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A computer model can be an effective tool in the master planning of a wastewater collection system. The model can be used to identify existing pipes that may be in need of replacement, and it can help identify future system needs as development occurs. Traditionally, sewer systems were analyzed under steady-state condition simulation only peak flow conditions. Sewer pipes would be identified for replacement if the pipe's full flow capacity were exceeded under the peak flow conditions. This is a conservative approach because it does not allow for conditional surcharging. In many instances, large sewer trunks are deep and do not have laterals connected to them. For these pipes, conditional surcharging under peak flow conditions may not pose a problem for the utility (i.e., manhole overflows or backups in laterals). The owner may not opt to replace these low-risk sewer trunks.

Another approach used in hydraulic modeling that allows for conditional surcharging is the fixed Q/Qfull method. Q is defined as the simulated peak instantaneous flow in the pipe reach and Qfull is the pipe's full flow capacity. The model identifies pipes to be replaced if the Q/Qfull ratio exceeds a specified value (typically 1.3).

This paper presents a new approach, referred to as the hydraulic grade line (HGL) status method, has been developed to make a more informed decision for selecting sewer line improvements with conditional surcharging. Using dynamic flow modeling, changes in the hydraulic conditions in the sewers can be evaluated over a 24-hour period. This HGL Status approach uses the simulated dynamic HGL output data to assess the system improvement needs.

Document Type: Research Article


Publication date: 2000-01-01

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