This report evaluates and assesses existing guidelines for the design, installation, monitoring, and maintenance of environmentally sensitive stream bank stabilization and protection measures, and develops quantitative engineering design guidance for selected treatments. Updated design guidelines for three widely used treatments are presented: live siltation and live staking with a rock toe, vegetated mechanically stabilized earth, and vegetated rip rap. For the engineer involved in the multidisciplinary design of an environmentally sensitive treatment, this report also includes current guidance from the Federal Highway Administration on the use of biotechnical treatments in proximity to transportation infrastructure. In addition, for the Professional Engineer (PE) on a design team, the report explores aspects of professional liability in environmentally sensitive design.

The majority of the bridges in the U.S. National Bridge Inventory (NBI) are built over waterways. Many will experience problems with scour, bank erosion, and channel instability during their design life. A number of studies in the recent past have attempted to quantify the uncertainty in predicting these erosional processes, particularly in pier scour, and developing probabilistic estimates of scour as a means of incorporating uncertainty. However, none of these studies have examined the overall uncertainty in local pier or abutment scour in combination with contraction scour. This study quantifies the model uncertainty in commonly used scour equations as well as parameter uncertainty. The overall reliability of the scour equations is then assessed for the individual components of scour and combined scour. The results lead to a set of scour design factors that are based on the reliability of the design estimate. (C) 2015 American Society of Civil Engineers.

NCHRP Project 24-34 was completed in September 2013 with the publication of NCHRP Report 761, Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction. The project accomplished its objectives of developing risk/reliability-based methodologies that can be used in calculating bridge pier, abutment, contraction, and total scour at waterway crossings so that scour estimates can be linked to a probability. The developed probabilistic procedures are consistent with LRFD approaches for bridge design used by structural and geotechnical engineers. As a necessary first step in developing the statistical parameters for risk and reliability analyses, the key bridge scour equations (HEC-18 pier scour, Florida DOT pier scour, HEC-18 contraction scour, and NCHRP 24-20 abutment scour) were tested against available laboratory and field data sets. The results which are summarized in this presentation include: · The pier scour equations (HEC-18 and FDOT) are design equations which do not under predict observed scour very often. Consequently, the probabilistic reliability indexes for pier scour compare favorably with those used by structural and geotechnical engineers in LRFD applications for bridges. · In contrast with the pier scour equations, the HEC-18 contraction scour equations are essentially predictive, given that they are derived from sediment transport principles and theory. Therefore, under predictions of observed scour are much more common, and the resulting reliability is very low compared to typical target values used in LRFD applications. · The NCHRP 24-20 equations for live-bed and clear-water abutment scour both use a calculation for contraction scour and then apply an amplification factor to account for the additional scour caused by local effects at the tip of the abutment. The scour predicted by this method is the total scour at the abutment. The reliability of the abutment scour equations was found to be intermediate between those of the pier scour and contraction scour equations. This presentation will be followed by Part II which will provide an example application of the basic risk-based methodology in a typical design situation.

Over the past 40 years, numerous studies have been conducted and equations developed to predict bridge-pier scour. Although it is desired to have a conservative approach to scour to increase our sense of safety and reliability, field experience often leads bridge and hydraulics engineers to suspect that the equations over-predict scour at a significant level. To date, few studies have been undertaken to quantify the reliability of the scour equations in terms of probability of exceedance. In this study, we used available laboratory data to determine the probability that the design scour will be exceeded using two pier scour equations, both recommended by the Federal Highway Administration: (1) HEC-18 equation and (2) FDOT equation.

This document is the fifth edition of HEC-18. It presents the state of knowledge and practice for the design, evaluation and inspection of bridges for scour. There are two companion documents, HEC-20 entitled Stability at Highway Structures, and HEC-23 entitled Bridge and Stream Instability Countermeasures. These three documents contain updated material from previous editions and continued research by NCHRP, FHWA, State DOTs, and universities. This fifth edition of HEC-18 also contains revisions obtained from further scour-related developments and the use of the 2001 edition by the highway community. The major changes in the fifth edition of HEC-18 are: expanded discussion on the policy and regulatory basis for the FHWA Program, including risk-based approaches for evaluations, developing Plans of Action (POAs) for scour critical bridges, and expanded discussion on countermeasure design philosophy (new vs. existing bridges). This fifth edition includes: a new section on contraction scour in cohesive materials, an updated abutment scour section, alternative abutment design approaches, alternative procedures for estimating pier scour, and new guidance on pier scour with debris loading. There is a new chapter on soils, rock and geotechnical considerations related to scour. Additional changes include: a new approach for pier scour in coarse material, new sections on pier scour in cohesive materials and pier scour in erodible rock, revised guidance for vertical contraction scour (pressure flow) conditions, guidance for predicting scour at bottomless culverts, deletion of the General Scour term, and revised discussion on scour at tidal bridges to reflect material now covered in HEC-25 (2nd Edition).

Load and resistance factor design (LRFD) incorporates state-of-the-art analysis and design methodologies with load and resistance factors that are based on the known variability of applied loads and material properties. These load and resistance factors are calibrated from actual bridge statistics to ensure a uniform level of reliability. LRFD allows a bridge designer to focus on a design objective or limit state; doing so can lead to a similar probability of failure in each component of the bridge. Bridges designed with the LRFD specifications should have relatively uniform reliability levels; such uniformity should ensure superior serviceability and long-term maintainability. Bridge hydraulics engineers should have the option of and ability to perform scour calculations that incorporate similar probabilistic methods. With this objective in mind, NCHRP Project 24–34 was initiated in April 2010 to develop a risk–reliability-based methodology that was based on risk and reliability and that could be used in calculating bridge pier, abutment, and contraction scour at waterway crossings so that scour estimates could be linked to a probability. The developed probabilistic procedures will be consistent with LRFD approaches used by structural and geotechnical engineers. This paper discusses sources of uncertainty in hydrologic estimates as those sources relate to bridge scour computations and summarizes a conceptual approach to the problem.

Waterborne debris (or drift) often accumulates on bridges during flood events. The effects can vary from minor flow constrictions to severe flow contraction resulting in significant bridge foundation scour. The results of National Cooperative Highway Research Program (NCHRP) Project 24-26, "Impacts of Debris on Bridge Pier Scour" represent a significant advance to predicting debris scour considering the variable geometry of debris clusters observed at bridge piers in the field. The study produced results on two related problems: predicting the accumulation characteristics of debris from widely varying source areas, and developing improved methods for quantifying the depth of scour at bridge piers. This paper highlights the observations from laboratory testing and the development of improved algorithms for predicting the depth of scour at debris-laden bridge piers.

This document contains Appendices A through C from the contractor's final report on NCHRP Project 24-26, which is published as NCHRP Report 653, Effects of Debris on Bridge Pier Scour. The appendices are (A) Debris Photographic Archive; (B) Survey of Practitioners, Instructions for Viewing NCHRP 24-26 Questionnaire Responses Databases, Survey Questionnaire, and List of Respondents; and (C) Field Pilot Study Report.

In Europe, partially grouted riprap (as contrasted with fully grouted riprap as historically used in the United States) has been successfully used for several decades to prevent erosion of the beds, banks, and shorelines in riverine and coastal environments such as rivers, canals, lakes, and harbors. This type of armoring treatment is also used to protect against local scour at structures such as bridge piers and abutments. Partial grouting in Europe is often performed under water, or in flowing water. Due to the concern for temporary water quality impacts during placement, this aspect may be a potential impediment to the acceptance and implementation of this technology in the US. Many sites cannot be dewatered during construction or rehabilitation activities; therefore, the ability to place grout under water or in "live stream" conditions while maintaining water quality parameters within acceptable limits is of paramount importance. Various degrees of grouting are possible, but optimal performance of the partial grouting technique is achieved when the grout is effective at "gluing" individual stones to neighboring stones at their contact points, leaving relatively large voids between the stones. This makes the partial grouting technique appealing for a number of reasons: 1) Because it is permeable, partially grouted riprap provides a more suitable alternative to total grouting because it alleviates the buildup of pore pressure beneath the system; 2) It remains flexible by allowing groups of stones to remain connected as an equivalent "conglomerate" particle; 3) Partial grouting allows the use of smaller rock compared to loose riprap; and 4) A thinner overall riprap blanket is achieved using the partial grouting technique. This paper presents selected aspects of NCHRP Research Project 24-07(2), "Countermeasures to Protect Bridge Piers from Scour" as related to partially grouted riprap. That project resulted in the publication of NCHRP Report 593 of the same name. Methods for placing partially grouted riprap in flowing water at prototype scale are presented, and include detailed results of water quality monitoring conducted under these conditions. The installation was representative of protection against local scour at a bridge pier in a riverine environment. Further testing of the installation demonstrated its inherent stability in the high velocity, highly turbulent flow field at the pier.

Partially grouted riprap consists of specifically sized rocks that are placed around a pier and "glued" together with grout. In contrast to fully grouted riprap, partial grouting increases the overall stability of the riprap installation unit without sacrificing flexibility or permeability. It also allows for the use of smaller rock compared to standard riprap, resulting in decreased layer thickness. The system typically includes a filter layer, either a geotextile fabric or a filter of sand and/or gravel, specifically selected for compatibility with the subsoil. Tests conducted under National Cooperative Highway Research Program (NCHRP) Project 24-07(2) confirmed the applicability of partially grouted riprap as a scour countermeasure for bridge piers. After placing the rock, the voids of the riprap matrix are then partially filled with a Portland cement based grout by hose or tremie, often under water. The final configuration results in an armor layer that retains approximately 1/2 to 2/3 of the void space of the original riprap. Hydraulic stability of the armor is increased significantly over that of loose riprap by virtue of the much larger mass and high degree of interlocking of the "conglomerate" particles created by the grouting process. NCHRP Project 24-07(2) "Countermeasures to Protect Bridge Piers from Scour" included investigation of partially grouted riprap installations in Germany and laboratory investigations at Colorado State University at prototype scale as a basis for developing guidelines applicable to U.S. practice for this technology. This paper summarizes Design Guidelines for partially grouted riprap as a bridge pier scour countermeasure.

Many methods and criteria are available for designing riprap for erosion protection of riverbanks, bridge piers and abutments, and other highway structures in riverine environments. Different design criteria for riprap can give significantly different results for protecting the same installation. In addition, the design procedure may be confusing to apply and can result in unsuitable gradations and ambiguous specifications. In the U.S. many state highway departments have developed their own specifications based on trial, error, and field experience. To provide adequate protection, riprap must be properly designed and specified. Equally important, the rock material must be produced and installed to satisfy the specifications requirements and the design intent. This paper provides an overview of the results of National Cooperative Highway Research Program (NCHRP) Project 24-23 which was a synthesis project to develop a comprehensive approach to riprap design, installation, and maintenance at bridges. This project provides design guidelines, material specifications and test methods, construction specifications, and construction inspection and quality control guidelines for riprap for bankline revetment, bridge pier and abutment protection, and for river training countermeasures such as guide banks and spurs. The study included an evaluation of riprap failure modes and development of inspection guidelines for consideration by the Federal Highway Administration (FHWA).

Waterborne debris (or drift), composed primarily of tree trunks and limbs, often accumulates on bridges during flood events. Debris accumulations can obstruct, constrict, or redirect flow through bridge openings resulting in flooding, damaging loads, or increased scour at bridge foundations. The size and shape of debris accumulations vary widely, ranging from a small cluster of debris on a bridge pier to a near complete blockage of a bridge waterway opening. The effects of debris accumulation can vary from minor flow constrictions to severe flow contraction resulting in significant bridge foundation scour. Currently, there is only limited guidance available on which to base critical public safety decisions during flooding on debris-prone rivers. There is a need for accurate methods of quantifying the effects of debris on scour at bridge-pier foundations for use in the design, operation, and maintenance of highway bridges. This paper provides a preview of the results expected from National Cooperative Highway Research Program (NCHRP) Project 24-26. The project was started in July 2004 and is scheduled for completion in December 2007. It is expected that this research project will produce results on two related problems: (1) predicting the accumulation characteristics of debris from potentially widely varying source areas, in rivers with different geomorphic characteristics, and on bridges with a variety of substructure geometries, and (2) developing improved methods for quantifying the depth and extent of scour at bridge piers considering both the accumulation variables and the range of hydraulic and geomorphic factors involved.

This report documents research on local scour at bridge piers resulting in the development and recommendation of a practical selection criteria for bridge-pier scour countermeasures, guidelines and specifications for design and construction of those countermeasures, and guidelines for their inspection, maintenance, and performance evaluation. Because of their critical role in ensuring bridge integrity and potentially high cost of these countermeasures, it is important that the most appropriate countermeasures be selected, designed, and constructed. The contents of this report are, therefore, of immediate interest to highway professionals responsible for planning, administrating, evaluating, designing, constructing, inspecting and maintaining bridges and other structures founded in erosive areas. The report is also of interest to those charged with specifying materials testing procedures and acceptable results, setting budget goals, and making policy.

Local scour at bridge piers is a potential safety hazard to the traveling public and is a major concern for transportation agencies. Bridge pier scour is a dynamic process that varies with water depth, flow angle, pier geometry and other factors. If it is determined that scour at a bridge pier can adversely affect the stability of a bridge, countermeasures to protect the pier should be considered. This paper provides an overview of the results from National Cooperative Highway Research Program (NCHRP) Project 24-07(2). The objectives of the research are to develop and recommend: (a) practical selection criteria for pier scour countermeasures, (b) guidelines and specifications for design and construction, (c) guidelines for inspection, maintenance, and performance evaluation. The countermeasures that were considered included riprap, articulating concrete blocks, gabion mattresses, rigid and flexible grout filled mattresses and partially grouted riprap. Extensive testing was performed for each of the countermeasure types in the 2.4 m flume at the Colorado State University (CSU) — Engineering Research Center hydraulics laboratory to provide guidance on countermeasure thickness, extent, filter requirements, edge treatment, toe down, flexibility, and interconnectivity. A summary of the current state of practice was combined with the results of laboratory testing to provide detailed guidelines on (1) Design and Specification, (2) Construction, and (3) Inspection, Maintenance, and Performance Evaluation for each of the five pier scour countermeasures.

This report presents the findings of a study to develop design guidelines, material specifications and test methods, construction specifications, and construction, inspection, and quality control guidelines for riprap at streams and riverbanks, bridge piers and abutments, and bridge scour countermeasures. Recommendations are provided on a design equation or design approach for each application. Filter requirements, material and testing specifications, construction and installation guidelines, and inspection and quality control procedures are also recommended for each riprap application. To guide the practitioner in developing appropriate designs for riprap armoring systems for these applications, the findings and recommendations are combined to provide design guideline appendixes for (1) Design and Specification of Rock Riprap Installations and (2) Construction, Inspection, and Maintenance of Rock Riprap Installations. This report will be particularly useful to bridge, hydraulic, and highway engineers, as well as bridge maintenance and inspection personnel responsible for design, construction, inspection, and maintenance of bridges and other highway structures.