Base Flood Elevations

By John Grego

FEMA uses the term Base Flood Elevations (BFEs) to refer to the water level of the 1% chance flood on the floodplain. A 1% chance flood is the new term for a 100-year flood, chosen to convey the possibility of risk from a major flood event year in and year out. The new terminology works well for a 100-year flood, a 50-year flood (2% chance flood), etc, but not so well for a 500-year flood (the .2% chance flood?). Base flood elevations are the most important source information used to map the regulatory floodplain for insurance purposes.

Some of the confusion on modeling base flood elevations for Lexington and Richland Counties has arisen because FEMA uses different scenarios for the two counties when modeling the 1% chance flood. FEMA assumes that initially the Columbia Venture levees won’t fail and that water will rise in Lexington County because all the flow will be carried in both the main channel of the Congaree River and the Lexington County floodplain. As a worst case scenario, FEMA maps the Lexington base flood elevations assuming that the levees won’t fail until the 1% chance flood reaches its crest. At the crest of the flood, the levees would break and the 1% chance flood would flow across the entire floodplain. FEMA has shown that the drop in floodwaters after the levees break will be as much as 4 feet in Riverland Park—this is why reinforced levees are so dangerous to Riverland Park and other Cayce property in the floodplain (not just the floodway)—they greatly increase risk from a 1% chance flood; the increase in risk is actually greater for even larger (though less frequent) floods.

In a nutshell, the Richland Base Flood Elevations are computed after the levees break, the Lexington Base Flood Elevations are computed before the levees break. FEMA computes BFEs under different assumptions because it is trying to protect both sides of the river from the greatest risk during a flood.

This is one reason why Riverland Park has consistently opposed Columbia Venture’s levee proposals in general, and Columbia Venture’s overtures to Cayce in particular. Residents recognize that the old levee system is very likely to fail in a 1% chance flood, and that the failure will provide them with a substantial amount of flood relief. A reinforced levee, or a new levee, has a smaller chance of failure, and a smaller chance of flood relief, and hence greater flood risk than the flood relief provided by the current agricultural levees.

The Floodway

The 2002 Flood Insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) placed almost all of Riverland Park outside the floodway (the neighborhood is now in the floodway fringe—that portion of the floodplain outside the floodway). Previous maps had placed it in the floodway, and homeowners were unable to make improvements to their property that could be considered to cause additional obstruction in a 1% chance flood.

To compute the floodway, FEMA used the same assumption on both sides, assuming the levees had already failed. Since the floodway designation can be so punitive, FEMA likes to assume equal conveyance on both sides of the river. Using the pre-levee break model would unfairly penalize Lexington, and likely place Riverland Park back in the floodway. Whether Riverland Park is placed officially in the floodway or not, placing the Richland floodway along the levees allows the levees to be reinforced. A reinforced levee system would provide a measure of protection for Richland County property, and allow development using the weaker rules for the floodway fringe. As mentioned before, a reinforced levee would also increase flood risk for Lexington County property, since levees would be less likely to fail, and thus increase the risk of severe flooding in Cayce.

Columbia Venture modeling

Columbia Venture has used up-to-date modeling methods, and spent a tremendous amount of money on modeling. However, their claims about flood risk and flood relief are consistently based on carefull controlled levee break scenarios.

Columbia Venture appears to have realized that reinforcing the current levees would be a hard sell in winning Cayce support—the increase in flood risk is simply too obvious. Incredibly, the increased flood risk was ignored by Cayce officials back in 2001—they actually proposed that FEMA should use the same model to map the floodway in Lexington County that FEMA used to map Base Flood Elevations; this action could well have placed Riverland Park and other Lexington County land in the floodway. There is a suspicion that the actual purpose of their support, of course, was to assist Columbia Venture in their attempts to modify the Richland floodway to Columbia Venture’s benefit.

Columbia Venture has proposed a new levee system set back from the current levees that it claims would not increase flood elevations in Riverland Park (it would increase flood velocities, another type of flood risk), even under the assumption that the new levees will not fail. This claim is not true if Columbia Venture compares the base flood elevations for the new levee (no failure scenario) to FEMA’s base flood elevations for Richland County computed after (old) levee failure. Columbia Venture has instead proposed a particular type of failure for the old levees to compute base flood elevations after the old levees fail. They modeled the levee failures as two long shallow breaks, which lets a substantial amount of water (35,000 cubic feet per second) flow through the breaks into the Richland floodplain. But the long shallow breaks mean that water levels on the Lexington side are carefully controlled so that flood relief on the Lexington side is not as great as flood relief caused by more realistic (shorter and deeper) breaks.

Realistic breaks would not support Columbia Venture’s claim that the new levee configuration does not increase flood risk, which may be why Columbia Venture focuses on the substantial amount of water flowing through the breaks into the Richland County floodplain, rather than their model for the way in which the levee fails. Note too that Columbia Venture’s model only introduces 35,000 cubic feet per second through the levee breaks. This amount of flow was chosen from one of FEMA’s levee break studies. Other studies (including earlier studies conducted by Columbia Venture) showed that as much as 50,000 cubic feet per second could flow through the Richland floodplain after levee failure. That amount of flood relief for Lexington County through the levee breaks would be difficult to compensate for with a new levee configuration—it would take the hypothetical “magic levee” to keep water levels low enough on the Lexington side in a 100-year flood so that they match the current Richland County BFEs.