K-State Researchers to Release Kansas River Dredging Study
Private in-channel dredging operations on rivers like the Kansas River cause deepening and widening of the channel and accelerate erosion of the banks. As a result, dredging lowers the water level of the river and the adjacent water table in the floodplain. This creates the risk for harm to public river uses (such as water treatment facilities, municipal wells, bridge footings, etc.) as well as to fish communities throughout the watershed, including endangered species.
These preliminary findings come from a study funded by the Kansas Department of Wildlife and Parks (KDWP) and carried out by Kansas State University researchers Melinda Daniels and Craig Paukert. The scientists have documented riverbed incision in dredged reaches, which is most likely also causing excessive bank erosion both upstream and downstream of dredge sites. The final study results will be released in late December.
The Army Corps of Engineers is considering a proposal from five private dredging companies to increase dredging on the Kaw close to 50%, from 2.2 million tons to 3.2 million tons. The local nonprofit conservation group Friends of the Kaw (FOK) recently interviewed Daniels for its public comment to the U.S. Army Corps of Engineers.
“If you take 3.2 million tons from the river bottom, then the river will take 3.2 million tons from the riverbanks, trying to balance the sediment load in the system,” Daniels said. “That’s the simple physics of how water works in river channels to transport sediment. Any riparian owner should be worried, particularly farmers with unforested river banks next to their fields. So should anyone with a water intake pipe or a creek in their backyard. The effects of in-channel dredging will propagate both upstream and downstream from the dredge site until a hard control point, like a dam or a bedrock outcrop, is reached. That means up tributary streams as well as the main river.”
Daniels surveyed major dredge holes on the Kansas River with a sophisticated new measuring technology, an acoustic Doppler instrument that mapped river channel topography and measured water velocity. The researchers discovered that while the Kansas River averages four to five feet deep, active dredge holes can measure up to forty feet deep.
The researchers also discovered that these deep dredge holes can migrate up and down river - sometimes very rapidly, depending on water conditions. Even during small flow increases, researchers documented the upslope lip of a dredge hole traveling upstream.
“People used to think the dredge holes just filled up, but now we know they don’t. The holes first cause erosion upstream and downstream and then eventually do fill in, but not before causing a net loss of sediment from the bed and banks of the channel, meaning the channel does not simply go back to its original state,” Daniels said. “If there’s no bedrock, or physical structure like the Bowersock Dam to stop them, those dredge holes cause channel erosion that will keep on going through the entire river network. Their effects can even travel up the tributaries.” Unless a bridge footing or other engineering infrastructure in the river is armored, then the migrating hole could erode that physical structure as well.
The technical term for this river phenomenon is a “migrating head cut.” Here’s how it works: The Kansas River is a sand bed river. Sand is a light sediment, and water transports it easily. When dredgers excavate into the riverbed, that hole creates a steep wall (or head cut) where the river depth suddenly increases. Water rushes rapidly over that wall, gaining speed and picking up sand from the upstream edge. At the same time, some sand falls into the hole. The water passing over the hole then picks up new sediment downstream, causing erosion there as well. The hole starts to expand, both upstream and downstream.
Part of the dredging proposal before the Army Corps is to re-open a closed dredge site above Topeka. The Army Corps previously shut down the site, operated by Meier’s Ready Mix, due to unacceptable bed degradation.
“Whatever happens above Topeka will eventually migrate upstream through the entire network, stopping only at the bases of Tuttle Creek and Milford and other dams,” said Daniels. “It could happen quickly, within one to two years. Dredging incisions set up cascading environmental effects – bed degradation, riverbanks become unstable and steep from accelerated erosion, etc. Change happens very quickly on a sand bed river.”
Over time, repeated dredging deepens and widens the river by removing sediment from the system. The result is that the river bottom lowers, too, along with the water level. This can leave the intakes for water treatment plants stranded. Dredging on the Missouri River has been scaled back recently because of similar problems propagating into the lower Kansas River and other tributaries to the Missouri.
When the river deepens, the water table in the floodplain lowers. Daniels said that this creates the potential for less water storage, which could affect the many municipal wells along the river. A lower water table also affects river vegetation and forests. For example, the cottonwood – the state tree of Kansas – can’t survive unless its roots can reach a good water supply.
The deep dredge holes may affect fish populations, too. “The river’s physical habitat is significantly different between dredged and un-dredged areas,” noted Daniels.
However, dredging’s most major environmental impacts for fish are not limited to the Kaw. Since migrating head cuts can also affect river tributaries, Daniels said the K-State study raises questions about risks to the habitat of endangered species (like the Topeka Shiner) that live in these smaller streams.
Daniels said that knowledge of the environmental impacts of dredging is incomplete without studying dredging’s impacts on the entire Kansas River system.
“We need a new environmental impact study that considers the impacts of dredging on fish that live in the tributaries as well,” said Daniels. Right now, the U.S. Army Corps of Engineers is depending on an environmental impact statement (EIS) dating from 1991.
Before Daniels and Paukert carried out their study, the effect of sand and gravel dredging on the Kansas River had not been seriously studied. This study was the first time such sophisticated measuring technology has been used.
“The Army Corps has studied similar conditions with sand dredging on the Missouri River,” said Daniels. “They are aware of the problems, and if dredging is a problem for the Missouri River, then it’s going to be a problem for the Kansas River. Simply shifting the problem from the Missouri to the Kansas is not a good strategy.”
How fast will the dredge holes move? Water movement on the Kaw is greatly influenced by how much water the Army Corps releases from upstream reservoirs. Extreme rains plus reservoir releases can add a lot of extra velocity to the Kansas River system. In some circumstances, this may mean the dredge holes have the potential for very rapid movement.
Daniels is seeking additional funding for a second phase of the study, to model dredge hole migrations under different flow regimes.
More information and a public comment letter template can be found on the Friends of the Kaw website.
By Christina Glauner