An inexpensive sewer model is helping the city of Live Oak in California conserve scarce improvement funds by focusing on repairs in areas that will have the most positive impact.
The city has experienced problems with excessive inflow and infiltration in its collection system since it was constructed in 1952. Since the problems were widespread, replacing the entire system at a cost of $9 million was considered by some to be the only viable alternative, even though the funds were not available.
Consulting engineers from Winzler & Kelly, Eureka, CA, created a dynamic sewer model designed to graphically illustrate the variations in flow through the system as a storm passes through at different times in relation to peak sanitary flows. This made it possible to identify a few choke points whose repair would solve the vast majority of the problems.
"The model helped us understand how we can solve the problems at the least possible expense," said Rob Hickey, City Manager.
Infiltration Problems"It was no secret that we had serious problems due to infiltration of groundwater through the original clay pipe," Hickey said. "Previous attempts to seal the pipes with chemical grout have provided only limited success. If funding was available, I would have not had a problem in recommending that the entire system be replaced. But, the money wasn't there. What I needed to know was whether we could make substantial improvements by repairing a few key areas and if so, what were they?The city hired Winzler & Kelly to simulate the entire system under a wide enough variety of conditions to gain an understanding of how it could be improved. Pat Kaspari, Consulting Engineer for Winzler & Kelly, was in charge of developing the sewer model.
"The first decision that I faced was what type of software package to use," Kaspari said. "Most simulation packages model flow under steady-state conditions, most commonly assuming that a peak rain flow event coincides with peak sanitary flow. The problem with this approach is that during the peak conditions the entire system may be backed up. This doesn't provide much help in looking for alternatives."
Kaspari selected the Hydra software package from Pizer Inc., Seattle, WA. A key advantage of the software is that it models flows over a period of time selected by the analyst. Sanitary flows rise and fall, normally following a diurnal pattern, while rain flow conditions can be adjusted to match actual conditions experienced during storm events.
Importing DrawingsAn AutoCAD drawing of the existing collection system was obtained from the city. This drawing was updated, cleaned up and imported into HydraGraphics, a Hydra Module designed for the entry of system data including the system pipe network and service areas. Properties including the pipe inlet and outlet invert elevations, ground surface elevations at the pipe inlet and outlet and the diameter of the pipe were assigned to the pipes in the collection system.The various sanitary service areas were then used to assign flows to the pipes in the system based on the zoning designation for each parcel. Diurnal curves were assigned to each service area. The three pump stations and the force mains were also entered into the model.
The model was then calibrated based on average dry weather flow data for the three pump stations. A per capita flow rate including wastewater flow and a dry weather infiltration factor was assigned for each service area and the total flows from the pump station were reviewed. The model flow rates were then adjusted until the total flow matched recorded flows. An infiltration factor was then assigned to the model to account for wet weather flows. Pump station flow rates and surcharge elevations for various manholes were obtained for a storm event of several years ago. An infiltration rate was assigned for the model and the level of the wastewater surcharge at various manholes was compared to the levels measured during the storm event. The infiltration rate was then adjusted to match the measured surcharge level.
Evaluating DeficienciesThe peak wet weather flow was then modeled to determine the surcharge level and capacity restriction points for the collection system. Flow of 3.991 mgd was divided among the drainage basins based on the ratios of their average wet weather flows. The capacity of the collection system was then analyzed under existing and future dry and wet weather conditions using the Hydra model. The model showed that even under dry weather flows several pipes are at 85 percent capacity or greater. The system has significantly more capacity deficiencies at various locations during wet weather flows.In the Ash Street sub-basin, the capacity of the new gravity main that runs south on Larkin Road is exceeded from Kola Street to the pump station. The capacity of various pipes in the Date Street sub-basin is also exceeded in a number of places.
A second model was created to take into account an estimated growth of approximately 2000 people in the next 10 years. This model showed flow surcharges throughout nearly the entire system.
Kaspari considered a number of different alternatives for repairing the system to reduce infiltration: 1) replace the entire existing system with new gravity sewer pipe; 2) replace the entire existing system with a new vacuum sewer system; 3) seal the known cracks with chemical grout; 4) slipline the existing pipe with polyethylene pipe, cured-in-place liner or a folded liner; 5) replace, seal or slipline portions of the existing system such as all pipes below 65 foot elevation; 6) replace, seal or slipline part of the system upstream of capacity-deficient sections; 7) seal manholes and; 8) seal laterals. Options for reducing inflow include: 1) raise low manholes; 2) raise low cleanouts; 3) review previously identified illegal lateral connections and determine if the connections have been removed; 4) provide lid inserts for manhole lids.
Economical RecommendationsKaspari recommended that, rather than focusing on reducing or eliminating infiltration and inflow, the city redesign the existing system to accommodate these additional flows. This will involve increased treatment, operation and maintenance expenses, but will avoid the need for the large capital outlay associated with extensive pipe rehabilitation and replacement. First of all, he recommended replacing Parshall Flumes used to measure flows at two pumping stations with Doppler or magnetic flowmeters. The model indicated that the flows at these pumping stations frequently exceeded the capacity of the flumes. In addition, head lost through a Parshall flow is substantial. Using the Doppler or magnetic flowmeters without any net loss will, for example, allow one particular 28-inch pipe to carry an additional 6.6 cubic feet per second.Another recommendation was to install a new sewer line to relieve capacity deficiencies on the Elm Street and Larkin Road sewers, which the analysis revealed to be a critical chokepoint. Kaspari also suggested installing a pump station in the city park at Kola and O Streets. This option would relieve the capacity deficiency along N Street and the alley between N and O Street north of Date Street. An additional pump station was suggested near the intersection of Larkin Road and Kola Street to relieve the capacity deficiency in the sewer along Larkin Road from Pennington Road north to the end of Larkin. An alternative is to install a single pump station near Kola and N Streets and construct a gravity sewer main to help relieve the same deficiencies as the previous two pump station suggestions.
"The model identified the areas that were the worst and Winzler & Kelly provided intelligent recommendations as to how the surcharges could be alleviated without repairing or replacing major sections of the system," Hickey said. "We are currently putting together a loan application for the funding required to implement the recommendations. The $140,000 required for the first year repairs will obviously be much easier to obtain than $9 million to replace the entire system." WW/
