U.S. Department of Defense
Date of this Version
2000
Abstract
The Corps was tasked by the National Park Service to detennine if erosion protection measures were needed to prevent further decline in cottonwood forest within the Missouri National Recreational River. The 1990's had three unusually high water years resulting in increased runoff and dam discharges, with resultant increased erosion. Concern about erosion of cottonwood forests was expressed by the public during public meetings held by the National Park Service and the Corps during 1999. Severe erosion was also evident in the comparison of aerial maps from 1985 to those from 1997. Forested habitat is considered a "wildlife value" for the Missouri National Recreational River and deserving of protection.
After site visits and an evaluation of potential erosion protection sites, an analysis of erosion rates, a detennination of an ultimate erosion line at each site, and an evaluation of habitat value at each site, it is the Corps' recommendation that erosion protection be pursued at five sites. The risk of not protecting the sites, in terms of habitat units lost due to erosion, was a primary factor in this recommendation. Without bank protection, the five sites combined stand to lose 3595 habitat units due to erosion over the next 25 years, based on HEP analyses. Three of the sites are private lands; the other two are state-owned lands. Two of the sites are in Nebraska, and the other three are in South Dakota. The Corps also recommends pursuit of conservation easements from willing sellers in addition to what is needed for construction.
Since sufficient funding is unlikely for simultaneous pursuit of the five sites, several factors (such as habitat value, erosion rate, etc.) were considered to prioritize the sites. Based on these factors, a private-lands site on the Nebraska side (site AI) has the highest priority. It is the Corps' recommendation that erosion protection at site Al should be the first project pursued, if funding is limited.
The most cost-effective way to protect the sites would be to construct pennanent projects using quarried stone. However, this study also includes an option to construct temporary structures to protect the sites. Temporary projects could be made pennanent, or allowed to erode, based on future studies. However, the cost for pursuing a temporary structure, followed by a pennanent structure, could be up to two times the cost of construction of a pennanent structure alone. There is also considerable uncertainty with regard to the ability of temporary structures to withstand ice movement in the Missouri River.
The Corps will first seek agency and local input on the proposed projects, then proceed with the detailed design for each structure (beginning with A1) in June. Additional opportunities for agency and public feedback would be available, should the projects proceed, through environmental compliance associated with each action.
Comments
Published by the U.S. Army Corps of Engineers, Omaha District (2000) 1-110