By Dwight Wayne Gemar, P.E., M.SAME, Adam Kenneth Brown, P.E., M.SAME, and Natalie Harkins, P.E., M.SAME
In March 2019, a “bomb cyclone” hit northeastern Nebraska and southeastern South Dakota, bringing an assault of snow and heavy rain that overwhelmed the downstream Missouri River flood control system. The cyclone was followed by a second round of precipitation in April, causing rapid snow melt and runoff and creating more damage throughout the Missouri River basin. Levees in Nebraska, Iowa, Kansas, and Missouri were overtopped, breached, and eroded by the high-water events—leaving devastating, chaotic after-effects. Communities were inundated and much of the 2019 planting season was lost.
Within the Missouri River basin, two of the largest levee breaches occurred within Levee Unit L550, an approximately 32-mi long levee embankment in Atchison County, Mo., managed by the Omaha District of the U.S. Army Corps of Engineers (USACE). The levee system protects small farming communities and 40,000-acres of primarily agricultural land. The two inlet breaches, Breach A and Breach B, measured 1,200-ft and 600-ft wide, respectively. Volumetric flowrate in Breach A was measured at 15,000-ft3/sec in July 2019, nearly one-third of the mean flow of the main Missouri River channel on a typical July day.
Approximately 15-mi downstream, the flooding led to severe erosion of the levee crest and slopes created by overtopping and wave action, including an outlet breach 1,500-ft wide where flood waters rejoined the main river flow.
Omaha District awarded a task order in mid-June 2019 to close the two inlet breaches and construct temporary levee repairs and provide temporary flood protection.
THE LAY OF THE LAND
To obtain the 700,000-yd3 of non-cohesive fill that was estimated to be needed to complete the work, pre-mobilization planning included the evaluation of different approaches. A pair of options were evaluated: dredge the Missouri River channel and pump the dredged sand slurry to the breach locations, or recover sand deposited on adjacent farmland inundated by the flood waters.
The project timeline was only three months, which was complicated by ongoing flooding and frequent, intense precipitation events. High river velocity from persistent flooding and lack of available river dredges would have delayed the start of repairs by two months or more. Therefore, river dredging was ruled out as a potential approach.
The landside farmland borrow areas were completely inundated with floodwater, and the recoverable sand volume was not fully quantified. To assess the sand volume available, a bathymetric survey of the breaches and landside borrow areas was performed immediately upon mobilization. Topography compared to pre-flood elevations indicated that enough sand volume was potentially recoverable.
In order to map areas by sand depth to prioritize and guide removal, and to visualize the breach geometry, the project team used 3D surface models. These models were key aids to adjust the initial closure berm alignment to avoid the 40-ft to 60-ft deep scour holes that had been created both riverside and landside of the levee alignment.
FOLLOWING A HYBRID APPROACH
Removal of the underwater sand deposited on the landside of the breaches began in July 2019 using hydraulic dredging. Large centrifugal pumps attached to the boom of excavators were mounted on several “marsh buggies” equipped with six-foot-wide tracks. The pumps vacuumed the sand from beneath the water surface on a 24-hour schedule. The slurry was pumped through individual pipelines to the breach area, where the water was decanted from the sand similar to how a conventional river dredge would operate. Dozers pushed the sand into the open-water breach area to begin constructing a closure berm.
Unfortunately, the sand recovery rate using this process was insufficient to maintain the project schedule. As a result, the team pivoted to a hybrid approach: re-purposing the excavators to mechanically dredge the underwater sand deposits in order to create stockpiles and haul roads above the flood waters, and to access the sand using tractor-pulled scraper pans (mostly supplied and operated by local farmers) for transport to the breach fill area. Approximately 16,000-yds3 per day were excavated, drained, and placed using this method.
As the project progressed, the team faced another challenge: managing excessive sand erosion of the closure berm’s leading edge due to increased velocity (about 7-ft/sec) as the distance to close Breach A decreased to 300-ft. Super sacks filled with sand were deployed, but they were unable to combat the accelerating river velocity.
Engineers with Weston, which was awarded the contract to repair the levees, prepared a River Analysis System model of the breach area and performed multiple scenario analyses of different closure options and geometries. Recognizing that erosive force is a combination of water velocity and water depth, quarry shot rock was used to armor the floor of the remaining channel. A combination of barge-mounted and land-based equipment was used to place the large rock from both sides of the channel to prevent erosion of the advancing sand closure berm and the opposite bank of the intact levee. The force of the water flowing over the last 100-ft segment of the closure berm was strong enough to open a 35-ft deep scour hole and displace the shot rock, so even larger stone was used.
Breach A closure was achieved ahead of schedule on Aug. 24, 2019. Following closure, additional sand was placed on the landside and riverside of the initial closure berm to complete a 500-ft wide foundation berm, and a 100-ft wide crest sand levee was constructed to full levee height.
With the initial closure at Breach A nearing completion, sand excavation and placement within the landside scour at Breach B proceeded. A rapid closure was then achieved at Breach B almost a month later, facilitated by the proactive placement of a rock revetment by river barge across a 100-ft-wide channel that was the primary influent for flood waters passing through. The revetment reduced the volumetric flowrate and velocity significantly. Closure with only sand was achievable at this breach. The sand levee at both breaches held through a second wave of elevated floodwaters in September 2019.
KEY LOCAL PARTNERSHIPS
Mechanical removal of landside sand deposits using locally sourced operators contributed to the resiliency of the community and restoration of agricultural land. The repairs for Breaches A and B leveraged a trusted subcontractor that scheduled and provided a large group of local farmers who were hired to operate their specialized equipment.
Many of these individuals had been directly impacted by the floods. Their common purpose and “farmer strong” approach energized the entire workforce and proved critical to project success. Daily scraper load count comparisons and safety recognition awards aided in reinforcing this partnership.
Employing the local farmers also stimulated the economy. Sourcing nearby equipment provided increased flexibility and cost-effectiveness in adapting to rapidly changing site conditions and mission priorities.
The partnership between local farmers and subcontractors, a cohesive and experienced project delivery team, and an engaged levee sponsor was fundamental in overcoming the many challenges faced to tame the power of the historical Missouri River flooding and restore properties and livelihoods in time for the 2020 planting season, leaving the levee system and surrounding community stronger and more resilient.
Dwight Wayne Gemar, P.E., M.SAME, is Vice President, Principal Project Manager, Adam Kenneth Brown, P.E., M.SAME, is Principal Project Engineer, and Natalie Harkins, P.E., M.SAME, is Project Engineer, Weston Solutions. They can be reached at email@example.com; firstname.lastname@example.org; and email@example.com.
[This article first published in the July-August 2021 issue of The Military Engineer.]