Preliminary Retention Study Results Presented

Preliminary Retention Study Results Presented

The preliminary results of the Halstad, MN Upstream Retention (HUR) study on upstream water detention options were presented to the Flood Diversion Board of Authority at its November meeting.  The results will assist the Diversion Authority in deciding how to prioritize and allocate $25 million the Authority has pledged for the development of upstream retention projects in the Red River Basin upstream of Halstad.  The HUR study is being carried out by the Red River Basin Commission (RRBC), with support from the State of Minnesota, the State of North Dakota, and the Corps of Engineers.  Projects will be developed by local water resource districts and watershed districts.

Among the preliminary findings: the study, the primary focus of which was on retention benefits to the Red River main stem, does not provide any evidence that distributed storage can replace the F-M Area Diversion Project’s upstream staging or provide the flood protection benefits that the Diversion Project will provide.

The Corps of Engineers has stated that the implementation of distributed, upstream storage could have the potential to reduce the frequency of diversion channel operation, thereby reducing how often the upstream staging area will be utilized.  In addition, distributed, upstream storage could help reduce the duration of the staging area during certain flood events.  However, the staging area is still necessary to offset the downstream impacts of the project for a variety of flood events, and is located in the most reliable, effective and efficient location.

“At the Water Resource District level, when we have flooding issues in the rural areas, we look to locate these projects immediately upstream of those flood areas because that’s where retention will do the best,” said Chad Engels, water resources engineer.  “The same rule will hold true no matter the location you are trying to protect with retention.”  This statement explains why the upstream staging area is located close to Fargo-Moorhead.

The modeling in the HUR study is based on an approximate 100-year flood, and based on uniform and standardized runoff assumptions.  The preliminary results from the HUR study are quite consistent with the results from the previous analyses including: the Fargo-Moorhead risk assessment, the Fargo-Moorhead Metropolitan Area Flood Risk Management Project Feasibility Report and Environmental Impact Statement (FEIS), and the RRBC long term flood study modeling efforts.

The output of the HUR study will be a valuable resource for use by local jurisdictions in selecting and implementing retention sites.  The HUR study contains tools which should be able to help move the basin towards achieving the long-term goal of 20-percent flow reduction for the Red River.  To achieve that goal, the HUR study modeled one scenario that identified 97 sites that would require around 108,000 acres, 169 square miles, or 4.7 townships of land for distributed, upstream retention.

“That is one thing that the public, I think, commonly does not understand: how many sites would be involved and how huge of an impact it would be.  97 sites would be a huge task for the Red River Basin to accomplish.  Yet, at that, it wouldn’t be enough,” said Nancy Otto, Diversion Authority member.  “To me, it just points out that if we are looking for permanent protection for the Fargo-Moorhead area, we are on the right path with the Diversion.”

Modeling completed during the FEIS estimated that 400,000 to 600,000 acre-feet of distributed storage upstream of Halstad would be needed to serve the same function as the Diversion Project’s upstream staging area.  Assuming $2,000/acre-foot for 580,000 acre feet of retention on these 97 potential sites, the estimated cost would be $1.16 billion.  It is important to note that this is one scenario and the 97 sites are conceptual.  In terms of development, nearly all of these sites are at ground zero.  Based on past projects, it takes years of study and development just to get a detention site permitted.

By comparison, the upstream staging that is currently proposed impacts approximately 35,000 acres, primarily in Cass and Clay counties, with an additional cost to the Diversion Project of approximately $200 million.

These preliminary results suggest that the staging area included in the Diversion Project is the most effective and efficient retention for mitigating the downstream impacts of the diversion.  The proposed staging area will cost less and impact fewer acres of land than distributed storage, and it will concentrate impacts in Cass and Clay Counties.

The final report is expected to be completed in December.  Detention sites in the report were funded and provided in Minnesota by the Red River Watershed Management Board and Buffalo-Red River Watershed District; and in North Dakota by the Red River Joint Water Resource District, Local Water Resource Districts, and North Dakota State Water Commission.

Similar Posts: