Apr 22, 2021

Aging Infrastructure: Looking at Storm Water Systems

With strained resources, those maintaining aging storm water systems can take proactive steps to ensure the systems continue to function. 

Aging storm water infrastructure

When municipalities began building separate storm sewers, they designed them mainly to relieve the burden that storm events put on sanitary sewer treatment plants. These separate storm sewers would take the water generated from precipitation events and run it to the local rivers and waterways, bypassing the treatment system. During the development of these separate storm sewer systems, many contractors and developers turned to the most popular material at the time for these underground systems: corrugated metal pipe (CMP).

Corrugated metal pipe was generally cheaper than other materials, available in a wide variety of diameters and could be laid out in a myriad of configurations. This helped designers to have the versatility and flexibility to make any design a reality. With all of these positives, storm water system designers neglected to give proper consideration to the potential drawbacks of this material. Corrugated metal is subject to corrosion (particularly with flowing water) and has a lower expected life expectancy than other materials. This means that the average life expectancy of these CMP lines range from 10-35 years, depending on conditions.  

The American Society of Civil Engineers (ASCE) recently gave storm water infrastructure a grade of ‘D’ during their annual Report Card for America’s Infrastructure. This grade was based on a combination of factors, including capacity, condition, operations/maintenance, public safety, funding and resilience.  With over 3.5 million miles of storm sewers in the United States, the study found that the overall age of the storm water system has exceeded or nearing the end of their useful lives, and many of these systems have not been maintenanced and maintained to prolong their lifespan.  The study also found that due to increased storm events, the majority of the storm water lines in the U.S. are estimated to be currently undersized to control the water flow.  All of these factors combine to show that between 2004 and 2014, ASCE found that urban flooding cost communities an average of $9 billion in direct damage and contributed to 71 deaths. 

Despite the public safety aspect of failing storm water infrastructure around the country, funding has been limited. The ASCE research shows that funding for the Clean Water State Revolving Fund (CWSRF) has decreased from approximately $589 million in 2012 to around $110 Million from 2013-2016. The good news is that this funding has increased to $387 million in 2019; however, only approximately 3% of all CWSRT funds have been allocated for storm water infrastructure. Studies have also shown that the growing annual funding gaps for fixing these systems are estimated to be $8 Billion. Municipalities have begun charging stormwater fees based on the total impervious area of property to help bridge the gap between available funding and failing storm water infrastructure.  

Storm water systems have unique attributes that make them more vulnerable to degradation. These systems have a variety of water and debris that flow into and through them on a routine schedule. As storm water flows off buildings, over streets, grass and any other surface, it picks up sediment, grass clippings, leaves, sticks, trash and other items on its march to the storm sewer. Over time, these materials can become lodged in the storm lines and accumulate, resulting in reduced volume capacity in the storm line, blockages or other issues. When the capacity of the pipe is reduced due to sediment and debris, the storm system cannot evacuate water from an area as designed, possibly leading to flooding conditions. If the debris accumulates to the point of creating a blockage in the pipe, the increased pressure in the pipe could lead to failures if not removed in a timely manner.    

In addition to the debris accumulating in storm lines, water flowing through metal pipes for decades can slowly erode the lining of the pipe, contributing to rust, perforations, and deterioration. The holes in the pipes can allow water to flow into the pipe bedding material, affecting the stability of the storm water line and even creating voids where material may wash away in these areas. Many of the news stories about large ‘sinkholes’ that suddenly open are a result of these voids created from storm water line breaks. During large rain events when a higher volume of storm water is flowing through the system, if there are breaks or holes in the pipe, the surrounding soil can be washed away quickly and a large void is created. The overlying soil collapses and creates an emergency situation on the surface. 

Another strike against storm water systems is that these lines are not considered normal utility lines, and as such, are not routinely inspected. If a water line develops a hole or has some sort of major defect, water pressure drops and the water utility typically immediately investigates to determine what the problem may be, hopefully catching it before something more catastrophic occurs. Storm water lines don’t have an early warning system for any potential issues, so owners are often unaware of any issues until it is too late. In addition to the issues already identified, other deficiencies can include utility lines penetrating through storm water pipes, the alignment/placement of the pipes can be shifted during earthwork operations or seismic activity or initial construction of the lines is not performed correctly and the pipes can separate from one another. These defects can go unnoticed for the life of the storm water line or until there is a large problem that unfolds under your feet.  

Although lines that have been in place for decades have plenty of potential problems, newly installed storm water lines can also pose a threat. It is vital that after installation, an inspection is performed to ensure that the joints are correct, there are no gaps between the pipe sections, the pipe has not been deformed or broken during installation and that there are no blockages left in the line. By performing a substantial completion inspection, owners can ensure that the product that is handed over meets the project specifications and will be able to perform the job it was designed to do.  

With all of these problems potentially lurking under your feet, what can owners do to proactively approach the maintenance of their storm water system?  To start with, have an updated storm  water utility plan showing the location, diameter and type of pipes in the storm water system. Once there is a complete plan of the storm lines, have a storm water utility professional run a Closed Circuit Television (CCTV) camera through the lines to determine their condition. This storm water professional will document debris accumulation, cracks or breaks and any other deficiencies. Some of these storm water professionals can gather this information together to grade the storm lines and rank the assets according to needs. This information can be accumulated to develop a Capital Improvement Plan (CIP) to plan/schedule maintenance and repair of these storm water lines.

By knowing the condition of these pipes now, owners can proactively approach the maintenance, reducing the chances of expensive emergency repair. It is also possible to rehabilitate pipes with repair methods rather than a complete replacement of the line. By proactively assessing storm water lines, you can get ahead of potentially catastrophic collapses.  


About the author

Nadean Carson, P.E., is a storm water program manager for Parker Design Group. Carson can be reached at NCarson@parkerdg.com