Little Tomahawk Creek Watershed

1
Little Tomahawk Creek Watershed

• Eric Anderson
• Cody The Crip
• Charles Jansen

2
The Effects of Landscape Alteration on a Watershed

• Logging
• Freeze/ Thaw
• Seasonal Changes
• Farming
• BMPs and stormwater management
• Development
• Residential
• Commercial
• Industrial

3
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

4
Introduction

• 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.

5
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

6
Introduction

• 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

7
Introduction

• 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

8
SCWTP
9
Methods

• 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

10
Methods

• 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)

1
2
5
3
4
11
Methods
12
Methods

• 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)

13
Methods

• 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

14
Methods
15
Methods

• 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

16
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

17
Uses/ Applications
18
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)

19
Project Limitations and Accuracy

• Probably not highly accurate for small
  watershedsours is borderline
• Soil survey data for hydrologic soil conditions
  is very accuratelots 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 didnt always match up

20
Why use GIS with this application?

• Faster
• Can be more accurate
• More organized
• Produce maps
• Helps make better decisions

21
Conclusion

• 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

22
Time for a
DEMO !