Factors Affecting Scour of Previously Captured Sediment from Stormwater Catchbasin Sumps
The sediment-retaining performance in conventional catchbasin sumps has been reported to be in the wide range between 14 and 99% (Metcalf & Eddy 1977); obviously, the higher performance is obtained by combining low flowrates, large particle sizes, and high specific gravities. Typically, about 30% of the total stormwater particulates are captured in properly designed catchbasin sumps during actual rainfall tests (Pitt 1985). The accumulation rate, or sediment-retaining performance, depends on the size and geometry of the device, the flow rate, sediment size, and specific gravity of the sediment. In the same way, scour phenomenon includes all those parameters previously mentioned, in addition to the depth of the water protection layer above the sediment and the consolidation of the sediment bed due to aging after each runoff event.
A series of tests was conducted to evaluate the importance of the parameters and their interactions on the phenomenon of scour and the migration of sediment out of a conventional catchbasin sump located at a stormwater inlet. A 2-dimensional computational fluid dynamics model (CFD), using Fluent 6.2, was used to conduct a full factorial experimental design experiment that examined four parameters: flow rate, sediment size, overlying water protection depth, and specific gravity of the sediment. The scour results were evaluated using predicted shear stress values for different depths in the catchbasin sump.
These modeling tests identified important differences in predicted shear stress values as a function of the flow rate, inlet geometry, and sediment elevation (overlying water depth). Flow rate and sediment size were the most important factors that explained sediment scour. The depth of the water over the sediment provided scour protection to the underlying material and was also identified as an important factor. However, specific gravity of the sediment material was not as important as the sediment particle size, or the water protection depth, on the prediction of sediment scour. Different inlet geometries also had a significant effect on the predicted scour conditions: in-line catchbasins having circular pipe inlets have much greater predicted shear stress values compared to a rectangular inlet associated with gutter flows.
The predicted rates of shear stresses at different elevations in a catchbasin sump are consistent with the development of the predicted velocity fields. The protecting water layer above the sediment bed is more important for the smaller flow rates; higher flow rates can cause large shear rates even with deep water layers due to the circulation pattern that is developed with continuous flows.
These scour observations are similar to what has been observed during field tests of catchbasins in the past. The next stage of this research program will directly measure the 3-D velocity fields in the laboratory using a full-sized catchbasin with a sump to confirm these calculations, followed by selected controlled scour tests for further confirmation. Finally, the results will be implemented in the WinSLAMM stormwater model to better consider sediment scour from small hydrodynamic devices.
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