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1. An Analysis of Erosion and Sediment Routing at the Botanical Gardens
Alexis McFadden, Cody Eimen, Michael Smith, & Dr. Tom Owino
Small Watershed Hydrology and Sedimentology, Clemson University, Clemson, SC, 29631
Abstract
In this project, a small watershed within the Botanical Gardens in
Clemson, SC was selected and analyzed. The soil loss of the 13.3-acre
area was calculated to be 1185.4 tonnes/year, while the sediment yield was
calculated to be 1115.6 tonnes/year. A 100 ac-ft reservoir added at the
outlet of this watershed would trap roughly 95% of the sediment that
leaves the watershed.
Site Information
Total watershed area: 13.3 acres, draining to one creek located on right side
of Area of Interest (AOI).
Location of watershed: Pickens County, SC Rainfall Factor: 300
Soil Composition:
• Cecil Sandy Loam: 6-10% slope, 12.5 ac (93.7% of total area)
• Length of flow path: approximately 600 ft
• Average slope of flow path: 8%
• Pacolet Fine Sandy Loam: 0.8 ac (6.3% of total area)
• Length of flow path: approximately 200 ft
• Average slope of flow path: 16%
Procedure
The first step of this project was to select a small watershed.
This area of the Botanical Gardens was selected because it has one
outlet, and the entire plot of land is sloped downward in roughly the
same direction, as shown in the topographical map in Figure 4.
Web Soil Survey was then used to find the specific plot of land being
analyzed along with the soil properties of this area. After gathering
this information, a calculation using Equation 1 was performed to
find the total yearly soil loss of the selected area.
The next step was to solve for the Sediment Delivery Ratio of the
selected area using Equation 2. This allowed us to calculate the
yearly soil yield of the plot of land using Equation 3.
Lastly, the trapping efficiency of a hypothetical reservoir that could
be installed at the outlet of our watershed was found. The reservoir
capacity was 100 ac-ft. The standard curve in Figure 3 was used to
determine the percentage of sediment that would be trapped by the
reservoir.
Conclusions
Overall, this area of land is experiencing a moderate level of erosion. The
increase in erosion from the extreme slope of the land is offset by the
decrease in erosion from the well rooted grass that covers this area. Using
resources available to us, we were able to solve for the predicted soil loss in
this watershed, as well as the soil yield. Due to the large volume of the
reservoir compared to the relatively small watershed, the trapping efficiency
of the reservoir was assumed to be 95%.
References
1. Owino T. (2021) Module 3. Unpublished Notes, BE 3220, Clemson University.
2. Soil Survey Staff, Natural Resources Conservation Service, United States Department of
Agriculture. Web Soil Survey. Available online at the following link:
http://websoilsurvey.sc.egov.usda.gov/. Accessed [04/13/2021].
3. U.S. Geological Survey, 2019, USGS National Map, accessed April 13, 2021 at URL
https://apps.nationalmap.gov/downloader/#/
4. Ward, Andy D, and Stanley W Trimble. “Soil Conservation and Sediment Budgets.”
Environmental Hydrology, Lewis Publishers, 2015, pp. 408–413.
Acknowledgements
We would like to thank our professor, Dr. Tom Owino, as well as Clemson University’s
Department of Biosystems Engineering for making this project possible.
Results and Engineering Calculations
Total Soil Loss (Ttotal):
T = ARKLSVM (Equation 1)
Tcecil = (12.5)(300)(3.52)(0.24)(0.35) = 1108.8 tonnes/year
Tpacolet = (0.8)(300)(4.56)(0.20)(0.35) = 76.6 tonnes/year
Ttotal = Tcecil + Tpacolet = 1185.4 tonnes/year
Sediment Delivery Ratio (SDR):
Area of watershed = 13.3 acres = 0.02078 mi2
log10SDR = 1.8768 - 0.14191[log10(a)] (Equation 2)
log10SDR = 1.8768 - 0.14191[log10(0.02078)]
SDR = 94.11% = 0.9411
Soil Yield = SDR Total * Ttotal (Equation 3)
(0.9411)(1185.4) = 1115.6 tonnes/year
Trapping Efficiency of a Reservoir:
ρCecil = 1.41 g/cm3 = 1,410 kg/m3
ρpacolet = 1.36 g/cm3 = 1,360 kg/m3
ρWeighted = 0.937* 1410 + 0.063* 1360 = 1407 kg/m3
Inflow (I) = 1115.6 tonnes/yr * 1 m3/1.407 tonnes = 793 m3/yr = 0.63 ac-ft
Capacity (C ) = 100 ac-ft
C/I = 158.7 → 95% trapping efficiency assumed
Trapped = 0.95*1115.6 tonnes/yr = 1059.8 tonnes/yr
Sediment that escapes reservoir = 0.05*1115.6 tonnes/yr = 55.8 tonnes/yr
Background
A small watershed within the SC Botanical Gardens was chosen. This
watershed drains to one creek that cuts through the Botanical
Gardens, which is located on the right side of the area of interest (Figure
1). This watershed is 13.3 acres and is composed of 2 different soil types,
which are Cecil sandy loam and Pacolet fine sandy loam (Figure 2).
Rainfall
Factor (R)
Soil
Erodibility
Factor (K)
Length-
Slope Factor (LS)
Vegetative
Mulching Factor
(VM)
Area [acres]
(A)
Cecil Soil 300 0.24 3.52 0.35 12.5
Pacolet
Soil
300 0.20 4.56 0.35 0.8
Figure 1: Watershed area of interest in the Botanical Gardens Figure 2: Soil composition in area of interest
Figure 3: Standard curve for trapping efficiency
Table1: Soil Loss Variables for each Soil Type
Figure 4: Topographic map of AOI