City Council ponders storm water report
WILLMAR -- The causes of flooding in the southeast quadrant of Willmar are complex and interconnected, ranging from very flat topography, increased runoff from impervious surfaces and under-sized storm sewers to high groundwater table, tight soils that restrict infiltration of water and few designated storm water detention ponds.
However, an engineering consultant said there are a number of approaches that the city can take to eliminate or reduce the city's flooding problem.
Steve Klein, vice president and senior civil engineer with Barr Engineering of Minneapolis, said his firm looked at seven options that range in price from $12.9 million and have low impact on flooding to $78.9 million and have a high impact on flooding.
During a more than 90-minute presentation and discussion session Monday night with the Willmar City Council, Klein made nine recommendations.
He said trying to develop storm water detention ponds where possible will be important.
He said one of the most ideal locations to maximize detention is the former wastewater treatment plant site.
He said trying to detain "every drop of water'' at the site will not only benefit flood control standards but could provide significant treatment to reduce storm water pollution.
Klein said the sizable, undeveloped site located just downstream of where Willmar Avenue Southeast discharges into Ditch 23A could provide the flood storage volume required to mitigate the effects of upgrading the storm sewer system to provide a 10-year level of service.
Other recommendations were:
- Complete a flood risk assessment feasibility study that would include, among other things, defining areas of nuisance flooding versus dangerous flooding, flood-proofing structures, abandoning roadways that have significant flooding, and buying and removing flood-prone properties.
- Prepare and update an emergency flood response plan that more formally defines the city's course of action to be used when a significant storm is forecast or during significant flood events.
- Review and revise the city's storm water ordinance to address required water quality treatment and flood protection for new development in the city.
- Continue upgrading the storm sewer system to provide a 10-year level of service. The city should plan for detention areas where downstream systems would not be capable of handling increased flows.
- Perform regular updates to city hydrologic and hydraulic models.
- Consider enrolling in the National Flood Insurance Program. Klein said city residents would be able to buy flood insurance at a significantly reduced cost. To participate, the city would need to develop a federal and state-approved floodplain ordinance.
Barr looked at flooding problems that occur in the southeast quadrant of the city, which is bound by Highway 12 to the north, the Highway 71 bypass to the south and east, and 18th Street Southwest to the west. The area was most likely developed within historically low areas or wetland areas that were drained by construction of a series of drainage ditches.
Like many other cities that developed during the same period, much of Willmar developed before management and detention of floodwater was common practice in the early to mid-20th century.
The flows leaving Willmar can be restricted because of the limited capacity of the conveyance system between the city and the Lake Wakanda outlet. For example, the normal water elevations in County Ditch 23A in the southeast quadrant, just upstream of the Highway 71 bypass crossing, are typically at nearly the same water level in Lake Wakanda, located about 5 miles downstream.
According to Barr, a very large drainage area that discharges into the same ditch system that serves the city can cause back-up in the ditch system during storms and causes conditions that restrict flows from the city.
The infiltration of water into the soils has little effect on reducing flood volume during large, intense storms due to the area's high groundwater levels. But the soils do provide water quality treatment and hold some runoff during smaller, less intense storms.
Beyond infiltration, according to Barr, increased piping sizes and ponding options could significantly reduce or eliminate much of the city's flooding. However, the extremely high costs associated with these options will likely prohibit their full implementation.
Because the southeast quadrant is nearly fully developed, there is limited undeveloped or open space to incorporate flood storage, and constraints posed by utilities, structures and buildings present logistical issues for construction of some projects.
Also, piping and ponding options require a combination of larger pipe capacity and additional flood storage, among other things, to address flooding problems. According to Barr, the ability for many of the options to reduce flooding depends on upgrading much of the storm sewer lines, most of which are below highly-travelled city streets.