Mapping Riparian Corridors using Hydrologic Landscape AnalysisTOM ROBINSONDirector of Conservation, Science, and InnovationBay Area Open Space Council

Synopsis

• There is a need in land conservation for more effective and efficientmapping of riparian zones

• Hypothesis: We can do a better job of mapping riparian zones by using new tools and data

• New methods could result in greater protection of riparian zones and their benefits to humans

Alluvial Riparian Zones

Watershed context Cross section perspectiveUSDA

Smith et al. (2008)

Fish and aquatic species habitatFlood water storage

and groundwater rechargeWater quality

Riparian Ecosystems: Time & Space

TIME: LONG PERIODS OF BOREDOM……….SPACE: MEANDER CORRIDORS

Riparian Conservation

Policy Definition

USFS Forest Practice Rules

Fixed-width buffer based on stream type (fish-bearing?)

County Zoning Ordinances

Fixed-width buffer based on stream order

NRCS RiparianManagement Zones

Fixed-width buffer based on objectives (e.g., erosion control)

California Wetland and Riparian Area Protection Policy

Variable width delineation method indevelopment (SFEI)

Two primary modes:1. Policy ($): One size fits all,

but cheap2. Voluntary, incentive-based

($$$): Can be comprehensive but expensive

Agriculture

Forestry

Riparian Easements

• Voluntary incentive-based approach

• $ to exclude development

• Long term needs of the river

• Need a boundary(a polygon)

• Gradation for negotiation

From Ilhardt, Verry, and Palik, 2000

Need: High quality delineations for easement structure

Riparian Corridor – Restoring Functional Width

Current Practices: Easements and Ordinances

100 meters1997 storm?

“Riparian-ness”

Figures from NRCS Forest Management

Guidelines

Riparian functions decrease as you move away from the river

How to characterize the ecotone?

?Build on previous

techniques to map this

zone

What are the map-able indicators of this zone?

New Riparian Mapping MethodsMapping the Area Affected Over Long Time Scales by Moving Water

From Dilts et al. 2011

Elevation Above Channel (EAC)

From Smith et al. 2008The Nature Conservancy

Active River Area (ARA)

From Benda et al. 2011

Regional Curves ofHydraulic Geometry

• Equations that relate Bankfull depth and width to drainage area

• Available for most of U.S.

From Collins and Leventhal (2013)

Drainage Area (mi2)

Bank

full

Dept

h (ft

)

y = 1.0195x0.3667

R2 = 0.92607

Bankfull Depth

2x Bankfull= “Flood-prone area” (Rosgen, 1994)3x Bankfull= “Predicted fluvial landscape” (Benda et al., 2011)

Case Study: Mark West CreekEvaluating these methods on the ground

Study AreaMark West Creek, Sonoma County, CA

Confluence of Mark West Creek and Porter Creek

Mapping the Fluvial Landscape

2x Bankfull= “Flood-prone area” (Rosgen, 1994)3x Bankfull= “Predicted flucial landscape” (Benda et al., 2011)

Width at BankfullWidth at 2x Bankfull

3x Bankfull

Regional curve equation of Bankfull Depth versus Drainage Area (Collins and Leventhal, 2013)Dbf = 1.0195x0.3667

where x = drainage area

+1-meter LiDARdigital elevation model

Fluvial Landscape Model Overview• 3 raster inputs:

– Stream mask raster (Arc Hydro Tools and SetNull) – Flow Accumulation raster (Arc Hydro Tools)– Elevation (e.g., high-resolution DEM)

Bfl_elev = ((1.0195 * (([FLOW_ACC] * 9) / 27878400)^0.3667) * %Bankfull multiple%) + [ELEVATION]

User-definedvariable

(> 0)

Extract Multiple Values to Points

Streamraster

Streampoints

Flowaccumu-lation

Elevation

FLOW_ACC ELEVATION Bfl_elev

25041980 467.78 476.56

25038770 468.82 477.60

24991310 474.09 482.86

y = 1.0195x0.3667

Field Calculator

Use of Inverse Distance Weighted (IDW) for Extrapolation

• Need: Give Bankfull depth value to closest associated cells within the riparian corridor

• Creates the Bankfull elevation raster to use with elevation raster• Output: all cells at or below Bankfull elevation

Elevation Riparian zone (4X Bankfull Depth)IDW Extrapolation Output<= =

Model results

Fluvial LandscapeAssumption: we’re 100% confident this area is riparian

Elevation Above Channel

1 x Dbf2 x Dbf3 x Dbf

Evidence of flooding

Evidence of channel migration

Avulsion

1942 centerline in bluePresent centerline in yellow

Going Beyond the Fluvial Landscape with Indicators of Riparian FunctionSoil Moisture/Depth to Groundwater Upslope Contribution Areas WetlandsShading

Indicators of “riparian-ness”

Topographic Wetness Index:Depth to Groundwater & Soil Moisture

Indicators of riparian function

Upslope contribution – Flow accumulation (MD∞ method)

From Seibert & McGlynn 2007

D8 method(ArcGIS)

MD∞ method

Indicators of riparian function

Palustrine Wetlands

scouring-rush horsetail (Equisetum hyemale)

Indicators of riparian function

Shading and Organic Material Inputs

75 feet

Reality Check!Creation of a ground-truthed riparian zone map for comparison

Gravelcobble

Roots, cobbles in soil,

bedrock

Cobble, sand, willow, alder, cottonwood, bay

laurel

Grassland, wetland, valley oak, bay laurelgood soil development

Mixed oak woodlands, conifer forests

Scarp Channel Floodplain Terrace Upland Fringe

Landform mapbased on field surveys and DEMAlternative name: The map that would take too long to create for every potential riparian conservation easement project

Upland Fringe Upland Fringe

Acquisition ScenariosGiving practitioners data to make decisions

Delineating the Functional Riparian Zone

Upland Fringe Upland Fringe

Delineating the Functional Riparian Zone

Blue areas =Area under 18x Bankfull Depth

Acquisition Scenarios

Binned EAC into 4 scenarios (A = 5xBFD, B = 6x, C = 10x, and D = 18x)

Conservation Scenarios

Acquisition

Scenario

Additional

Shading

Canopy

(ha)

Additional

Wetlands

(ha)

Additional

High Soil

Moisture (%

of High TWI)

% of Functional

Riparian Zone

Covered

(cumulative)

Total

Land

Area

(ha)

Acquisition

Cost*

($)

A 6.1 0.32 31.0 26 11.1 $244,200

B 2.2 0.20 26.6 46 19.4 $426,800

C 3.4 0.03 31.7 77 32.4 $712,800

D 0.9 0 10.7 100 42.2 $932,800*Based on $22,000/ha estimate from Sonoma County Agricultural Preservation and Open Space District.

What’s it worth to you?

Marginal Gains

Where are the break points?

Different indicators but approximately the same

Benefit-to-Cost Ratio

Acquisition

Scenario

Relative

Increase of

Indicators of

Function (%)

Additional

Land Area

(ha)

Acquisition

Cost of

Additional

Area ($)

Benefit-to-

Cost Ratio

A 46 11.1 244,200 1.88

B 27 8.3 182,600 1.47

C 21 13.0 286,000 0.75

D 6 9.8 220,000 0.27

𝐵𝐵𝐵𝐵𝐵𝐵 = (𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑜𝑜𝑜𝑜 𝐹𝐹𝐹𝐹𝐼𝐼𝐼𝐼𝐹𝐹𝐹𝐹𝑜𝑜𝐼𝐼𝐼𝐼 × 10,000) ÷ 𝐴𝐴𝐼𝐼𝐴𝐴𝐹𝐹𝐹𝐹𝐼𝐼𝐹𝐹𝐹𝐹𝐹𝐹𝑜𝑜𝐼𝐼 𝐵𝐵𝑜𝑜𝐼𝐼𝐹𝐹

More land doesn’t always mean more benefit…

Significant breakpoint

Scaling to the Catchment Watershed-scale conservation planning

Catchment-Scale Conservation Planning18xBFD

Evaluate for:• Level of protection• Impacts/development

A

B

Protected

Protected

Protected

Protected

Site EvaluationA

B

Conclusion New methods can mean more conservation

Implications: Riparian easement

Future Research

• More sensitive indicators (biological, groundwater). Example: Is there a correlation between the alluvial hotspots and fish/roe counts?

• Repeated study with multiple sites stratified by climate, watershed gradient

• Test performance using lower resolution terrain data (e.g., 5, 10, or 30-meter) and lower point density LiDAR (1-3 points/sq meter)

• Is there a threshold that characterizes riparian zones throughout a catchment? (creation of a credible watershed-wide alluvial riparian zone map)

Thank youTom Robinson | tom@openspacecouncil.org

Collaborators:Dr. Jerry Davis, San Francisco State UniversityDr. Nancy Wilkinson, San Francisco State UniversityKaren Gaffney, Sonoma County Agricultural Preservation and Open Space DistrictDr. Ralph Dubayah, University of MarylandMark Tukman, Tukman Geospatial LLC

Funding provided by:Sonoma County Agricultural Preservation and Open Space DistrictRobert & Patricia Switzer FoundationCalifornia State University Council on Ocean Affairs, Science, and Technology (COAST) Program