Early indications point to new detention basin in northwest Waukon serving its purpose

by Joe Moses

Area residents or passers-by may have noticed heavy machinery and activity within the northwest corner of the Waukon city limits over the past couple months of summer. With that machinery and activity now gone, what remains is what may look to many like the makings of a pond, and following a heavy rainfall takes on an even greater resemblance to such a body of water.

If so, then that work done throughout this summer is serving the exact purpose it was intended for.
Located between Ninth Street NW and Fifth Street NW and to the north of Third Avenue NW, the area where the work has been completed is referred to as the Northwest Detention Basin and it is intended to be an important part of the City of Waukon's storm water management plan. Designed by Fehr-Graham Engineering and contracted out to Rusty Berger Masonry, with earthmoving and other work being done by Mahr Excavating, the basin is intended to help manage heavy rainfalls that could exceed the Waukon community's storm sewer capacity - and have proven to do so in the past.

Over the past several years, Waukon has experienced some serious flooding. The flood of 2013 was especially memorable for devastating residential neighborhoods and the downtown business community in Waukon. Considering Waukon's topography, it was determined that the most cost effective approach to try and prevent further flood damage of that caliber in the future would be to better manage water flow before it even gets to the storm sewer. The resulting detention basin is intended to assist in that task, and early indications are that the basin is living up to its intended purpose.

Frank Mahr of Mahr Excavating was involved in a majority of the construction of the basin. He commented that the construction went well without unexpected challenges other than wet conditions and hitting a bit of bedrock during the excavation process. The project also involved rerouting a section of storm sewer to allow the basin to reach its desired depth at its most optimal location.

Lyle TeKippe, Senior Project Manager with Fehr-Graham Engineering, headed the project and explained how the basin helps manage a large volume of water that would otherwise immediately flow through the City's storm sewer during a heavy rain event. TeKippe said the basin can hold a maximum of seven million gallons of water at a time before overflowing its supporting dike, the structure being designed to handle up to a 6.4-inch rainfall event within a 24-hour period, which TeKippe said would be an historically rare and exceptional amount of rainfall.

TeKippe said the basin works by collecting surface run-off from approximately 137 acres north and west of the basin dike, including from the Waukon High School and West Elementary School campuses, surrounding farm fields, and home and business lots that drain into the area due to the topography of that location. During a heavy rain, TeKippe said an estimated 146,580 gallons of inflow or run-off per minute may enter that basin area, that flow historically washing through that area prior to completion of the basin and often overloading the City's storm sewer intakes along Fifth Street NW, as area residents could attest to.

Now that the basin is complete, TeKippe said that drainage flow is much more controlled and limited to approximately 25,500 gallons per minute by the basin's outflow pipe in the southeast corner of the structure, allowing the City's storm sewer system a better chance to keep up with such a heavy rainfall. When operating at capacity, the basin's outflow is just 17% of the overall inflow or run-off that enters the basin, TeKippe calculated.

The outflow pipe of the structure measures 12 inches in diameter. If the water level in the basin rises to within three feet from the top of its supporting dike, the overflow moves into an outlet control structure and fills an 18-inch diameter outfall pipe that helps with the overflow but still allows it to be released in a controlled manner. If the water level within the basin were to rise within one foot from the top of the dike, the overflow will travel across the structure's emergency spillway. TeKippe said a basin filled to capacity could take up to 24 hours to completely drain, releasing the water in a flow much more manageable for the storm sewer.

The Northwest Detention Basin has already had the opportunity to prove itself with a couple of heavier rainfalls that have soaked the area, one of those measuring approximately four inches in mid-July and another falling during the first week of August. City of Waukon Water/Sewer Superintendent Bob Campbell shared that during those recent rainfalls wastewater flow at the Waukon wastewater treatment plant has not been substantially higher, as it has traditionally been in the past with similar heavy rain totals, providing a positive indication that the basin is doing what it was designed to do.

To bring the project to full completion, metal grates are being constructed to fit over any open intake or outflow locations, and the silt fences used initially to help prevent erosion while the structure was established will also be removed. The basin area has also been seeded down, with some reseeding required following the early August heavy rain.

Additional information available in the August 24 print edition or e-edition. You can subscribe to The Standard's print edition or e-edition by clicking here.