SSO AVOIDANCE IN LOS ANGELES: RECOGNIZING THE ROLE OF SURCHARGE
Abstract:In 1998, severe El Niño rainstorms inundated portions of the City of Los Angeles' (the City's) sewer collection system causing overflows. The spills led to a legal challenge by the EnvironmentalProtection Agency (EPA), State Regional Water Quality Control Board, and other environmental organizations. A negotiated settlement was reached in 2004, which required the City to producea collection system capacity report and plan (Plan). The Plan was to examine, among other things,hydraulic relief criteria, and recommend alternative approaches to conveyance management if necessary.
The City confronted an issue familiar to many agencies: how can the City eliminate avoidable spills while maximizing system conveyance and minimizing Capital Improvement Program (CIP) expenditure? An approach to collection system capacity assessment relying in part on the concept of 'allowable surcharge' was developed for exploration. The approach recognizes the realistic dynamic nature of collection system performance, and will enable a risk management strategy to be used either for targeted (basin-specific) hydraulic relief parameters and I/I reduction, or across the entireCity.
The City's current design criteria require that a sewer be relieved by the time the peak dry weather flow (PDWF) d/D level in the pipe reaches 0.75. This is termed the relief level, and assumes that peak wet weather flow (PWWF) will be conveyed within the full-flow capacity of a pipe.
The trigger flow level concept (or simply trigger level) is a planning tool that proactively monitors and measures the hydraulic conditions of the sanitary sewer against a predetermined hydraulic relief level, and initiates the planning and design of capacity relief. The trigger level is lower than the relief level to allow for an increase in flow over the duration of relief project implementation. The trigger level is a project-specific function of sewage flow rate increase over time (factoring in population growth, land use changes, etc.), and expected project implementation duration. Trigger level depends upon relief level, and relief level depends upon I/I rates.
Because I/I rates can vary considerably across the City, it is reasonable to apply different relief levels to different areas of the City based on boundaries defined by I/I rate. More than 200 such basins have been defined across the City. City-wide gauging data (PDWF) and standard City design event (PWWF) results were analyzed using GIS tools to produce weighted average relief level values. These relief levels assume that wet weather flow can be conveyed within the fullflow capacity of a pipe. Twenty-four hypothetical relief levels were developed which spanned the City.
An approach was then developed to consider the effects on conveyance capacity of different amounts of upstream surcharge: 1.5 feet, 3 feet, and 5 feet. While hydraulic analysis was done using the Bernoulli and Manning's equations, several small-scale hydraulic models were developed in MOUSE(DHI Inc.) to verify the assumed performance of the typical pipe. Model results matched manual calculations to within a few percent. GIS analyses were then repeated with the three levels of maximum surcharge assumed, and results were weighted and compared with base-case results.
Results were then translated into rough estimated cost impacts on the existing CIP program by further GIS analysis using PDWF gauged d/D values from over 700 metering locations across the City.
Depths below grade of all collection system pipes 16-inches in diameter and larger were extractedthrough GIS. Results, when overlaid on the relief level results and I/I data, clearly indicate locations where surcharge alone may not provide the solution. This helps to screen areas likely to need rehabilitation or relief projects.
Comparison of base-case results with those of the surcharged conditions showed clearly that recognizing and utilizing some degree of surcharge in the collection system can reduce future CIP costs, while allowing the City to meet its objective of no avoidable SSOs.
An expanded 650-mile dynamic hydraulic model of the City's Primary (16-inch diameter and above) Sanitary Sewer Collection System is being developed. When calibrated with adequate and accurate wet weather gauging data, the City will have the analytical and performance monitoring tool that can assess how much surcharge is acceptable in specific portions of the collection system. Further analysis of model results combined with other risk factors can then be considered, which will greatly assist in locating and prioritizing hydraulic relief projects.
Document Type: Research Article
Publication date: January 1, 2006
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