County-wide Flood Insurance Rate Maps (FIRMs)

During the flooding of 2008 the State of Iowa became aware of the need for up-to-date flood risk data for emergency response and flood preparation for communities. Nearly a third of the counties in the state were without Flood Insurance Rate Maps (FIRMs) while nearly half were working with data that was 20 - 30 years old. This data was out of date due to development, changes on the landscape and increases in stream gage (flow) information. This made growth difficult for those communities with older mapping as it was unreliable for permitting and floodplain management.

The Iowa Department of Natural Resources (Iowa DNR) responded to this need by applying for a Housing and Urban Development (HUD) grant and began responding to the lack of flood risk data with the initiation of the Statewide Floodplain Mapping Program in 2010. This was possible because Iowa had already begun collecting Light Detection and Ranging (LiDAR) data in 2007. Collection of LiDAR data was completed in 2010. The Federal Emergency Management Agency (FEMA) requires LiDAR for all new regulatory mapping. Ten years later the main goal of this program, to provide digital flood risk data and updated mapping based on LiDAR to all communities in the state, is nearing completion. Preliminary mapping has been completed for all county-wide FIRMs for the entire state. Statutory review and appeal periods remain before some counties can be issued FIRMs.

Two-Dimensional Base Level Engineering (2D BLE)
Goals

This year the Floodplain Mapping Program will begin a project for developing a new kind of flood risk data known as two-dimensional base level engineering (2D BLE). As development and other changes occur, flood risk data requires updates in order to provide accurate, up-to-date risk awareness. Mapping methodologies also change over time. Flood risk awareness data is shifting from binary (in the floodplain vs. not in the floodplain) to a graduated risk analyses paradigm. This will help to better understand flood hazards, to more adequately rate flood insurance, and to improve planning for mitigation. Graduated risk means utilizing a greater range of event recurrence and/or storm variability in order to better define flood hazards and associated risk, as opposed to simply evaluating based on the 1% (100yr) risk. Development of base 2D BLE hydraulic modeling will provide the foundational data that will be used for future FIRMs, probabilistic analysis for graduated risk, and data bundling that allows others to work with the data to answer “what if” questions. Instead of providing modeling related to flood insurance requirements only, this data will deliver both predictive capabilities and can be used to regulate if a community so chooses. Goals of the 2D BLE mapping project are listed below:

  • Improve Risk Reduction through Mitigation
  • Increase Resilience
  • Improve Floodplain Management
  • Enhance Natural Resources/Quality of Life
Methodology

2D BLE is a major improvement over 1-dimensional (1D) modeling although some portions of the inputs remain similar. The major difference is that 1D methodologies are tied to a profile baseline and cross sections to determine risk while 2D BLE is not. The profile baseline is restricted to a drainage area of greater than 1 square mile and some semblance of a stream to collect water. The models show what happens when the water from a flooding event reaches the profile baseline (stream). This makes modeling for lakes difficult and does not represent overland flooding scenarios.

2D BLE is not confined to the profile baseline and cross sections. It instead takes a whole surface into account. This modeling uses 2D rain-on-grid made possible by HEC-RAS 6.0 software, which relies on a user defined mesh and high-resolution terrain data to capture the flow of runoff through an entire watershed. By using newly acquired LiDAR (2019, 2020, 2021) a more up-to-date picture of flood risk can be realized. 

 

Figure 1. 1D mapping uses a profile baseline and cross sections on left. In areas of confluences this can make drawing appropriate cross sections difficult and can greatly influence the results of modeling. The grid produced by 2D mapping is shown on the right. This grid can be adjusted to be more resolute for areas of development to more accurately represent risk and eliminates the effect of poorly drawn or difficult to place cross sections on modeling results.

Figure 1. 1D mapping uses a profile baseline and cross sections on left. In areas of confluences this can make drawing appropriate cross sections difficult and can greatly influence the results of modeling. The grid produced by 2D mapping is shown on the right. This grid can be adjusted to be more resolute for areas of development to more accurately represent risk and eliminates the effect of poorly drawn or difficult to place cross sections on modeling results.

The result of taking the whole surface into account is a more comprehensive understanding of flood risk due to pluvial flooding (overland flooding not tied to streams) as well as fluvial (stream/river) flooding. In vast flat areas where there is not a defined stream there are paths the water follows that experience flash flooding. While not as costly as fluvial flooding that goes through cities (often located near rivers), pluvial flooding is nonetheless a risk for road access and emergency response related issues. This information can be used for stormwater management strategies by communities as well.

Figure 2. 1D mapping is confined to an actual stream being present on the surface to help decide where to start mapping. The blue shading shows the 1% annual chance flood risk modeled using the 1D approach. 2D mapping, (green shading), is not confined to a stream being present to help decide where to start mapping. Instead the whole surface is used to map flood risk in areas over 1 square mile.

Figure 2. 1D mapping is confined to an actual stream being present on the surface to help decide where to start mapping. The blue shading shows the 1% annual chance flood risk modeled using the 1D approach. 2D mapping, (green shading), is not confined to a stream being present to help decide where to start mapping. Instead the whole surface is used to map flood risk in areas over 1 square mile.

The new risk data will benefit communities by providing more comprehensive understanding of the flood risk in areas under consideration for future development, both in cities and rural housing developments. Through smart planning and coordination, development can start or continue in an area with an eye to the future - risk can be planned for and mitigated. This information will be free to all of Iowa and can be used as best available data by jurisdictions, if they choose to use it for regulatory purposes. In cases where a jurisdiction decides it would like its regulatory mapping updated with the 2D BLE product Iowa DNR will work with them to help with that process. Risk data products listed below are developed as part of the 2D BLE process:

  • Raster format data for analysis through GIS
  • Water surface elevation (WSEL) grids for floodplain management/emergency response
  • Potential for cumulative rise/change calculations during development planning
  • Velocity grids for better understanding of flood risk
  • Distance times velocity calculations for floodway determination
  • Shear stress calculations for determining areas at risk for soil or bank scour
Outreach

Outreach is an essential part of the success of this project. Robust outreach has been effective in limiting appeals and will be important in promoting stakeholder acceptance of the 2D BLE data for wise planning and development. A series of webinars focusing on mitigation and risk awareness will be held by geographic footprints for local stakeholders as well as a separate series for larger statewide or federal partners. These meetings will serve as an introduction to the project to communities and other organizations and a first review of the data with no emphasis on floodplain regulation. As modeling is completed and mapping results start to be available for review Draft Map Distribution and Partner Review will take place that will be very similar to Draft Flood Hazard Meetings that the Iowa DNR already performed for new mapping. This outreach is planned to start in early 2021 with products becoming available for review in the Fall of 2021. These meetings will provide an overview of mapping results and inform partners on how to access any data that is available for review or use.

Floodplain Mapping Status

Map of Iowa, Floodplain Mapping Status

Outreach

FEMA Cooperating Technical Partner for Excellence in Outreach and Communications Story Map.

Draft Flood Hazard Outreach Program Increases Disaster Preparedness and Promotes Local Mitigation Investments

The Iowa DNR learns of the need to restudy streams, and do new studies where they don’t exist, through outreach events attended by local officials. When the Iowa DNR learns of a need they put it in the annually updated CTP Business Plan in order to ask for grants from FEMA.

Floodplain Mapping Contacts

Scott Ralston, P.E., CFM, Floodplain Mapping Coordinator/CTP Program Manager, Scott.Ralston@dnr.iowa.gov, 515-537-3181

Chris Kahle, GIS Analyst/Technical Program Lead GIS, Chris.Kahle@dnr.iowa.gov, 319-300-5719

National Flood Insurance Program (NFIP) Contacts

The Iowa DNR is the state coordinating agency for the National Flood Insurance Program (NFIP) and provides assistance to local communities that participate in the NFIP.

Bill Cappuccio, State NFIP Coordinator, Bill.Cappuccio@dnr.iowa.gov, 515-725-8342

Jason Conn, Local Floodplains Program, Jason.Conn@dnr.iowa.gov, 515-725-8333

Ken Bouma, Local Floodplains Program, Ken.Bouma@dnr.iowa.gov, 515-725-8352

History of the Program

The State of Iowa was awarded $15.0 million in Community Development Block Grant (CDBG) Disaster Recovery Funds in 2009 as a result of the devastating floods of 2008. This funding was allocated to the Iowa DNR for the development of new floodplain maps and updating of existing maps for all of the 85 Iowa counties listed in the federal Disaster Declaration of 5/27/2008 (Declaration FEMA-1763-DR). Iowa had begun collection of LiDAR in 2007 before the 2008 flood; having that dataset available was one of the keys to the program’s success. This $4.3 million project was funded by several state and federal agencies and completed in 2012.  At the time, Iowa was one of a few states in the country to accomplish this significant data acquisition task.

In 2010, the Iowa legislature mandated that the Iowa Flood Center (IFC) at the University of Iowa contract with the Iowa DNR for $10 million of the $15 million of CDBG grant funds to map the 85 disaster declared counties. In March 2015, the contract with the Iowa Flood Center was amended to increase the contract amount to $12.5 million, this contract ended in December 2016. To update floodplain mapping for the remaining 14 non-disaster declared counties state funding and leveraging of state resources with the Army Corps of Engineers Section 22 Public Assistance to States (PAS) program was used. Two Section 22 PAS agreements between the Iowa DNR and the U.S. Army Corps of Engineers have been executed: September 21, 2011 (for eleven countywide floodplain work maps); and June 28, 2012 (for one countywide floodplain work map). Development of flood hazard data for the two remaining non-declared counties was funded with non-grant state funding. The $15 million CDBG grant has been completely expended as of December 31, 2016 – and has been closed out by the State of Iowa Economic Development Authority.  

The map below shows the status of 2D BLE mapping
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