Model Results

To evaluate the performance of the combined sewer system, the model will be simulated for a minor and a major design storm. A 0.1918 inch design storm, and the design storm with 100 year return period will be used as the minor and major one respectively. Model behaviors for these two storms are represented below.

Design Storm: 0.1918 inch

The model is first simulated for the 0.1918 in. storm event. The Link Flow Summary of the resulting Status Report shows that there is no flow in any of the links (i.e. the weirs) that divert water from the combined sewers into the stream. Thus, no water flows to the storm water conveyance pipes and CSOs do not occur for this size storm.

Table 2.10 represents the summary of the interceptors’ flow behaviors. The maximum flow, except in the force main pipes, occurs in the pipe P1and quite understandably it is located in the most downstream direction through which the wastewater flows are conveyed to the pump station. It also shows that the ratios of the maximum depth to the full depth of all the interceptors are less than one that means the pipes are well-sized. Figure 2.7 shows the flows through the sections of some of the interceptors for the 0.1918 in. storm. The figure reveals that the more the position of a link towards the downstream, the more its peak flows and the volumes. The flow patterns in the other links are pretty much the same.

Table 2.10 : Interceptor flow summary for 0.1918inch design storm

Design Storm: 100-yr return period

The model is now simulated for the design storm of 100-year return period. The occurrence of flow in the weirs implies that they start discharging flow into the storm water conveyance pipes once the capacity of diversion into the interceptor has been reached and thus CSOs occur for this design storm. Figure 2.8 shows the flow behavior through the each section of the weirs. It appears that except the weir Wr2, the peak flow in all the weirs occurs almost in the same time period.

Figure 2.9 represents the interceptor flow summary for the 100yr design storm. Comparing with the 0.1918 inch design storm’s result (i.e. Table 2.10) the maximum flows in the interceptors are higher which reflects the increase in the rainfall intensity. The table also shows that for some of the conduits the ratios of the maximum depth to the full depth are equal to 1 which means that they were surcharged. Ideally those conduits might require resizing, but standard drainage criteria define a maximum allowable velocity in pipes and culverts of 15 to 18 ft/sec (CCRFCD, 1999; Douglas County, 2008). The highest velocity occurs in the conduit P2 (16.19 ft/sec) which is less than the above limit. So, no re-sizing of the pipes is required.

Figure 2.9 represents the flows through the same sections of the interceptors, which were represented in the previous figure, for the 100-yr design storm. In comparison with the 0.1918 in storm’s result, quite reasonably, the flows are higher for this design storm. The irregular fluctuations in flow are caused by fluctuations in the flow pumped into the force main at the pump station.

Table 2.11 : Interceptor flow summary for 100yr design storm

Figure 2.10 shows the comparison of flows through an interceptor and the weir that is used to convey storm conveyance pipe CSO when its capacity is reached. This interceptor flow is from P8, while the CSO discharge is from weir Wr3. Note that the durations of CSO is smaller than that of the discharge through the interceptor; however, the peak discharge is larger.

Pump Station Behavior

Figure 2.9 shows how the pump in this combined system behaves under the 0.1918 inch event. Part (a) of the figure plots the water depth in the pump’s wet well and part (b) shows the pump’s discharge flow rate. The startup and shutoff depths are shown with solid red lines (i.e. part (a)). At the start of the simulation the water depth is 3 ft and the pump is initially off (Point 1). The pump turns on once the startup depth of 5 ft is reached (Point 2). The water depth immediately moves down to the shutoff depth and the pump stops operating (Point 3).  After the runoff flow ceases some 3+ hours into the simulation, only wastewater flows are received at the wet well, and the water depth increases slowly again until reaching the startup depth (Point 4); the pump turns on but rapidly stops when the shutoff depth is reached (Point 5). From here after pumped discharges fluctuate between the startup and shutoff limits.

Major Outcomes

The major outcomes from the analyses carried out in this tutorial are presented below:

• For minor storm the wastewater flows do not reach the capacity of the interceptors, while for major storm they do and CSOs to the storm water conveyance pipes occur
• The maximum water depth in the wet well for the 0.1918 inch ¬and 100- year storm is 5 and 5.03 ft respectively which is practically the same
• The CSOs to the storm water conveyance pipes occur for a shorter time in comparison to the flow through the interceptors
• The flow regulators work in a way such that all the flows above the diversion capacity are directly discharged into the water body.