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101 CHAPTER 4: CONCLUSIONS AND SUGGESTED RESEARCH The main purpose of this research was to collect field data from which processes affecting scour magnitude in contracted openings could be identified, to support verification of physical- and numerical-model studies, and to improve guidelines for applying scour-prediction methods at contracted bridge sites. Field data collected from 15 sites in this study were added to the National Bridge Scour Database (BSDMS). These data, available through the World Wide Web, can currently (2004) be accessed by researchers and practitioners from a link on the USGS Kentucky District world wide web page (http://ky.water.usgs.gov) and eventually (2005) from a link on the USGS Office of Surface Water world wide web page (http://water.usgs.gov/osw/techniques/bs/sed.bs.html). CONCLUSIONS Conclusions about current HEC-18 scour-prediction methods, the use of hydraulic and sediment-transport models, and scour at piers with debris were developed through analysis of the field data. The most important finding is that the main sources of error in the abutment scour- prediction methods presented in HEC-18 (2001) are the scour prediction equations and not the hydraulic parameters typically obtained from one-dimensional models. Specific conclusions from this study are provided in the following sections.
102 Factors Not Included In Laboratory Models (1) To date (2004), laboratory research has failed to capture the complexity of typical field conditions, rendering the resulting equations unreliable for field applications. The concepts and methods for evaluating scour derived from these studies may not accurately account for scour processes in the field, because of the simplifications inherent in these studies. The inability of current (2004) scour-prediction methods to accurately predict scour at abutments is a result of the simplifying assumptions on which the research is based and the complexity of abutment scour in field conditions. (2) Channel alignment and, in particular, channel bends upstream of bridges can have an appreciable effect on the depth and distribution of scour, including the location of maximum scour. Scour Components (3) When compared to field data, contraction- and abutment- scour equations predict scour depths greater than those observed and often this error can be 2 to 40 times the measured scour depth; however, some comparisons indicate that there are conditions under which some equations will predict scour depths less than those observed. These comparisons indicate that the current (2004) methods for predicting contraction and abutment scour at bridges are unreliable. (4) Field observations of scour at many bridges indicate that conceptual separation of contraction and abutment scour as described in HEC-18 (Richardson and Davis,
103 2001) is problematic because the hydrodynamic mechanisms that induce the individual scour components work together. Consideration should be given to an alternative provided by Benedict (2003) to the superposition procedure (addition of independently calculated contraction- and abutment-scour components) recommended in HEC-18. Benedict (2003) recommends that regions of high-flow curvature (abutment scour) and low-flow curvature (contraction scour) be separated and computation of scour made independently but not added. Development of contraction and (or) abutment scour is highly dependent upon the site and approach flow conditions. (5) The presence of a contracted bridge opening does not guarantee that either or both types of scour will occur. Bed-material size and gradation, cohesion, armoring potential, and road overflow are common factors that can limit or prevent contraction scour. In addition, highly site-specific factors such as channel alignment, large abutment-scour holes, obstruction in the bridge opening, embankment skew, wide floodplains in the bridge opening, and bed protection also can limit contraction scour. Contraction Scour (6) Clear-water contraction scour prediction is highly sensitive to the critical conditions of the bed material; therefore, accurate representation of the bed material is essential. Bed-material samples should represent both the surface and subsurface material.
104 (7) The clear-water scour equations grossly over-predicted field observations by 2 to 40 times the measured scour depth. Soil cohesion at most sites where clear-water scour was observed and the lack of a method to account for the increased soil resistance in currently (2004) accepted HEC-18 scour methods is considered to be the reason for the large range of over-prediction. (8) The longitudinal location of contraction scour is highly variable. The longitudinal location of contraction scour can be dependent upon factors such as the configuration of scour protection, guide banks, bridge length, channel alignment and bed material, and does not follow the patterns typically reported from laboratory experiments. (9) Judgment should be used in selecting the location of the approach cross-section used in the analysis of contraction scour. Site-specific characteristics such as flow structures, geomorphic setting, floodplain topography and land cover, and upstream channel configuration should be considered in the location of this analysis cross- section. Abutment Scour (10) Although the length of the bridge and channel geometry are important factors that have been examined extensively in previous research and in the development of existing abutment-scour equations, there are a multitude of other parameters that must be considered when evaluating and (or) predicting scour in the abutment region including the following: cohesion of soils; geometric contraction ratio; approach flow velocity distribution including the effects of channel bends; floodplain
105 roughness and topographic variation; floodplain flow obstructions; valley geometry, including valley width variation and slope; roadway crossing geometry â roadway profile, embankment geometry and orientation, and bridge length; embankment protection; and duration and frequency of flood flows. (11) Inspection of the data compiled for the NCHRP Project 24-14 sites revealed that approach and contracted flow velocity, geomorphic setting (i.e. upstream channel alignment and valley configuration), bed material cohesion and size, and geometric- contraction ratio are the factors that have the most affect on the measured abutment scour. (12) Comparison of measured abutment scour depths and computed abutment scour depths by several of the methods (Sturm, Froehlich, modified Froehlich, and HIRE equations) provided in HEC-18 (2001) indicates that all methods can appreciably overpredict scour when used in combination with one-dimensional models for the selected hydraulic parameters. The comparisons of predicted versus observed scour in this report show that although geometry and the one-dimensional modeling approach can cause variations in the depth of predicted scour, the accuracy of the selected scour equations are currently (2004) the largest source of error in abutment scour predictions.
106 Numerical Models (13) The flow conditions and resulting scour patterns at contracted bridges are often too complex to be represented accurately by one-dimensional hydraulics and current (2004) scour equations. (14) Current (2004) guidelines in HEC-18 indicate that when using one-dimensional models and the HIRE equation, it is acceptable to use the conveyance tube closest to the abutment for determining the velocity and depth at the abutment. Comparison of field measurements with one-dimensional modeled flow shows that the velocity in the conveyance tube closest to the abutment is always low compared to the rest of the flow field around the abutment. (15) The use of the field-measured velocity as defined by HEC-18 for the HIRE equation, results in gross over-prediction of abutment-scour depths. A more representative field velocity measurement for use in the HIRE equation should be taken upstream of the abutment tip near the approach section. (16) Although the current (2004) amount of field data in the approach sections of the surveyed bridges were inadequate to provide a comprehensive evaluation of the ability of a one-dimensional model to represent complex two-dimensional flow fields, the comparisons that could be made showed the limitations of the one- dimensional modeling approach. Where conveyance dominates the hydrodynamics, such as for fully developed scour-hole conditions, a one-dimensional model is able to provide a reasonable estimate of the velocity distribution; however, where two and three-dimensional effects caused by flow accelerations dominate the flow field, such
107 as at the beginning of a flood and during the scouring process, the one-dimensional model is severely limited in its ability to accurately distribute the flow. (17) Default HEC-RAS hydraulic parameters used for abutment-scour calculations can provide erroneous predictions based on incorrect projection of the bridge opening to the approach section. (18) Despite the calibration complexities induced by geometry uncertainty and three- dimensional flow, the two-dimensional model was able to reproduce the hydraulics in the bridge opening for the conditions measured more accurately than the one- dimensional model for the early flood condition; however, for fully developed scour- hole conditions, the one-dimensional model provided a slightly better representation of the velocity distribution than did the two-dimensional model. (19) Using HEC-18 equations, scour computed using the hydraulics generated from a two-dimensional hydraulic model was less accurate (more conservative) than scour computed using a one-dimensional model. (20) Scour computed from a two-dimensional sediment transport model was found to be less accurate (more conservative) than scour computed using HEC-18 methods and hydraulics generated by a one-dimensional model. Scour at Pier with Debris (21) The method for predicting scour with debris proposed by Melville and Dongol (1992) accurately predicted scour over several events and the method proposed by Deihl (1997) for estimating the width of debris rafts on piers accurately predicted the
108 observed width of debris accumulations on the observed pier; however this was based on data from only one site. RECOMMENDATIONS Although additional real-time data collection would be beneficial, the collection of large volumes of field data sufficient for developing regression or semi-empirical equations for abutment and contraction scour would be difficult and expensive. Therefore, the development of techniques for estimating scour at contracted bridges must be based on a combination of detailed field data sets and the study of these data using laboratory or multi-dimensional numerical models. A major limitation to physical models is the scale effect on the depth of scour, especially in the case of modeling cohesive soils. Numerical models should not have this same limitation, but are limited by the available sediment-transport algorithms. Regardless of the modeling approach selected, the model must be calibrated to field conditions, considering all of the complexities of channel alignment and the geometric and hydraulic configuration of the main channel, floodplain, and bridge crossing. Once these complexities are properly modeled, changes then can be made to the alignment and geometric and hydraulic conditions to study the corresponding effects on the depth of scour. It is unlikely that a simple computational equation could be developed, except for gross-envelope curves; however, it is likely that a procedure could be developed based on a series of computations or multidimensional models that would provide a more accurate estimate of scour than is currently (2004) available. Post-flood field data collection and analysis similar to the approach used in South Carolina by Benedict (2003) also would be valuable. As shown by the research for NCHRP 24-
109 14, this approach is only appropriate for regional applications; however, a large data set with a broader range of conditions may provide a more detailed assessment of the limits of scour given a variety of different conditions. The result could be a simple method or family of curves that would provide the maximum observed scour for various site conditions. While this approach does not directly account for the site complexities and is only empirical in nature, where applicable, it would provide a conservative and low-cost approach for estimating scour at contracted bridges. Additional research is needed to obtain information on flow patterns near bridges to improve numerical representation of flow conditions and to verify numerical models. Because of hazardous conditions that include debris in forested floodplains, submerged obstacles, and partially submerged superstructures, collection of approach flow velocity data from a manned boat requires very specific conditions that may not be representative of the wide spectrum of conditions where information is needed. Specific recommendations developed from this research include the following: (1) Additional field observations are necessary to develop a definitive classification system based on site characteristics that could indicate the expected scour pattern. The classification system could be the basis for methods to determine the position of scour holes, especially in cases where the scour is likely to occur sufficiently far from the bridge such that it does not affect foundation stability.
110 (2) No detailed real-time measurements were collected during conditions where the bridge superstructure is partially or completely submersed, because of the difficulty and hazards in measuring scour these conditions. Additional research is needed to evaluate this common condition. (3) Contraction scour in gravel-bed streams where sediment sorting occurs may affect the transport to the bridge opening. Limited field data is available for this common condition. (4) Only four sites had sufficient data to allow computation of contraction scour based on field data that includes measured upstream flow velocities. Additional research in which approach distribution of flow velocity upstream and within the bridge opening is measured is needed to improve understanding of flow contractions. (5) Detailed data was collected at only three sites in this study because of the difficulties and hazards of collecting data in floodplains upstream of bridges during flood events. If scour equations are to be based on hydraulic models, and meaningful improvements in flow modeling are a key component to accurate prediction of scour, then methods for collecting flow-velocity data in these difficult conditions are necessary. Additional approach flow data and research on methods for collecting flow velocity during flood events is needed. (6) This study indicates that distribution of the flow by conveyance used in one- dimensional models may lead to overestimating the depth of contraction scour. Methods for adjusting one-dimensional models to overcome this problem would increase the accuracy of computed scour depths.
111 (7) Additional research should be conducted on the geometry of debris accumulations to extend and support the methods proposed by Diehl and Bryan (1997). Additional field data are necessary to verify the method by Melville and Dongol (1992) for predicting scour depth around debris accumulations. MODIFICATION TO STRATEGIC RESEARCH PLAN This research project was one of 37 projects recommended in the strategic research plan developed under NCHRP Project 24-8 (Parola et al, 1996) to improve scour-prediction methods. The findings of NCHRP 24-14 emphasize the need for information on scour related to soil cohesion, vegetation, channel alignment, addition of abutment and contraction scour, the location of scour holes, and debris effects. Studies similar to those recommended in the strategic plan that will address these issues are underway and include the following: NCHRP Project 24-15: Abutment Scour in Cohesive Soils NCHRP Project 24-20: Prediction of Scour at Abutments NCHRP Project 24-24: Criteria for Selecting Numeric Hydraulic Modeling Software NCHRP Project 24-26: Effects of Debris on Bridge Scour Research on the effects of vegetation on scour, although recommended, was not given a high priority in the strategic plan for scour research. For low gradient systems, the combined effect of vegetation and fine-grained soil behavior may prevent the initiation of or limit scour at a
112 large number of bridges. The research on the effect of vegetation should be given a higher priority. Other studies that should be considered are (see strategic research plan for details) Total scour at bridge contractions Post-flood evaluation of bridges (similar to Benedict, 2003) Enhancement of one-dimensional modeling Enhancement of two-dimensional modeling and sediment transport. Bridges with superstructures partially or completely submersed are a common occurrence during design flood events. A project that should be added to the strategic research plan is one that includes both field and laboratory research on scour at partially and completely submersed bridges. Little is known about this common design condition.
