Quick Contact

Water Resources Solutions • 5000 W. 95th St. • Suite 290 • Prairie Village, KS 66207
(913) 302-1030






Search Our Site

Bismarck, North Dakota

When erosion in the mixed loess/alluvium soils along the left descending bank of the Missouri River began threatening the main site of the historic Double Ditch Indian Village, a large earth lodge village inhabited by Mandan Indians from AD 1490 to 1785, Water Resources Solutions was named to direct a geotechnical investigation program that included multiple borings and lab tests to ascertain the underlying issues.

Over the past several years, the left descending bank of the Missouri River had been eroding and threatening the main Double Ditch Historic Site. The erosion was actively occurring in the form of rotational slumps. Rotational slumps were evidenced by the bottom (toe) of the streambank sliding outward into the stream channel and the top of the bank breaking away and coming to rest at a lower elevation. On this site, the vast amount of weight (from the high bank) pushed the bottom of the bank into the river. The observed changes became more dramatic, as large areas of river bank slumped into the river. Nine documented burials were exposed and/or lost due to this erosion. The old county road, which ran between the village site and river, had been turned into a hiking and biking trail. Access through this trail was closed due to the erosion migrating into the trail. Two additional feet of trail width was lost in a six-month period.

Ideally when solving streambank erosion issues, having sufficient time and adequate space is a luxury, as space provides time. The more space that exists between active erosion and existing infrastructure, the more time we have to solve the problem. On this site, space was limited and the erosion observed at this location did not show any signs of slowing or stopping.

Based on the investigation, WRS completed a stability analysis for 2,500 feet of river bank slope and used the analysis to complete the stable slope design and the associated recreational trail. WRS also assisted with the river engineering to protect the toe of the slope. This protection consisted of bendway weirs and longitudinal peaked toe stone protection. WRS completed opinions of probable construction costs for those design elements. The overall project included coordination with the local tribal representatives, cultural resources permitting, environmental permitting and construction oversight.

Based upon the materials obtained from the borings and test pits, the slopes typically have a mantle of 5 to 20 feet of River Terrace Deposit, which is a mixture of loess (wind deposited silt) and alluvium (river deposited silts, sands and gravels). Typically underlying the River Terrace Deposit is up to 13 feet of Glacial Till typically consisting of lean clay with variable contents of sand and gravel. Based on the investigation and stability analysis WRS designed improvements, including the preferred recommendation comprised of a combination of stabilizing the riverbank toe and slope. It included installation of riprap at the toe and installation of bendway weirs, removal of mass from the upper slope of the north section, installation of pipe pile walls within the north section, stabilizing the lower slope of the south section by adding fill and vegetative plantings.

The Preferred Design Solution was not chosen due to regulatory agency resistance. The chosen design alternative solution investigated during the value process included the installation of a rock key, removal of mass from the upper slope of the north section, installation of pipe pile walls within the north section and south section, stabilizing the lower slope of the south section by adding fill, and vegetative plantings.
Cost: $310,000 (fee); $2.5 million construction (est.)