Project Location: The U.S. Army Corps of Engineers (USACE), Rock Island District (District) proposed project, dredge material placement, is located in Pool 11 of the Upper Mississippi River (Figure 1). The proposed location is on the right descending bank at approximately River Mile (RM) 605.4-606.0.
- Upper Mississippi River, Pool 11
- Section 17, Township 91 North, and Range 1 West
- Clayton County, Iowa
- Latitude 42.698568°, Longitude -90.983231°
Background: A high water event on the Mississippi River has caused significant shoaling within the 9-Foot Navigation Channel in the District’s area of operation. Water levels are falling rapidly, and actual depths are forecast to fall below nine feet by 11 June 2023 in the Cassville Lower Reach. The activity is considered an imminent closure in accordance with the GREAT II, Channel Maintenance Handbook dated December 1980. Barge groundings are imminent and navigation delays are likely with elevated risks for channel closures. With water levels continuing to fall, the District anticipates these areas are likely to result in channel closures without channel maintenance dredging.
The District is declaring this situation an emergency. USACE procedures associated with implementing emergency actions prior to completion of environmental coordination are addressed in 33 Code of Federal Regulations (CFR) § 337.7 (Practice and Procedures, Emergency Actions) and 33 CFR § 230.8 (Procedures for Implementing National Environmental Policy Act, Emergency Actions). The District has initiated environmental coordination with Federal and State agencies but has not completed all environmental compliance requirements for dredged material placement. The District intends to satisfy as many requirements before dredging as possible, but some requirements may be met after-the-fact. The emergency action is not considered major in scope, and the District does not anticipate it will result in significant environmental impacts.
Project Description: In order to maintain the 9-foot Navigation Channel, dredging needs to be performed at various locations on the Upper Mississippi River within the District. There is currently an immediate need to dredge 30,000 to 50,000 cubic yards from the navigation channel between RM 603.9 - 605.1, referred to as the Cassville Lower Reach. Dredged material is removed from the channel by hydraulic cutter-suction dredges or floating mechanical plants. Hydraulic cutter-suction dredges use a cutterhead to agitate the material and large centrifugal pumps to suspend dredged material via a floating pipeline to the placement location. Floating mechanical plants scoop material from the bottom, typically using cranes with clamshell buckets or excavators, and place material on barges for transport to the placement location. Hydraulic cutter-head dredges are typically more efficient than mechanical plants for larger volume jobs. While hydraulic cutter-head dredges require more set up, the amount they dredge per day is much higher than mechanical plants.
This dredge cut area has not required maintenance since 1965. The shoal initially deposited following high-water events of 2019 and 2020. The District and resource agencies have been working to address this area and find suitable placement for the recently deposited material. The total amount of material that has deposited is now in the 100,000 to 150,000 cubic yard range. These are unprecedented volumes in this area and existing placement sites are lacking to adequately perform the maintenance that is needed to return the channel to navigable dimensions.
Hydraulic dredging is the preferred method to remove approximately 30,000 to 50,000 cubic yards cubic yards from the channel. Material would be placed on a bankline between RM 605.4 and 606.0. A secondary option would be mechanical dredging plant placing in the same location. Dredged material would be placed up to but not exceeding the Ordinary High Water Mark (33 CFR 328). The material would be graded after placement. Side slopes of the placed material would fall at approximately 3H:1V slope. Bankline placement would be approximately 3,200 ft long x 75 ft wide. Placing dredged material on the bankline to an elevation of 606.15 ft NAVD88 has no impact on flow velocity. Placement of dredged material at this site would present no-rise in the water surface elevations at this site or upstream of this site.
Dredging to address the channel condition and ensure a safe, reliable navigation channel may begin as early as 11 June 2023.
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