WATCH: City looks at upstream detention to mitigate downtown flooding

Whittington Creek flows through the entrance of its tunnel system Friday. The detention system proposed for the creek's watershed would include a wider and deeper entrance. - Photo by Richard Rasmussen of The Sentinel-Record
Whittington Creek flows through the entrance of its tunnel system Friday. The detention system proposed for the creek's watershed would include a wider and deeper entrance. - Photo by Richard Rasmussen of The Sentinel-Record

Upstream detention of Whittington and Park creeks is the latest strategy for mitigating downtown flooding, a hydrological conundrum that's long frustrated stormwater managers.

Unlike previous plans, which have included diverting Hot Springs Creek and boring a new underground tunnel, detention has practical merit, officials have said. Developed by Hot Springs World Class High School's Miles Hermann while he was a student at the school's EAST Lab, the detention concept is ready to assume tangible form.

The Hot Springs Board of Directors authorized the city earlier this week to apply for a $375,000 federal flood mitigation assistance grant and allocated funds to begin acquiring property for a detention basin in Whittington Creek's watershed. The grant requires a $125,000 match from the city.

An earlier FMA grant helped fund the $103,425 study the board authorized in late 2018 to determine the feasibility of an upstream detention system. It recommended building a detention area upstream of the Whittington Creek tunnel entrance behind First Presbyterian Church.

Public Works Director Denny McPhate said the tunnel entrance would be widened and deepened as part of the initial phase of the detention project.

"The ultimate goal is to obtain 12 parcels from the tunnel entrance southwest to Ozark Street," McPhate told the board. "As we obtain those properties, and if we get the grant application, we'll begin the first phase of excavation and the dirt work will begin for the retention."

The detention system will include baffles that gradually release water downstream. The Whittington basin is expected to reduce the level of a 100-year flood downtown by 2 feet. A 100-year flood has a 1% chance of occurring in any given year.

"We can release water over a period of time and hold it until the significant flood is over and then release that as we can without flooding the downtown area," City Manager Bill Burrough told the board.

Whittington and Park creeks swell when rain accumulates quickly and can overwhelm the capacity of their tunnel systems. Their combined flows dovetail underneath the fountain at the intersection of Park, Whittington and Central avenues, where they merge into Hot Springs Creek.

But stormwater managers have said the elevation change from downtown to the mouth of Hot Springs Creek at Lake Hamilton causes the creeks to subside as quickly as they rise. According to the city's GIS map, there's close to a 200-foot drop from the creeks' downtown confluence to the lake, making a system that can contain and incrementally release fast-rising water a promising solution.

The board adopted ordinances earlier this week authorizing the purchase of four of the 12 properties for the roughly 80-by-925-foot detention basin proposed for the Whittington Creek watershed. The basin should fit inside the 12 properties and the 50 feet of stormwater utility right of way the city has on Water Street, McPhate said.

The $15,900 for the acquisition of the four properties the board authorized earlier this week will be allocated from money carried over from the stormwater/drainage fund's 2019 budget. St. Mary's Church agreed to sell a 0.39-acre parcel adjacent to the tunnel for $2,950, and the owner of a 1.06-acre parcel near the intersection of Water and Ozark streets agreed to a $5,000 sale price.

The 0.05- and 0.34-acre properties acquired on Ozark Street aren't in the creek's flood plain, but the city said they were necessary to access other parcels in the proposed detention basin. Their owners agreed to sell them for $2,550 and $5,400, respectively.

McPhate told the board prices were based on assessed values and what the city paid for similar floodplain properties on the Hot Springs Creek Greenway Trail. He said development along Park Avenue will make building a Park Creek detention system more cost-intensive.

"The difference between this area and the Park Avenue area is there's not any big parcels vacant on the Park Avenue watershed," he told the board. "It's going to be expensive. You're going to have to buy property and tear buildings down. That leg is going to be expensive. The safe thing to do in the early phases was go after the least amount of impact as far as property goes."

Local on 01/26/2020

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