Douglas County leaders hear they’ll have to keep a close eye on development and soil health to mitigate severe flooding | News, Sports, Work

photo author: Entered

A drone screenshot shows severe flooding in rural Douglas County on August 1, 2019. Washington Creek, where the floodwaters came from, is within the trees behind their homes.

As Lawrence expands southward with projects like the proposed New Boston Crossing, the landscape in the urban growth area is expected to change significantly with new residential and retail development.

But five years ago, the landscape southwest of Lawrence changed in a much more unexpected—and frightening—way.

In late July 2019, 8 to 10 inches of rain fell in just eight hours, flooding Washington Creek, which runs north from Lone Sear Lake to the Wakarusa River. The nearby countryside turned into a muddy swamp, and more than 100 houses were damaged.

Clearly, local leaders do not want these two types of change to happen in the same place and at the same time. They also want to protect rural residents who currently live in the county’s watersheds. And to do that, the consultant says, they need to be smart about where growth is allowed and take a hard look at the county’s soil health.

Douglas County commissioners were presented Wednesday with a Stantec study on potential flood prevention measures for Washington Creek, taking into account both existing and future development. The Kansas Department of Agriculture has engaged an engineering, architectural and environmental consulting firm to study Kansas’ increased flood risk and identify solutions to reduce flood risk throughout the state, including the urban growth watershed south of Lawrence.

The flood of 2019 was the topic of discussion at the commission meeting. Although numerous watersheds were affected, the Washington Creek watershed from Lone Star Lake to the Wakarusa River was where the county experienced the most flooding.

It is clear from the details that this flood was a rare event. If the rain fell within 24 hours, it would approach the 100-year rainfall threshold used to determine Douglas County’s FEMA floodplain maps, maps that show areas at risk of flooding in a community. However, because the rain actually fell in just eight hours, historical data suggests that the storm is comparable to a 500-year flood, a flood that has only a 0.2% chance of occurring in any given year.

While flooding of this magnitude may not happen often, Stantec Director Will Zung said it’s part of a bigger picture of flood mitigation practices across Kansas.

Even during smaller floods, according to meeting agenda materials, there are homes and properties in the watershed that are often affected, and several roads have been impassable during flash floods.

photo author: Douglas County Public Works

Flooding forced the closure of Interstate 458 west of US Highway 59 after heavy overnight and morning rains on Thursday, August 1, 2019.

Flooding issues have been brought up repeatedly as developers look to expand south of Lawrence. As the Journal-World reported, the New Boston Crossing project proposed just south of the South Lawrence Trafficway has raised concerns among some Lawrence residents and planning leaders because of its proximity to the Wakarusa River.

“Frankly, I can’t get past the rise of the floodplain,” said Planning Commission member Sharon Ashworth. at the meeting in February. “I don’t think we should grow in the floodplain. This is not where our development should go. We’re limited by that, and I think we have to respect the floodplain.”

The Stantec study uses this approach for the Washington Creek watershed. This suggests that because the catchment is now largely rural and has an abundance of natural vegetation along the waterways, retaining the floodplain during development will help direct growth to safer areas with less flood risk and reduce downstream impacts. Plan 2040 and the Lawrence Development Code set priorities for limiting development in the riparian corridor — or the land and waterways adjacent to a larger body of water — indicating that the community now has a vested interest in doing so.

In addition to the damage they cause to property, floods can carry pollutants and sediment that can settle and disrupt fragile ecosystems. They can also change the landscape by creating more impervious surfaces and compacting once-healthy soils.

The need for soil restoration and improvement is an important finding in the Stantec study. Healthy soils improve water infiltration, reduce runoff, reduce peak floods and increase drought tolerance.

Some of the practices Stantec suggests in its report include supporting living roots in the soil to maintain structure and fertility, minimize soil disturbance and maximize ground cover.

County Commissioner Karen Willey said Wednesday that soil remediation practices are not being used much on Douglas County’s farmland. Willie asked if there was a way to model research into regenerative farming practices such as cover crops, no-till or rotational grazing.

Zung said using cover crops is a young science, but he stressed that clay soil can be prone to flooding.

Clay soil has a high capacity to store water, but has low drainage. These heavy clay soils hold water for long periods of time and can leave yards, gardens, and other landscapes full of standing water over time.

Commissioner Shannon Reid asked Zung about what the Agriculture Department intends to do with the statewide reports and whether the reports will create some potential statewide policy opportunities. But Zung said it’s up to local governments to decide what to do with the data.

Zoning and Coding Director Tonya Voigt said the county initially wanted it to be an educational tool for everyone who currently lives in the watershed.

“Maybe the first step is just knowing,” Voigt said. “It’s also a small step in a much bigger picture. This is the first recent hydraulic study we’ve done in the watershed, so the next one we hope to do will be Wakarusa.”

“We would take information from a hydraulic study of Wakarusa and Washington Creek and all the tributaries that flow into Wakarusa Coal Creek,” Voigt said. “A lot of these creeks affect a lot of unincorporated landowners and then come up with some ideas, tools, solutions (and) create a great picture. It’s just a very small step.”

photo author: Mike Yoder

Floodwaters fill Naismith Valley Park on 27th Street in south Lawrence, Thursday, Aug. 1, 2019.