FLOOD04125 - Laserfiche WebLink

Home Browse Search

(b) Bridges that serve as vital links in the transportation network and whose failures could adversely effect area or regional traffic'operations. The ultimate objectives of 'this scour evaluation program are (1) to review all bridges over scourable streams in' the National Bridge Inventory; (2) to determine those foundations which are stable for estimated scour conditions and those which are not; (3) to provide for frequent inspections of scour critical bridges during and after flood events until adequate scour countermeasures are installed; and (4) to install scour. countermeasures in a timely manner. C. CONDUCTING SCOUR EVALUATION STUDIES An overall plan should be developed for conducting engineering bridge scour evaluation studies. An example of this type of a plan prepared by the Minnesota DOT is provided in Appendix B. It is recommended that each State develop its own plan for making engineering scour evaluations based on its own particular needs. The FHWA offers the following recommendations in regard to conducting these studies: 1. The first step of the scour evaluation study should be an office review of available information for purposes of assessing the stability of the strea. and the adequacy of the bridge foundations to withstand a superflood (a Q500 flood or a flow 1.7 times Q100 flood as recommended by the FHWA.) The Minnesota'worksheets include lists of items to consider in collecting information. 2. The use of worksheets along the lines of those prepared by the Minnesota DOT is encouraged, since they serve to provide a consistent frame of reference for making field and office reviews and for documenting the results of the investigations. 3. In order to develop an efficient process for properly evaluating a large number of bridges, a logical sequence needs to be established for conducting the evaluations. This sequence should serve to screen out those bridges where scour is clearly not a problem. For example, sufficient information may be available in the office to indicate that the bridge foundations have been set well below maximum expected scour, and that a field inspection is not necessary for determining that the bridge is not at risk from scour damage. However, a field inspection is generally recommended for bridges over, streams that have one or more of the characteristics listed under Step I of the evaluation process, Section B. Where adequate hydraulic studies have been prepared and kept for the original bridge design, the scour estimates can be checked or recalculated from this information. Where hydraulic data is not available, it may have to be recalculated. For such cases, a .worst case analysis. is suggested. If the bridge foundations are adequate for worst case conditions, the bridge can be judged satisfactory. Where the worst case analysis indicated that a scour problem may exist, further field and office analyses should be made. 48 I I (' I I I I I I I I I I I I I I I I I