International Dredging Review

International Dredging Review

Brennan’s dewatering plant at the left of the photo separated the slurry into two flows that ran through identical water removal processes then fed onto a common system of conveyor belts.  At the stockpile D6 dozers man-aged the 500000 cubic yards of sand that will be used for Iowa’s highway construction.  Brennan’s job site trailers also occupied this area.

J.F. Brennan’s Environmental Group has completed a sensitive lake dredging project in Lake Manawa near Council Bluffs Iowa for the Iowa Department of Natural Resources (IDNR).

The purpose of the project was to improve water quality by increasing the depth of a 60-acre area the lake and to acquire 500000 cubic yards of construction grade sand for Iowa Department of Transportation (DOT) road construction projects while preserving the top mud layer that forms a hydraulic seal between the lake and the nearby Missouri River.  

Lake Manawa is an oxbow lake in the Missouri River floodplain with a surface area of 747 acres. IDNR is engaged in an effort to increase the depth of the lake for water quality purposes.  Deepening the entire lake will entail dredging more than four million cubic yards of material.  With funding available for a small part of this IDNR awarded the 500000 cubic yard contract to Brennan early this year.

The contract was to dredge 60 acres of the lake in the vicinity of a peninsula in the north corner of the lake and to dewater and stockpile the sand. The targeted sand was dredged from various locations in the dredging area which was adjacent to a small peninsula on the north end of the lake. 

To avoid removing the top layer of mud the Victor Buhr crew removed the sand by digging into the bottom and dredging under the mud which then caved onto the bottom behind the dredge. The final step of levelling the area was to re-create the solid mud seal on the bottom by moving it to the bare spots. The final depth varied from eight to 16 feet.  

Brennan began mobilization and site preparation in March bringing all the equipment including the dredge pipeline booster jet boat dewatering plant Flexifloat barges and Case Mini Excavator from LaCrosse Wisconsin in 40 semi loads. By April 11 all equipment assembly was complete and the crew began dredging. Nine crew members 18 total worked 12-hour shifts for six 24-hour work days a week. Dredging was completed on August 27 and demobilization was complete in early September. 

Mike and Tim Tolvstad were shift superintendents and Casey Evenson was the project manager.

The 12-inch dredge Victor Buhr was used on the project because it had the required horsepower to pump the material through 7300 feet of 16-inch HDPE pipeline. It is a hydraulic cutterhead dredge with a swinging articulating ladder and a complete RTK GPS system. A 12-inch booster set on Flexifloat barges worked in conjunction with the dredge. The jet boat was used to push the support/fuel barge a 30- x 40-foot Flexifloat barge equipped with a Case Mini Excavator. 

The sand slurry was pumped by floating pipe-line to the stockpile and Brennen-designed de-watering plant. It entered a velocity box then dropped onto the main shaker to remove debris and material larger than 3/8 inch. The slurry was then divided and channeled through two parallel dewatering systems each including a liquefying tank a set of cyclones that removed the majority of the water and two shakers that removed any remaining water. The sand from all four shakers was then discharged onto a common conveyor and moved to the stockpile via five additional conveyors. The stockpile was maintained by two CAT D6 dozers.

The discharge water flowed down an effluent canal to a retaining basin then moved by a 10-inch booster pump back to the lake via another 16-inch pipeline.

The project specifications allowed for up to 25 percent fines and organics in the sand after dewatering but the Brennan system produced sand with no more than nine percent with the majority being less than five percent said Vic Buhr Brennan vice president and namesake of the dredge used for this project.

Buhr recalled that he was on the Robers dredge crew that performed the first Lake Manawa dredging project in 1982 when he was “just a young whippersnapper.” When J.F. Brennan acquired Charles Robers’s dredging equipment in the late 1980s Buhr joined the Brennan team. The staging area for dewatering and stockpiling the sand at the south end of the lake was the placement site for the dredged material from the 1982 project.

Lake Manawa was created in 1881 as a “cutoff” or “oxbow” lake when the Missouri River channel changed during a storm. When it became obvious that the lake was not just a transient body of water a New York real estate developer purchased land on the south shore to create a bathing resort named Manhattan Beach and a developer from nearby Council Bluffs created a subdivision on the north shore. Until the mid-1920s Lake Manawa was said to be the most popular lake resort west of Chicago. A competing park at Carter Lake which had streetcar access took Manawa’s business and by 1935 the buildings were collapsed or destroyed by fire.  

The Iowa Fish and Game Commission (now the Department of Natural Resources) acquired the land in the 1930s converted it to public use for boating swimming and fishing and deepened it with a major dredging project in 1982. Lake Manawa State Park is now one of the most popular recreation locations in the Council Bluffs area. 

Lake Manawa was one of the Iowa lakes identified as eligible to receive funding for restoration through the Iowa 2006 Infrastructure Bill (HF2782) which provided additional funding and required IDNR to use a science-based approach to achieving lake water quality improvements. The program was continued in 2007 as a result of status quo funding from HF911 through the Restore Iowa Infrastructure Fund (RIIF). 

In the spring of 2007 the Iowa Department of Transportation suggested the possibility of acquiring construction sand from the lake. IDNR hired Tetra Tech to conduct a diagnostic and feasibility study and review the option of dredging as a potential lake restoration activ-ity. Tetra Tech also completed a Jurisdictional Wetland Delineation for Lake Manawa Pi-lot Dredge Spoil Site and finalized a dredging plan that would reduce the risk involved both in providing the materials to the specifications required and in the ability to control additional seepage from areas along the lake bottom. Tetra Tech determined the feasibility of conducting dredging activities at Lake Manawa identified risks with respect to dredging and identified locations and methods of in-lake dredging to obtain sand as building material. 

In addition Tetra Tech conducted a Phase I Archeological Survey for a Dredging Project at Lake Manawa to provide IDNR with additional information required for compliance with Iowa’s State Historic Preservation Office (SHPO).

The Iowa DOT and IDNR have met periodically since 2007 to discuss opportunities to obtain highway building materials from Lake Manawa sediments and in 2014 IDNR hired FYRA Engineering of Omaha Nebraska to provide design and construction plans for dredging 500000 cubic yards of sand from the lake and stockpile it on state land adjacent to the lake.   The contract was advertised in 2015 and Brennan’s bid of $4.2 million lower than the $5.2 million IDNR engineering estimate was the winning bid.