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Storm sewer systems are designed to ‘fail’: Shirley Clark

Posted 8/16/17

When it rains, stormwater will go two places.

First it will try to infiltrate into the soil to replenish the groundwater. However, if the rainfall rate is higher than the infiltration rate, flows …

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Storm sewer systems are designed to ‘fail’: Shirley Clark

A water mark on this house shows how high the water got just south of the Wood Street underpass in late July.
A water mark on this house shows how high the water got just south of the Wood Street underpass in late July.
Posted

When it rains, stormwater will go two places.

First it will try to infiltrate into the soil to replenish the groundwater. However, if the rainfall rate is higher than the infiltration rate, flows exceeding the infiltration rate will become surface runoff.

Surface runoff will run downhill following the path of least resistance, and it is this surface runoff that caused many Middletowners to suffer damage on Sunday, July 23.

Many people have asked what should have been done to prevent it. We have a drainage system that is designed to capture this runoff and safely transport it away from our streets … up to a certain point.

Storm sewer systems are designed to “fail.” The state and borough decide how often they can “live with” this “failure.” This creates a level of risk that is assumed in the system.

For example, the decision may be made that we can accept that there is, in any given year, a 10 percent chance that your local street’s storm sewer system’s capacity may be exceeded. The excess water will either pool on the street or run over the curb into your yard. These levels of risk are determined based on the quality of the road for emergency services.

So as a borough, we want main streets to have a flooding probability that is smaller than the side streets. This probability of the system “failure” is based on recommendations that have been in place for decades and have come from the Federal Highway Administration.

Second, system “failure” means that the pipe capacity and gutter inlet capacity (flow rate) is exceeded. It does not mean that the pipe fails or breaks but that it is full. The system “failed” if the water backed up in the street beyond a certain point. For example, on the local side streets, the goal is to have the equivalent of a lane open with the crown of the road in the center.

For a major road such as Main or Union street, the parking spaces would flood, but the two driving lanes would remain open. For the interstate, the driving lanes would remain open, but the shoulders could flood. In those cases, the system capacity has not been exceeded.

That rainstorm on July 23 dumped 4.5 to 5 inches in an hour. For Middletown, that is a storm that had 1 to 4 percent probability of us getting that much rain in that short of a time. That storm level, also called a 25-year to 100-year storm, is greater than the design requirements for side streets for most locations with side streets, which are usually designed with a 10 percent probability of failure. It was not surprising that the system “failed.”

Now, what do we do about it, especially since these intense storms will become more frequent due to climate change? When I moved up here 15 years ago from Alabama, in a heavy rain, I could see across the street. Now, it reminds me of the South where I couldn’t see across the street.

When bad weather is predicted, we can go out shortly before a rainstorm and check the gutter inlets and clean them out if needed. If we put leaves in the street that wash down to and block the gutter inlet, then we have restricted the capacity of the gutter inlet in the street to take in water and it does not matter how big the pipe is, the inlet can’t get the water into the pipe.

When it is good weather, we can walk around our property and look for the flow paths and places that water will pool, such as along your foundation. Groundwater will “see” your basement as an open area that is easy for water to flow through, rather than pushing through small soil pores. It is called a preferential flow path.

Did you build up your yard in a certain area to put in a garden, which changed the surface flow path? Did you put in a patio that covered up the land where water infiltrated? When this borough was designed, it was assumed, based on the grading plans, that water would flow in a particular direction and will end up in the street in a specific location. If you change that planned flow path with anything that affects the slope of your land and/or the drainage path, then your water may miss the inlet where the engineer thought it would go and it has to go one inlet further down the street, flooding that inlet and backing up water in the street.

New development designs incorporate in their calculations whether there is sewer system capacity in the current buildout of Middletown, including the small yards and impervious surfaces which generate a lot of runoff. Many new developments have implemented stormwater management devices to reduce their flow rates.

While there is room for improvement, a large part of our problem is the older areas of town, which have a lot of pavement and buildings that cover the land. Very little of the older areas have stormwater control practices to help retain water on the land. It just wasn’t part of the design then.

The bottom line is this — the system was not designed to deal with a 25-year storm, which is, at a minimum, what we got. It is designed for a 10-year storm. The decision was made that a 10 percent failure level was acceptable. The decision was made that the cost of the pipe size to protect for a more intense storm was too costly.

Our neighboring towns have the same level of service. If we want a bigger pipe that is rarely used, then we as a borough can make that decision and agree to pay the taxes to dig up the old pipe and replace it. We also can prepare for these storms by making sure that drains are uncovered and we don’t alter substantially, without a drainage plan, the flow path on our land.

But we also have to accept that these systems will “fail”. At some point, we will have a storm like July 23 that will exceed the design of the system; and even the preventive measures that we each can do at home are not going to prevent all flooding. They will help if you didn’t get a lot of flooding. If you were in a major flash flood area, it will make the water level less deep, but not zero. It is a balance between the level of service we wish to pay for and the risk we are willing to assume. And that is a conversation we should have as a community.

Dr. Shirley Clark is a professor of Environmental Engineering at Penn State Harrisburg.

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