This document analyzes runoff at the entrance to an apartment complex called The Ridge. Initial calculations using the SCS Curve Number method estimated 7.58 inches of runoff from a 100-year storm for the current design. A bioretention cell is proposed to reduce runoff and mitigate pooling at the entrance. With the cell, runoff is estimated to decrease to 5.45 inches. The bioretention cell is deemed the most feasible option due to its estimated $5,000-$15,000 cost and ability to significantly reduce runoff without disrupting road access.

1. Analyzing the Entrance to “The Ridge” Using the SCS
Curve Number Method
Aaron Reeser, Patrick Cusack, Rachel Cron, & Dr. Tom Owino
BE 3220, Biosystems Engineering, Clemson University, Clemson, SC, 29631
Abstract
Introduction
Materials and Methods
Data & Governing Equations
Conclusions
References
1. Purvis, J. C., Tyler, W., Sidlow, S. (1988) Maximum Rainfall Intensity in South Carolina By County. South Carolina Department of Natural
Resources. Retrieved on April 30, 2019 from http://www.dnr.sc.gov/climate/sco/Publications/max_rain_intensity.php#chart
2. Jarrett, A. (2016). Rain Gardens (Bioretention Cells) – A Stormwater BMP. Pennsylvania State University. Retrieved on April 30, 2019 from
https://extension.psu.edu/rain-gardens-bioretention-cells-a-stormwater-bmp
3. Low Impact Development Center, Inc. (2007) Bioretention Costs. Retrieved on April 30, 2019 from https://www.lid-
stormwater.net/bio_costs.htm
4. Ding, B., Rezanezhad, F., Gharedaghloo, B., Van Cappellen, P., Passeport, E. (2019) Bioretention cells under cold climate conditions: Effects
of freezing and thawing on water infiltration, soil structure, and nutrient removal. Science of The Total Environment. Vol. 649 (749-759)
5. WebSoilSurvey. United States Department of Agriculture Natural Resources Conservation Service. Retrieved on April 30, 2019 from
https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx
6. Massachusetts government. Demonstration 3: Permeable Paving Materials and Bioretention in a Parking Lot. Retrieved on April 30, 2019 from
https://www.mass.gov/service-details/demonstration-3-permeable-paving-materials-and-bioretention-in-a-parking-lot
Acknowledgements
We would like to thank Dr. Tom Owino for his assistance and knowledge about surface water runoff.
Additionally, we would like to thank the Clemson University EES department for the equipment and lab space.
Results and Discussion
Curve Numbers
Ground type CN Source
Impervious
ground
98 Table 9-5
Brush, forbes,
grass; B; Poor
67 Table 9-1
Woods/grass 73 WinTR-55
<50% grass 79 WinTR-55
Soil Data
Hydro Soil Group B
Area [acres] 179.2
Soil Type
Hiwassee clay loam
(severely eroded)
K factor 0.28
Slope 2-6%
The Ridge is a new apartment complex located near Clemson University. As shown in
Figure 1, the entrance to The Ridge experiences a significant amount of pooling at the base
of the driveway. Since this is the only entrance into the complex, this pooling becomes
problematic during colder months. Several cars have been observed sliding on the frozen
water. If a car accident were to occur at the site, the construction company that built the road
could be liable for the damage. Additionally, as severe storms become more frequent, the
amount of runoff at the site may increase. Using the amount of runoff from the area,
calculated using the SCS Curve Number Method and WinTR-55, a solution to reduce the
amount of runoff to the bottom of the entrance can be designed. The design solution must
account for the types of soil cover and surfaces in the surrounding area, the rate of
precipitation during a major storm event, and general economic and logistical feasibility.
The area of interest is shown below in Figure 2. Using the Curve Number Method, and the
values contained in Table 1, the amount of runoff from a 100 yr, 24h storm, at the current site was
calculated to be 7.58 inches. The calculated peak discharge from the surrounding area was estimated
to 3.519 cfs. Figure 3 shows the unit peak hydrograph for the current site. The current design
reduces runoff from the surrounding areas by only 21.04%. The group chose to reduce the amount of
runoff to the base of the driveway by theoretically placing a bioretention cell at the base of the straw
and brushy hills, as shown below in Figure 2. This bioretention cell would need to be designed for
the specific characteristics of the site. Since the soil type of the area, Hiwassee clay loam soil, has
low permeability, the soil must be excavated approximately 2 to 4 feet deep in order to install the
underdrain system of the bioretention cell. Additionally, a rock bed, PE diaphragm, and native
vegetation, such as New England Aster and the Water Canna, could be added. The cost of this
bioretention cell would range from $5,000 to $15,000. The bioretention cell was intentionally
designed to be over 10 feet wide in order to preserve the health of the vegetation in the cell. The
length of the bioretention cell was designed to create an area equal to the size of the area the cell
would collect water from divided by 7.5; from literature, this ratio would result in the retention cell
reducing runoff by 90%.
After performing a second analysis of the site including the bioretention cell, using the values
shown in Table 2, the amount of runoff was calculated to be 5.45 inches, while the peak discharge
was estimated to be 2.744 cfs. This new design reduces runoff from the surrounding areas by
42.38%. This bioretention cell does not completely eliminate runoff flowing to the base of the road.
Ways to completely eliminate ponding at the base of the hill are not logistically nor economically
feasible for this site. Examples include filling the base of the road with concrete or adjusting the
shoulders of the road to divert water to storm drains. Not only are these adjustments expensive, but
since the street shown is the only entrance/exit from the complex, repaving the area would inhibit
residents from coming and going. The group chose to add a bioretention cell to the base of the steep
hill because it reduced the amount of runoff to the base of the driveway significantly while also
being the most feasible option logistically and economically. Over the course of performing
calculations and modeling the system, sources of error likely included approximating the shape of
each subarea, limited length input in WinTR-55, and idealizing the bioretention cell.
• WinTR-55 Modeling Software
• Web Soil Data website
• US Department of Natural
Resources website
• Tape measure
• iPhone Compass Application
• Charts and tables from class notes
The group visited the site to measure the dimensions
of the brushy hill, the straw hill, and the road with a
tape measure. Additionally, the slope of each section
was approximated using the compass app. The
group used Web Soil to determine the characteristics
of the site’s soil, US Department of Natural
Resources website to determine historical
precipitation data, and class notes to determine the
curve numbers for the site. The group then
calculated runoff using SCS Curve Number Method
equations and WinTR-55.
This project focuses on runoff at The Ridge apartment complex, aiming to reduce the amount
of water pooling at the entrance. Based on the initial calculations using runoff estimation
methods, like the curve number method and modeling the current design using WinTR-55, it
was concluded that an excess amount of water is being directed to the entrance. The amount
of runoff from a 100 yr, 24 hour storm at the current site was calculated to be 7.58 in, while
the peak discharge was calculated to be 3.519 cfs. Due to this flaw in the initial design, a new
layout that reduces the amount of runoff into the entrance is required. After reviewing several
possible solutions, a bioretention cell to contain and reduce the runoff was determined to be
the best option. It was the cheapest option, with a construction cost ranging from $5,000 to
$15,000. With the addition of a bioretention cell, the amount of runoff from a 100 yr, 24 hour
storm was calculated to be 5.45 inches, while the peak discharge was estimated to be 2.744
cfs. This option will reduce the amount of runoff to the entrance without shutting down the
road, which is the only access point in or out of the apartment complex. Since this issue was
not solved during the initial construction, building a bioretention cell is the best option to
reduce the amount of runoff that pools at the entrance to The Ridge apartment complex.
Certain technical questions may come to
mind when designing this new layout such
as: How much runoff is coming off the hill?
What is the surrounding soil comprised of?
How is this new design better than the old
one? The main objectives of this project are
to calculate the current amount of runoff on-
site using the SCS Curve Number Method &
design site adjustments to reduce the amount
of water pooling at the entrance of The
Ridge apartment complex in the most
environmentally, socially, and economically
beneficial way possible.
𝑆 =
1000
𝐶𝑁
− 10
𝑄 =
𝑃 − 0.2𝑆 2
𝑃 + 0.8𝑆
𝑊𝑒𝑖𝑔ℎ𝑡𝑒𝑑 𝑄 =
𝑄 ∗ 𝐴
𝐴
% 𝑅𝑢𝑛𝑜𝑓𝑓 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛 − 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑅𝑢𝑛𝑜𝑓𝑓
(𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛)
∗ 100
𝐿 =
(𝑙0.8
∗ 𝑆 + 1 0.7
)
1900 ∗ 𝑌0.5
𝑡 𝑐 =
0.6
𝑙𝑎𝑔 𝑡𝑖𝑚𝑒
𝑄 𝑝 = 𝑞 𝑝 ∗ 𝐴 ∗ 𝑄
Figure 1: Pooled runoff at the entrance to The Ridge after a rain storm event
Figure 2: Above view of
surrounding areas that
contribute runoff to The
Ridge entrance.
Table 1: Weighted Q data, used to calculate runoff, for the current area design
Figure 3: Unit peak
hydrograph for a 100-yr
storm for the current area
design
Table 2: Weighted Q data, used to calculate runoff, for the proposed area design
Runoff at the entrance to The Ridge could have been initially avoided had the base of the
driveway been filled and shaped to divert water to the storm drains. Implementing a
bioretention cell is the most feasible option for reducing runoff to the base of The Ridge’s
entrance, as compared with other options such as repaving the base of the driveway, adding
additional storm drains, or improving the shoulders of the driveway to carry water more
efficiently. The reason reducing runoff at the site is important is because it would reduce
the risk of an accident occurring during the cold seasons when the water may freeze. An
accident caused by a design flaw could bring up a lawsuit between the resident and the
construction company. In addition to a bioretention cell being the most economically and
logistically feasible solution, it is also designed to be comprised of native vegetation and
environmentally-friendly, which is an over-arching goal of biosystems engineering.