City’s concentration on flood mitigation continues
By Correne Martin
The city of Prairie du Chien continues its efforts to address flood and flash flood conditions on the east side of the community.
A contract with Gerke Excavating, of Tomah, has been approved to remove silt from Nommenson pond this fall. The pond affects water south of the Godden Pit watershed at the intersection of 22nd Street and Campion Boulevard, according to City Administrator Aaron Kramer. During its regular meeting Tuesday night, the common council voted to move forward with the $18,000 project, with about $8,000 in funds coming from excess money in the 2014-2016 capital projects fund and the remaining $10,000 coming from reallocated funds previously earmarked for the Old Rock School.
This project is in addition to a number of steps the public works department has done to alleviate flash flooding on the city’s east side at the base of the steep bluffs, where a large residential area has been battered by numerous flood events over the years. The most significant project, a stormwater basin, proposed by the city and its consultant, Vierbicher Associates, is hoped to be completed yet this fall also. Vierbicher has stated it is designed to safely pass a 500-year storm event, which is a requirement of the Wisconsin Department of Natural Resources.
“This project (removing the silt from Nommenson Pond) was brought to the attention of the mayor and I by the adjacent property owner following a recent flash flooding event,” Kramer noted. “It seemed like a reasonable and affordable solution in another area of the city where we have been dealing with flash flooding.”
Initial discussions the common council had regarding the need for attention there took place after the June 20-22, 2013, flash flooding. The council decided to develop a plan and to examine potential solutions that would help ease water backup on streets and in residents’ homes.
“We brag about our geography and our scenery in this region, but it can also be a curse from time to time. We treated 2013 as a learning experience, and we looked at every possible solution, including a big pipe to the river,” Kramer said. “We understand it sure was painful for homeowners. Now, we have much better data, including pictures and testimonials. We appreciate everyone’s comments and suggestions.”
The first phase of mitigation, as he explained, involved dredging and cleaning out Godden Pit.
“We had to increase its capacity, otherwise it made no sense to do much else,” he stated. “Godden pit can hold an estimated 15 million gallons of water and, for the first major rain event after that work, it held up. After a recent deluge in July, it was brought to my attention that, if we had not done the removal of the material from Godden Pit, we would have a serious situation on our hands, so it was definitely a good ‘first step’ in this entire process.”
Not long after the 2013 problems, Crawford County field workers did a great job of clearing debris from the upstream watershed, on public property at least. Certainly, private property is not accessible to the city or the county. “If they didn’t contribute, and if we didn’t share resources and manpower, we wouldn’t be able to be successful in our efforts,” Kramer said.
Additional phases of mitigation have aided in lessening the flood issues, such as cleaning out and deepening the drainage ditch and putting in place an emergency response system, which involves having a backhoe and operator at the Fillmore Street storm sewer during storms to remove debris that gets washed downstream.
However, the major project entails:
a) a stormwater basin adjacent and upstream of Highway 27, with two trash racks designed to catch debris by aid of steel I-beams and cross braces, bottom drains and riprap spillways with clay cores. The basin will provide storage and is designed to attenuate peak flow for smaller storms.
b) a stormwater basin at Jackson Street, with one trash rack, built to reduce debris reaching the box culvert there.
c) a bypass to divert stormwater away from the Fillmore Street culvert.
d) improvements to the existing ditches downstream from the Jackson Street box culvert, the Fillmore Street culvert and the bypass.
e) a redesign of the headwall to the Fillmore Street storm sewer pipe.
In early 2014, the city applied for and received a state grant to help with these mitigation projects. The money from that grant must be spent yet in 2016, Kramer noted.
History
The city of Prairie du Chien is located on a flat plain in the Mississippi River valley between steep bluffs to the east and the river to the west. A 790-acre watershed in the steep bluffs east of the city discharges its runoff along Highway 27 to the flat plain that is the east side of the developed portion of the city. Over 50 years ago, a drainage way was constructed to convey the stormwater runoff to the discharge point, which is about one mile away. The discharge point is known as Godden Pit. In order to maintain a grade that would drain through the flat terrain, the ditch was constructed above the level of the surrounding land. This was done by building up two earthen berms about 40 feet apart, creating a drainage way that was above the adjacent land surface. Since then, the city has developed around the drainage way.
In 1999, a large diameter storm sewer interceptor sewer was installed to replace the downstream portion of the drainage way on Fillmore Street. This interceptor sewer is a 72-inch diameter reinforced concrete pipe, which was installed deeper than the original drainage way.
In 2004 and again in 2013, heavy rains caused large amounts of runoff and debris to come out of the watershed into the drainage way. Due to high flows and debris blockage, the stormwater backed up and overtopped the drainage way berms. This resulted in water, mud and debris filling the adjacent yards, garages and basements, resulting in damage to private property. The water, mud and debris also filled multiple blocks of city streets.
In conjunction with the most recent flood, and in conjunction with the presidential disaster declaration, the city is taking steps to plan and implement a flood mitigation project to prevent future flooding.
