Flow equalization analyses were based on the methodology employed by the Storage, Treatment, Overflow, Runoff Model (STORM), which was developed by the US Army Corps of Engineers Hydrologic Engineering Center (1977). STORM was used to predict infiltration/inflow (QI/I) generated in the CSDOC wastewater treatment plant sewershed for the period from 1976 to 1995 based on a modified Rational Formula method and rainfall records from the National Climatic Data Center, as discussed below. For each hourly time step, STORM routes the predicted QI/I to the headworks of each treatment plant and subsequently predicts the flow at the outfall structure. As long as QI/I does not exceed the assumed plant wet-weather treatment and discharge capacity (QT), then all flow is routed through the plants and outfall. If QI/I exceeds the wet-weather treatment capacity of the plants, then the flow that cannot be treated is diverted into equalization storage at a rate QE = QI/I -QT. Once the QI/I falls below the wet-weather treatment capacity, then the model returns flow from equalization storage through the plant at a rate QE = QT -QI/I.
If the flow equalization storage is full and QI/I influent to the plants exceeds the wet-weather capacity, then an "excess flow event" occurs. These excess flows need to be routed through the plant at a rate exceeding the assumed wet weather treatment and discharge rate (QT) to avoid plant bypassing or localized overflows. The outfall was modeled and its hydraulic capacity was simulated in the same manner as the wet-weather treatment and discharge capacity of the treatment plants. If QI/I exceeds the hydraulic capacity of the outfall, an “excess flow event” occurs. The model was run under several scenarios that simulated increased system equalization storage upstream of the outfall.
Two initial scenarios were examined to initiate calibration of the model based on available information. Both scenarios were examined under infiltration and inflow (R) values of 0.007 and 0.01. The first scenario assumed that the 120-inch outfall was the primary discharge structure for he system and that it had a 480 mgd flow capacity (design capacity). The 78-inch outfall was considered as the discharge structure for flows exceeding 480 mgd up to 650 mgd. Above 650 mgd, the Santa Ana River overflow weir would be utilized.
The second scenario was identical to the first with the exception that the 120-inch outfall had a 550 mgd capacity (estimated peak flow through the outfall in January 1994) and the 78-inch outfall handled flows above 550 mgd and below 720 mgd.
Using 19 years of hourly rainfall records for Orange County, taken from the National Climatic Data Center monitoring station in Long Beach, STORM simulated a continuous time series of infiltration/inflow entering the CSDOC wastewater treatment plants. For this period, STORM tabulated the total number of excess flow events and volume of excess flow that could not hydraulically pass through the treatment plants and the outfall at the assumed hydraulic capacities. Model simulations were performed for varying outfall capacities and for varying flow equalization storage volumes. The equalization storage volumes were chosen over a range of values in order to create a graphical presentation of the information.
Because the CSDOC sewer system contains two treatment plants, interplant pipelines, and various outfall structures, a modified version of STORM was required for this study to allow excess flow diversion and flow splitting. These modifications, along with the database pre-and post-processor software developed to facilitate STORM input setup and output analysis, are described in further detail below.
Since 1991, CDM has been developing and refining NetSTORM, an enhanced STORM model that incorporates the basic solution algorithms of the original 1977 HEC-STORM model into a FORTRAN program customized to interface with a Windows shell that facilitates model input setup and output interpretation.
NetSTORM’s principal enhancement to the original model is a program loop that allows the model to sequentially process multiple overflow structures. Each model node (an overflow regulator, other relief/diversion structure, or a flow split) can have up to two downstream nodes that receive overflows. Flows exceeding treatment rate and storage capacity may either be reported as an overflow, or may be routed to another structure. Treated flows (e.g., flows less than the wet-weather capacity of the structure) can also be routed to as many as two downstream structures. When a structure splits flow to two downstream nodes for treatment, the flows are allocated according to a fixed ratio for that structure specified in the input file. These enhancements facilitate accurate representation of the CSDOC sewer system, in which influent flows to Plant 1 are consistently bypassed to Plant 2, according to plant flow data, and effluent from both plants flows to the outfall structure.
Test results for NetSTORM have been verified against those generated with the original model and by manual calculation of test cases, to confirm that the modified version produces the same results as those of the original version for any set of hydrologic parameters. Significant modifications were made to the model output and extensive error checking procedures were implemented. The program is also capable of using data from multiple rain gauges which facilitate the simulation of spatial variation of rainfall throughout the collection system drainage basin.