Bottomless Culvert Scour Study: Phase I Laboratory Report
1. INTRODUCTION
Bottomless (or three-sided) culverts use the natural channel bed and are environmentally attractive alternatives to traditional closed culverts. Moreover, they are considered by many highway agencies to be economical alternatives for replacing short bridges. They are typically placed on spread footings, and the issue of scour and the depth of footing must be addressed. The scour problem is analogous to abutment and contraction scour in a bridge opening and can be treated in much the same manner.
The study described in this report was conducted at the Federal Highway Administration's (FHWA) Hydraulic Laboratory at the request of the Maryland State Highway Administration (SHA) in a partnership arrangement in which the Maryland SHA shared the cost of the study. Two suppliers, CONTECH® and CONSPAN®, agreed to provide models of the typical configurations used for highway applications. Part of the study objective was to compare the results from a simple rectangular shape to the results from shapes that are typically available from the suppliers.
Since abutment scour estimates at bridge openings are often quite large, a scour protection task was included to determine the sizes of rock riprap that might be required to reduce scour in the most critical zones.
A major consideration in estimating scour and riprap sizes is the flow distribution at the entrance of the culvert, especially when there is side flow that is being contracted to pass through the opening. Although the analysis was aimed at simple one-dimensional (1D) approximations for this flow distribution, some 2D numerical simulations of the laboratory experiments were conducted to demonstrate how this could be used if they are available to a designer. As numerical models become user-friendly and computers become more powerful, 2D and even 3D numerical results are likely to become readily available to designers.
In presenting status reports to drainage engineers at American Association of State Highway and Transportation Officials (AASHTO) meetings and at hydraulic conferences, we found that there was widespread interest in this topic. This report is an attempt to share the results of this study with a larger audience.
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