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Mitigating Flood Risk in the East River

Wed, 02/24/2016 - 1:53pm -- David S. Liebl

June 1990 was a wet one in Brown County with a total of 14.2 inches of rainfall recorded at the Austin Straubel Airport. But it was the 4.9 inches falling on June 22 that forced the abandonment of an estimated 200 homes as the East River overtopped its banks, causing about $11 million in damage. While there had been similar heavy storms in 1954, 1969, and 1975, no subsequent rainfall has been as severe.

Yet, since 1990, property development in the 1447-square-mile East River watershed has continued apace, both on the floodplain and along the slopes feeding runoff into the river. This trend, combined with the clearing of forested land and tiling of agricultural fields high in the watershed, is raising the question of what flood damage will occur during the next extreme rainfall.

In 2011, the consulting firm Resources for the Future published the results of a flood risk assessment for the East River watershed. The report, The Role Of Land Use In Adaptation To Increased Precipitation And Flooding: A Case Study In Wisconsin’s Lower Fox River Basin, predicts that flooding in the area could cause damages between $33 million (30% annual risk of occurring) and $124 million (annual risk of 0.2%).

Meanwhile, water quality studies of the Lower Fox River and Green Bay have identified the East River corridor as a contributor to poor water quality due to agricultural runoff and soil erosion. The 2015 Outagamie County report Nonpoint Source Implementation Plan for the Upper East River Watershed describes a number of agricultural and runoff best management practices meant to reduce these downstream water quality impacts.

Are erosion, agricultural runoff, extreme rainfall, and flood damage risk somehow related to one another? Is there anything that can be done that will co-benefit downstream communities’ flood preparedness and water quality in Green Bay?

An East River Watershed study group is beginning an in-depth look at these questions. Formed with representatives of Brown County, UW-Green Bay, NEW Water, Fox-Wolf Watershed Alliance, Baird Creek Preservation Foundation, Oneida Nation, The Nature Conservancy, Wisconsin DNR, Bay-Lake Regional Planning Commission (RPC), Wisconsin Sea Grant, and others, this group aims to identify approaches and resources that can both reduce flood risk and improve water quality. Some example approaches include:

Extreme Storm Risk Analysis

The June 22, 1990 rainfall of nearly 5 inches is considered what’s called a “50 year storm” (i.e. a storm that has a 1-in-50 or 2% probability of occurring in any year). The annual number of these extreme events in Wisconsin has doubled since 1950, with some rainfalls of greater than seven inches in 24 hours. When combined with high soil moisture and preceding days of rain, these extreme storms can lead to disasters like the June 2008 Baraboo River flooding, which caused $34 million in damage across 800 square miles. It is now possible to simulate such an extreme storm over the East River Watershed to show how extreme rainfall would affect flooding in the watershed. By estimating the extent of flooding and the resulting damage, decision-makers can develop cost-benefit scenarios for runoff best management practices that could reduce flood impacts.

Land Use Management

According to Resources for the Future, 60% of the East River Watershed is currently farmed, while the area of developed land is projected to increase by 46% by 2025. How this conversion takes place can have a substantial effect on future flood risk. Setting aside flood prone areas as wetlands, natural areas, or parks can reduce the number of properties at risk to flood damage. As development in the watershed takes place, limiting the amount of impervious surface (rooftops, parking lots, sidewalks, and roadways), especially on steep slopes, can retain stormwater until flood peaks have moved downstream.

Flood Peak Attenuation

Holding runoff high in a watershed limits the severity of downstream flood impacts. Approaches such as reducing the amount of tiled farmland, establishing buffer areas around streams and drainage ways, and increasing forested land and perennial pasture can all serve to hold water in the soil until it discharges as groundwater long after a storm event. Creating stream peak flow diversion and storage basins can also hold water high in the watershed until after flood peaks have passed.

By evaluating the feasibility and cost of these and other approaches, and building on existing water quality monitoring and runoff management planning, communities in the East River Watershed have the opportunity to better understand the risk from extreme rainfall and floods, and what can be done to reduce future vulnerability, damage, and costs. Hopefully these same strategies will also lead to reducing the East River’s role in contributing sediment and nutrients to the Fox River and Green Bay.

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David S. Liebl is a Faculty Associate in the Department of Engineering Professional Development at the University of Wisconsin-Madison, and a Statewide Stormwater Management Specialist for the University of Wisconsin-Extension.

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