Final Draft - Wastewater Collection System Capacity Analysis Apprendix A April, 1999

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Final Report City of Paris, Texas Wastewater Collection System Capacity Analysis assumption is that if the pipe has the capacity to convey the flow at minimum slope it is not really necessary to know the actual slope. In common field practice the slope is almost always greater than this number and the corresponding capacity greater as well. In some instances this slope was increased to calibrate the system to observed peaks in the collection system. For example if a modeled reach of minimum slope showed a peak occurring hours after that observed in the flow metering program, the slope was increased to calibrate the model so that the peak more nearly occurred as observed. All assumptions are subject to a certain element of risks. In this program all assumptions that were made were within acceptable standards. Design Storms Another underlying assumption is the design storm. This is the predicted quantity of rainfall that is used as the basis of the defect flows that enters the system. The design storm used for the system is described as a 5-year 1-hour storm. This is a storm that would produce an overall depth of 2.5 inches. The storm event of May 10/11, 1996 was quite significant (1.5 inches) and created an overall pronounced wet weather response in the system. For this reason the overall defect response of the system during this event was used as the basis to predict the effects of the design event. Modeling and Capacity Analysis The overall goal of the modeling program is to determine the ability of the system to adequately convey the flows that are imposed on it. By incorporating the flow and system descriptions described above, the operations of the system can be observed. In this process the system is calibrated such that the system behaves as observed during known conditions. T'his process results in a model that portrays as accurately as possible the conditions at the time of the flow measurement. By observing the predicted depth of flow in the pipes, each segment in the system can be evaluated to determine if the system has adequate capacity. If the segment has inadequate capacity, a replacement diameter that would insure proper operation (non- surcharge conditions) is calculated. This can then be used as the basis of preparing an estimated cost to provide the capacity required. The model is typically calibrated based on observed wet and dry conditions. Once these calibrations are complete the design storm effects are imposed on the system. The system is then inspected for hydraulic capacity during the projected event. Rehabilitation Impact on Line Replacement Hydraulic capacity may be restored by line replacement to provide adequate pipe capacity to convey the defect flows. Another way that capacity may be provided is by the reduction of upstream defect flows. The method employed will be based on a number of criteria. For the purposes of this report, we will establish the 14