Tables and Figures
ArcGIS Assessment of Site
The team used Esri's ArcGIS software to get a better understanding of the watershed contributing runoff to the wetland. The group began by downloading a Digital Elevation Model (DEM) of Fayette County. The group used this elevation data, along with a map of all the outfall areas contributing runoff to the lake, to create a stream network image. The group used this stream network to create a map layer of all the 45 sub-watersheds that empty into the lake. Figure 6 allowed us to make an informed decision when choosing our area of interest. The group continued to use ArcGIS in order to take advantage of its powerful hydrology tools. These tools allowed for a better understanding of the complex ways water moves through our selected sub-watershed. The group obtained National Land Cover Data (NLCD) (Figure 7) and soil type data (Figure 8) from United States Geological Survey databases. The group was able to intersect these two data sets into a single layer (Figure 9). This single layer provided the group with all the information we needed to determine the curve number for our sub-watershed. The curve number is a necessary parameter that allowed us to predict the runoff we might expect for our wetland.
Figure 6
Map of the 45 sub-watersheds that provide runoff to the lake.
Figure 7
Land Cover map provides data that describes the physical attributes of the surface of our area of interest.
Figure 8
Soil data gives an idea of the permeability of the soil. Each color represents a soil type.
Figure 9
The layer that represents an intersection of the land cover and the soil data. Each color represents a curve number specific to that area.
Hydrologic Analysis
Residential runoff calculations were performed using the Soil Conservation Survey (SCS) "Curve Number" method. This method involves selecting a curve number for various surfaces, both permeable and impervious to flow, and averaging these number over an area. Impermeable surfaces will have a curve number of 98, and all permeable surfaces will have lower curve numbers. For this watershed, the curve number was calculated as 74.4. This indicates that the majority of the watershed is developed, and that most rainfall will become runoff.
Once the curve number was calculated, the group chose a design storm. This wetland is designed to treat a 1 year, 24 hour storm. Additionally, the wetland has an emergency spillway in the event of a larger storm. This will prevent flooding and damage to the wetland.
The group continued the hydrological analysis by calculating runoff, and the S value. These parameters were calculated using the following equations:
- Q= (P- IS)^2/(P+0.8S)
- S= (25400/CN)-254
Where Q is the runoff depth, P is the rainfall depth, and I is the initial abstraction. The initial abstraction in this case was 0.05, as advised by Carmen Agouridis, Ph.D.
Once these runoff calculations were completed, the group was able to determine the volume of flow that the wetland would need to convey. This allowed the group to begin sizing and designing the wetland.
