GENTRY — Improvements in the city’s drainage system will first have to be made on the outskirts of town where runoff water is received before major improvements can bemade in the developed portions of the city, according to a recently-completed drainage study report.

Ron Homeyer, of Civil Engineering, Inc., Siloam Springs, presented the summary report to the city council in a special March 15 meeting.Homeyer’s firm was hired by the city to do the study so that informed decisions could be made as the city considers ways to improve its storm drainage system.

Because of the limited capacity of drainage ditches and culverts downstream from the main portion of the city - with many of them already being inadequate for runoff during heavy rains - to improve drainage in town without first making improvements downstream would only worsen the flooding in some areas during rain events, Homeyer said.

Six drainage basins wereidentifiedin the study, with two emptying into the Little Flint Creek and SWEPCO Lake, and the other four entering Flint Creek on the south side of the city.

Runoff rates were calculated based on statistical rainfall amounts within 24 hours during 2-year, 10-year, 25-year, 50-year and 100-year floods. In addition to providing capacities of water flow for ditches, culverts and bridges in cubic feet per second, the study provided calculated flow capacity percentages for them based on rainfall in a 100-year flood event. Many of the exist-ing ditches, culverts and bridges are already inadequate for a major rain event, the report revealed.

A 100-year storm is 9 inches of rain in a 24-hour period, Homeyer said. A 25-year flood is 6.33 inches in the same time period. A 100-year flood is not limited to occurring only once in 100 years; rather there is a 1 percent statistical probability of such a storm in any given year.

Should the city make major improvements to its intown drainage infrastructure, the runoff water would cause more flooding in the downstream areas.

“To do things right, you need to start at the bottom of the system and work your way up,” Homeyer said. “Making improvements in the top three drainage basins will get the water to the problem areas faster and cause flooding before it getsto the creek,” he said.

The study detailed points where improvements are needed to help overall drainage for the city. An additional list of prioritized improvements will also be completed as a part of the study.

Improving drainage near the city’s water and sewerdepartment shop would help move water out of the main part of the city more quickly, Homeyer said. Increasing the capacity of the cement culvert under Arkansas Highway 12, or adding a second would help too, he said.

Homeyer and council members discussed the possibility of seeking drainage easements and using wider, shallower ditches on the outskirts of town to move more water out of the developed area quickly. Clearing out some brush and trees in the runoff area was also suggested.

A channel from Marion Lee Road to Flint Creek was suggested as a way to alleviate water running across the road and flooding properties at Pioneer Lane and Marion Lee during heavy rains.

Homeyer estimated the cost to install underground drainage systems with curb and gutters on all the city streets at $15.8 million - roughly $100,000 per block. But should the city build such a system first, it would worsen the flooding downstream, he said.

Though detention ponds were not in the plans for Gentry, wide ditches or some detention areas would permit more chemicals and pollutants to settle out of the water before it reached the creek, Homeyer said. With concerns over water qualityin Flint Creek and the Illinois River basin, this may be an issue the city will consider before seeking to streamline drainage through cement storm drains and paved drainage ditches.

“There are water-quality benefits to detention basins,” Homeyer said.

A number of council members raised concerns about the safety of children should the city ever use a detention area.

“There is a right way to build detention ponds,” Homeyer said, explaining that they should be shallow on the edges and slope down toward the center so thatchildren and people can get out of them easily.

A number of immediate improvements which the city could make at very little cost involved cleaning out and, with permission, extending some culverts under the railroad which are currently blocked.

This would greatly improve drainage in that portion of the city where runoff water must drain under the railroad, Homeyer explained.

A map detailing the drainage basins and their features is posted on the wall in the city hall conference room.

News, Pages 1 on 03/24/2010