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Freshwater & Oceans

Tools for evaluating flood mitigation

Wetland Prioritization Study Main Page

 

Single-objective tools

 

Multi-objective tools

 


 

The Nature Conservancy and Environmental Law Institute Duck-Pensaukee Watershed Approach Pilot Project (TNC-ELI DPWAP) Flood Abatement Tool:1 A planning team assessed the ability of individual Potentially Restorable Wetlands (PRWs) and preservation wetlands to perform flood abatement functions by evaluating each site for three types of criteria using a GIS-based approach. These included:

  1. "Opportunity criteria," representing the possibility of provision of flood abatement benefits, given the landscape context of each site evaluated (e.g., wetlands in large catchments with large amount of impervious surfaces have the opportunity to receive large amounts of runoff during storms).
  2. "Effectiveness criteria," representing the capability of wetlands to provide flood abatement benefits, given the specific characteristics of each individual site (e.g., wetlands in topographic depressions are more effective at storing floodwaters than those on slopes)
  3. "Social significance criteria," represented the extent to which flood abatement benefits for a site provided clear societal benefits (e.g., wetlands that lie upstream of developed, flood-prone areas may have a high social significance for flood abatement functions).

Using GIS analysis, the team counted the number of opportunity and effectiveness criteria satisfied at each wetland site, with PRWs and existing wetlands analyzed separately. These counts were divided by the total number of opportunity or effectiveness criteria that could possibly have been satisfied to obtain final scores for each PRW or existing wetland. The team added 0.1 to the resulting scores if any social significance criteria had been met for the site. Assessing PRWs and existing wetlands separately, the team designated the highest scoring quarter of all sites to be "exceptional" priorities and the next quarter of sites to be "high" priorities. The lowest half scoring sites were considered "low" priorities for PRWs (i.e., restoration) and were not considered priorities at all for existing wetlands (i.e., preservation). Factors and data sources used to assess flood mitigation are listed below:

Factor used in analysis

Data source(s)

Wetland reestablishment opportunities (PRWs)

See above

Wetland preservation opportunities

See above

Opportunity criteria

Impervious surfaces cover › 10% of the site's catchment

2001 NLCD Impervious Surface Dataset from USGS via Data Basin, NHDPlus Catchments (14-digit), National Wetlands Inventory (NWI), Potentially Restorable Wetlands (PRW)

Slopes within the site's catchment exceed 15%

demgw930 (30m DEM distributed by WDNR), NHDPlus Catchments (14-digit), NWI, PRW

Site receives point source inflow

NWI+, WI DNR 24k hydro layer, NWI, PRW

The catchment area is large relative to the site (›82% of the catchment is upland)

NHDPlus Catchments (14-digit), NWI, PRW

Effectiveness criteria

Dominant vegetation is dense and persistent (forest, scrub-shrub, emergent marsh)

NWI+, NWI, PRW

Site is in a topographic depression or floodplain setting

NWI+, Historic Wetland LLWW

Flow through the site is not channelized or incised

NWI+, Historic Wetland LLWW

Social significance criteria

Developed flood-prone areas occur downstream, within 5 miles or above the nearest dam

NHDPlus Catchments (14-digit), NWI, PRW, Minor Civil Divisions, 24k hydro

Acronym definitions

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The Nature Conservancy and Environmental Law Institute Duck-Pensaukee Watershed Approach Pilot Project (TNC-ELI DPWAP) Shoreline Protection Tool:1 A planning team assessed the ability of individual Potentially Restorable Wetlands (PRWs) and preservation wetlands to perform shoreline protection functions by evaluating each site for three types of criteria using a GIS-based approach:

  1. "Opportunity criteria" represented the possibility of providing shoreline protection services given the landscape context of each site evaluated.
  2. "Effectiveness criteria" represented the capability of wetlands to provide shoreline protection services given the specific characteristics of each individual site (e.g., wetlands containing densely rooted vegetation are more effective at storing floodwaters than those on slopes).
  3. "Social significance criteria" represented the extent to which shoreline protection services for a site translated into clear societal benefits.

Using GIS analysis, the team counted the number of opportunity and effectiveness criteria satisfied at each wetland site, with PRWs and existing wetlands analyzed separately. These counts were divided by the total number of opportunity or effectiveness criteria that could possibly have been satisfied to obtain final scores for each PRW or existing wetland. The team added 0.1 to the resulting scores if any social significance criteria had been met for the site. Assessing PRWs and existing wetlands separately, the team designated the highest-scoring quarter of all sites as "exceptional" priorities and the next quarter of sites as "high" priorities. The lowest-scoring half of the sites were considered "low" priorities for PRWs (i.e., restoration) and were not considered priorities at all for existing wetlands (i.e., preservation). Factors and data sources used to assess shoreline protection are listed below:

Factor used in analysis

Data source(s)

Wetland reestablishment opportunities (PRWs)

See above

Wetland preservation opportunities

See above

Opportunity criteria

Adjacent to or containing a river, stream, or lake

24k Hydro WI DNR, Wetlands, PRWs

Effectiveness criteria

Adjacent waterbody is large (lakes › 10 acres; streams › 2nd order)

24k Hydro WI DNR, Wetlands, PRWs

Densely rooted emergent or woody vegetation (EM, SS, FO)

NWI

Social significance criteria

Located between (adjacency to) developed area and open water

Wetlands, WROC 2010 18" air photography

Acronym definitions

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The Louisiana Coastal Protection and Restoration Authority Coastal Master Plan (LACPRA CMP) Coastal Louisiana Risk Assessment (CLARA):2 CPRA used its CLARA model to estimate flood depths and damage for each of the approximately 35,500 census blocks that make up coastal Louisiana. CPRA calculated flood elevations for protected areas within each of its "basic hydrologic units", which it used to estimate flood depths within each census block. Flood depths for unprotected or semi-protected areas were then calculated based on surge and wave input values for each census block. CPRA calculated the total economic damage and risk within each census block due to flooding based on storms of category 3 or higher. Various scenario analyses were used to address uncertainty in the model and capture a wide range of possible outcomes. Factors and data sources representing input variables for the model are listed below:

Factor used in analysis

Data source(s)

Flood depth module

Surge hydrographs

Arcadis; Storm Surge/Wave Model

Wave period

Arcadis; Storm Surge/Wave Model

Significant wave height

Arcadis; Storm Surge/Wave Model

DEM of Louisiana

U.S. Geological Survey (USGS); Wetland Morphology Model

Wave free crest height

Arcadis; Storm Surge/Wave Model

Foreshore armor of protection structures

State of Louisiana/USACE

Presence of floodwall

State of Louisiana/USACE

Floodwall geometry

State of Louisiana/USACE

Length of protection structure's foreshore

State of Louisiana/USACE

Geotechnical data regarding protection system

State of Louisiana/USACE

Pumping rates for each BHU

Sewerage and Water Board of New Orleans

Rainfall

Arcadis; Storm Surge/Wave Model

Economic module

Inventory

Number of structures

GNOCDC, ACS, LACPR, Hazus MH MR4

Number of structures

LACPR, Hazus, U.S. census

Acreage of agricultural crops

LACPR, NASS, LSU AgCenter

Number of vehicles

LACPR (adjusted by ACS)

Inventory of roads and bridges

LACPR

Square footage

LACPR, Hazus

Valuation

Structural characteristics for each asset class

Hazus

Replacement cost per square foot

Hazus

Proportion of structures by construction class (economy, average, custom, luxury)

Hazus

CSVR

LACPR

Value of inventory per square foot

Hazus

Repair costs per mile

LACPR, Hazus

Agriculture valuations

LACPR

Proportion of structures by construction method (e.g., wood frame, masonry)

Hazus

Flood elevations

Calculated by model

Damage

Structural elevation above grade

LACPR, Road Home, HMGP

Depth-restoration time curve

Hazus

Depth-damage curves for structure

Hazus

Depth-damage curves for contents

Hazus

Depth-damage curves for inventory

Hazus

Costs dependent on displacement time: lost income, lost wages, lost sales, disruption costs, relocation rental costs

LACPR, Hazus

Costs dependent on displacement time: evacuation and subsistence costs

LACPR

Post-flood response costs: landscaping repair, debris removal, other cleanup

LACPR

Acronym definitions

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The Louisiana Coastal Protection and Restoration Authority Coastal Master Plan (LACPRA CMP) Relative Elevation Sub-Model:3 As part of its Wetland Morphology Model, LACPRA measured the effect of aquatic resource restoration and conservation (among other flood mitigation measures) on land building using its relative elevation sub-model. This model redistributed the simulated sediment supply value from LACPRA's Eco-Hydrology Model across the landscape by multiplying total sediment in each area by a sedimentation redistribution weighting surface. Vertical accretion within each 30m2 cell is calculated and the updated surface elevation compared to mean water level to calculate land building. Factors and data sources representing input variables for the model are listed below:

Factor used in analysis

Data source

Rate of vertical accumulation of sediment (cm/yr)

Eco-Hydrology Model

Mineral sediment accumulation rates (g/m2/yr)

Organic matter accumulation rates (g/m2/yr)

Soil bulk density (g/cm2)

Acronym definitions

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The Louisiana Coastal Protection and Restoration AuthorityCoastal Master Plan (LACPRA CMP) Storm Surge/Wave Attenuation Potential Suitability Index:4 This tool ranked each 500m2 cell in terms of the project's effect on the attenuation of storm surge/waves that would otherwise impact populated areas. Areas near zones designated for a 100 or 500 year level risk reduction, containing a low percent coverage by water, containing a high coverage by vegetation types that increase friction with waves (e.g., dense woods and brush), and having high water depth/land height were ranked highest. Factors and data sources representing these variables are listed below:

Factor used in analysis

Data source

Distance from an area designated for 100- or 500-year level protection (km)

N/A

Percent water

Wetland morphology model

Water depth/land height

Vegetation type rated based on ability to increase friction with incoming waves

Vegetation model


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The New Hampshire Department of Environmental Services Wetland Restoration Assessment Model ( NHDES WRAM) Flood Protection Tool:5 Flood protection is determined as the potential for a site to act as a natural flood control buffer. Factors used to assess flood protection are storage (e.g. the amount of water that the wetland can hold), the outlet flow rate, the percentage of the site located within a FEMA floodplain, and the dominant wetland class (listed below).

Factor used in analysis

Data source(s)

Upslope watershed area

USGS DEM

Wetland Control Length (i.e., restriction of outlet flow from wetland based on proximity to bridges, dams, and roads)

NHD waterbodies and flowlines

Flood zone area

FEMA/GRANIT

Dominant wetland class

NWI

Acronym definitions

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The National Oceanic and Atmospheric Administration's (NOAA) Habitat Priority Planner (HPP) Mississippi-Alabama Habitats Tool (MAHT) - Intertidal Marshes and Flats (Flood Hazard Protection) Tool:6 The Coastal Habitats Coordinating Team (CHCT), which consisted of more than 60 state and local scientists, non-profit staff, environmental professionals (consultants), and local/state officials, identified priority habitats for protection using the Habitat Priority Planner (HPP) tool. The HPP was designed by the NOAA Coastal Services Center (CSC) to readily incorporate stakeholder input into planning and was applied by the Mobile Bay National Estuary Program ( MBNEP) in the following steps to identify priority habitat areas:

  1. The CHCT identified ten focal habitat types for which prioritization analyses should be completed - four of these represented aquatic resources.
  2. Staff from The Nature Conservancy and the CSC compiled data from local sources for each of these focal habitat types.
  3. CHCT members were provided the list of available data for each focal habitat type in addition to a list of possible metrics (e.g., perimeter-to-area ratio, proximity to other habitat patches, etc.) that could be applied to each. CHCT members used the available data to decide on metrics that could be used to prioritize habitat patches for each focal habitat type.
  4. Using the metrics identified for each habitat type by the CHCT, the CSC used the HPP tool to identify priority habitat areas. After the results were presented, the CSC engaged the CHCT in validating or modifying the results to produce a final set of HPP priority habitat maps.
  5. HHP priority habitat maps were incorporated into the Habitat Mapper tool.

CHCT members prioritized intertidal marshes and flats for flood hazard protection using the factors and data sources listed below:

Factor used in analysis

Data source

Classified as Mississippi Sound salt marsh or brackish tidal marsh

Alabama GAP data

Salt marsh is 30m or less from developed areas

Salt marsh is 500 ft or less from the 100-year flood zone

Federal Emergency Management Agency's 100-year floodplain

Acronym definitions

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Wisconsin Department of Natural Resources (WDNR) Flood Storage Decision Support Tool:7 WDNR is currently developing a method for determining which individual subwatersheds (HUC-12s) and catchments (HUC-14s) would benefit most from wetland restoration and protection. The method accomplishes this by quantifying several parameters indicative of the capacity of wetland restoration or protection to improve flood abatement for each subwatershed or catchment (listed below). These would then be combined to obtain final scores for potential flood abatement benefits for each subwatershed or catchment.

Factor used in analysis

Data source

Percent area of remaining wetlands

N/A

Percent of wetlands in headwaters areas

N/A

Percent of wetlands in isolated depressional areas

N/A

Percent of wetlands in floodplain areas

N/A

Percent impervious land cover

N/A

Number of wetland acres lost

N/A

Percent loss of PRWs

N/A


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Wetland Prioritization Study Main Page


1Miller, N., T. Bernthal, J. Wagner, M. Grimm, G. Casper, and J. Kline. 2012. The Duck-Pensaukee Watershed Approach: Mapping Wetland Services, Meeting Watershed Needs. The Nature Conservancy and Environmental Law Institute, Madison, Wisconsin.
2 Fischback JR, Johnson DR, Ortiz DS, Bryant B, Hoover M, Ostwald J. 2012. Risk assessment (CLARA) model technical report. Appendix D-25. Louisiana's Comprehensive Master Plan for a Sustainable Coast. Coastal Protection and Restoration Authority of Louisiana. Baton Rouge, LA.
3 Holm G, Perez B, Stagg C, Wamsley T, and Snedden G. 2012. Wetland morphology model technical report. Appendix D-2. Louisiana's Comprehensive Master Plan for a Sustainable Coast.. Coastal Protection and Restoration Authority of Louisiana. Baton Rouge, LA.
4 Reed D. 2012. Storm surge/wave attenuation (potential for) technical report. Appendix D-23. Louisiana's Comprehensive Master Plan for a Sustainable Coast. Coastal Protection and Restoration Authority of Louisiana. Baton Rouge, LA.
5 Vanasse Hangen Brustlin, Inc. 2009. Merrimack River Watershed Restoration Strategy. Prepared for New Hampshire Department of Environmental Services.
6 The Nature Conservancy, National Oceanic and Atmospheric Administration, and Mobile Bay National Estuary Program. 2009. Prioritization guide for coastal habitat protection and restoration in Mobile and Baldwin counties, Alabama. Accessed from: http://habitats.disl.org/HabitatMapperGuide.pdf.
7 Kline J, Bernthal T, Burzynski M, Barrett K. 2006. Milwaukee River Basin Wetland Assessment Project: Developing Decision Support Tools for Effective Planning.