Flood Stage Change for the West Fork Des Moines River Jeff Zogg, Senior Hydrologist The Weather Whisper May 2019 Volume 1, Issue 1 NWS Des Moines Cover Photo Credit: NWS Des Moines Damage Survey at the Iowa Operator Engineers Training Facility, northwest of Lacona, Iowa. The morning of August 20th saw severe storms across the state pro- duce 3 tornadoes. Extremely strong straight-line winds also produced widespread damage within Lacona that was just as intense as was seen in some of the tornado impacted areas. Through coordination with local officials, on July 19th the NWS in Des Moines changed the flood stage for the West Fork Des Moines River at Estherville from the old value of 8 feet to the new value of 10 feet, or an increase of 2 feet. The NWS periodically works with local officials to evaluate flood stages—and changes them if necessary—to ensure they accurately reflect flood- related impacts. The NWS considers various factors in evaluating exist- ing flood stages and making recommendations for changes including a statisti- cal analysis—analysis of past crests on the river—and more importantly, flood- related impacts. A good rule of thumb is for the river to reach flood stage around once every other year, or from a statistical perspec- tive, an annual exceedance probability of around 50%. In the case of Estherville, past crests on the river were reaching the previous flood stage multiple times per year. This frequency of oc- currence was a red flag that the flood stage may be set too low. Statistical analysis showed that for past crests to reach flood stage around once every other year, the flood stage would have to be raised by around 2 feet. Despite what the statistical analysis may show, the ulti- mate decision in a change to flood stage rests with flood- related impacts and local officials such as emergency management. Ideally, flood stage represents the stage on the river at which “significant impacts begin to life and property within the reach of the streamgage.” The “reach of the stream- gage” is a NWS-specified distance up and down- stream from the streamgage within which the readings, forecasts, and flood warn- ings are considered to be valid. When “significant impacts” occur, water from the river typically affects access and egress, and/or may flood properties. The NWS often relies on local emergency management and community officials to provide this information. In the case of Estherville, local flood-related impacts sup- ported raising the flood stage by 2 feet, because im- pacts at the previous level were minimal to none. River forecasts and flood warnings for Estherville will continue to be valid along the reach of the West Fork Des Moines River extending from the Iowa-Minnesota border to near Wallingford, reflecting the new flood stage value. The gauge has been in operation since 1986, is maintained by the USGS Central Midwest Wa- ter Science Center, and funded cooperatively by the U.S. Army Corps of Engi- neers and the USGS . As with other NWS river forecast points, the NWS will provide river forecasts when the observed or fore- cast stages go above the Action Stage. For Esther- ville, the Action Stage was also changed on July 19th; it was raised from 6 feet to 8 feet. In addition, the NWS will issue river Flood Warn- ings and follow-up river Flood Statements when the observed or forecast stages go above the Flood Stage. River forecasts for the above locations will continue to be available on the NWS Ad- vanced Hydrologic Predic- tion Service (AHPS) Web- site. River forecasts for these locations are updated as needed while the stream remains at or above the Ac- tion Stage. “Ideally, flood stage represents the stage on the river at which ‘significant impacts begin to life and property...’“ Flooding on the West Fork Des Moines River at Estherville, Iowa. Images courtesy of Mike Ehret and where taken on April 26, 2018.
3
Embed
The Weather Whisper...2018/04/26 · The Weather Whisper May 2019 Volume 1, Issue 1 NWS Des Moines Cover Photo Credit: NWS Des Moines Damage Survey at the Iowa Operator Engineers
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Flood Stage Change for the West Fork Des Moines River J e f f Zo g g , S e n i o r H yd r o l o g i s t
The Weather Whisper
May 2019 Volume 1, Issue 1 NWS Des Moines
Cover Photo Credit:
NWS Des Moines Damage Survey at the Iowa Operator Engineers Training
Facility, northwest of Lacona, Iowa.
The morning of August 20th saw
severe storms across the state pro-
duce 3 tornadoes. Extremely strong straight-line winds also produced