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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-026
Date: February 2007
Bottomless Culvert Scour Study: Phase II Laboratory Report
Chapter 1: Introduction
Bottomless (or three-sided) culverts use the natural channel bed and are environmentally attractive alternatives to traditional closed culverts. Moreover, they are often promoted as alternatives for replacing short bridges. These structures are typically founded (supported) on spread footings, and the issue of scour and the depth of footing must be addressed as part of their design. Many State highway agencies will not allow bottomless culverts unless they can be founded on solid rock formations. Therefore, there is a need to formulate a defendable procedure for estimating scour depths in other types of soil formations (e.g., sands). The scour problem is analogous to abutment and contraction scour in a bridge opening and can be treated in much the same manner.
This report describes a two-phase study conducted at the Federal Highway Administration’s (FHWA) J. Sterling Jones Hydraulic Laboratory at the request of the Maryland State Highway Administration (MDSHA) in a partnership arrangement. Phase I was a preliminary investigation focused on measuring maximum scour depths at the culvert entrance and developing equations for estimating inlet scour.(1) Phase II was a follow-up investigation to include scour measurements at the outlet, submerged entrance scour measurements, detailed velocity and depth measurements with a prescour fixed bed to refine the equations, and evaluation of various potential scour countermeasures to reduce scour at the culvert inlet and outlet.
One of the objectives of the Phase II study was to compare the MDSHA methodology for determining scour at bottomless culverts with physical modeling data from various culvert configurations. Data from both phases were included in the comparison. In Phase I, models of the typical configurations used for highway applications provided by two commercial suppliers of bottomless culverts were compared to simple rectangular models to gain insight about the effect of culvert shape.(1) In Phase II, simple rectangular shapes were used for the experiments.
Since abutment scour estimates at bridge openings are often quite large, a scour protection task was included to investigate possible scour countermeasures. Various inlet and outlet wingwall configurations were tested. Equations to determine the sizes of rock riprap (rough stones placed to prevent scour) that might be required to reduce scour in the most critical zones were developed. Cross vanes (upstream angled lines of boulders, connected by sections of smaller rocks) and pile flow dissipators (arrays of circular piles buried below the channel bed) were also investigated as scour countermeasures.
While presenting status reports to drainage engineers at American Association of State Highway and Transportation Officials (AASHTO) meetings and at hydraulic conferences, FHWA officials found widespread interest in this topic. The intent of this report is to share the results of this study with a larger audience.