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Utah Floodplain Mapping History

Utah Floodplain

Mapping Eras

Over the years, flood maps have taken on different forms and methods and have been referred to by different names. They have always been produced using state-of-the-art methods and data.

Early Floodplain Maps

Utah’s first flood maps were released in 1973 in response to the passing of the National Flood Insurance Act of 1968. Called Flood Hazard Boundary Maps, these maps showed the extent of flood-, mudflow-, and erosion-prone areas. Over xxx Utah communities were mapped before the end of the decade. Many of these maps are still effective but they may not have been digitized. Thus they cannot be used within a GIS.

From the late 1970s to the end of the 20th Century, the Flood Maps were referred to as Flood Insurance Rate Maps, or FIRMs, and Flood Boundary and Floodway Maps, or FBFMs. The FBFM is a version of a flood map that shows only the floodway and flood boundaries. FBFMs are no longer produced. FIRMs depicted flood risk zones and their boundaries, and may also show floodways and Base Flood Elevations (BFEs).

Throughout the years, FEMA has produced three types of flood maps: Flood Insurance Rate Map (FIRM)s, Flood Boundary and Floodway Maps (FBFM), and Flood Hazard Boundary Maps (FHBM).  The FIRM is the most common type of map and most communities have this type of map. At a minimum, flood maps show flood risk zones and their boundaries, and may also show floodways and Base Flood Elevations (BFEs). The FBFM is a version of a flood map that shows only the floodway and flood boundaries. The FBFM is no longer produced; current FIRMs include all of this information. The FHBM is an older (the oldest) version of a flood map and is based on approximate data.

Regardless of mapping era, maps were based on best available data at the time. However, much of the data and many of the tools we use today did not exist. Many of the landform features were derived from topographic maps dating back to the 1960s. Accurate elevation was not available until the invention and adoption of LiDAR.

Accuracy of the paper maps: These early maps were made from the best available data at the time. Often they made use of paper topography maps. (Accuracy of old topo maps) If your community uses flood risk data from this period we highly recommend considering an update. Currently the Utah’s Division of Emergency Management is working together with FEMA many Utah jurisdictions to update maps of the populated areas of the state. However, it will take decades to complete as funding is in short supply. In those cases where a municipality or a homeowner believes their land or property has been misidentified as a floodplain (or is not identified as a floodplain!) there is a process to amend the official maps.

Digital Flood Risk Information

In 2002 Congress appropriated funding for FEMA to implement a five-year “Flood Map Modernization” effort. Map Modernization was a multi-year effort to upgrade the paper FIRM inventory into seamless flood hazard data publicly available in a geographic information system (GIS) format nationwide. How much of Utah digitized? The following counties have digital floodplains dating back to the Map Modernization Program.

Many of the digitized maps produced during this period did not involve geographic updates from earlier maps. In some cases, older maps were merely digitized. The Flood Insurance Study report will have details on the year and method of data collection and creation.

Risk MAP Era

Risk MAP is FEMA’s latest and most accurate effort to provide floodplain information in the Untied States. Since 2009, the Risk MAP process has created high-quality flood maps and information, and tools to better asses the impacts of risk from flooding. Each Risk MAP flood risk project is tailored to the needs of the community in question and may involve different products and services. Services may include assistance with mitigation planning and/or outreach support.

Risk MAP operates at two different levels of geography and data detail:

1. BLE or (Base Level Engineering)

BLE or (Base Level Engineering) takes automated riverine modeling approach to produces a baseline understanding of community flood risk. It is recommended for sparsely populated areas where no full-scale mapping of Special Flood Hazard Zones has occurred previously. BLE can cover a lot of ground for a reasonable cost ($/square km). so it is often used to delineate risk in larger watersheds or even across entire counties. To create it, engineers use state-of-the-art hydrologic and hydraulic models with LiDAR-derived elevation,  National Hydrography Dataset (NHD) and precipitation records as inputs. BLE products include floodplain boundaries, flood depth and water surface elevation grids.

Below are areas where Utah Risk MAP is engaged in BLE modeling projects:

2. Individual stream reach studies:

A more data intensive approach is recommended when delineating floodplains in populated areas where manmade structures and flood control practices play a large role in diverting natural water flow. As with BLE projects, input information includes an elevation model, precipitation records, hydrology and hydraulic modeling.  In addition, local engineers/floodplain managers share knowledge and data which are incorporated into the final results.  Field reconnaissance uncovers additional information. The resulting flood map delineates the 1-percent-annual-chance (100-year) floodplain (typically termed Special Flood Hazard Areas or SFHA); and the 500-year floodplain (between 1- and 0.2-percent-annual-chance. The remaining areas above the 0.2-percent-annual-chance floodplain are termed areas of minimal flood hazard.

Below are areas where Utah Risk MAP is engaged in individual stream reach studies:

See corresponding projects below

Individual stream study project websites:

Cache County pictures are good. But I am still waiting for info.

Click below to see a catalog of Risk MAP products available for stream study projects.

3. Lake studies:

Considerable flooding can occur when high lake levels are combined with strong winds that drive water and waves onshore. When large waves are paired with elevated lake levels, the waves are able to reach farther onshore, eroding the backshore, and potentially causing damage to
developed shoreline areas.

Since 1900, lake floods have occurred several times around the Great Salt Lake (1923, 1952, 1983-84) and Utah Lake (). Risk MAP’s shoreline studies delineate V zones to show areas that have at least a 1 percent annual probability for a flood event with additional hazards associated with storm-induced waves.