Components of a Basement Flooding Protection Plan: Sewer System Improvements. November 2000

Transcription

1 Components of a Basement Flooding Protection Plan: Sewer System Improvements November 2000

2 Components of a Basement Flooding Protection Plan: Sewer System Improvements November 2000 SEMCOG 2000 Prepared by SEMCOG, the Southeast Michigan Council of Governments, with assistance from the Basement Flooding Liability Task Force Abstract This document describes the technical issues related to basement flooding from sewer backups. Discussed are how sewer systems are designed and operated, causes of sewer backups, and potential engineering remedies. Preparation of this document was financed completely through local membership dues. Permission is granted to cite portions of this publication, with proper attribution. The first source attribution must be SEMCOG, the Southeast Michigan Council of Governments; subsequently, SEMCOG is sufficient. Reprinting in any form must include the publication s full title page. SEMCOG Southeast Michigan Council of Governments Information Services 660 Plaza Drive, Suite 1900 Detroit, MI fax

3 iii Table of Contents Page v vii Preface Executive Summary 1 Purpose 3 Background 3 Types of Sewer Systems 5 Sewer Design and Construction 7 Why Basements Flood 9 Basement Backup Control Measures 9 Transport and Treat 10 Detention 10 Source Removal 11 High-Water Management 11 Implementing Cost of Control Measures 13 Conclusion

4 v Preface Citizens in Michigan are paying out more and more tax dollars for the increased costs of lawsuits related to basement flooding. As a result of a recent court decision, municipalities are now being held to a much stricter liability standard for basement flooding than ever before, as well as for damages caused by events beyond their control. Numerous multi-million dollar lawsuits have been filed and more are expected. Left unresolved, citizens will be required to pay the cost of these lawsuits either through increased premiums for liability coverage or higher taxes. This paper discusses the issues related to municipal sewer systems that lead to basement backups and improvements needed to help prevent the occurrence of flooding. SEMCOG has prepared two additional background papers on the related policy and insurance issues, Components of a Basement Flooding Protection Program: Legislation on Liability (November 2000) and Components of a Basement Flooding Protection Program: Insurance Protection (November 2000).

5 vii Executive Summary A combination of public policy actions in three areas is necessary to adequately protect property owners by either avoiding basement flooding or providing remuneration for damages when flooding does occur. These policy actions include legislation on municipal liability, sewer system improvements, and continued and improved insurance protection. This paper discusses the issues related to sewer system improvements needed to mitigate the likelihood of basement flooding. Recommended Action Step: Improvements to sewer systems are necessary to help reduce the likelihood of the occurrence of flooding in the first place. These improvements may include a combination of upgrades to municipal sewer collection systems and interceptors, proper maintenance of municipal collection systems and interceptors, proper maintenance of individual building sewers by property owners, and implementation of prevention mechanisms by property owners. Hundreds of millions of dollars in upgrades to systems have already been spent, and projects are in various stages of implementation. The need for additional infrastructure improvements should be continually evaluated. However, as noted above, implementation of all of these measures does not ensure complete elimination of backups.

6 1 Purpose There are several possible sewer system contributors to basement backups. Some of these include rain events that exceed required design conditions, deteriorating sewers, mechanical failures, operator errors, and accidents. Other contributions may result from sources on private property. This includes such things as downspout, footing drain and sump-pump connections, and blockages in private leads. It is difficult to determine how much these private-propertybased sources contribute to a sewer backup. It is also difficult for municipalities to control them. This paper describes the engineering and technical issues surrounding the design and operation of municipal sewer systems and how they relate to basement flooding. Background information is included on how the design capacity of a sewer system is established, the permitting process under which sewers are constructed and operated, why sewers backup, and potential remedies to eliminate or reduce basement flooding.

7 3 Background Municipalities constructing sewers in Michigan since the early 1900s have been required to obtain a permit from the state under part 41 of PA 451 of 1994 (previously Act 98 of 1913). The permit ensures that design and construction of municipal sewers meet state requirements. As part of the permit process, municipalities must certify to the state that adequate sewer capacity exists to transport the anticipated wastewater volume to a treatment plant. These certifications are typically based on a theoretical rain event. State and federal grant and low interest loan programs have been instrumental in providing municipalities with the capital necessary to construct both sanitary sewer systems and wastewater treatment plants. However, these grant programs stipulated design standards in order to be grant eligible. Increases in facility sizes is not considered cost-effective and is not allowed, even with local revenue for the incremental cost. Therefore, the state standards effectively became the only size constructed. Further confusion has arisen because the state has periodically changed its design standards. Types of Sewer Systems There are two basic types of municipal sanitary sewer systems combined and separate sewer systems. Combined systems (usually in older urban areas) use a single pipe to carry both sanitary waste and storm water. Combined sewers tend to be very large because they are designed for large amounts of storm water. For example, these systems were typically designed to carry, at a maximum, the amount of water associated with a 10-year storm. Separate systems have different pipes for sanitary sewage and storm water. Storm water is collected through catch basins and transported directly to a stream, drain, ditch, or wetland where it is discharged. Under state and federal law, the discharge of raw sanitary sewage is illegal. Sanitary sewers are designed to transport the sewage to a wastewater treatment plant where it is treated before being discharged to the environment. Because even relatively new separate sewer systems contain sources of infiltration and inflow (I/I), they have higher peak flow rates during large rain events. During very large rain events, there can be more flow than the sewer system was designed to handle and backups will occur, unless an overflow, or bypass, is allowed. The state has aggressively pursued elimination of all such bypasses, as required by the federal Clean Water Act. A significant component of the overall sewer system is privately owned lateral sewer lines located on private property that connect to the municipal system near the property line. Connections to municipal sewers are regulated by sewer use ordinances. However, because these laterals are on private property, they are not directly controlled by the municipality.

8 4 Prior to the mid-1970s, footing drains (also known as weep tiles) were connected to sanitary sewers. It was expected that their flow contribution during rain events would be minimal. Numerous studies have since shown that footing drains actually are very large contributors of flow, but their removal is extremely difficult and disruptive. Therefore, municipalities have tried to account for them in sewer upgrades.

9 5 Sewer Design and Construction One of the biggest challenges for municipalities is determining a methodology for designing sanitary sewers in systems where sewer flows respond strongly to rainfall. This is a challenge because flow rates in sanitary sewers depend not only on the amount of rainfall, but also on a large number of other factors, such as: intensity and duration of rainfall, groundwater elevation, presence of snow on ground prior to rainfall, moisture in soil prior to storm, types of soils in sewer system, condition of sanitary sewer and appurtenances, amount of infiltration/inflow contributed from private property, and adequacy of surface drainage. The State of Michigan has a statutory responsibility to ensure that sewer improvements are designed to a standard of care. However, as the understanding of the wet weather response in sanitary sewers has grown, design standards for sewers have changed. Prior to the 1970s, the standard in many parts of Michigan for design of sanitary sewer systems was to accommodate a peak flow of 0.4 cubic feet/second (cfs) per 1000 people. In the 1970s and early 1980s, the majority of trunk sewers were constructed with the federal construction grant program. This program required that communities study the wet weather flow in their sewer system and construct improvements that most cost effectively used the grant monies. Generally, this approach led to projects where some infiltration/inflow (I/I) sources were removed, but also some I/I sources remained and sewers were designed to transport flow generated by these remaining sources. The Michigan Department of Natural Resources (MDNR) reviewed all of these projects and issued construction permits for them. Many major systems were designed based on the 10- year, one-hour storm. For example, the estimated peak flow rate in the City of Livonia was 90 cfs. This equates to a flow rate of approximately 0.8 cfs per 1000 people, or twice the old design standard. The program to update the City of Livonia and Rouge Valley systems to this new standard was completed in Since the late 1980s, the majority of large sewer projects have been constructed with financing from Michigan s state wastewater revolving fund loan (SRF) program. This program also requires that the money be used to manage wet weather flows in the most cost-effective manner. The Michigan Department of Environmental Quality (MDEQ), formerly part of MDNR, reviews and approves sewer project applications. In Southeast Michigan, the most recent standard from MDEQ s district office requires that sanitary sewers be designed to

10 6 accommodate peak flows generated from the 10-year, one-hour storm and accommodate the total volume generated from the 25-year, 24-hour storm. If a bypass point is desired, a 100- year storm must be used. Communities must make many assumptions for the factors mentioned above to assign a peak flow resulting from these rainfalls. In some districts, the MDEQ specifies storms smaller than the 10-year, one-hour storm be used for designing sanitary sewers. In at least one district, MDEQ specifies storms larger than the 10-year, one-hour storm be used. For the City of Livonia, the peak flow rate from a 25-year, 24-hour storm is estimated to be 165 cfs, which is 1.5 cfs/1000 people. This is 1.8 times the peak flow used in design of the grant-funded improvements and almost four times the state s design standard used prior to the 1970s. A community may design a sewer system to accommodate a rainfall that is larger than the rainfall used by MDEQ in the review of SRF loan applications. However, if a community elects to design for a larger storm, the additional cost of doing this will be ineligible for funding via the SRF program and must be financed separately by the community, usually at a much higher interest rate. Regardless of the design storm selected, the chances are that during the life of the sewer, storms experienced in the community will be larger than the design storm. This can cause flow depths in the sewer to rise high enough to reach basements. Sewer system alterations can take a long time to implement. It is common for large, complex projects to take 10 years to complete. The long time periods are needed to gather data necessary to understand the existing problems, rehabilitate sewers, remove sources of I/I, design cost-effective improvements, and construct those improvements in a manner that is not too disruptive physically or financially.

11 7 Why Basements Flood When flow in the sewer exceeds the system s capacity, it will relieve itself at the first opportunity. If an overflow is not available, sewer pipes will hold the sewage only until it reaches a level greater than the lowest point of relief. Since basement drains are below grade, sewage will often overflow into basements before overflowing at manholes. The primary cause of widespread basement flooding is excessive flow from infiltration/inflow (I/I) in the sewer system. This I/I can be attributed to a number of sources, including, but not limited to downspouts, footing drains, sump pumps, house leads, infiltration, cross connections of storm sewers to sanitary sewers, and sanitary flow exceeding design capacity. Studies conducted in Michigan demonstrate that it is not unusual for 50 to 90 percent of I/I to originate on private property. However, there are several causes other than wet weather capacity exceedances that can result in, or contribute to, basement flooding. These include factors such as sewer blockage, mechanical failures, sewer breaks, treatment facility malfunctions, and operator errors. Properties which experience more frequent backups are often found to have a property lead deterioration problem. Additionally, areas that contribute flow to a sewer system may experience considerable new development. This development can not only increase the amount of wastewater entering the sewer system, but also the I/I entering the system from private sources, further reducing the pipe capacity available to transport wastewater. In cases where several municipalities share a common interceptor, a municipality may not be able to use all of the pipe capacity designated for it, resulting in backups within their system even though the excess flow originated in another municipality.

12 9 Basement Backup Control Measures Controlling the occurrences of basement backups associated with wet weather requires controlling the infiltration/inflow (I/I) that is present in sewer systems. There are three general techniques for controlling infiltration/inflow: Increasing system capacity to transport and treat the peak flow. Detention of peak flow in storage facilities for treatment at a later time. Controlling infiltration/inflow sources to reduce peak flows. Because the origin of infiltration/inflow, age of the sewer system, construction materials, land uses, home construction methods, and maintenance activities are unique to each community, the optimal means of controlling basement backups must also be unique to each community. No single control measure is appropriate for all communities. Many communities elect to implement a combination of the three control measures. Each community affected by basement backups must study the origin of the infiltration/inflow in their community and the specifics of each control measure to choose a solution that is appropriate and cost effective for that community. These techniques will be further explained in detail below. Further information can also be found in the SEMCOG report, Implementing Sanitary Sewer Overflow Corrections: An Action Strategy (August 2000), which deals with the related issue of bypass prevention. Transport and Treat Description of method Transport and treat consists of constructing sewers, pump stations, and treatment works large enough to accommodate peak flows produced by infiltration/inflow. By having conveyance facilities large enough for wet weather peak flows, the wastewater will be transported away and will not back up into basements as a result of storms less than the design capacity. Advantages of method Little disruption on private property. Disadvantages of method Does not address sources of infiltration/inflow. Could become undersized as sewer system ages and deteriorates. Must be designed to function for a given set of climatological conditions. Capacity will be exceeded for more extreme conditions. Does not reduce peak flows. High peak flows harm the efficiency of treatment works. Construction causes disruption to streets.

13 10 Design and implementation may take years to complete. Detention Description of method Detention consists of constructing storage facilities that fill when wastewater flow rates exceed the capacity of the existing sewers and pump stations. By constructing storage, flows can be maintained in the sewers at a depth below that of basements and control basement backups. This technique normally requires construction of pump stations to lift wastewater into the storage facility. Advantages of method Little disruption to private property. Reduces peak flows so that they have a minimal impact on treatment works. Disadvantages of method Does not address sources of infiltration/inflow. Could become undersized as sewer system ages and deteriorates. Construction causes localized areas of disruption. Must be designed to function for a given set of climatological conditions. Capacity will be exceeded for more extreme conditions. Design and implementation may take years to complete. Source Removal Description of method Source removal consists of identifying the sources of infiltration/inflow (I/I) and removing the I/I from the sewer system. If enough I/I is removed, the existing sewer system will be able to transport wastewater to treatment without backing into basements. This method involves detailed study to locate the sources of I/I and localized construction to remove I/I sources. Because more than 50 percent of the peak I/I may originate on private property, this method generally involves working with property owners to remove I/I sources on private property. Advantages of method Removes source of infiltration/inflow. Less road disruption. Provides a flow stream that optimizes treatment works. Disadvantages of method Requires long-term commitment to rehabilitate system as it ages. May disrupt private property. May take a long time to implement.

14 11 High-Water Management In some instances, communities and property owners attempt to manage high depths of flows in the sewer system so that they do not create property damage. This is done by modifying private plumbing within individual buildings and includes such things as changing to abovegrade plumbing (hung plumbing) or installing devices (i.e., such as backflow preventor valves) in basement floor drains to protect basements from high water. Advantages of method Low cost. May protect homes for the largest storms. Disadvantages of method May push basement backup to neighboring property. Does not control high water depths in sewer. Requires each property owner to install and pay for modifications. Devices may fail if not properly maintained by homeowner. May damage foundation of structure if not properly installed. Implementation Cost of Control Measures Because every sewer system is unique, there is no single way to assign costs for implementing control measures. However, costs can range up to thousands of dollars per sewer customer. The major financing vehicle for implementation is Michigan's wastewater revolving fund loan program. However, the program's current level of funding cannot support implementing control measures within each community experiencing basement backups. Additionally, the revolving fund loan program requires a rigorous application procedure, including detailed sewer studies.

15 13 Conclusion Flow rates in sanitary sewers can be strongly influenced by precipitation. As this report demonstrates, sewer backups into basements can result from a variety of causes associated with wet weather events. Under severe conditions, flows can exceed sewer capacity, causing sewer backups into basements. However, it is not possible to design and construct a sewer system capable of accommodating all natural wet weather events. Sewer systems are constructed to state design standards. The design standards for sewers have changed over the years as the understanding of how sanitary sewers respond to precipitation has advanced. However, sewers in the ground today were constructed to the engineering standard in place at the time. And at every step in the process, the State of Michigan reviewed and approved sewer designs and standards. Proper sewer maintenance and operation, along with the control of infiltration and inflow can help minimize the occurrence of basement flooding. Some municipalities can do more to address their sewer backup problems, while others experience backups even though their systems are designed, operated, and maintained to acceptable engineering practices. Legislation to define when municipalities are liable for basement flooding is needed in order for municipalities to determine what they need to do to plan appropriate infrastructure improvements.