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Little Tomahawk Creek Watershed. Eric Anderson Cody “The Crip” Charles Jansen. The Effects of Landscape Alteration on a Watershed. Logging Freeze/ Thaw Seasonal Changes Farming BMPs and stormwater management Development Residential Commercial Industrial. Some Real-world applications.

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Little Tomahawk Creek Watershed

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little tomahawk creek watershed

Little Tomahawk Creek Watershed

Eric Anderson

Cody “The Crip”

Charles Jansen

the effects of landscape alteration on a watershed
The Effects of Landscape Alteration on a Watershed
  • Logging
  • Freeze/ Thaw
  • Seasonal Changes
  • Farming
    • BMPs and stormwater management
  • Development
    • Residential
    • Commercial
    • Industrial
some real world applications
Some Real-world applications
  • Problems associated with pollution from runoff (flow direction and accumulation)
  • Reservoir holding capacity
  • Wastewater and water treatment plants
  • Hydroelectricity
  • Flooding
  • Predictive Analysis
  • Overview: Use land use and land cover with soil characteristics to determine the runoff of the watershed
  • Purpose: Determine the amount of runoff to accumulate at a particular point during a storm
  • Application: Used GIS to combine the land use and soil data layers. Found hydrologic soil groups and land use types to determine local CN values. Used CN value in equation to determine discharge for local area using arbitrary rainfall values.
the watershed
The Watershed
  • Little Tomahawk Creek watershed is a part of the larger Swift Creek Reservoir watershed (a drinking watershed for the Midlothian, VA area)
  • Size about 2366 acres
  • Located about 20 miles west of Richmond
  • This project is based on research being performed by Eric Anderson who is studying the affects of Route 288 bypass construction runoff on the Swift Creek Reservoir
  • Concerns from the Swift Creek Water Treatment Plant (SCWTP) and local advocacy groups over sediment load into Swift Creek Reservoir
  • Eric hopes to use the information gained from this project to help calibrate a model that estimates “pre-construction” conditions in the watershed
  • This will enable a more accurate measurement of the total affects of the enormous Route 288 construction project
  • First, data was found for elevation, soil characteristics, and land use/land cover
  • The Little Tomahawk Creek watershed was delineated based on a combination of sub-watersheds obtained by using the “watershed” function in the Hydrologic Analysis extension
  • Little Tomahawk Creek Watershed can been seen in red with the other watersheds in black
  • Here we have a flow chart representing the process for determining the amount of runoff
  • The hydrologic soil group and land use/cover are GIS layers that are combined
  • Random Rainfall values were created to show different runoffs at different intensities of storms (we chose 1”, 2”, and 5” rainfall amounts distributed evenly over the watershed)






  • We combine the soil and land cover layers by unioning them in the Geoprocessing Wizard
  • The union gives us unique combinations of hydrologic soil groups and land cover
  • These combinations are then referenced on a chart (Table 2.1) to determine the curve number (cn)
  • Next we use the given equation to find the total water available for runoff (Q):
          • Q = water available for runoff
          • P = precipitation
          • CN = curve number
  • The curve number (c) was determined from the chart and arbitrary precipitation values were chosen: 1, 2, and 5 inches
  • Finally the Q values are combined with the Flow direction DEM
  • This gave us the accumulated water available for runoff (in) in a visual form
problems we encountered
Problems We Encountered
  • Delineating Watershed
    • Combined many sub-watersheds
  • Sorting and “cutting out” data we needed
    • Most data collected was at the quadrangle and county scales
    • Had to convert lots of data back and forth between raster and vector depending on the operation we were performing
    • Most of our operations were performed in vector with the Geoprocessing wizard, but some were performed in vector with the Raster Calculator
  • Lots of field data had to be entered manually
    • LU/LC field
    • CN numbers from reference table
    • If more time allowed, we could have made an algorithm to perform this work
review what info does our analysis provide
Review: What info does our analysis provide
  • Flow direction
  • Flow accumulation
  • Qualitative Potential Runoff Analysis (using CN map)
    • Land use
    • Soil type (A, B, C, D)
    • Storm event
  • Quantitative Potential Runoff Analysis (Point Discharge)
project limitations and accuracy
Project Limitations and Accuracy
  • Probably not highly accurate for small watersheds—ours is borderline
      • Soil survey data for hydrologic soil conditions is very accurate—lots of detail
      • 30 meter DEMs and LU/LC (10 meter would be better)—not so accurate
      • Assumes spatially equal distribution of rainfall
      • Most accurate for antecedent soil moisture of 0.25
      • Some rough estimations of CN from the reference table
        • LU/LC and reference table didn’t always match up
why use gis with this application
Why use GIS with this application?
  • Faster
  • Can be more accurate
  • More organized
  • Produce maps
  • Helps make better decisions
  • Watershed analysis with GIS can be accurate and relatively easy
  • Helps make existing watershed models and equations more accurate
  • Allows visual analysis of a solution
  • Allows predictive temporal analysis
  • Can be applied to any watershed with existing data