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Spring brings threat of river flooding

(Note: Although the following information from the Ada City County Emergency Management office was produced for the Treasure Valley, some of it is applicable to other regions of Idaho as well.)

Flooding is a natural and inevitable part of life along any river. Nationwide, 75 percent of presidentially declared disasters are due to floods. The Boise River is a beautiful amenity in our community, but we must never forget that it still is force of nature. March 16-20 is Flood Safety Awareness Week. Now is the time to heighten our awareness of the flood history and flood potential of our community.

What Causes Flooding?

Heavy rains, winter storms and spring thaws
Heavy snowpack in the mountains can be a mixed blessing. When the Treasure Valley receives warmer weather, coupled with long periods of significant rain, localized flooding can occur. This year the snow pack is slightly below average, but that does not mean flooding will not occur. Significant weather events during spring can create flooding in the foothills and perhaps even along the river.

Overburdened or clogged drainage systems
Water will take the path of least resistance. If storm drains or culverts are clogged with debris or pushed beyond capacity, water will spill out over the adjacent area. This type of flooding can occur both within and outside of a floodplain.

Construction and new development
Changes made to the environment by development can affect natural drainages and create new flood risks.

Recent Flood Events

May 2006
Above average snow pack along with warm wet weather brought on high river flows during spring. The river bank breeched and required repair. No homes were flooded, but a septic tank failure did effect 8-10 homes.

May - June 1998
Two weeks of rain fell on melting snowpack causing flooding along Boise River drainages. A levee break near Eagle Island caused flooding of nearby homes. Sixty residents were evacuated. A mobile home park and some farm lands were flooded.

January 1997
Boise River flows were increased in order to make room in the reservoirs. A dike near South Eagle Road broke, flooding the road. Two homes were flooded and others were evacuated. Parts of the Greenbelt were closed.

What river flows will create flooding?

The river flows measured at Glenwood Bridge in cubic feet per second (cfs) are the best indicator of potential flooding. The Boise River has not flowed through town at the 100-year flood rate of 16,600 cfs since the completion of Lucky Peak Dam in 1955. It’s high flow of 9,850 cfs occurred in 1983 and caused significant flooding. The river is considered to be at flood stage at 7000 cfs. Below are a couple of examples of potential flood scenarios according to the National Weather Service.

Flow rate potential effect

7000 cfs Large sections of the Greenbelt Path adjacent to the river will be submerged. Erosion of the riverbanks may become a significant problem. Minor flooding will be observed on sections of Eagle Island and other low spots along the river.

10,500 cfs Flooding near the river will occur in low areas of Boise, Garden City, Eagle and Star. Portions of Eagle Island will be submerged. Access in/out of some neighborhoods may be limited by high water.