Floodplain mapping modernization: a case history
Curt Edwards, Vice President, Psomas, San Diego, California, and member, APWA Emergency Management Committee
Teresa Scott, P.E., Director of Public Works, City of Gainesville, Florida, and member, APWA Emergency Management Committee
Alice Rankeillor, P.E., Principal Engineer, City of Gainesville, Florida
Background and Rationale
In the 1940s the United States Geological Survey (USGS) began developing a series of maps known as the USGS topographic maps. These maps were then used for the development of the original floodplain mapping. By today's standards, these maps are relatively inaccurate for identifying vertical elevations, a key data point in the development of floodplain boundaries. These maps typically show elevation contours with intervals of 10 feet or in many cases 20 feet. This means that the accuracy of the original topographic data is +/- 5 feet at best, and in many cases, +/- 10 feet.
These USGS maps were then used to identify and depict flood hazard areas in communities and for setting flood insurance rates. Current regulations use these maps to determine zoning, restrict developments, identify hazard areas and determine flood insurance rates. In many cases, these actions are developed using regulated flood elevations that are =/- 1 foot or less (i.e., maximum allowable 1' rise in the Base Flood Elevation [BFE]). Approximately 20 states have more stringent regulations allowing from 0.001 to 0.5 foot of rise. By applying this level of accuracy to inaccurate topographic mapping, it is clear that many flood fringe area definitions may also be inaccurate. In addition, the data used to produce these original maps is 15-30 years old and, in many cases, does not reflect development during that period.
The current modernization program involves taking the original graphical floodplain data and digitizing it into a database. In theory, having these digital versions available allows for state and local governments to have more accurate hazard warnings and to have more adequate emergency plans in place. It also aids in the management of floodplains for future loss reduction. These digital versions are often incorporated into the community's GIS database.
Lessons learned by one community along the way
The City of Gainesville recently went through the Federal Emergency Management Agency (FEMA) flood map update and learned some very important lessons along the way. By sharing these lessons we wanted to assist others in avoiding some of the pitfalls that can be deep and troublesome.
Flood maps were developed for the urbanized area in and around the City of Gainesville in the early to mid 1970s. The topographic and hydrologic data provided at that time provided a much more accurate estimation of flood areas than the USGS flood maps. A tremendous amount of development occurred over the years, so in the mid to late 1980s the City and County partnered again to have the hydrologic study updated; however, the base maps were not. When FEMA came in to update the FEMA flood maps, these maps were provided to them by the City and County. FEMA digitized these maps and identified areas as flood zones A, with no elevation data provided because they did not concur with the hydrologic analysis used in the development of flood elevations. This has led to the addition of hundreds of properties in flood zones that had not previously been identified as such and in reality should not be identified in a flood zone.
When the draft maps are submitted to a community for the review period it is very important that the maps be reviewed thoroughly by local staff, and every effort is made to reach out to the community to educate them on the ramifications of the new maps on their individual property. The draft maps will most likely contain errors and may incorrectly identify properties in flood zones. In our case, because the base flood maps used in Gainesville were established in 1971 and much development had occurred, there were large parts of newer subdivisions identified in flood zones that were not accurate.
Secondly, some discrepancies when digitizing can create zone limits being offset from actual flood limits by 40' to 50' resulting in properties being identified as a flood zone when they are in fact several feet higher in elevation than the actual flood elevation.
Thirdly, previously issued Letters of Map Revisions and Letters of Map Amendments may be overlooked in the map updates.
Fourthly, the ability to have the digital Flood Insurance Rate Map (FIRM) and the FEMA Geographic Information System (GIS) coverage in place at the time of final adoption of the maps is very important. It has been five months since our effective date of the updated maps and we still do not have the GIS coverage from FEMA. The City developed GIS coverage from preliminary information but are not able to disseminate this tool for use by the public because of data discrepancies with the new maps. Being able to provide a tool that allows the public to access information assists in expediting responsiveness to the public's inquiries.
The comparison of FIRM flood zones to local information in the geographic information system indicates that misalignment has occurred during the processing of the paper maps to the new digital format.
The appeals process is the time to identify those properties that staff and the property owner believe are not accurately identified. While we made efforts to reach out to the community by having articles in the newspaper and providing locations for property owners to come review maps, we did not reach out to specific neighborhoods where we should have known there were errors. The appeals process was undertaken only in the event it was initiated by a property owner.
Once the maps are adopted the information is widely disseminated to insurers and mortgage companies. The property owners are immediately put on notice of changes and need for flood insurance. There will be an immediate rush on city hall for an explanation of the changes and why people were not notified in advance. Hundreds of dollars will have to be expended by property owners to obtain surveys to establish flood elevations in areas where base flood elevations are available to file Letters of Map Revisions. In areas without established base flood elevations, thousands of dollars will have to be spent to establish the base flood elevation to file Letters of Map Amendments. All of this effort becomes necessary to save a property owner potentially thousands of dollars each year for unnecessary flood insurance.
Our lessons: 1) providing up-to-date topographic data will allow for a more accurately defined floodplain boundary; 2) providing new hydrologic data could further refine the floodplain boundaries, but may not prove to be as cost effective as the new topographic data; 3) devoting sufficient staff time to review draft maps during the appeals period is important to ensure the quality and accuracy of the final maps; and 4) public outreach targeted to specifically impacted properties during the appeals process is time and money well spent.
Continued funding for these flood map updates, along with funding for improvement of the FEMA review process, will ultimately increase the speed at which these revisions are completed, saving processing costs and reducing risk which will also save recovery and response costs.
Curt Edwards can be reached at (619) 576-9200 or email@example.com; Teresa Scott can be reached at (352) 393-8801 or firstname.lastname@example.org; and Alice Rankeillor can be reached at (352) 334-5072 or email@example.com.