Preliminary Discussion Paper Number 2 - Call for Data
Capacity Requirements for Wet Weather Induced Peak Flows
in Sanitary Sewer Collection Systems
Draft - October 5, 1998
Introduction
EPA is evaluating industry and State practices and guidelines which address the requirements for peak hydraulic capacity of sanitary sewer collection system components that undergo rehabilitation.
Sources Evaluated
Several information sources are being evaluated, including:
- Industry Guidelines
- This includes design manuals and guidelines from EPA, WEF, ASCE, 10-State Standards, TR-16- State Standards
- This involves a review of State requirements, including information from indepth discussions on sewer review/approval processes with 9 States.- Individual municipals
- Information from 60 municipalities who have or are undergoing major rehabilitation projects.A discussion paper is being developed that will summarize information from these sources.
Individual municipalities
Sixty communities which have developed SSO abatement programs that identified abatement goals in terms of design storms were identified by EPA contractors. Population data, existing treatment plant capacity data, SSO abatement goals and actual cost data were collected for each municipal sanitary sewer collection system (see Table 2-1). The SSO abatement goals that were applied in actual SSO abatement programs were identified. For the sixty calibration communities, the median design storm was 5 years and the mode (most frequently applied) design storm was 5 years.
Relationship between Design Storms and Overflows per year
Sanitary sewer collection systems that are designed to not overflow when a given design storm occurs, may nonetheless experience wet weather induced overflows as the result of conditions other than the design storm. These overflows occur, in part, because peak flows in a sanitary sewer collection system can depend on a number of factors (e.g. ground water height, snowmelt, freeze/thaw conditions, soil saturation, consecutive storms, stream heights, storm drain capacity, etc.) in a complex manner. In addition, it is difficult to relate peak sewer flow to precipitation events because of uncertainty in some of the parameters considered.* Finally, lack of effective routine maintenance can limit the effective capacity of a system.
* A significant amount of uncertainty can come from measuring rainfall data, measuring resulting flows in a sewer, the lack of uniformity in rainfall events, the difficulty associated with predicting the efficiency of estimating I/I removal measures and the difficulty in predicting the effect of rehabilitation on exfiltration.
The discussion paper being developed will outline different approaches to characterizing the performance of sanitary sewer collection system under peak, wet-weather induced flow conditions. Two approaches that will be considered are the use of design storms and the use of a performance standard that is expressed in terms of how often wet weather induced overflows occur per time period (e.g. overflows per year).
Eleven of the 60 communities listed in Table 2-1 provided estimates of the number of wet weather events that would occur after the SSO abatement design storm goals were implemented. This information is presented in Table 2-2. In all communities, a greater number of annual events producing wet weather overflows are anticipated than the number predicted solely based upon the return frequency of the design storm used in the SSO abatement plan. Four communities predict average annual wet weather overflow events under one per year, but estimates of a specific return frequency could not be made.
Additional Data
Additional data in the following areas would assist in developing a discussion paper:
- Design storms used by communities other than those listed in Table 2-1.
- Estimates of the number of wet weather events that would occur after the SSO abatement design storm goals were implemented.
- States often play a critical role in reviewing plans for major sewer expansions and/or rehabilitation efforts. Descriptions of the approach a specific State uses when evaluating the peak capacity of a rehabilitation project would support the analysis.
Key Issues
- In a regulatory context how does a design standard related to a design storm compare to a performance standard that is in terms of number of overflows per year? What are the advantages and disadvantages of both approaches?
Discussion Paper
A draft paper will be developed and updated periodically to reflect new data and comments. The most recent version of the paper will be posted at the following internet site:
http://www.epa.gov/OW-OWM.cfml/wet.cfm#sso
Data and comments related to the discussion papers can be sent to:
Kevin Weiss
U.S. EPA (4201)
401 M Street SW
Washington DC 20460
or
e-mail address: Weiss.Kevin@EPA.govTable 2-2. Relationship between Design Storm and Estimated Number of Overflows
Community
Design Storm
Average Annual Wet Weather Overflows1
Average Annual Dry Weather Overflows
Covington, Louisiana
10-yr, 24-hr
<1
5
Crowley, Louisiana
5-yr, 1-hr
1
<5
Buena Vista, Michigan
25-yr, 24-hr
2
<1
Downriver Communities, Michigan
100-yr, 24-hr
0.22
(1 in 5 years)
not readily available
Jackson, Michigan
25-yr, 24-hr
3
not readily available
Midland, Michigan
25-yr, 24-hr
3
not readily available
Fairfield, Ohio
10-yr, 24-hr
<1
not readily available
Enid, Oklahoma
25-yr, 24-hr
2
25
Norman, Oklahoma
2-yr, 24-hr
3
15
Greenville, Texas
2-yr
1
50
Kerrville, Texas
10-yr, 24-hr
<1
not readily available
1
Based on actual data collected over 18 months or less of record
2 Based on model estimates