Flood Facts and Initiatives
In the summer of 2018, flooding occurred in many Dane County communities due to heavy rains and other factors. This webpage provides information on frequently asked questions and topics including:
- Factors that contributed to the flooding
- Next steps/initiatives reduce future flooding and improve Dane County's ability to respond to flooding
- Additional information and contacts
Yahara River Watershed
The Yahara Watershed (see map) covers a quarter of Dane County and connects Lakes Mendota, Monona, Waubesa, and Kegonsa. Each lake has a minimum and maximum target lake level goal set by Wisconsin Department of Natural Resources (DNR) in 1979. In the summer of 2018, large amounts of rainfall and inefficient movement of water out of the system led to flooding in communities surrounding the Yahara chain of lakes.
In May 2018, the Dane County Regional Airport (located in the Yahara River Watershed) received the second highest amount of rainfall on record. Despite all four lakes being at summer minimum at the beginning of the ‘summer’ season, all four lakes rose above their summer maximum levels with Kegonsa reaching a historic high in July. The flow from Tenney Dam was decreased to minimize flooding in Lake Kegonsa. In August, the watershed received higher than average amounts of rain causing waterlogged soils. This culminated in intense rain event on August 20-21 with parts of the watershed receiving over 10 inches of rainfall. This was followed by many more rain events in August, September, and October.
The following information describes many of the factors that contributed to the flooding and challenges moving water through the Yahara River and out of the system. This information was presented to the Dane County Board of Supervisors, Committee of the Whole on September 20, 2018.
Challenges with Moving Water Efficiently
The following factors reduce water flow, which contributed to the 2018 flooding:
SlopeDuring normal conditions, the slope of the Yahara River is very flat with a drop of only 1.5 feet over 4 miles (0.007% slope) between Lakes Waubesa and Kegonsa. In comparison, the slope between Monona and Waubesa is 0.4 feet over 2 miles (0.004% slope). Because the landscape is so flat, water moves very slowly through the river and out of the system.
Constriction PointsThe Yahara River channel narrows or becomes shallower in several locations which slows down water flow. Examples of these constriction points include narrow bridges and shallow riverbeds due to layers of sediment accumulation from decades of erosion. Debris in the river, such as tree trunks and boulders, causes friction and slows water flow. In July 2018, 31 dump truck loads of debris (rock, pieces of metal, train parts, and trash) were removed from the river bed at the railroad trestle in Stoughton. The removal of debris increased water flow about 20%.
Aquatic PlantsAquatic plants cause friction and reduce water flow. The location and volume of aquatic plant growth is highly variable from year to year and must be continuously evaluated throughout the growing season. Every year aquatic plants are harvested in the Yahara River between Lake Waubesa and Lake Kegonsa. During the 2018 flooding, Dane County obtained special permission from DNR to harvest aquatic vegetation in the Yahara River south of Lake Kegonsa. This was the first year that there has been abundant plant growth in that section of the river. Between 5/21 to 9/14, 628 dump truck loads of aquatic plants were removed in the Yahara River. Over a 5 day period of 12-14 hour work days in mid August, 95 dump truck loads were removed which doubled the flow.
Clogged Storm DrainsDebris, such as leaves and trash, can clog storm drains which reduces water drainage and can cause localized, urban flooding.
Other Factors Contributing to Flooding
In addition to the factors listed above, other factors contributed to the 2018 flooding including:
UrbanizationCurrently, the Yahara River Watershed has approximately two times more urban area than it did in 1970. Urban areas have more roads, parking lots and other impervious surfaces which reduces the amount of water that soaks into the ground and increases surface water runoff. The amount of urbanization in the lake’s watershed impacts the amount of water the lake receives. The land around Lake Monona, for example, is highly urbanized so the lake receives increased amounts of surface water (stormwater) runoff while the land that drains to Lake Mendota has a higher percent of agricultural land. To learn more about stormwater runoff, watch this short video (YouTube).
Rainfall PatternsHeavy downpours are increasing nationally, especially over the last three to five decades. Since 1900, average annual precipitation in the Midwest has increased by roughly nine percent. Rainfall events have become heavier and more frequent (National Climate Assessment, 2017). In 2018, areas in Dane County experienced the second wettest May on record and an intense rain event on August 20-21. As of October 15, Madison has received 46.30 inches of precipitation which is 6.33 inches away from the record annual rainfall amount (52.93 inches in 1881). In 2018, heavy rainfall resulted in lake levels that were approximately 12-18 inches above summer maximum goals prior to additional rainfall in August 2018 which further raised lake levels to new historic highs.
Wetland LossThe watershed lost over 30% of its original wetlands in Dane County which reduces areas previously available for flood water storage. Photo to the left shows wetlands in the Pheasant Branch Conservancy (Middleton area) surrounded by development.
Saturated SoilsSoils that receive large amounts of rainfall can become waterlogged which reduces the amount of water that soaks into the ground and as a result more rainfall runs off the ground. In 2018, due to the rainfall received (see Rainfall Patterns above), many soils had waterlogged conditions.
Western Dane County
On August 20-21 2018 heavy rainfall impacted western Dane County including parts of Madison, Middleton and communities along Black Earth Creek (see map). Large amounts of rainfall occurred over a short period of time which overloaded the capacity of stormwater drainage infrastructure, resulting in rapid or flash flooding. The flash flooding damaged roads/bridges (e.g. Highway 19 in the Village of Black Earth), residential and commercial property, and recreation trails (e.g. Middleton).
Factors Contributing to Flooding
On August 20-21, a low pressure weather system rotated over the area producing 11-15 inches of rain in areas of Western Dane County. (National Weather Service)
Developed areas contain roads, buildings, parking lots, and other impervious surfaces. These surfaces reduce the amount of water that soaks into the ground and therefore increases water runoff. The water runoff travels through storm sewer drains and pipes. The storm sewer drains, bridges, and other infrastructure have limited capacity and are often not designed to accommodate water from extreme events such as August 20-21.
Resources for Additional Information
- Flood Alert Signup
- Help, Prevention
Information and resources about reducing stormwater runoff and protecting Dane County lakes, rivers and streams.
Learn more about initiatives to reduce future flooding and improve Dane County's ability to respond to flooding.
2019 County Budget to Address Flooding
On October 1, 2018, Dane County Executive Joe Parisi released the 2019 budget, which contains several new areas of funding directly related to flooding.
The Dane County Board adopted 2018 Res-227 that calls for a technical report to identify ways to address flooding on the Yahara chain of lakes and the creation of a task force to make policy recommendations.
The Land & Water Resources Department is developing flood forecast models with the goal of reducing the number of flood disasters and recovering quickly if a flood disaster occurs. These models help better predict flooding and prepare the community. A model will eventually be part of the Yahara INFOS system.