.0
Rabun clay 195 20 bare, rough 67 87.77 1219.0
sum for bare, rough surface scenario 187 192.89 2679.0
Rabun clay 75 32 grass, 4 yrs old 36 31.54 438.0
Rabun clay 195 20 grass, 4 yrs old 31 40.61 564.0
sum for established grass scenario 67 72.15 1002.0
Sediment delivery, tons/year
200 — Wherever the land cover is
dense grass, there is minimal
160 soil loss. Where the slope is
120 -- —_-_.____-.-_ -.- _..___ _-_. _--. disturbed, even when that
disturbance is in the past, the
80 soil erosion is much higher.
40 _ _ _ .__
0
dense grass bare, rough grass,4 yrs old
Water Runoff:
To provide a means of quantifying the volume of runoff, the James River Association in partnership with
Timmons Engineering developed a web based tool that estimates the runoff volume and nutrient loads
when provided with site and house characteristics and lawn management practices.10 Three scenarios
were modeled through this tool to simulate a case where there is no house on the land, a typical house
on the site, and a 'green' house or a house built to be as environmentally as possible. All scenarios are
for a single family house on a single lot, representing any one of the planned houses from C8 to C3. Lot
and house size are based on information available on real estate sites for houses that are for sale in the
subdivision.The exception is the 'green house'for which is fabricated. In each case the results are based
on stormwater runoff generated by a 1-inch rainstorm.
No House Typical House 'Green' House
Lot Size 11,222 ft2 11,222 ft2 11, 286 ft2
% house 0 20 0
driveway 0 4 9
lawn 0 51 4
trees and shrubs 100 25 53
Green Roof 0 0 34
Rain Barrels 0 0 575 ft2
Rain Garden 0 0 1 garden, 200 gallons
# Downspouts 0 4 4
#spouts to impervious 0 0 0
Fertilizer application never 1/year Never
Runoff from 1 storm 209 gallons 2,430 gallons 1,686 gallons
Annual Runoff 8,961 gallons 103,801 gallons 72,008 gallons
Nitrogen 0 5.5 lbs/acre/year 3.71 lbs/acre/year
Phosphorus 0 0.77 lbs/acre/year 0.51 lbs/acre/year
The comparison quantifies a dramatic increase in runoff due to a typical house.The inclusion of a 'green'
house scenario demonstrates that no matter the care taken during construction to include stormwater
BMPs, there will be an increase in runoff from a development on this hillside. It is this increase in runoff
that will increase the rate of erosion from the hillside and the rate of deposition into Willow Lake.
Annual Runoff Runoff from 1 storm
120000 3000
100000 2500 Imo
80000 2000
° 60000 - I
To
00 40000 - 1000
20000 - 500
No House Typical 'Green' No House Typical 'Green'
House House House House
Summary:
Development of the hillslope around Willow Lake will have an unavoidable consequence of increased
the amount of sediment and nutrients delivered to Willow Lake with each storm.The lake will begin to
fill in and as it shallows, and as the water column becomes saturated with nutrients, algae will grow. In
large storms, the lake will be more likely to overflow its downstream berm as it will not have a large
storage capacity any longer. Outflows from the lake will increase in volume, suspended sediment
concentration, and nutrient load. These outflows will travel into Cow Branch Creek and then into the
nearby Moore's Creek.
Nutrients from one house
6
5 .
*Nitrogen
m ■Phosphorus
J _IIIIIIIIIi_
111
0
No House Typical House 'Green'House
. 1
Endnotes
'Lamb, M.P., Dietrich,W.E., and Venditti,J.G. 2008. Is the Critical Shields Stress for Incipient Motion
Dependent on Bed Slope?Journal of Geophysical Research 113: F02008
Luino, F. 2005 Sequence of Instability Processes Triggered by Heavy Rainfall in Northern Italy.
Geomorphology 66(1-4): 13-39.
Oostwoud Wijdenes, D.J., and Ergenzinger, P. 1998. Erosion and Sediment Transport on Steep Marly
Hillslopes, Draix, Haute-Provence, France:An Experimental Field Study. Catena 33 (3-4): 179-200.
2Chatterjea, K. 1994. Dynamics of Fluvial and Slope Processes in the Changing Geomorphic Environment
of Singapore. Earth Surface Processes and Landforms, 19(7): 585-607.
3Oppenheimer,J.R. 1988. A Review of Slope management and Precipitation in Relation to the Serpentine
Hillside on Staten Island and New York City Local Law 7" Report to the N.Y.C. Department of City
Planning. http://www.librarv.csi.cunv.edu/dept/as/ces/sirept.htm
4Southern Environmental Law Center, Rivanna Conservation Society, and UVa School of Law's
Environmental Law and Conservation Clinic. 2008. Before the Storm: Reducing the Damage from
Polluted Stormwater Runoff, Recommendations for Albemarle County. 22p.
5County Engineer's Commentary Number 3: Extra Erosion and Sediment Control. Provided to Albemarle
Bounty Department of Community Development. Dec. 3, 2007.
6Places 29 Update at Albemarle County Board of Supervisors Meeting on June 6, 2007.
http://www.albemarle.org/upload/images/Forms Center/Departments/Board of Supervisors/Fo
rms/Agenda/2007files/20070606/Places29ExecSu mma ry.htm
'Beckmann, E. and Perez-De La Rosa, A. 2010. Sustainable Management of Deer Lake Pond. UVa School
of Engineering and Applied Science, Department of Civil and Environmental Engineering. 14p.
8Natural Resources Conservation Service (NRCS). Custom Soil Resource Report for Albemarle County,
Virginia,Willow Lake Subdivision. USDA. 82p.
9Revised Universal Soil Loss Equation,Version 2. USDA-Agricultural Research Service.
'°James River Association Runoff Calculation Mapping. Runoff Calculator.
http://www.jamesriverassociation.org/get-involved/runoff-calculator/