Countermeasures to Protect Bridge Abutments from Scour (2007) / Chapter Skim
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191 10.1 Introduction This chapter presents design guidelines for scour countermeasures for use at bridge abutments. The guidelines use the findings of the laboratory experiments discussed in Chapters 7 through 9.
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192 Abutment Scour Concern Countermeasure Concept Construction Option General bed degradation Use a bed-control structure 1. Place weir across channel to maintain bed level at bridge waterway 2.
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10.3.1 Low Weirs In recent years, considerable effort has been devoted to developing low weirs that do not block fish and aquatic creatures from moving along channels. The structures typically have replicated the form and flow features of rock riffles, like small-scale rapids.
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The following construction and constructability issues should be considered for sheet pile weirs: • A sheet pile weir will need scour protection along its downstream side because of flow passage over the weir. • The channel bank adjoining the sheet pile weir will need concrete or riprap scour protection.
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• The sheet-piling should extend around the front and sides to the end of the bridge. • The sheet-piling should be a short distance out from the toe of the face slope of the abutment.
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• Vanes are an alternative to spur dikes, bendway weirs, or barbs for use in improving approach channel alignment, as shown in Figure 10-8. These flow-control structures are used in rather specific applications that often have to be tailored to fit local conditions of channel alignment and morphology, as well as bridge extent and alignment.
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• If the spur points upstream, the scour hole is shifted away from the bank, and flow accordingly is deflected away from the bank. A deflecting spur is illustrated in Figure 10-9c.
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2. Hardpoints.
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y depth of flow in the contracted bridge section, Fr Froude number in the contracted bridge section, Ss specific gravity of the riprap material, g gravitational acceleration, and Ks shape factor. Riprap size selection is appropriately based on stability against shear and edge failure, although consideration of the possibility of winnowing or bed-form undermining is also important in design.
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(10-4) Where: cb block density, fluid density, and U approach-flow velocity.
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The minimum required block height, H, can be calculated as follows: (10-5) Where: p volume fraction pore space within the mattress.
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riprap are available (e.g., U.S. Army Corps of Engineers, 1989; Richardson and Davis, 1995; Austroads, 1994)
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defined as the setback distance divided by the average channel flow depth. For SBRs less than 5, U is evaluated as the total discharge divided by the total flow area of the contracted section.
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Cable-Tied Blocks The design parameters for cable-tied blocks as a scour countermeasure at spill-through abutments are block size, block shape, filters, layer extent, edge protection, and scour hole geometry. Cable-tied block aprons are subject to two observed flowinduced failure modes, as described by Parker et al.
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(10-19) Design steps.
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Dike Length The top length of the dike (perpendicular to the flow) should be equal to the abutment length, La (perpendicular to the flow)
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Dike Width Dike width is determined by the dike face angle, which should be less than the angle of repose of the rock used to construct the dike. 10.7 Relation to Existing HEC Guidelines The set of HEC guidelines that currently address bridge scour and stream instability countermeasures is HEC-23 (Lagasse et al., 1997)
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