Effects of Bottom-Up Blockage on Entrance Loss Coefficients and Head-Discharge Relationships for Pipe Culvert Inlets: Comparisons of Theoretical Methods and Experimental Results

Abstract

Culvert blockage is a recognized problem known to increase the risk of cross-drainage failure. Presently, the effects of bottom-up inlet blockage can be estimated using the theoretically derived energy loss method (ELM) and reduced area method (RAM). Both methods imply that hydraulically efficient inlets are more resilient to blockage effects but have not been verified experimentally for bottom-up blockages. In this study, a physical culvert model was used to determine the entrance loss coefficients and head-discharge relationships for commonly used pipe culvert inlets under different combinations of bottom-up blockage ratio, shape, and roughness. The experimental results confirm that hydraulically efficient inlets are more resilient to bottom-up blockage. Under submerged outlet control conditions, it was found that both blockage ratio and shape significantly influence the entrance loss coefficient and that ELM overestimated the entrance loss coefficient by up to 124%. Under inlet control conditions, it was found that only the blockage ratio significantly influenced the head-discharge relationship and that RAM underestimated the blockage discharge ratio by up to 38%. Comparisons to the experimental results show that ELM and RAM do not account for the increased efficiency of the unblocked part of the inlet under conditions of bottom-up blockage. Comparison to embedded inlets shows that they result in significantly lower entrance loss coefficients than partially blocked inlets under outlet control but yield similar discharge capacity ratios under inlet control. Uncertainties and estimation errors are given for the results, and validity for use in minimum performance design frameworks is evaluated for the different flow types used in hydraulic culvert design. The results show the importance of evaluating blockage effects as part of hydraulic culvert design and verify the general predictions of the energy loss method and reduced area method; that hydraulically efficient inlets are more resilient against blockage effects. The results are valid for type 1 and 5 (inlet control) and 4, 6, and 7 (outlet control) flows and can be used with existing design methods in the following ways: (1) informing the choice of inlet design, for both unblocked and blocked conditions; (2) including a blockage ratio in the design of culverts, based on registered blockage data or risk assessments; (3) evaluating necessary maintenance regimes based on acceptable blockage ratios; and (4) evaluating the effects of blockage on stream continuity. Given the large variation in in situ inlet blockage conditions, it is recommended to use the maximum values of entrance loss coefficients (keb) and the minimum discharge ratio (Qb/Q) as a conservative approach for bottom-up blockage effects. Recommended design blockage ratios given in design manuals should be supplemented with registered data on blockage history for specific locations. For data registration and monitoring, the blockage ratio (Ab/A) should be recorded.