Sediment movement in storm sewer systems

  • Author / Creator
    Tang, Yangbo
  • Sediment in storm sewers attracts increasing attentions in recent years due to two major issues including the sewer blockage and the environmental contamination. Pollutants attached to fine sediment surface may adversely affect aquatic life and natural water bodies. Excessive sediment deposition can increase the possibility of urban flooding during storm events. Implement applications of sediment source control, deposition growth prediction, and sewer sediment erosion could help to reduce the severity level of above problems.

    A review about sediment in storm sewers was made to identify knowledge gaps about the sediment source control, the deposition growth prediction, and the sewer sediment erosion. Specifically, the review summarized characteristics of storm sewer sediment, studies about sediment removal efficiency in storm sewer inlets, and sediment movement in storm sewers including erosion, transport and deposition processes. In general, knowledge gaps were identified as following: (1) a general prediction method on sediment removal efficiency in storm sewer inlets; (2) the deposition growth in submerged sewer pipes; and (3) the erosion process on storm sewer deposition in the presence of cohesive materials. In this research, one analytical technique, one field investigation and two sets of experimental studies were conducted to extend the knowledge and to fill above gaps.

    For the sake of providing a guidance to control sediment source and predict sediment removal efficiency for different source control devices, an analytical technique was developed based on experimental and field studies. In the analytical technique, a comprehensive summary on sediment removal efficiency in different commercial products was conducted, which presented the need to develop a consistent criterion to evaluate the performance of different devices. Therefore, a general prediction method on sediment removal efficiency considering scaling effects was developed and it approved to be a consistent criterion for different laboratory and field data. The significance of this generalized prediction method was to be used as a preliminary performance indicator for Oil-Grit Separator units that have not yet been subjected to rigorous laboratory testing.

    In order to understand deposition growth processes and predict the deposition height, a laboratory experiment was conducted in a submerged pipe under different experimental conditions (i.e., flow rate, pipe slope, particle size, and sediment loading rate). The general process of deposition growth was obtained, which included two stages (rapid growth stage and equilibrium growth stage). The bed shear stress varied from 1.8 to 8.7 N/m2 under different conditions in this study, which could carry 0.01 to 0.50 kg/m/s sediment loading. The shields diagram and bed load transport equations were proved valid in submerged pipes. A general prediction method on equilibrium height was developed, which could be applied practically in the deposition prediction.

    As for erosion processes, a laboratory experiment and a field investigation were implemented to understand erosion patterns, erosion rates, cohesive deposition characteristics, and real deposition characteristics. Deposition erosion patterns were classified as four typical scenarios including: (1) ripple surface deposition, (2) dune surface deposition, (3) flat surface deposition, and (4) rugged surface deposition. Methods to calculate the bed shear stress, the critical shear stress, and the erosion rate were developed and compared to previous studies. The cohesive deposition had more resistance to erosion compared to the non-cohesive deposition, since the critical shear stress was much higher. Real sediment characteristics and the deposition profile were obtained through a field sampling program. Field data validated the method regarding the calculation on bed shear stress and critical shear stress, which proved its feasibility in practical uses.

  • Subjects / Keywords
  • Graduation date
    Fall 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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