evaluation of overland flow paths generated from multiresolution dems j.m. shawn hutchinson 1, stacy...

Evaluation of Overland Flow Evaluation of Overland Flow Paths Generated from Paths Generated from Multiresolution DEMsMultiresolution DEMsJ.M. Shawn HutchinsonJ.M. Shawn Hutchinson11, Stacy L. Hutchinson, Stacy L. Hutchinson22, and I.J. Kim, and I.J. Kim22

11Department of Geography and Department of Geography and 22Department of Biological and Agricultural EngineeringDepartment of Biological and Agricultural EngineeringKansas State UniversityKansas State University

Manhattan, Kansas 66506-2907Manhattan, Kansas 66506-2907

AAG 2005 - Denver Department of Geography, Kansas State University 2

The Problem: NPS Pollution

Largest contributors to surface water impairment are nonpoint source (NPS) pollutants – sediment, nutrients, pathogens, other chemicals

Percent of Impaired Waters (1998) HUC 8 Scalewww.epa.gov/iwi/2000aug/iv22_usmap.html

“40% of streams in US areconsidered contaminated..”


One Solution: VBS

Vegetated buffer systems (VBS) – common and sustainable BMP for mitigating NPS pollutant transport


VBS Illustrated

Zone 1Zone 3 Zone 2Grass – control runoff, sediment

Shrubs – nutrient removal

Trees – bank stabilization

Design standards from USDA Farm Service Agencyand Natural Resource Conservation Service

Source: NRCS Photo Gallery


Assumptions…

W

L

Uniform Sheet FlowA = W x L

Concentrated FlowA = W x L x β

β = A – Ineffective Area

Hydrologic and Vegetation Conditions


Overall Objectives

To determine optimal DEM spatial resolution to model soil erosion processes

To evaluate the efficiency of different VBS designs

False-Color CompositeFalse-Color CompositeLandsat TM 5; June 7, 1997Landsat TM 5; June 7, 1997

NN


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


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

1

2

6 8

4 3

7

5

10 m 3 m

30 mUSGS

30 m


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


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] >> [tov-computed]


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 = Manning’s coefficient L = slope lengthS = slope i = rainfall intensity


When Sheet Flow?

Ignoring impervious surfaces…

Izzard (1946): iL (in/hr x ft) < 500 Not fully verified (Rogan and Duru 1972)

SCS TR-55 (1986): Slope length < 300 ft Lacks justification (McCuen and Spiess 1995)


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


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 Manning’s n


Parting Shot…

Flow paths from cell accumulation routines don’t 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…


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


0

10

20

30

40

50

60

70

80

90

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Slope (-)

Lim

iting

She

et F

low

Len

gth

(m) n=0.15

n=0.24

n=0.41

Applying the nLS-0.5 Criteria


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


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 it’s 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)

evaluation of overland flow paths generated from multiresolution dems j.m. shawn hutchinson 1, stacy...

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