Watershed Study Impact on Your Flooded Basement

This effort may or may not help your flooded basement. It depends. The watershed studies are designed to quantify flooding risks throughout the City due to stormwater runoff from large storms. They will not include a detailed look at specific site grading or home / building design issues, groundwater conditions, groundwater-surface water interactions, or flooding that could occur due to a series of continuous, low-level storm events. Basement flooding at any given property may or may not be due to surface water runoff, and therefore may or may not be solved by the solutions laid out in this study.

Difference between Watershed Study and FEMA Floodplain Map

FEMA Flood Insurance Rate Maps (FIRMs) show areas subject to flooding from primary flooding sources, typically major rivers and their tributaries. Flooding within an urban watershed may be caused by local issues, which would not be included in a FEMA floodplain analysis. Local issues are things such as undersized storm sewer pipes, not enough inlets, ponding at intersections, etc. For instance, a FEMA floodplain map will show what areas are at risk when the Yahara River, Starkweather Creek, Lake Mendota or Monona floods, but not what areas are at risk when a large rainfall floods a specific roadway or property or when the storm sewer system capacity is exceeded.

Flood Insurance, Especially outside FEMA Floodplain

Having flood insurance is a decision each property owner makes for their situation. FEMA Floodplain Maps show flooding from rivers and lakes and can be used to determine what flood zone a property is located.

Much of the flooding that occurs in parts of Madison is because of the local surface drainage and the storm sewer system, not the rivers and lakes. Many homes flood because excess stormwater cannot drain into a storm drainage system fast enough to prevent localized flooding. Also, many homes are in high groundwater areas where seasonal basement flooding can occur without rain.

Homeowners can purchase flood insurance through the National Flood Insurance Program even if they are not in a FEMA floodplain. Private flood insurance may cover sump pump failures and sanitary backups. To learn more, visit the National Flood Insurance Website, and the City’s Understanding Flood Insurance webpage or contact your insurance agent to determine what your options are.

New Development/Re-Development Flood Prevention

The City of Madison recently revised its stormwater ordinance.  The ordinance describes the local rules and regulations for stormwater management in the City.

The ordinance allows the City to set low building openings in areas where flooding is a risk for new development and re-development.  The low building opening is the lowest opening on a structure where flood water could overflow and enter the structure. Where available, the information for the low building opening is taken from the watershed study computer model results which can help determine the appropriate elevation. However individual owners need to make decisions and do their own analysis on how to protect their buildings based on their specific levels of risk.

Setting this low building opening above the estimated flood elevation helps prevent new structures from flooding in the future. One item to note is that the elevations are generally set based on the 1% chance event. Therefore, if that structure experiences an event larger than that, it could still be at risk of flooding.

The stormwater ordinance also has requirements for reducing flows and volumes from development.  This helps reduce the risk of flooding for properties downstream of the development.

For more information see the Stormwater Ordinance webpage and Development Standard for Stormwater webpage.

Lake Level Information and Questions

The Dane County Land & Water Resources Department maintains lake levels in Lakes Mendota, Monona, Waubesa, and Kegonsa. Target lake levels were set by the Wisconsin Department of Natural Resources in 1979. Information on Madison lake levels can be found at Dane County's Flood Facts and Initiatives  webpage.

Climate Change Impact

Recent studies indicate the Midwest is experiencing more rainfall in general.  Additionally, the studies show the Midwest is experiencing more 3”+ storm events in the last 20 years. The City has acknowledged this vulnerability and frequently reviews its design standards and Stormwater Ordinance to address the more extreme storm events, as was done in 2020. The current design standards can be found on the Development Standards for Stormwater webpage or in Chapter 37 of the Madison General Ordinance .  While there are many goals for the watershed studies, the primary goal is no flooding of private structures during the current Atlas 14, 1% chance event (not including groundwater issues).  This goal cannot be met in all cases for multiple reasons discussed throughout this webpage.

The watershed studies include storm events that are larger than typical flood study events.  This was done to understand the impact of larger storms on the stormwater drainage system. Additionally, where possible, the solutions were increased in size to pass the larger storms.  For example, the goal is to size pipes that go under the road between greenways to pass the 1% chance event without water flowing over the road.  If there was room under the road and no other design constraints, the greenway crossing could be made bigger for the 0.2% chance event.