Coastal Flood Mapping Using Customized GIS Layers

by Jeff Zanotti

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National Flood Insurance Program

NFIP established in 1968

Community participation is voluntary

Participation allows for chance to buy federal flood insurance in exchange for community floodplain management regulations

Buildings constructed in compliance with NFIP standards suffer around 80% less damage annually

Over 350 communities in Alabama participate

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Map Modernization

Majority of Flood Insurance Rate Maps (FIRMs) have become outdated in Alabama

FEMA assumes primary ownership of creation of the new flood maps with Alabama’s Office of Water Resources (OWR) being responsible for the Alabama Flood Map Modernization Program

“The mission of the program is to make Alabama and its citizens less vulnerable to the impact of flooding through statewide floodplain management and provide local communities with the tools and

resources for managing, assessing and planning for development in flood prone areas to reduce the loss of life and property”

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Coastal Issues

Storm surge plays an important role in controlling flooding along the coast

Unlike riverine flooding, coastal flooding also takes into account the impact of waves in designated special flood hazard areas

The impact of waves can be affected by numerous variables: sand dunes, barrier islands, type of vegetation, buildings, etc

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AMEC Tool

AMEC created a tool that would efficiently and accurately take into account those variables while modeling

Developed using VB.NET

Works as a toolbar within ESRI’s ArcGIS Desktop 9.3

Utilizes WHAFIS 4.0 and RUNUP 2.0 which are required to be installed on the computer running the Coastal Tool

Requires a combination of customized GIS layers

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GIS Layers

Transects

Survey Points

PFD Crest

PFD Heel

Buildings

Vegetation

Surface DEM

Surge DEM

Over Water Fetch

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Transects

Polylines that run from the body of water inland

Drawn from scratch in GIS by engineers and water resource specialists

Similar to cross sections in riverine studies

Evenly spaced throughout the entire study area

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Survey Points

Point survey data containing ground elevation

Also can be coded for toe, peak, and heel of dune

Points are taken along transects

This layer is optional dependent upon the quality of the surface DEM

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PFD Crest and PFD Heel

Polylines of the primary frontal dune’s crest and heel

These will not always be used as not all coastal areas have dunes

Assist in profiling the level of dune erosion and whether or not that will affect wave impact

These lines can be constructed using survey, obtained from data of local gov’t, or made from high resolution Lidar

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Buildings

Polygons of buildings digitized from aerial imagery

Not individual buildings; groups of buildings with similar attributes (i.e. width, layout, spacing, etc)

Calculations are made or estimated (based on quality of aerial and size of the structures) to assess the “open space ratio” of each polygon

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Buildings (continued)

Calculations involving row width and building width are done in GIS and reflective to properties perpendicular to the coast line

Polygons must be a certain length, dependent upon the spacing of the transects in that area

Google Street View and in field data collection allowed for the percentage of houses on stilts to be taken into account

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Vegetation

Polygons representing areas of similar vegetation type

These polygons were determined by using a combination of online datasets from gov’t agencies, remote sensing data, and aerial photography

Data had to be put into a format that WHAFIS could read

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Vegetation - Reclassification

Land use reclassification values Marsh Vegetation

–Herbaceous Wetlands Rigid Vegetation

–Deciduous Forest

–Evergreen Forest

–Mixed Forest

–Woody Wetland

–Shrubland Other

–Agriculture

–Barren

–Urban

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Vegetation - GAP

GAP national land cover data was used as a helpful tool in reclassifying land use into WHAFIS accepted values

Pixel resolution was too low to be able to use in its current

Digitization had to be done to make smoother polygons that more effectively matched aerial photography

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Vegetation - FIA

From WHAFIS model input parameters similar plant characteristics included: Drag coefficient Mean wetted height Mean effective diameter Mean horizontal spacing

Spatial join with Forest Inventory and Analysis (FIA) data was performed on the reclassified polygons to get a representation average for each of the sub groups

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Vegetation – Marsh Grass

WHAFIS input model incorporates information based on marsh grass type

2 main marsh grass types on the Alabama coast Spartina alterniflora Juncus roemerianus

A guiding shapefile was used from the U.S. Fish & Wildlife Service’s National Wetland Inventory

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Vegetation – Marsh Grass (continued)

National Wetland Inventory’s (NWI) habitat shapefile came with various extraneous polygons as well that had to be weeded out

Areas associated with marsh grass were determined by the corresponding habit based on Cowardin classification

NWI’s shapefile needed to be reshaped to match up with aerial photography

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Surface DEM

Digital Elevation Model depicting the topography of the study area

The better the DEM the more accurate the model will be

AMEC obtained local Lidar as well as detailed coastline Lidar from NOAA’s Digital Coast website

There was a concern of edge matching the DEM and Finite Mesh Development for storm surge modeling by FEMA and the Northwest Florida Water Management District

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Surge DEM

Raster DEMs

10 year surge

100 year surge with wave setup

100 year surge without wave setup

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Over Water Fetch

Fetch is area of open water over which wind can blow (i.e. sounds, oceans, bays, etc)

Transfer of energy from the wind to the water caused by frictional drag

Larger the fetch the bigger the waves that can be generated by the wind

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Data QA/QC

Field visits were made to select transects for data verification

Data and photos were recorded from these locations using a smart phone application

Many transect locations required coordination with other local and state agencies in order to record field data

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Advantages of Smart Phone App

Data is uploaded immediately to server No need to unload and then sort through large amounts of data once

returning from the field Data can be used instantaneously If the phone is damaged or broken the data is not lost

Less equipment needed out in the field Can be more efficient Can go more places Utilizes a variety of tools normally inaccessible out in the field

Data is uniform regardless of who is sent out in the field

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Coastal Inspection Data Entry

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Coastal Inspection Data Entry/Viewing

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Ready to Run the Coastal Tool

Data is formatted and compiled, now we are ready to run the Coastal Tool

9 steps to the tool

Tool guides you step by step

Some of the 9 steps are optional dependent on the study area

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Step 1

Create Transect Database(s) Sets up individual databases for

each transect Requires Surface and Transects

shapefile

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Step 2

Integrate Survey Optional step Set Search Radius for distance

from transects Requires Survey Points

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Step 3

Dune Erosion User chooses method of erosion:

Retreat or Removal Step is dependent on the

presence of dunes in the study area

Requires PFD Crest, PFD Heel, and Surge DEMs

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Step 3 (continued)

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Step 4

Populate Transect Nodes This step utilizes the most

customized GIS layers Loads layer data into transect

databases Requires vegetation, buildings,

over water fetch, PFD Crest, and Surge DEM layers

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Step 5

Generate WHAFIS Input Transforms data into a WHAFIS

models

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Step 6

Run WHAFIS WHAFIS input and output files

are placed in the transect databases

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Step 7

Generate RUNUP Input Creates RUNUP model User chooses a roughness

coefficient

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Step 8

Run RUNUP Models Puts outputs into transect

databases

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Step 9

Generate Summary Transects Integrates all the previous

modeling steps to generate a summary of all relevant locations along each transect

This step analyzes where to determine zone breaks for mapping

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Questions?