Enhanced Abutment Scour Studies for Compound Channels
CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS
The results of this research on bridge abutment scour in compound channels show that the discharge distribution factor is the appropriate variable to use rather than abutment length to measure the effects of flow contraction and flow redistribution in the contracted section on local scour depth. Sediment size is incorporated into the proposed scour-prediction technique in terms of the critical velocity required to initiate motion of the sediment. It is shown that the unconstricted flow depth in the floodplain should be used as the reference depth in the scour-prediction formula when bridge backwater is significant. The abutment shape is demonstrated to be important for shorter abutments, while no experimental differences in the abutment shape effects could be detected as the abutment increased in length and caused more contraction with encroachment on the main channel. Attempts to relate local abutment scour to local hydraulic variables near the abutment face were successful only for the shorter abutments; however, a relationship was developed between the approach hydraulic variables and the local contraction hydraulic variables. It is recommended that the proposed scour formula (equation 56) (which depends on the approach hydraulic variables predicted by WSPRO) be used. The suggested scour formula is for clear-water scour for both setback and bankline abutments. A second formula (equation 58) was proposed for bankline abutments that experience threshold live-bed scour conditions in the main channel; however, experiments in a compound section specifically designed for live-bed scour are needed. Testing of five current abutment scour formulas has shown that they all significantly overestimate or underestimate the scour data measured in this research. Field evaluation of the proposed method provided good agreement on one bridge, but an overestimate of scour on a second bridge. Armoring may have occurred in the latter case; however, inadequate data on the field sediment size distribution precluded a definite conclusion. It must be emphasized that the experimental results for bankline abutments that were used to develop the proposed scour formulas herein do not distinguish between contraction and abutment scour. Thus, the method of superposition of contraction and abutment scour for bankline abutments as though they were independent may be overly conservative.
It is concluded that the experimental results and methodology developed from this research can be used to estimate the depth of bridge abutment scour. It has been determined that for a compound channel over a much wider range of variables than was previously available that the effects of discharge distribution, sediment size, and time development on scour depth can be predicted from the relationships proposed herein. The comparisons of the WSPRO results, the experimental results, and the results from a two-dimensional numerical turbulence model have shown that the results from WSPRO are adequate for estimating the independent parameters needed for abutment scour prediction as long as bridge approach hydraulic conditions are used as predictor variables. It is recommended that the proposed procedure for abutment scour estimation be used alongside current FHWA procedures subject to the limitations on the ranges of dimensionless variables given in table 3, and that it be tested on additional field data as it becomes available. It is emphasized that sediment properties and their changes with depth must be known to adequately evaluate clear-water scour depths.
It is recommended that further research be conducted on abutment scour in order to evaluate and protect scour-critical bridges that are subject to possible failure. Suggested areas for research are:
- Laboratory study of scour countermeasures is needed in order to design the most efficient abutment scour-protection schemes. The study should consider: (a) the extent, size, and placement of riprap at abutments, and (b) the effectiveness of spur dikes.
- A three-dimensional numerical model with advanced turbulence closure schemes needs to be applied to the laboratory model used in this research for several selected cases of abutment scour. This effort should include three-dimensional velocity and turbulence measurements in the scour-hole area at different stages of scour-hole development with respect to time by temporarily fixing the bed in the scour-hole area.
- A well-planned, detailed field study of a bridge subject to abutment scour in cooperation with the USGS is needed. The bridge should be instrumented and scour determined over a 3-year period with detailed field measurements of velocity and bed elevation changes. A laboratory model of the instrumented field bridge should be constructed and tested to compare scour predictions based on the laboratory data with those actually measured in the field to finally resolve laboratory scale-up issues.
- Further experimental investigation of the live-bed scour case with the abutment at the edge of the main channel is warranted. This will require special design of the compound-channel geometry so that a careful selection of a combination of flow rates, sediment size, and discharge distribution will result in sediment transport in the main channel without scour exceeding the physical limits imposed by the finite depth of the sediment.
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