STORM WATER MANAGEMENT CHOICES LIMITED
In light of recent flooding in Munster and surrounding communities, it seems appropriate to discuss how the town storm water system is supposed to work and to explain how it is designed. Munster relies on Schoon Ditch, Hart Ditch, the Lincoln-Lansing Ditch, and the Little Calumet River. None of these waterways have much slope or capacity to move large amounts of water. Also, except for the area just north of Ridge Road, most of Munster is flat, which means that it is difficult to control storm water at its source. Since storm water does not flow very freely on its own, the town must install artificial high and low points to drain streets and yards during storm events. Hart Ditch drains property in Dyer and as far as Illinois. Many times Hart Ditch will be near capacity before it even crosses under Main Street into Munster. This ditch drains about half of the town, and unfortunately communities to our south imposed storm water detention requirements on developers well after Munster requirements.
One of the first questions that comes to mind is: "Why don't we dig Hart Ditch wider and deeper to increase the flow capacity and eliminate flooding"? If Munster did this, Hammond and Lansing would then experience increased flooding. As a result, we are legally prohibited from doing so. The town can remove downed trees and conduct erosion protection efforts, but we cannot increase the flow downstream. With all these constraints (flat terrain, poor drainage waterways, high flows from communities to the south and legal prohibitions) what can we do? The answer is detain, retain, and store. We need to use every street, every yard, and every open swale or close pipe to store water and release it slowly.
The Midwest Industrial Park houses the first detention pond installed over 30 years ago. Briar Creek, designed in the mid-1980s, was the first residential subdivision in which a detention pond was required. All subsequent developments including subdivisions, office buildings, retail stores and shopping centers have been required to install detention. Detention is sometimes provided by digging a pond, allowing parking lots to flood temporarily, installing oversized storm sewers under a parking lot, or even roof storage. In some cases retention ponds (permanent ponds) serve as detention when they are allowed to increase in depth and then slowly return to the normal elevation.
How do we decide how much detention is enough? Storm water engineers have debated for decades about this issue. The answer is by compromise. Each time a utility is overbuilt it wastes resources and results in added cost to the resident or business owner. The towns design standards call for storage of the 100-year rain event with a 2-year storm release rate. This is a common standard used by most communities. A 100-year storm can be a very misleading term. It means that there is a 1 percent chance that such a storm will occur each year. It does not mean that it is a storm which will occur once every 100 years. We could have multiple 100-year storms in the same year. Another problem with the term is that it looks at storm events in isolation, without taking into account the influence that one storm has on another. For example, if rain has occurred for three or four days in a row, and the ground is saturated, a 25-year storm might have the same impact as a 100-year storm which occurs on dry ground. Frozen ground will also have the same effect.
It could be argued that detention ponds waste property, walking through a parking lot full of stored water is unacceptable, and water in backyards could kill the grass, prevents mowing, and could cause mosquito problems. Let's consider the alternative. If we did not require detention and tried to get every drop of water out of every yard and off of every street as fast as possible, the result would be more frequent and more serious floods for residents who live near the Little Calumet River in Munster, Hammond, and everyone downstream. Water that sits in the back yard or on the street for a few minutes or even hours is usually an inconvenience. Water overflowing the levy and flooding basements and perhaps even first floors of homes and businesses, or preventing emergency vehicles from responding to calls for assistance is a tragedy.
Storm water is regional. It is much different and much more frustrating than sanitary sewer or water main issues. In the case of the later two utilities, the town is in control of what happens once the water reaches the town limits and up until the sewage leaves. Not so with storm water. We rely on upstream communities to refrain from dumping excessive amounts of storm water into ditches in order to keep their community dry. We rely on downstream communities to keep streams and ditches maintained by removing sandbars and downed trees, so they are functioning at capacity. If you have questions contact James Mandon, PE, town engineer, at 836-6995.
One is able to monitor the progress of drainage within the area with information provided at regular intervals from the US Geological Society. If you would like to monitor the level of Hart Ditch, click on the following link: http://water.usgs.gov/waterwatch/?m=real&r=in . Click on the area of the map where Munster is located.
