By Gregg Thielman and Lee Beauvais, Houston-Moore Group

The Federal Emergency Management Agency (FEMA) defines a floodplain as: “Any land area susceptible to being inundated by flood waters from any source”. A role of FEMA is to map the regulatory floodplain, which is often referred to as the 100-year floodplain. The 100-year floodplain correlates to a flood having a 1% chance of occurring in any given year. FEMA statistics show that homes in the 100-year floodplain have a 26% chance of flooding during a 30-year mortgage.

Properties located in the 100-year floodplain that carry a federally backed mortgage are required to carry flood insurance. The National Flood Insurance Program (NFIP) provides insurance to communities that adopt and enforce floodplain management ordinances to reduce future flood damage. The Cities of Fargo and Moorhead, Cass and Clay Counties, and a number of other communities and Townships in the Project area are all NFIP participants and have adopted floodplain management ordinances which regulate development in the floodplain.

Other goals of NFIP are to reduce the loss of life and property damage caused by flooding and to reduce rising disaster relief costs caused by flooding. While the regulatory 100-year floodplain is often used to establish building elevations and for determining whether mandatory flood insurance is required, there are flood risks for properties located outside the 100-year floodplain as well. This was the case for the 2011 floods on the Missouri and Mouse (Souris) Rivers where the flood events were in excess of a 100-year frequency. In both cases, a majority of the structures that were damaged were not carrying flood insurance, presumably because it was not required. Nationally, NFIP statistics show that last year, one-third of all claims paid by NFIP was for policies in low-risk communities.

Many FEMA Flood Insurance Rate Maps (FIRMs) were developed in the 1970’s and early 1980’s. Typically, these maps are updated every 20 to 30 years. In 2003, FEMA initiated the map modernization program that more aggressively sought to update FIRMs and convert them to a digital format (Digital Flood Insurance Rate Map (DFIRM). Map Modernization has since transitioned into the RiskMAP program. Risk MAP builds off the Map Modernization program and has a vision to deliver quality data that increases public awareness and leads to action that reduces risk to life and property.

Following the historic flood of 1997, FEMA Region VIII initiated a detailed flood insurance restudy (FIS) for the Red River and Wild Rice River in southern Cass County, ND and Clay County, MN. This FIS was subsequently expanded to include the Red River through the communities of Fargo and Moorhead. The new FIS maintained the existing hydrology from the prior FIS for the Red River, which was originally developed in 1971 (29,300 cubic feet per second (cfs) 100-year discharge). The hydraulic models were updated as part of the FIS using new topographic data (LiDAR) and calibrated to the 1997 flood. This resulted in an increase in flood stage at the USGS streamgage at Fargo from 38.5 feet to 39.4 feet. These stages assume no emergency flood protection measures are implemented by the communities adjacent to the Red River.

A number of delays have been experienced in the development of the FIS, including the addition of new studies on the Sheyenne and Maple River; the transition to FEMA’s DFIRM format; and re-accreditation of existing levees in the communities. Ultimately, the preliminary FIS and DFIRM for the City of Fargo and southern Cass County are expected to be distributed later this summer. The FIS and DFIRM for the City of Moorhead and Clay County became effective on April 17, 2012. The adoption of the new FIS and DFIRM will result in a significant number of new properties entering the 100-year floodplain. In addition to modifying the 100-year flood stage elevation, the new FIS and DFIRM also provide a new tool for managing development in the floodplain for the communities using their adopted floodplain ordinances.

Following the historic 2009 flood on the Red River, the U.S. Army Corps of Engineers – St. Paul District (USACE) fast-tracked an ongoing feasibility study to evaluate long-term flood protection options for the cities of Fargo, ND and Moorhead, MN. As part of the feasibility study, hydrology for the Red River was updated and a new, highly sophisticated hydraulic model (unsteady HEC-RAS) was developed. The hydrology utilized the recommendations of an expert panel that concluded the region is in a wet cycle. Based on this recommendation and analysis using a wet cycle period of record from 1942 to 2009, the 100-year discharge at Fargo based on the USACE hydrology was determined to be 34,700 cfs. The USACE hydraulic model was calibrated to the 2009 flood event and verified using other historic flood events, including the 1997, 2006, and 2010 floods. This resulted in an increase in flood stage at the USGS streamgage at Fargo to 41.1 feet, assuming no emergency protection measures are constructed. This equates to a flood stage of approximately 42.4 feet if emergency flood protection measures are implemented similar to those used during the 2009 flood event.

Due to the timing of the FEMA and USACE efforts, the updated USACE floodplain information will not be incorporated into the FEMA FIS and DFIRM updates at this time. However, it is anticipated that similar new information may be adopted by FEMA in the future.