Quality and use of sewer invert measurements

Sewer replacement and rehabilitation is the most capital-intensive issue in urban drainage. Currently visual sewer inspection is the primary investigation technique. This information source, however, has a large uncertainty and does not provide the necessary information; therefore, for areas with soft soil conditions (e.g. river delta areas) the potential of a new information source, namely settlement data, is assessed. To analyse settlement, the vertical position of a sewer invert is determined by measuring the position of the manhole cover and the distance between cover and sewer invert. Measurement of the sewer invert is a common practice in the city of Amsterdam, the Netherlands. The use of the measurement data to assess settlement, however, is new. When multiple measurements of the same sewer invert are available over a period of time, the settlement process can be described and future positions can be estimated. Based on these estimations, the occurrence and location of stagnant water zones in the system were predicted for a case study area. The formation of these locations reduces the storage capacity and increases the accumulation of sediments. The settlement model can be used to develop and assess suitable rehabilitation strategies.