ABSTRACT: The existing Manhattan, Kansas, Flood Risk Management Project consists of a single 5.4 mile levee unit along the left bank of the Kansas River and the right bank of the Big Blue River. The existing project provides flood risk management for 1600 acres of urban industrial, commercial and residential area including a population of 6,900, 2,300 structures, and over $1.1 billion investment. The existing project was authorized in the Flood Control Act of 1954 and constructed in the 1960’s.
The non-Federal sponsor for the levee unit is the City of Manhattan, Kansas. The existing levee unit withstood the Flood of 1993, but was seriously challenged as the flood crested. This flood experience raised a concern that the levee may provide less than the level of performance for which it was authorized.
The Feasibility Report is the conclusion of a study to update and verify data on the level of performance and benefit provided by the existing project. The study evaluated whether one or more plans for reducing flood risk and increasing levee performance is likely to be technically viable, economically feasible and environmentally acceptable. The Feasibility Report presents recommendations for performance and reliability improvements to the existing project.
The recommended plan is the National Economic Development (NED) Plan, and includes a partial levee raise and reliability improvements to provide nominal flood risk management up to the 0.33% (1/300) annual chance flood event. Targeting this level of performance is consistent with the original project authorization. Implementation of this plan would include raising approximately 2.77 miles of the levee an average of 1.5 feet, and as much as 3.3 feet, above its current height, primarily on the Big Blue River. Additional underseepage control measures will be installed to accommodate the raise and improve performance. Reliability improvements will be made to existing structural and drainage control features. The recommended project will reduce flood risks and hazards in the community; minimize impacts to human safety, health, and welfare; and have minimal impact to the natural environment.
The total estimated first cost of the NED Plan is $23,754,000, to be cost-shared $15,440,000 Federal and $8,314,000 Non-Federal (October 2014 Price Level). The non-Federal sponsor is responsible for operation, maintenance, repair, rehabilitation, and replacement at an estimated annual cost of $54,000.
Based on the current 3.375 percent discount rate and a 50-year period of analysis, the Recommended Plan generates annual flood risk management benefits of $4,074,440 at an annual cost of $1,177,660. Net average annual equivalent benefits are $2,896,780 and the benefit to cost ratio is 3.5 to 1. Annual economic damages are reduced by 59% over the future without project condition.
REPORT DOCUMENTATION: Pertinent documentation on the project, the results of the Civil Works Review Board, and subsequent Washington-Level Review Actions, are listed below (items not linked will be provided when available):
- CWRB Agenda
- Project Map/Placemat
- Project Summary
- CWRB Briefing Slides
- CWRB Lessons Learned
- CWRB Meeting Record
- State & Agency Review Comment Letters
- Documentation of Review Findings
- Signed Chief of Engineers Report
- Advance Copy to Congressional Committees
- ASA(CW) Memo to OMB
- OMB Response
- ASA(CW) Transmittal to Congress
- Signed Record of Decision