Water Environment Federation (WEF)

Modeling of Sedimentation in Stormwater Detention Basins

To minimize the non-point source pollution from roadway surface, the Texas Department of Transportation (TxDOT) initiated a research project for design of a small footprint non-proprietary detention basin, which consists of pipes and box culvert sections with specialized inlet and outlet system. A conceptual model was developed to estimate particle removal efficiency of the rectangular detention basin for the treatment of stormwater runoff by extending ideal horizontal tank theory under the condition in which water level is varied. A physical model was built in 1/5 scale of the prototype to measure its particle removal performance and verify the conceptual model. Several experimental runs were conducted using the physical model with steady inflow conditions, but various inflow rates, durations, and suspended solid concentrations (SSC). Measured time series outflow SSC and particle removal efficiency conformed closely to the calculated results from the conceptual model although the measured particle removal efficiency in the physical model was always few percent better than the predicted result using the conceptual model.

Treatment of stormwater runoff is required in many areas to reduce the discharge of pollutants. One of the most common facilities installed to accomplish this treatment is an extended detention basin. In this research project funded by the Texas Department of Transportation (TxDOT), a simple and cost effective stormwater detention basin has been developed to treat polluted runoff from highways. The detention basin is a dry basin composed of off the shelf materials including pipe and box culvert sections, the schematic of which is shown in Figure 1.

Water level is increased while highway runoff is flowing into the detention basin and decreased by draining through outlet orifice at the other end of the basin. Water overflows an outlet weir structure if the runoff volume is larger than basin capacity. Sedimentation occurs in the sedimentation basin by gravitational force.

It is important to be able to estimate particle (or Suspended Solids (SS)) removal efficiency to determine appropriate design parameters such as dimensions and drainage time. Many models were proposed for stormwater facilities such as extended detention basins and wet ponds. A methodology was suggested by EPA in 1986 for the analysis of detention basins for the control of urban runoff pollution (Urbonas 1993). The methodology combines probability concept with theoretical removal efficiency modeling concepts. For wet ponds, Su (2006) presented a removal efficiency model using plug flow reactor (PFR) and continuous-stirred tank reactor (CSTR) model to estimate the characteristics of first flush effect reduction. Luyckx (2005) proposed a normalized particle efficiency and showed its strong relationship with Hazen number for several CSO treatment facilities such as storage sedimentation tank, high side weir overflow, and vortex overflow. However, none of these models deal with the change of water level in the basin throughout a storm.

Therefore, the research objective was set as follows:

  1. Develop a conceptual model for estimating particle removal efficiency for a rectangular basin with vertical side slopes that incorporates unsteady inflow and outflow rate and corresponding water level fluctuation.
  2. Build a physical model to evaluate whether the conceptual model predicts the removal efficiency.
  3. Use the conceptual model to address typical design questions.

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