Review Spring Harbor Flood Model Results

The City of Madison Engineering Division has draft flood model results and information that summarizes the second Public Information Meeting

City of Madison Engineering

September 11, 2020

Stormwater flowing through drainage channel (greenway) in Owen Park

The City of Madison began the Spring Harbor watershed study in Spring of 2019.

This story map will:

• Briefly explain how the Spring Harbor flood model was built

• Present what the model shows flooding looks like throughout the watershed

If you were unable to attend the first (April 25, 2019) or second (February 24, 2020) Public Information Meetings for this project, please visit the Spring Harbor Watershed Study website to get a background of the study's goals, and progress to-date.

The Watershed

Spring Harbor is located on the west side of Madison. The watershed spans from the West Towne Mall area to Owen Park and Bordner Park, and lake-ward to Spring Habor.

Stormwater from this watershed flows primarily from the southwest corner northeast towards Spring Harbor where it reaches Lake Mendota.

Map of the Spring Harbor watershed showing how stormwater generally flows through the watershed. The shading shows higher ground in the south and west (red) to lower ground in the north and east (blue). Greenways and ponds are in lower areas to help hold and convey stormwater. — The blue arrows show how stormwater generally flows thru the watershed. The shading shows higher ground (red) to lower ground (blue). As you can see, our stormwater greenways and ponds are in lower areas to help hold and convey stormwater.

Data Collection

Data collection can take many forms. Our data collection effort involved collection of traditional survey data and outreach to and information gathering from impacted residents. The colors and symbols below show where data was collected.

The study area has a series of points that show where monitoring, survey, focus groups and flood reports occurred. Flood reports and focus groups are primarily near greenways and are the areas that were targeted for focus groups. — Data collection that occurred within the Spring Harbor watershed

Monitoring Location – At this location the City used monitoring equipment to measure the amount of rain, how high a pond would rise during a storm, and/or how fast water moved through a pipe or channel.
Survey Location – At this location, the City collected the elevation of the ground or elevations describing the City’s stormwater infrastructure.
Flood Report – At this location, a resident submitted a flood report on the City’s online system. You can provide information on flooding you've experienced via the City's Flood Report portal .
Focus Group – At this location, the City held an in-person focus group. Residents in the Focus Group area walked around and described the flooding they’ve experienced.

Level logger equipment in a pond that records the level of the pond during a rain event. — Example of monitoring equipment in a pond that records the level of the pond during a rain event.

Inputs to the Spring Harbor Flood Model

The Spring Harbor watershed is a complex, urban area with many drainage components. The table below shows the drainage components that went into the model.

There are 2,400 acres of area modeled, 471 subcatchments, 1653 structures, 23.4 miles of pipe, 5.3 miles of open channels, and 62 ponds or storage area. — A snapshot of inputs that are used to build the flood model.

Because of all the drainage components, the model takes a long time to build.

Model Calibration

Model calibration involves comparing the model results at key points in the model to real-world observations collected from the field equipment we installed in 2019. Modeling inputs are adjusted so that the flows and water levels at those locations more closely match the observed values.

Although no model is 100% accurate for every storm event, model calibration allows engineers to develop a tool that is useful for making decisions about potential infrastructure improvements in the future.

The level of the West Towne Pond South is shown on a graph as it actually rose and fell during a monitored storm in 2019. The model prediction level is also depicted in a different color. The lines on the graph match quite closely showing that the model is predicting what happens in reality in a way that tells us that it is properly calibrated. — The blue line represents how the ponds near West Towne Mall (across from Memorial high school) rose and fell throughout a rain event in October 2019. The orange line represents how the model predicted the pond would rise and fall with the same rain pattern. For a flood model, these lines match very closely.

Preliminary model results for storm that has a 1% chance of occurring annually

Disclaimer:

The intent of the inundation maps are to assist individuals in quickly finding general flood risk information for the incorporated and unincorporated areas of the City of Madison. Inundation maps do not necessarily identify all areas subject to flooding. The City of Madison provides the maps as an advisory tool for flood hazard awareness. Individuals should not use inundation maps as their primary resource for making official flood risk determinations for insurance, lending or other related purposes. This is not an official flood map.

The City of Madison assumes no liability for any errors, omissions, inaccuracies, completeness or usefulness of the information provided regardless of the cause or for any decision made, action taken, or action not taken by the user in reliance upon any of the maps or information provided.

Spring Harbor 1% Annual Chance Inundation Max Water Depth

Map description

This map shows the model's best prediction of the peak water elevation during a storm that has a 1% chance of occurring each year (~6.6" in 24 hours).

This is a smaller storm, and therefore shows less flooding, than the August 2018 flood event.

This event is similar to a storm that occurred on June 16, 2018.

The map shows flooding based on the depths in this legend, so the dark blue could just mean water is flowing over the surface, or barely noticeable in the grass (0-3" deep).

We asked people to provide feedback on this map via a survey in the summer of 2020. We incorporated the survey feedback into the flood model.

Many areas with deep flooding are within stormwater features designed to hold or convey water, so, for clarity, these features have also been added to the map.

Please zoom in to areas you are familiar with using the "+" button, as seen below, located on the side of the map, and see if the flooding looks approximately correct.

Thank you for your time!