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Abstract
Rectangular storm sewers are conduits delivering water to outfall locations and have sizes that are typically larger than standard pipes, by which they become subject to more sediment deposits during operation and thus require higher flow strengths to maintain the bed clean. This study suggests a design procedure of self-cleansing rectangular sewers based on maintaining a lower limit of Shields stress and an upper limit of dimensionless bedload transport capacity. The lower limit of Shields stress is proposed under two considerations: to sustain equal sediment mobility at the channel bottom instead of selective transport and to avoid progressive deposition of finer grains due to low and reducing flows. The upper dimensionless bedload capacity is determined rationally and confirmed by using experimental data obtained from the literature. An existing bedload transport equation developed under equal sediment mobility is modified to provide a basis for the design method. It is shown that the proposed design procedure can practically be applied for a channel to estimate sediment concentrations by setting the required flow strength of Shields tress and particle size. Charts are given as an example for determining explicitly the channel design parameters. The study demonstrates that despite the high flows imposed, the design specifications determined according to this criterion can reasonably be achieved in practice for a given project.
Highlights
Rectangular storm sewers are conduits delivering water to outfall locations and have sizes that are typically larger than standard pipes, by which they become subject to more sediment deposits during operation and require higher flow strengths to maintain the bed clean
This study suggests a design procedure of self-cleansing rectangular sewers based on maintaining a lower limit of Shields stress and an upper limit of dimensionless bedload transport capacity
The current design practice of rectangular storm sewers is based on the common recommendation of a single minimum water velocity as a generalized rule, which has been found for pipe flows to be inadequate to prevent the accumulation of sediment deposition in the long term (Ackers, 1991; Vongvisessomjai et al, 2010)
Summary
Rectangular storm sewers are conduits delivering water to outfall locations and have sizes that are typically larger than standard pipes. The current design practice of rectangular storm sewers is based on the common recommendation of a single minimum water velocity as a generalized rule, which has been found for pipe flows to be inadequate to prevent the accumulation of sediment deposition in the long term (Ackers, 1991; Vongvisessomjai et al, 2010). One essential factor that makes obvious distinction in the design concept between the two sewer types is the shape of the conduit cross section This factor can affect the assumption of active bed width in generating sediment movement. A well defined procedure taking into account design factors such as sediment particle size and concentration is necessary for rectangular sewers to maintain self cleansing systems capable of preventing deposition in the long term. A model based on these considerations will be developed and discussed
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