Evaluation of Overland Flow Paths Generated from Multiresolution DEMs - PowerPoint PPT Presentation

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Evaluation of Overland Flow Paths Generated from Multiresolution DEMs

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Title: Evaluation of Overland Flow Paths Generated from Multiresolution DEMs


1
Evaluation of Overland Flow Paths Generated from
Multiresolution DEMs
J.M. Shawn Hutchinson1, Stacy L. Hutchinson2, and
I.J. Kim2 1Department of Geography and
2Department of Biological and Agricultural
EngineeringKansas State University Manhattan,
Kansas 66506-2907
2
The Problem NPS Pollution
  • Largest contributors to surface water impairment
    are nonpoint source (NPS) pollutants sediment,
    nutrients, pathogens, other chemicals

40 of streams in US are considered
contaminated..
Percent of Impaired Waters (1998) HUC 8
Scale www.epa.gov/iwi/2000aug/iv22_usmap.html
3
One Solution VBS
  • Vegetated buffer systems (VBS) common and
    sustainable BMP for mitigating NPS pollutant
    transport

4
VBS Illustrated
Design standards from USDA Farm Service
Agencyand Natural Resource Conservation Service
Source NRCS Photo Gallery
Shrubs nutrient removal
Zone 1
Zone 3
Zone 2
Grass control runoff, sediment
Trees bank stabilization
5
Assumptions
Uniform Sheet Flow A W x L
Concentrated Flow A W x L x ß ß A
Ineffective Area
L
W
Hydrologic and Vegetation Conditions
6
Overall Objectives
  • To determine optimal DEM spatial resolution to
    model soil erosion processes
  • To evaluate the efficiency of different VBS
    designs

7
N
False-Color CompositeLandsat TM 5 June 7, 1997
8
DEMs and Hydrology
  • Zhang and Montgomery (1994)
  • 10 m resolution is recommended
  • Kuo et al. (1999)
  • Hillslope curvature an important factor
  • Runoff volume is affected most by DEM resolution
    during dry seasons
  • Moglen et al., (2001)
  • Coarser resolution DEMs tend to overestimate peak
    stream discharge

9
DEM Creation
  • GPS data collected on-site at 3m intervals using
    DGPS unit
  • Raster DEMs created from interpolated point
    datasets of varying density 30m, 10m, 3m
  • ArcHydro tools
  • Flow paths - D8 method

7
6
8
5
1
4
2
3
10
30 mUSGS
30 m
10 m
3 m
11
Hmmmm.
  • Best resolution DEM for representing flow
    processes and ability to capture/assess small
    (sub-pixel) BMPs
  • Traditional flow accumulation flow paths not
    sufficient
  • Compounding assumptions effective VBS relies on
    sheet overland flow
  • Observed channelized flow (rill erosion) need
    to identify where channelized flow begins

12
Time of Concentration
  • Time of concentration
  • twatershed toverland flow tchannel flow
    tdetainments
  • tov equilibrium time when rainfall intensity
    equals runoff discharge rate
  • Differences in the observed and computed tov
    results from the transition from sheet to
    concentrated flow
  • Sheet tov measured tov-computed
  • Concentrated tov measured gtgt tov-computed

13
Kinematic Wave Equation
  • Federal Aviation Administration (1970)
  • tov 1.8(1.1-C)L0.50/S0.333
  • C rational method runoff coefficient
  • Morgali and Linsley (1965) Rogan and Duru (1970)
  • tov 0.93(nL)0.6/i0.4S0.5

Primary Variables n Mannings coefficient L
slope length S slope i rainfall intensity
14
When Sheet Flow?
  • Ignoring impervious surfaces
  • Izzard (1946) iL (in/hr x ft) lt 500
  • Not fully verified (Rogan and Duru 1972)
  • SCS TR-55 (1986) Slope length lt 300 ft
  • Lacks justification (McCuen and Spiess 1995)

15
Kinematic Wave Criteria
  • McCuen and Spiess (1995)
  • tov 0.93(nL)0.6 / i0.4(S0.5)0.6
  • Assessed measured and computed tov from 59 field
    sites and lab experiments

16
So What?
  • Where calculated values of nLS-0.5 on a hillslope
    (or plot) is less than 100, sheet flow is the
    dominant flow type.
  • Apply this measure to guide VBS placement within
    a watershed
  • Good Independent of i (rainfall intensity)
  • Problem Spatial estimates of Mannings n

17
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20
Parting Shot
  • Flow paths from cell accumulation routines dont
    address energy required for initiating rill
    erosion
  • Identifying nLS-0.5 threshold will help place VBS
    where they can be most effective, and where model
    assumptions are valid sheet flow dominated
    slopes
  • Extend nLS-0.5 method to identify gully erosion
    sites

21
Acknowledgements
  • Strategic Environmental Research and Development
    Program
  • Assessing the Impact of Maneuver Training on NPS
    Pollution and Water Quality
  • Project CP-1339
  • Geographic Information Systems Spatial Analysis
    Laboratory, Kansas State University

22
Applying the nLS-0.5 Criteria
23
Operational Requirements
  • Hydrologic Condition
  • Maintaining uniform sheet flow through buffer
  • Overland flow often becomes concentrated (Dillaha
    et al. 1986 Fabis et al. 1993 Dosskey et al.
    2002)
  • Vegetative Condition
  • Maintaining healthy dense vegetation growth
  • Vegetation condition varies within and between
    years

24
  • Models assume sheet flow
  • Best resolution DEM for representing flow
    processes and ability to capture small BMPs
  • Traditional flow accumulation flow paths not a
    sufficient for
  • Assuming VBS effective in removal but its
    assumption is overland flow
  • Observed channelized flow need to understand
    where channelization initiates and not violate
    model assumptions
  • Rill erosion (up to 200 mm deep) is onset of
    channelization biggest cause of sediment
    transport (enough water with enough energy to
    move dirt)
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