Current Conditions
Sand Hill River near Climax, Minnesota

(Source: USGS and NWS)

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Project # 29 - Rock Riffles


Project Photos

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Project setting

Historically, the Sand Hill River near the community of Beltrami, MN had limited capacityand resulted in frequent Riffles Pictureoverland flooding throughout the region. In an effort to reduce overland flooding, portions of the Sand Hill River were straightened and channelized in the 1950s as part of a US Army Corps of Engineers (USACE) flood control project. The channelization project was completed in portions of Hubbard Township, Scandia Township, Reis Township, and Liberty Township. This area is characterized by moderate to steep landscape slopes along the Lake Agassiz Beach Ridge in the east, with slopes flattening further west. The USACE channelization project provides an outlet for primarily agricultural lands within the Sand Hill River Watershed.

Need for action

Channelization of the Sand Hill River by the USACE resulted in a steeper channel slope by removing meanders and replacing with a less sinuous channel. The project also resulted in less floodplain storage, with more flow being contained within the constructed channel. In an attempt to mitigate the potential for channel erosion resulting the steep gradeline and reduced floodplain storage, the USACE channelization project also included the installation of four concrete drop structures. These structures allow for a controlled drop for flows in the channel to reduce velocities in open ditch portions of the channel. While these structures assisted in stabilizing the channel, a steep gradeline was still left between each drop structure, and downstream of the western most drop structure. This has resulted in much of the constructed channel experiencing significant erosion since construction of the USACE project. One area of significant channel erosion the portion of the channelization project located east of Beltrami, MN. Through this region, approximate 5-7 feet of head cutting along the channel has occurred. This head cutting has led to bank failure at many locations, with sluffs encroaching on adjacent private land. Over time this bank sluffing has resulted in formation of a vegetated floodplain within the eroded channel. However, without stabilization of the developing stream thalweg it’s likely that the channel will continue to erode at an unnatural rate. The current channel profile and cross sections as compared to the USACE project design channel profile and cross sections are presented in the attached Construction Plans.

Proposed project

In order to stabilize head cutting along the Sand Hill River east of Beltrami, MN, installation of 16 rock riffles are proposed along the Sand Hill River east of Beltrami, MN. These locations are illustrated in the attached Construction Plans. In total, these 16 rock riffles will provide a controlled drop that will mitigate 21 vertical feet of fall from the existing Sand Hill channel. Each riffle is designed to provide approximately 0.1 feet of backwater on the next upstream riffle. This design configuration will minimize future head cutting along the Sand Hill channel and reduce to occurrence of bank sluffing currently experienced.

A secondary objective of the rock riffles is to ensure they do not become a barrier to fish passage along the Sand Hill River. The USACE, working in collaboration with the MN Department of Natural Resources (MN DNR) and the SHRWD, are in the process of modifying the four (4) concrete drop structures that were installed as part of the USACE’s channelization work in the 1950s. This process involves retrofitting the concrete drop structures with rock riprap at a flat enough slope to allow fish passage upstream across each structure. The rock riffles proposed for grade stabilization in the Sand Hill River are also designed to ensure upstream fish passage across each. The proposed slope of each rock riffle varies from 0.03% for riffles greater than one foot of vertical fall, to 0.04% for riffles less than one foot of vertical fall. Vertical fall is when measured to the crest of the next downstream rock riffle. Additionally, for rock riffles greater than one foot of vertical fall, rock arches will be installed to provide necessary resting areas for fish as the climb the riffle. These rock arches will be installed at 26 foot intervals along the riffle, starting 26’ downstream of the riffle crest. The arches will be constructed of large boulders (36 – 72 inch diameter; 48 inch typical) that will be set to leave one foot exposed above the rock riffle.

Each rock riffle was designed to fully contain the bank full flow event, or approximately the 1.5 year recurrence flow rate. In order to convey this flow rate, each riffle will have a maximum depth of six (6) feet, a bottom width of 22 feet, and a top width 46 feet. Based on field survey collected by the MN DNR in 2015, this appears to match the existing channel and ties in well with the developing floodplain that has resulted from bank sluffing. The upper layer of rock riprap will be two (2) feet thick and will be constructed of MN Department of Transportation (MN DOT) Class III Riprap. This gradation consists of round field stone of 18 inch maximum diameter. This size of rock riprap will withstand velocities expected within the Sand Hill channel. Some channel excavation will be required where the existing channel is less than two (2) from the finish grade of the proposed rock riffles. Excavated material will be placed along the upstream end of the rock riffle in the Sand Hill River channel to assist in plugging voids within the placed rock riprap. In areas where the existing channel is greater than two (2) feet lower than the finish grade of the rock riffles, MN DOT Class I Riprap will be used to provide fill. The gradation of MN DOT Class I Riprap consists of round field stone with a maximum six (6) inch diameter. More details on the rock riffle geometry is available in the attached Construction Plans.

Hydraulic impacts to the usace channelization project

The Sand Hill River Grade Stabilization Project will have no adverse impacts to the hydraulic capacity of the USACE Channelization Project. The layout of the rock riffles is below the design gradeline and cross sections of the USACE Channelization Project. The riffle crest elevation, existing channel gradeline, and the USACE project design gradeline are presented in the attached Construction Plans. Additionally, the existing cross sections, rock riffle cross sections, and USACE project design cross sections are also available in the attached Construction Plans. Because all work is being done below the USACE design finish grade for the constructed channel, the design channel capacity for the USACE Channelization Project will remain unaffected.

Permitting

The SHRWD worked with agencies to acquire the proper permits for the project. These permits include:

  • US Army Corps of Engineers 404 Permit
  • US Army Corps of Engineers 408 Permit
  • Minnesota Department of Natural Resources Public Waters Permit
  • Minnesota Wetland Conservation Act

Funding

The primary funding source for the Proposed Project is Minnesota’s Clean Water Land and Legacy Amendment. A portion of funds generated from this Amendment are administered through the MN Board of Water and Soil Resources (BWSR) Clean Water Fund grant program. The West Polk Soil and Water Conservation District (SWCD) was successful in attaining a $475,000 grant from the BWSR Clean Water Fund for the Sand Hill River Grade Stabilization Project. These funds require a 25% minimum match of awarded grant dollars, which is being provided by the Sand Hill River Watershed District, and a $100,000 Enbridge Ecofootprint Grant

Construction timeline

The SHRWD Project No. 29, Sand Hill River Grade Stabilization Project, was completed in the fall of 2016.