Scour in Stormwater Catchbasin Devices – Experimental Results form a Physical Model

Humberto Avila and Robert Pitt

ABSTRACT

This paper presents experimental results of scour of pre-deposited sediment in catchbasin sumps obtained by conducting tests with a full-scale physical model. Hydrodynamics (velocity measurements), Turbidity, Total Suspended Solids (TSS), Total Dissolved Solids (TDS), and Particle Size Distributions (PSD) of the scoured material leaving the device were analyzed. The scour experiments were conducted at Lake Lurleen State Park, Alabama, near Tuscaloosa. A full-scale physical model was constructed for these tests. A sediment mixture was prepared to simulate a typical particle size distribution of pre-deposited sediment in catchbasin sumps based on field data obtained by Pitt (1997), Valiron and Tabuchi (1992), and Pitt and Khambhammuttu (2006). Scour was evaluated at four different sediment depths below the outlet (overlaying water depths): 10, 25, 46, and 106 cm. Each sediment elevation was tested with a series of successive steady flow rates: 5, 20, 50, 100, and 160 GPM, each for a 25 min duration. Additionally, an impacting test was performed for each depth by applying four successive periods of high flows at 160 GPM that lasted 3 min.

A decreasing exponential pattern was found in the turbidity time series when a constant flow rate is applied. The initial values are high at the beginning of each flow step, but then dramatically decrease with time, due to a build up of an armoring layer on the surface of the sediment. The Total Suspended Solid concentration for the tests of sediment at 46 and 106 cm below the outlet did not show any significant change in relation to the initial TSS of the lake water. However, at 25 and 10 cm below the outlet, the TSS concentration increased progressively as a function of both increasing flow rate and decreasing water layer thickness below the outlet. Field observations have shown that the scour depth in catchbasins is generally about 30 cm below the outlet (Pitt 1985 and EPA 1999). The particle size distributions show that when the sediment layer is at 106 cm below the outlet, no significant difference was found between the effluent water and the initial PSD of the lake water. However, as the overlaying water depth decreased, the scoured particle size progressively increased until reaching particle sizes greater than 1,200 mm when the flow rate was 160 GPM and the overlaying water was only 10 cm.

In addition, observations during the tests revealed the importance of air entrainment and armoring in reducing the ability of the plunging water jet to suspend sediment. Prior tests (Avila, et al. 2007) examined the effects of inlet geometry and plunge fall on the resulting shear stress for different catchbasin conditions. Additional figures and explanations of TSS, TDS, and PSD for these tests will be presented in the final paper.


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