Letter: Water Conservation Board clarifies floodplain issue

EDITOR’S NOTE: We received the following in response to an article from Feb. 28, 2019, requesting an update/correction of information presented in the Town of Rangely’s work session Feb. 26. As the email was too long for a correction, we are printing it in its entirety here.

Dear Editor:

The Colorado Water Conservation Board (CWCB) and Federal Emergency Management Agency (FEMA) recently received information regarding an article titled, “Rangely board pushes back on FEMA’s flood plain classifications” in the Feb. 28, 2019, publication of the Rio Blanco Herald Times. Both agencies were concerned about misinformation presented and potential future negative impacts that could result from incorrect information.  Therefore, we have prepared a corrective statement to clarify information contained in the article. The main points in the article that needed to be addressed included:

1. The intent of a FEMA restudy is not to see if areas can be reclassified as non-floodplain. They are to reevaluate flood risk to determine if it is greater or lower than previously mapped.

2.  Culverts already exist under White Ave. They are partially clogged, which causes additional overtopping over White Ave.

3.  Another engineer was hired by the Town to review the floodplain analysis.  The recommendation from him would reduce some of the anticipated flooding, but not remove it. Conversations are still ongoing as to whether this is an acceptable approach.

FEMA and the CWCB were asked by Rangely and Rio Blanco County to re-study floodplains in and around the community as they had not been updated in more than 30 years. The 1 percent annual chance of flooding (commonly referred to as the 100-year floodplain) is used throughout the country to inform flood insurance rates and development regulations. Re-studying flood risk typically involves engineers evaluating historic rainfall and stream flow records, creating models of streams, and incorporating ground and structure elevations to determine where runoff will occur during large flood events. The results are then mapped on Flood Insurance Rate Maps (FIRMs) so that homeowners can determine if they are at risk of flooding. Floodplains and flood risk information does change over time and may increase or decrease in areas when re-studied.

As a participant in the National Flood Insurance Program (NFIP), updates to these designations are typically the responsibility of the Town/County.  However, FEMA and CWCB often partner with communities to support mapping updates.  At the outset of and throughout this study, CWCB has held multiple meetings with community officials to apprise them of results and methodology.

This re-study included numerous streams totaling more than 300 miles in Rio Blanco County, including College Canyon Draw. This drainage originates south of Rangely and is diverted to the west, around the town, by a berm. This drainage is included on the existing FIRMs and shows flow overtopping at White Ave and flowing north into town. While restudying this stream with updated methods since it was last studied in the 1970s, CWCB confirmed that flow does in fact overtop at this road crossing, which is increased by partially-clogged culverts. FEMA regulations also require berms that are not designed as levees for flood protection to be analyzed to show what would happen if they did not withhold flow, since there is a reasonable possibility of them breaking and causing a flood. 

The results of the College Canyon Draw analysis were presented to the Town in December 2018 showing the anticipated flood risk. Working with the Town, engineers were able to identify areas where flood depths would likely be deepest. The results show that approximately 20 homes are located in the 100-year floodplain but are not currently mapped in a floodplain. Many of these homes are located next to the berm. Upon review of this data, the Town hired a third-party engineer to review information presented by CWCB. This review focused on the hydrology, or amount of water, anticipated during the 1 percent annual chance event, but did not involve additional modeling efforts.

In an effort to assist Rangely in their evaluation, FEMA and CWCB also presented several options for reducing flood risk in the Town. One option is to replace the berm with a designed levee, which would be an expensive endeavor as it requires stringent design and construction of infrastructure meant to withstand and reduce flooding. Cleaning of the culverts was also discussed and would allow better flow conveyance under White Avenue and reduce some of the overtopping. However, it would not solve the flood risk posed by the potential for berm failure resulting in flooding downstream.

FEMA and CWCB are continuing to work with the Town so that the flood risk data presented is as accurate as possible. Since the Town participates in the NFIP, flood insurance is available to anyone who purchases it. Flood insurance is required for anyone with a federally backed mortgage located in the 100-year floodplain.  It should be noted that homes mapped outside of the mapped risk could still be at risk of flooding and should still consider their risk. 

Thuy Patton, MPA, CFM

Floodplain Mapping Coordinator

Colorado Water Conservation Board

Denver

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