flood risk datasets & products in greenville county, south carolina agenda greenville co, sc...

May 19, 2011

Application of Risk MAP Flood Risk Datasets and Products in Greenville County, South Carolina

Andy Bonner, PE, CFM – AECOMDaryle Fontenot, PE, CFM – AECOMMaria Cox Lamm, CFM – SC DNR

ASFPM 2011 Annual Conference

Flood Risk Datasets & Products in Greenville County, South Carolina

Agenda

• Greenville Co, SC Overview• Process, Examples, Lessons Learned, and Community

Feedback– Changes Since Last FIRM– Flood Depth & Analysis Grids– Flood Risk Assessment– Flood Risk Report, Map, and Database

• Conclusions

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Greenville County Overview

• Risk MAP Early Demonstration Project

• Purpose is to develop Flood Risk Datasets and Products for selected flooding sources in Greenville County

• Projected funded under CTP Early Demonstration MAS No. FY10.18

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• Most highly populated county in South Carolina (10% of State population)

• First SC County modernized under Map Modernization

• First SC County to be studied under Risk MAP

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• All streams included within the Demonstration Project area are within the Saluda HUC-8 Basin (03050109)

• 3 Communities affected:– City of Greenville– City of Travelers Rest– Greenville County

Unincorporated Areas

• 40 Miles of Stream Identified for Flood Risk Datasets & Products

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Project Scope – Included Elements

• Flood Risk Datasets– Changes Since Last FIRM– Flood Depth Grids (10%, 4%, 2%, 1%, 0.2% Annual Chance Events)– Flood Velocity Grids (1% Annual Chance)– Percent Annual Chance of Flooding Grid– Percent Chance of Flooding over a 30-yr Period Grid– Water-Surface Elevation Change Grid (1% Annual Chance)– Flood Risk Assessment (Hazus Results)

• Flood Risk Products– Flood Risk Database– Flood Risk Map– Flood Risk Report

• Community Meeting (WebEx held on 5/11/2011)

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Community Meeting Format

• WebEx meeting held on May 11, 2011• Discussed the purposes of the data, goals of Risk MAP, and

mitigation actions that could be undertaken• Data within the Flood Risk Database at several key locations

was presented to the community within ArcGIS• Discussion centered around:

– What the data means and how it can be used– Feedback from the community– Tie-ins to the community’s mitigation efforts

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Changes Since Last FIRM


Greenville County: Changes Since Last FIRM• Process

– GIS processing (Union of Effective and New floodplains) and table attribution

– Manual identification of Contributing Engineering Factors– We developed custom tools to automate portions of the GIS

processing

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• Examples

Greenville County: Changes Since Last FIRM

Added to SFHA

Removed from SFHA

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• Examples

Greenville County:Changes Since Last FIRM

Added to Floodway

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Greenville County:Changes Since Last FIRM• Lessons Learned

– Populating the CSLF database was slightly more complex than originally anticipated (Contributing Engineering Factors) – once tools are fully in place and the database design is “locked down”, this shouldn’t be an issue

• Community Feedback– Thought this dataset would be very helpful– Asked whether or not there was information on the vertical change

as well as the horizontal change (WSEL Change Grid)– Seemed to appreciate the chance to review the results in certain

locations ahead of preliminary issuance

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Flood Depth & Analysis Grids


Greenville County:Flood Depth Grids• Process

– Subtraction of Ground Elevation from Water Surface Elevation grid– We used in-house automated tools for mapping backwater fingers

and around confluences – based on attributed cross sections and terrain data

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Greenville County:Flood Depth Grids• Some awareness is needed when using these grids along

with specific buildings/homes

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First FloorElevation

Depth reported in Depth Grid

Depth within Structure


• Examples

Greenville County:Flood Depth Grids

10% Annual Chance (10-yr)Flood Depths


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Greenville County:Flood Depth Grids• Examples



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Greenville County:Flood Depth Grids• Lessons Learned

– Production process fairly straight forward, but need to make sure confluences and backwater fingers are addressed

– 25 foot grids were too coarse in populated areas, and on smaller streams – recommend allowing for smaller grids if terrain accuracy supports

• Community Feedback– Felt the data would be beneficial to community officials, planners,

emergency responders, etc.– Flood depth information at roads (road access) is an important part

of community’s planning efforts– Had some reservations of having this information openly accessible

to the general public – concerns about the devaluation of property that it could lead to

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Greenville County:Percent Chance of Flooding Grids• Process

– Computation of percent annual chance of flooding based on Water Surface Elevation curves and intersection with ground

– Projection of probability of flooding during 30-yr time period– We used in-house automated tools for producing this dataset

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Greenville County:Percent Chance of Flooding Grids• Examples (Percent Annual Chance)

Home with greater than 10% Annual Chance of Flooding

Outside SFHA but with Flood Risk

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Greenville County:Percent Chance of Flooding Grids• Examples (Percent Chance over 30 years)

Greater than 90% chance of flooding at least once during 30 years

Nearly 20% chance of flooding at least once during 30 years

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Greenville County:Percent Chance of Flooding Grids• Lessons Learned

– Production process fairly straight forward, but need to make sure confluences and backwater fingers are addressed

– 25 foot grids were too coarse in populated areas, and on smaller streams – recommend allowing for smaller grids if terrain accuracy supports

• Community Feedback– None received, other than a question on whether the

mortgage/insurance industry would have access to this data

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Greenville County:Water Surface Elevation Change Grid• Process

– Generate 1% WSEL Grid based on new study results– Generate 1% WSEL Grid based on effective study results– Subtract the effective from the new– Clip to the Changes Since Last FIRM areas that were SFHA on the

effective maps and are also SFHA on the new maps

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Greenville County:Water Surface Elevation Change Grid• Examples

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Greenville County:Water Surface Elevation Change Grid• Examples

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Greenville County:Water Surface Elevation Change Grid• Lessons Learned

– Having effective GIS data for the models is helpful. This would eliminate the need to ‘reverse engineer’ the spatial information.

– The more effective cross sections (all – not just the lettered cross sections) you have enhances the elevation change grids – an accurate WSEL Change Grid requires you to be able to accurately recreate the effective profile from the digital data

• Community Feedback– Very helpful to have this data in conjunction with the Changes Since

Last FIRM dataset

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Greenville County:Velocity Grids• Process

– We used HEC-GeoRAS and its velocity mapping tools– Velocity distributions were specified within HEC-RAS as follows:

• 20 on each overbank• 5 within the channel

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Greenville County:Velocity Grids• Examples

Structures Potentially Subject to High Velocities

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Greenville County:Velocity Grids• Examples

Structures Potentially Less Vulnerable to High Velocities

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Greenville County:Velocity Grids• Lessons Learned

– Velocity grids produced from a 1D model should be considered approximate at best, unless cross section spacing is very close and very detailed overbank n-values have been incorporated into the models (velocities sensitive to n-values)

– 2D models would produce velocity grids that are more accurate and defendable

– Backwater areas are a challenge – would need to supplement separately or spend time manually adjusting cross sections

• Community Feedback– Velocity results over roads would be particularly beneficial to support

the community’s efforts to identify where road closures should occur during a flood

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Flood Risk Assessment(Hazus Results)


Greenville County:Flood Risk Assessment• Process

– Input depth grids from study into Hazus and compute flood losses– Calculate annualized losses based on the calculated results from

each flood event (10%, 4%, 2%, 1%, 0.2% Annual Chance)

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Greenville County:Flood Risk Assessment• Examples

Medium Flood Risk

Low Flood Risk

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Greenville County:Flood Risk Assessment• Examples

High Flood Risk (although closer inspection is warranted)

Medium Flood Risk

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Greenville County:Flood Risk Assessment• Lessons Learned

– Currently, somewhat cumbersome to export tabular results from Hazus and incorporate into Flood Risk Database format (although future tools should hopefully streamline this)

– Message needs to be carefully crafted in areas where a census block reports flood losses, but most/all of the homes are outside the floodplain (due to the way Hazus assumes the distribution of buildings with a census block) – site-specific analyses will be much more accurate

• Community Feedback– Community has gone through an effort to collect information at

specific structures to help them identify potential buyout properties – census block information may not provide that much benefit in these cases

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Flood Risk Products:Flood Risk Database, Report, & Map


Greenville County:Flood Risk Database, Report, & Map

Flood Risk Map• Community boundaries• Potential losses from 2010

AAL study supplemented with new Hazus runs from new modeling areas

• Community per capita losses• Additional data available

through FRD, FIRM and National Flood Hazard Layers

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Greenville County:Flood Risk Database, Report, & Map• Lessons Learned

– Database production will be easier in the future once the database format and standards are locked down

– With so many different ways to look at the data within the database, a community user guide should be provided that recommends various attributes, or combinations of attributes, to look at depending on the situation

– Flood Risk Map may be too “zoomed out” to be effectively used, especially if Areas of Mitigation Interest are not part of the project

• Community Feedback– Can see some use in the Flood Risk Map to help initially identify

other areas for further investigation

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Conclusions


Greenville County:Conclusions• The Water Surface Elevation Change Grid should be delivered along

with the Changes Since Last FIRM – they will be used hand-in-hand• It is critical to have discussions early on at the Discovery meeting about

the best way to optimize the balance between the community’s and FEMA’s resources to produce the flood risk data

• Although there is recognition that mitigation actions are easier to identify when the flood risk assessment data is provided at the building-specific level rather than census block level, the strategy for the dissemination of that information needs to have buy-in from the communities

• Although not a part of this project, the Areas of Mitigation Interest dataset should be considered in future Risk MAP projects – community had interest in undersized culverts and information about overtopping

• A community user guide should be provided that contains suggestions on how to view and symbolize the data for various purposes

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Flood Risk Datasets & Products in Greenville County, South Carolina 41

Questions?

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flood risk datasets & products in greenville county, south carolina agenda greenville co, sc...

Documents

populated county

goals of risk map

greenville countyprojected

south carolina changes

south carolina andy

south carolinaproject

sc overviewprocess

firmflood depth grids