Collection System Analysis Methods
Collection System Analysis Methods
Engineering Analysis of Flow Quantities and Hydraulic Conditions
One of the most frequent analyses performed is the dry and wet weather analysis of flow — often to determine levels of RDII in a sewer system. One of the goals of such work is to correlate rainfall with RDII in such a way that this rainfall-to-flow relationship is used to characterize the performance of a sewer basin. Capital planning and sewer rehabilitation is guided by the severity of the rainfall-to-flow relationship of each basin within a collection system. Being able to monitor the rainfall-to-flow relationship for each basin is key to getting an early warning that an invisible problem is lurking.
Every collection system manager has a good idea of the rainfall needed to cause problems in their system. But some rains just don’t follow the rules. “That 1.7-inch rain we had yesterday sure caused unexpected problems.” This ADF display (Accumulation Duration Frequency) allows the manager to immediately “see” how the storm behaved. In this case, the graph reveals that what appeared to be a small 2-year storm when looking at the storm total was actually a 30-year, 2-hour storm. Knowing this information can help explain why the sewer responded poorly, and it can even become a defense in an enforcement action. With the help of software, this analysis can be performed in seconds for each gauge and for each storm.
The scattergraph is a great “human viewing speed” graphic that can reveal the hydraulic conditions in a sewer. Every SSO must be accompanied by a downstream restriction, whether it is a pump station, a treatment plant, or a problem with the pipe itself. A scattergraph can quantify the restriction and can often determine the type of restriction. This scattergraph is equipped with iso-Q lines (lines of constant flow rate) and shows that this pipe actually carries 60 percent of its design capacity and is surcharged to a depth of 70 inches. This operational capacity of a pipe can change over time, and being able to quantify operational capacity is key to spotting “lurking problems.” This problem could be the result of tree roots or a pending sewer failure. [Note: For more information on Scattergraphs and how they can help us identify ‘lurking problems’ check out the information at www.adsenv.com/scattergraphs.]
Q-to i Relationships
The rainfall-to-flow relationship is the key performance indicator of the wastewater production side of the collection system. It measures the “yield” or the amount of I/I generated in a basin. Tracking this yield can reveal one of the first “indicators of change” that a manager may see in the system. The manager of the Longmont Creek system was able to spot a serious problem in the MS04 basin by analyzing the Q-to-i relationship for just the first four storms in 2004 as shown in Figure 2.
The dramatic shift indicates that new I/I was showing up in this basin. Investigation quickly discovered that a road construction contractor had incorrectly tied a storm sewer into the sanitary. Many collection system managers may sheepishly admit that such an occurrence could continue for years without being discovered. Managers also rely on this rainfall-to-flow relationship to quantify the effect of rehabilitation. All too often, managers try to compare I/I volumes before and after rehabilitation by selecting one or two storms that are similar in magnitude. Inevitably there are other variables, such as antecedent rain or a different season of the year that interfere with the comparison. These Q-to-i relationships for several storms before and after make this evaluation much more effective.