Interception capacity of curb opening inlets




Muhammad, Muhammad Ashraf

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Curb inlets are sized and placed along a road to maintain a safe spread of water from the curb to reduce the chances of vehicle hydroplaning and flood hazards at assets near the roadway. The accepted curb inlet design standard is the Hydraulic Engineering Circular No.22 (HEC-22), which contains the FHWA's guidelines and recommended design procedures. However, the source and assumptions in the design equations are not well-documented in the HEC-22 report. This dissertation uses full-scale physical model and experimental data reported in the literature to evaluate the HEC-22 design approach and prepare updated guidance for the sizing of curb inlets. The studies presented herein provide key insights into the hydraulics of curb inlet flow. The first study provides a detailed derivation and discussion of the assumptions in HEC-22 equations for depressed and undepressed inlets, at 100% interception and bypass flow conditions. The derivation shows that the 100% interception equations from HEC-22 deviate from their supposed theoretical basis through significant rounding up of numerical coefficients in the equations and by introducing a parameter to model depressed inlets without providing any justification. The inconsistencies in the 100% interception equations led HEC-22 to deviate from theory once more by introducing a bias in the partial interception equation in an attempt to compensate for the bias in the 100% equations. The second study shows that HEC-22 significantly underestimates the 100% interception of undepressed inlets. Experimental data reported in the literature was used to provide a new design equation that reduced the relative error by a factor of 2 compared to HEC-22. The data was used as well to propose a modification to the partial interception equation and to show that Froude scaling in physical models of undepressed inlets only provides accurate results at smooth roadway surface with minimal effects of friction. In the third study, a full-scale model is modified and operated to assess the assumptions in HEC-22 equations for depressed inlets, such as the inaccurate assumption of a linear water profile along the inlet length. Data from this study was then combined with data from five other studies to provide a correction factor for the 100% interception as computed by HEC-22, and the correction factor reduced the RMSE by a factor of 3.75. A new approach was proposed as well for partial interception condition that has the advantage of providing better predictions and being structured in a way that facilitate checking and updating it using experimental data. The Fourth study modifies the depressed inlet model to test a 10 ft model of an inlet with a channel extension. The inlet was found to be robust towards flow restriction when installed on-grade. However, the capacity of the inlet extension when submerged regresses into only 23% of the expected capacity. Experimental data from this study and data reported in the literature was provided in the Appendix to facilitate future research on curb inlets hydraulics. Finally, an analysis is provided for the interception of recessed inlets in the Appendix as well. All these studies have important implications in the safe and economical design of urban stormwater drainage.


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