Determining Rehabilitated Sewer Flow Capacity

The flow capacity of existing sanitary and storm sewers is a significant issue when making decisions on the optimal method of rehabilitation. At times the resulting flow capacity is an important factor during the design of a sewer rehabilitation program. In the United States, the flow capacity is usually determined using the Manning formula, which includes the empirical parameter n. This committee report discusses the principal elements of the Manning formula, coefficient and field variables that contribute to the selection of values for n, and provides typical values for n for mature sewers in need of rehabilitation and for mature sewers rehabilitated with many currently available rehabilitation materials. The report then discusses a rationale for selecting a design n within a range of values and predicting the resulting change in flow capacity of the sewer. A field example is given where n values were measured. The report is intended to provide guidance to engineers making decisions related to rehabilitation of gravity flow sewers.