Manhattan and Riley County are looking at the possibility of building a series of 18 dams around the county — two of which could be on Fort Riley land — that based on preliminary estimates could reduce peak flows on Wildcat Creek by 30 percent during a 100-year flood.
According to Joe File of Wood Environment and Infrastructure Solutions, who conducted the feasibility study on the dams and presented at Thursday’s joint meeting between representatives from Manhattan and Riley and Pottawatomie Counties, the proposed detention and retention dams could provide flood control on approximately one-third of the nearly 100 square mile drainage area of Wildcat Creek and reduce likelihood of flash flooding in areas like Garden Way or Plaza West while also reduce damage to county roads during high rain events. He did note that the studies are all still in their preliminary stages and more concise data is pending.
“With a 30 percent reduction of a 100-year event — not the last September event — […] we get down close to a 25-year event and close to a level where it’s certainly an inconvenience and you’ve your emergency response people on-call, but you’re probably not in that danger zone of evacuating a whole bunch of people,” File says.
If all proposed dams were built, no flooding is projected to occur on Garden Way in a 100-year event. In a 500-year event, closer to what was experienced in the 2018 Labor Day flash flood of Wildcat Creek — in which one-third of the annual rainfall for the area came down in around six hours — flooding impact on Garden Way is projected to decrease from an average flood depth of 4.5 to 3 feet and from 6.5 feet to 4.2 feet in Plaza West. An event identical to 2018, the elevation at Scenic Drive could be decreased from 28.3 feet to 24.8 feet, below the top of road elevation. A 100-year flood does not mean they will not happen only once in 100 years, but that there is only a 1 percent chance that such an event will take place any given year.
Adding up construction and easement acquisition for all 18 dams, they would cost a little more than $28 million. Riley County Commissioner Ron Wells says they would be more able to handle the costs if the State would fund the Local Ad Valorem Property Tax Reduction Fund and the County City Revenue Sharing fund — both of which haven’t been funded since 2003 despite state statute requiring it. Wells estimates Riley County loses $5 million to $6 million per year as a result.
“The federal government’s going to be our main hope and then I’m going to still pursue our two statute money here in the State of Kansas,” Wells says. “In that case we could probably use some of that money each year […] and earmark it for these type of projects.”
The study assumed the two Fort Riley dams would be dry dams that can retain “approaching 1000” acre feet of water, while the dams dotting the county range from holding 91.6 to 706.6 acre feet of water and are envisioned to retain some portion of water year-round. File says that that is not set in stone, but that wet ponds may provide an incentive for landowners to agree to having the dams on their land.
“We believe that that provides a benefit to the landowner,” File says. “Whether it be for stock water — because a lot of these site are in pasture area — or whether it be for recreation or fishing or hunting.”
Wells says as they continue through the progress, they will likely find many landowners are amenable to the idea and even increase their size.
“I’ve talked to one landowner and she has several watersheds — 300 acres — and she’s willing to put that up,” says Wells. “And is even willing to spend some of her own money for the pond retention part.”
When selecting potential locations for the 16 dams not on Fort Riley, File says they tried to place them throughout the watershed to protect against rainfall over the entire drainage area with an eye toward existing pond locations that could be modified and up-sized rather than relying on only constructing new dams while minimizing impact to prime cropland.
“There’s a few of them that still have a little bit of actual cropland there that might be periodically flooded if these were built,” says File. “It’s not that they couldn’t farm it, but once in every five years, once in every 10 years, it may be inundated and if gets inundated it’s probably going to be inundated for five to seven days. So that’s just something that would have to be considered.”
Riley County Commissioner Marvin Rodriguez asked what total amount of acreage would be taken up by the ponds if these dams were constructed. File didn’t have the numbers on hand, but told Rodriguez he had the data and would provide it by Friday.
File says their studies and cost analysis assumed the dams would have overflow spillways are 200 to 300 feet wide and they’d be designed under the highest design standard assumptions from the Army Corps of Engineers, National Resources Conservation Services and the State of Kansas, though the city and county could consider to reduce those standards to reduce costs.
“That was the assumption we made off the bat is that all these dams were going to provide 100-year, 24-hour flood control, so the only water that would be passing through these dams during that 100-year, 24-hour flood would be what can go through the primary spillway,” says File.
Wood is also working on a 2D modeling software for the City of Manhattan and Riley County and once that is fully completed they will be able to get much more specific and accurate numbers on the overall impact of the proposed dams and how much they’d reduce the impact of various levels of rainfall.
File says going forward they recommend also developing an easement acquisition plan for the dams, a master plan for additional flood reduction measures and to continue to consider buyouts in the floodplain.
“No matter what we do there are a few low-lying structures along Wildcat Creek that even if we reduced the flows by 50 percent or 60 percent there’s some that are just reoccurring flooding and there’s probably not any solution that will be able to be developed that will solve everybody’s problems.”
More dams and further reductions of peak flow are possible, though File notes it will come at a higher cost, potentially decreased cost-benefit radio and more impact to farm land.