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<br />Transverse river training structures are countermeasures which project into the <br />flow field at an angle or perpendicular to the direction of flow. <br /> <br />Longitudinal river training structures are countermeasures which are oriented <br />parallel to the flow field or along a bankline. <br /> <br />Areal river training structures are countermeasures which cannot be described as <br />transverse or longitudinal when acting as a system. This group also includes <br />countermeasure "treatments" which have areal characteristics such as <br />channelization, flow relief, and sediment detention. <br /> <br />3.2.1.2 Group 1.8 Armoring Countermeasures <br /> <br />I <br />I <br />I <br />1 <br />, <br /> <br />Armoring countermeasures are distinctive because they resist the erosive forces caused <br />by a hydraulic condition. Armoring countermeasures do not necessarily alter the hydraulics <br />of a reach, but act as a resistant layer to hydraulic shear stresses providing protection to the <br />more erodible materials underneath. Armoring countermeasures generally do not vary by <br />function, but vary more in material type. Armoring countermeasures are classified by two <br />functional groups: revetments and bed armoring or local armoring. <br /> <br />Revetments and bed armoring are used to protect the channel bank and/or bed <br />from erosive/hydraulic forces. They are usually applied in a blanket type fashion for <br />areal coverage. Revetments and bed armoring can be classified as either rigid or <br />flexible/articulating. Rigid revetments and bed armoring are typically impermeable <br />and do not have the ability to conform to changes in the supporting surface. These <br />countermeasures often fail due to undermining. Flexible/articulating revetments <br />and bed armoring can conform to changes in the supporting surface and adjust to <br />settlement. These countermeasures often fail by removal and displacement of the <br />armor material. <br /> <br />j <br /> <br />Local scour armoring is used specifically to protect individual substructure elements <br />of a bridge from local scour. Generally, the same material used for revetments and <br />bed armoring is used for local armoring, but these countermeasures are designed <br />and placed to resist local vortices created by obstructions to the flow. <br /> <br />3.2.2 Group 2. Structural Countermeasures <br /> <br />Structural countermeasures involve modification of the bridge structure (foundation) to <br />prevent failure from scour. Typically, the substructure is modified to increase bridge stability <br />after scour has occurred or when a bridge is assessed as scour critical. These <br />modifications are classified as either foundation strengthening or pier geometry <br />modifications. <br /> <br />Foundation strengthening includes additions to the original structure which will <br />reinforce and/or extend the foundations of the bridge. These countermeasures are <br />designed to prevent failure when the channel bed is lowered to an expected scour <br />elevation, or to restore structural integrity after scour has occurred. Design and <br />construction of bridges with continuous spans provide redundancy against <br />catastrophic failure due to substructure displacement as a result of scour. <br />Retrofitting a simple span bridge with continuous spans could also serve as a <br /> <br />4 <br /> <br />1 <br />I <br />, <br />