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Nez Perce County Comprehensive Plan

SECTION 4

Hazardous Areas

PURPOSE OF THE HAZARDOUS AREAS ELEMENT

Flood image

Nez Perce County encompasses more than 800 square miles, but not all areas are suitable for intensive development. Certain areas have been designated in the county land use plan for future non-agricultural development, and there are a number of other areas currently showing residential growth or having growth potential.

There are, however, specific limitations on development in some places because of potential natural hazards. The two most notable natural hazards in Nez Perce County are known flood plains and geologically unstable land. Construction in such areas, without careful investigation and precautionary measures, can result in loss of life and property damage.

INVENTORY AND FUTURE NEEDS ANALYSIS

Flood Hazard Areas:

The Lapwai Valley and the Big Canyon Creek Valley around Peck had extensive flood damage in 1965 and 1996. Lapwai, Big Canyon, Sweetwater, and Mission Creek valleys have historically been the scenes of extensive flooding. U.S. Army Corps of Engineers stream re-channeling and diking projects have minimized the flood threat in the Lapwai Valley.

Other areas designated as flood prone are:

  • The Potlatch River valley from Kendrick to the confluence with the Clearwater River. This valley in most areas is quite narrow, with limited acreage available for development. The largest housing concentration is adjacent to the northern corporate limits of Juliaetta.
  • The lower reaches of Hatwai and Cottonwood Creeks.
  • The Cow Creek valley, in the northwest corner of the county south of Genessee.
  • Lindsay Creek, running through a narrow valley along the eastern boundary of Lewiston, serves as the major drainage for most of the east and central Lewiston Orchards. Many homes have been built along Lindsay Creek, regardless of the possibility of flooding. Increased runoff, and constricted floodway in the lower portion, make frequent flooding probable in the lower reaches.

The major flood danger in Nez Perce County results from warm winter rains on a heavy snow cover, where the drainage basin is extensive and the stream carrying capacity is relatively low. Tammany Creek to the south of the Lewiston Orchards is an example of a stream with low carrying capacity considering the large drainage basin that it serves.

The potential of damaging floods in the Clearwater River Valley has been reduced by the completion of Dworshak Dam, which controls the flow of the North Fork of the Clearwater River as it enters the main river near Orofino. The Clearwater River is still subject to considerable stream fluctuation and although its watercourse has a good carrying capacity, there are low benches that could be inundated during periods of heavy winter or spring runoff.

The risk of flooding in Lewiston has been all but eliminated by the completion of the flood control levees. The federal government has made detailed maps of all streams and rivers in the nation, identifying flood hazard areas based on a 100-year flood potential. Please refer to the Flood Insurance Rate Maps (F.I.R.M.) published by the Federal Emergency Management Agency (F.E.M.A.) for detailed information concerning floodplain elevations in Nez Perce County.

Geologically Unstable Areas:

Residential development often takes place on hillsides and steep slopes for the added view provided, or to use sites which would otherwise be of limited value. Such sites may be susceptible to geological instability, such as slides and soil slumping. Even areas that have been historically stable can deteriorate and become unstable through infiltration by wastewater, surface irrigation of lawns, and entry of surface runoff at road cuts. Extensive surface irrigation and infiltration of water down to the basaltic base rock has enhanced the slide potential of the soil mantle along the west edge of the bench. Development of hillsides must include measures to prevent development of such conditions in the future.

The removal of vegetation along hillsides or steep slopes during development increases the susceptibility of surface soils to wind and water erosion. Other ground disturbing activities that increase soil erosion include tillage, quarrying, etc. If structural and temporary controls and Best Management Practices are not used, the cumulative effect of erosion from site material can devastate surface water bodies and create hazardous conditions for surrounding properties.

Two landslides occurred recently in Nez Perce County. The McGary Grade landslide across from Juliaetta occurred Wednesday, May 13, 1998 destroying a mobile home, with injuries to two persons and the other on Snake River Avenue in Lewiston (between the Lewiston Elks Club and the highway) occurred on Tuesday, May 4, 1998. Both of these slides caused significant damage and federal emergency funds were requested for repairs.

No comprehensive study has been undertaken to map potential landslide areas. A map of this nature would be helpful should the county desire to implement special requirements for construction of property within potential landslide areas.

Earthquakes:

Idaho is ranked fifth in the nation for earthquake risk. Two of the largest earthquakes in the last 40 years occurred in Idaho. They were Hebgen Lake in 1959, which measured 7.5 on the Richter scale, and Mount Borah in 1983, which measured 7.3. Earthquakes of significant magnitude have not been recorded in Nez Perce County in recent years.

HAZARDOUS AREAS GOAL AND POLICIES

Goal Statement:

To manage development in known hazardous areas with care and foresight to prevent disastrous loss of life or property.

Policies:

  1. Nez Perce County should regulate construction of structures on identified floodplains.
  2. Nez Perce County should review and update floodplain zoning when necessary for public safety.
  3. Nez Perce County should continue enforcing a building permit system for construction within floodplains in accordance with the Uniform Building Codes and the National Flood Insurance program, as adopted by Nez Perce County.
  4. Nez Perce County should regulate development in areas identified as geologically unstable.
  5. Nez Perce County should develop within the zoning ordinance procedures by which the potential for natural hazards can be reviewed before approval of development, and special development criteria applied if necessary.
  6. Nez Perce County should develop construction standards applicable to development in potentially unstable areas.

Hazardous Strategy Chart

 

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