Flooding

Approach to Storm Sewer Design is not a One-Size-Fits-All

Designing storm sewers to prevent all flooding would be cost-prohibitive and not feasible. The City’s current design target for newly constructed storm sewer is to fully convey stormwater in the storm sewer from rain events that have a 10% chance of occurring in any one year (10% Annual Exceedance Probability). This is the “10-year storm event” and in Madison is equivalent to about 4 inches of rain over a 24- hour period.  Not all storm sewer can be designed to meet this criteria, especially as part of reconstructions in older areas of the City that have been developed with lower/limited design standards. During the August 2018 storm event, some parts of Madison received 12 to 15 inches of rain in a 12-hour period. This amount of rain is three to four times more rain than the system is designed for. Storm sewers that can carry this much water can be too big to physically fit within and under the road in many instances.

In new construction, the City’s design approach is to construct overflow pathways to direct the larger events safely to public-owned land or water bodies. In fully developed areas there is not always an option to provide an alternate overflow path so storm sewer must be sized to try to accommodate areas that cannot drain before flooding properties.

Purpose of a Stormwater Pond

Stormwater ponds typically have two jobs: they provide temporary storage for stormwater and they remove sediment and phosphorous from stormwater before it drains to our lakes and rivers. By storing the water in a designated pond it will slow it down enough to allow sediment to drop to the bottom where it can be removed by routine dredging. Some sediment is too fine to settle out and will stay suspended in the water but most large particles will settle out.

Please visit the City’s Stormwater Ponds webpage for more information.

Removing Sediment from a Pond Does Not Help Reduce Flooding

Ponds with standing water in them are referred to as retention ponds. Generally speaking, the pond’s ability to hold water is referred to as “storage” is located in the part of the pond above the permanent water level. The permeant water level is set by the elevation of the pipe or structure that drains the pond.  Digging a pond deeper or dredging out the sediment generally only removes the sediment below the permanent water level. If the permanent water level does not change, the pond cannot store more water because the areas where the sediment was removed will just have standing water in its place since the normal water level remains the same. Pond storage can only be changed by either adding taller walls to the storage area to allow it to hold more water or by lowering the permanent water level, which is usually done by modifying the outgoing pipe or discharge or outlet structure elevation.

Learn more about the City's stormwater drainage system.

Purpose of a Greenway

The purpose of a greenway is to convey, and if possible, treat stormwater. At times, greenways can have ancillary uses such as walking trails, bike paths, habitat for native species, etc.  Because the main purpose is to convey stormwater, the stormwater utility manages them with that main purpose in mind.

Greenways that are heavily wooded or heavily eroded cause issues with the management of stormwater. Greenways that have a dense canopy or invasive trees sometimes have issues getting healthy understory vegetation established that holds the soil in place. This can lead to a cycle of erosion and undercutting by erosion and can actually damage trees that are in the greenway. Downed trees or debris can cause blockages that impede water while heavily eroded channels usually move sediment that can be deposited elsewhere in the system or find its way to our lakes. This sediment deposition in the lakes can cause water quality issues in addition to increased phosphorus loading and algae blooms.

Learn more about the City's stormwater drainage system.

Difference between a Park and a Greenway

One way that land is categorized is by its intended use.  The intended use is what the land was originally supposed to be used for when the City obtained it. They City typically is dedicated land as a requirement of development and the platting process. In other instances land is sometimes gifted, purchased or even condemned by the City for a specific use. There are other instances where lands also have deed restrictions on them that dictate the intended use.

For instance, a park could be used for only a park, or, it could be used for both a park and a stormwater practice depending on the way in which it was acquired by the City. Blackhawk Park in the Blackhawk Neighborhood on the far west side of Madison is an example where the lands were intended to be only used as a park and were dedicated that way as part of the subdivision process.  But, right next to it is Blackhawk Pond.  Blackhawk Pond is intended to only be used as a stormwater pond. Because Blackhawk Park and Blackhawk Pond were designed at the same time, it can be hard to understand they are categorized differently. There are many locations across the City where parks and stormwater management properties abut each other to allow for larger areas of open space so parsing out the different uses and ownerships is not always easy unless you have more information.

A stormwater practice, like a greenway could be used for only stormwater conveyance, or, it could also have park-like uses, like walking trails. For example, Sauk Creek Greenway’s intended use is for stormwater conveyance. But, the current maintenance path for the sanitary sewer along the greenway is used for a walking trail. Just like the Blackhawk Park and Blackhawk Pond example, it can be hard to tell the difference.

So, how would you know? Currently the best clue is to find out which agency in the City owns it. The City’s Park and Open Space Plan has an inventory of the public park lands and is a good place to start.  Public land in the park inventory are typically considered parkland, and, may occasionally have greenways or other stormwater treatment facilities within them.

Learn more about the City's stormwater drainage system.