MPC
Research Projects (2008-09)

Identifying Number

MPC-307

Project Title

Maximum Velocity and Shear Stress in Flow Fields around Bridge-Abutments in Compound Channels

University

University of Wyoming

Project Investigator

Robert Ettema

Description of Project Abstract

Many bridge abutments, especially spill-through abutments, are built in compound channels, as depicted schematically in Figure 1. Though general information exists regarding the flow field around an abutment in single, rectangular channel (without a floodplain), and on flow in compound channels (unpunctuated by a bridge abutment), little is known about the flow field formed around an abutment in a compound channel. Of particular interest are the maximum values of boundary shear stress, depth-average velocity, and unit discharge around abutments. This information is of central importance for estimating scour depth at abutments, and thereby evaluating abutment stability against scour.

The problem of scour at bridge abutments has remained little understood, let alone adequately well addressed by scour-estimation relationships. A major weakness is the lack of insight into the flow field associated with scour; notably, information is substantially lacking regarding the maximum values of boundary shear stress, depth-average velocity, and unit discharge in the flow field. Some studies have illuminated several of the processes causing scour, notably scour attributable to flow contraction through a bridge waterway. These studies have described certain parametric trends associated with flow contraction, and have developed tentative design relationships for estimating contraction scour depth. However, few situations of flow, boundary erosion, and alluvial-sediment transport are more complex than those associated with scour of compound river channels at bridge abutments. Therefore, it is not surprising that crucial information on the scour flow field has remained practically unknown.

Project Objective

The overall goal for the project is to define comprehensively the flow field around common configurations of abutment in compound channels. In particular, the information would be used to determine maximum values and locations of boundary shear stress, depth-average velocity, and unit discharge. The information obtained would aid significantly in several ways – in the diagnosis of scour processes at the test abutments; in identifying how best to characterize the flow field at the test abutments; and, in the selection of representative velocities for use in predictive equations.

The overall goal comprises two individual goals:

1. Build a laboratory flume for use in obtaining data on laboratory-scale abutment flow fields; and,
2. Complete numerical simulations and laboratory tests on abutment flow fields.

Project Approach/Methods

Work presently is underway (with in-house funds) to achieve the goal by means of a commonly used numerical model (a 2D, depth-averaged flow). However, a suitably sized laboratory flume is needed to obtain the laboratory data needed to validate the data and insights obtained from the numerical model. Accordingly, the first year of work will be to build a flume. The second year of work will be conduct lab tests with the flume. The numerical simulations (funded from a separate in-house source, will continue during both years.

The numerical-modeling of abutment flow fields is being done using a commercial or public-domain code that is in wide use and readily available, so that the results from the task can be readily replicated, digested, and directly used by practicing engineers using such codes. Though several codes could be used for this task, the PIs will use the model FESWMS-2HD, developed and extensively used by the U.S. Geological Survey. FESWMS-2DH has been developed to analyze flow at bridge waterways where complicated hydraulic conditions exist, although it also may be applied to many types of steady or unsteady flow problems. Froehlich (1989) and Lee and Froehlich (1989) document the capabilities of FESWMS-2DH.

The following activities will be carried out for this task:

1. YEAR 1: Complete construction of the flume and hydraulic models of compound channel and spill-through abutments.
2. YEAR 1 AND 2: Continue use of FESWMS-2DH to simulate flow through bridge waterways in compound channels. The geometry of the channel and the abutment length will be varied over a range encompassing many fairly common bridge waterways.
3. YEAR 2: Run series of laboratory tests to obtain the measured data needed to validate the numerical data.
4. YEAR 2: Compare flow field simulations from the flume with those from FESWMS-2DH.
5. YEAR: 2Prepare plots, curves, or functional relationships indicating how maximum values of shear stress, depth-average velocity, and unit discharge vary with abutment and floodplain configuration

The flume to be used for the study will be built during June and July 2008 in a hydraulics laboratory at the University of Wyoming's College of Engineering and Applied Science. Half the funds exist for the flume. Funds for the remaining half are sought as part of the budget for this project. The flume essentially comprises a 75ft-long, 12ft-wide, and 4ft deep box to which a pump and flow-circulating pipe is fitted. Figure 2 shows the overall layout intended for the flume. Flow field measurements will be made during the ensuing months, leaving adequate time for a thorough final report to be written and submitted to the MPC program.

MPC Critical Issues Addressed by the Research

1. Infrastructure Longevity (better bridge abutments).
2. Improved Infrastructure Design (better and safer abutments).
3. Environmental Impacts of Infrastructure (abutment effects of flow and erosion).
4. Low-Cost Safety Improvements (effective placement of scour countermeasures).

Contributions/Potential Applications of Research

The research findings facilitated by the flume, aiding the numerical model results, will lead to improved estimation of scour depth at bridge abutments, and thereby to improved design of bridge waterways. Also it will help in the design and placement of effective scour countermeasure methods.

Technology Transfer Benefits

The insights and design information obtained from the project will be of interest to bridge designers, as well as to bridge inspection personnel, working for State Departments of Transportation or other entities.

Time Duration

July 1, 2008 – June 30, 2009

Total Project Cost

$50,000

MPC Funds Requested

$25,000

TRB Keywords

Transportation systems, bridges, scour, infrastructure

References

Froehlich, D.C., (1989). "HW031.D - Finite Element Surface-Water Modeling System: Two-Dimensional Flow in a Horizontal Plane--Users Manual." Federal Highway Administration Report FHWA-RD-88-177, Reston, VA.

Lee, J.K., and Froehlich, D.C., (1989). "HW031.X." Research report: Federal Highway Administration Report, Reston, VA.