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Role of impervious surfaces in increasing runoff discussed
rain garden

DRIFTLESS - When the subject of land use, water runoff and flooding is discussed, more often than not attention is focused on agricultural lands. However, members of the conservation community are increasingly expanding the scope of the discussion to include both rural and urban areas with increasing amounts of ‘impervious’ surfaces at the forefront of the discussion.

The word ‘impervious’ is defined as “not allowing entrance or passage” by the Merriam Webster dictionary. Examples of impervious surfaces include roofs of homes, businesses and agricultural buildings, paved parking lots, roads, driveways, and basically anywhere that the soil is covered with a surface that does not allow water to infiltrate into the ground.

As Monroe County Conservationist Bob Micheel noted recently at a gathering of citizens to discuss the breached dams in the headwaters of Coon Creek, the increase in ‘impervious’ surfaces in the watersheds is contributing to the problem with runoff in the area’s increasingly large rain events. Impervious surfaces, according to Micheel, produce 100 percent runoff. 

Karl Green, a professor with the Community Development Institute, UW-Extension LaCrosse, recently spoke to a group of interested residents in Viroqua about what urban residents can do to help control runoff from their homes and businesses in the city. The presentation was part of the ‘Conservation on Tap’ series put on by Valley Stewardship Network.

You might think that a ridge top community like Viroqua wouldn’t experience problems with flooding, but that is not exactly true. Given the increase in impervious surfaces in the community, the city can experience brief flash flooding events during a large rain event. 

As City of Viroqua Engineer Sarah Grainger submitted comments at the FEMA/DNR meeting held in Viroqua in March of 2018 to discuss evaluation of the floodplain maps and mitigation planning needs in the Kickapoo River Watershed.

“One third of the City of Viroqua drains to Cooks Creek. The water comes from three main branches and reaches confluence at the edge of town near Highway 56,” Grainger wrote. “We have a homeowner right at this location, and the water starts backing up trying to go through culverts. Storm water detention would likely benefit the city and those downstream.”

Further, in the seven-year study on runoff conducted by UW-Discovery Farms in the headwaters of the West Fork of the Kickapoo River, one of the sites studied was the City of Cashton. Their report revealed that there was approximately two-to-three times more volume of water runoff from the city site than from the agricultural sites included in the study. However, the report went on to specify that the quantity of nutrients running off from the city site were approximately equal to those from the agricultural sites because the concentrations of nutrients at the city site were much lower.

“Did you know that a one-inch rain on an acre parking lot produces 27,154 gallons of water,” Green asked. “Where I’m from in the LaCrosse area, we receive around 43 inches of precipitation annually. An average residence there has approximately 2,850 square feet of impervious surfaces, which generates about 1,775 gallons of water per one-inch rain fall event, or 76,400 gallons of runoff annually.”

Green discussed the history of the Driftless Area, how it had avoided being covered by the glaciers, and its fractured underlying karst geology. This geology means that urban runoff can not only contribute to flooding problems, but also has potential to wash contaminants off the city landscape that could penetrate into our groundwater aquifers.

Rain gardens

What Green proposed in his talk is that citizens have the ability to take responsibility for their landscapes by creating structures, such as rain gardens, in their yards. Rain gardens can increase the ability of water to infiltrate into the soil and reduce runoff.

“A rain garden is a depression in a yard that focuses runoff waters for infiltration,” Green said. “Infiltration amounts are based on cubic footage of your rain garden – so that would be the length x width x depth.”

The Department of Natural Resources recently released ‘Rain Gardens: A Guide for Homeowners and Landscapers,’ which gives detailed instructions about siting and constructing a rain garden. The guide can be downloaded for free at

Rain gardens are landscaped areas planted with wild flowers and other native vegetation that soak up rain water, mainly from a roof of a house or other building. The rain garden fills with a few inches of water after a storm, and then the water slowly filters into the ground rather than running off to a storm drain.

Prior to human development, our natural areas absorbed and filtered rainwater. Some water slowly returned to rivers and streams as it filtered through wetlands, while other water slowly infiltrated into groundwater.

As cities and suburbs grow and replace forests and grasslands, increased stormwater runoff from impervious surfaces becomes a problem. Stormwater runoff from developed areas increases flooding because there are fewer places where rainwater is slowed and infiltrated. Stormwater runoff also carries pollutants from streets, parking lots and lawns into local streams and groundwater, and leads to costly municipal improvements in stormwater treatment structures.

By reducing stormwater runoff, rain gardens can be a valuable part of changing these trends. While an individual rain garden may seem a small thing, collectively they produce substantial neighborhood and community environmental benefits.

Choosing a site

Home rain gardens can be in one of two places – near the house to catch only roof runoff or farther out on the lawn to collect water from the lawn and roof.

Wet areas in your yard are actually an indication of where water infiltration is poor, and are often not a good place to site a rain garden. Observing how the water flows off your property in a rainstorm can be a good guide about where a rain garden can be placed to best effect.


The guide from the DNR provides specific calculations you can make in order to determine the size rain garden you will need to achieve runoff control on your property.

Any reasonably sized rain garden will provide some stormwater runoff control. A typical residential rain garden ranges from 100-300 square feet, though ultimately you will need to calculate not only the width and length but also the depth of your garden. Your calculations will provide a size for your rain garden expressed in cubic feet – length x width x depth.

Three main factors will determine the size of your rain garden:

1. how deep the garden will be;

2. what type of soils the garden will be planted in; and

3. how much roof and/or lawn will drain to the garden

A typical rain garden is between three and eight inches deep. If the garden is greater than eight inches, it could cause ponding, and if shallower, could reduce the benefit.

You will also need to take into account the slope of the land where your rain garden will be placed. The DNR’s guide contains a simple way to calculate slope. On slopes greater than eight percent, you may need to consider terracing your garden.

The type of soil you have will influence the size rain garden you need to construct. Water infiltrates into sandy soil at the rate of about 2.5 inches per hour; into silty soil at 0.5 inches per hour; and into clay at 0.3 inches per hour. The DNR’s guide contains a simple way to test the infiltration rate of the soil where you plan to build your garden.