Riprap protection at bridge piers

Although riprap is the most commonly employed countermeasure against scouri ng around bridge piers, few studies exist of riprap performance under live- bed conditions. In this study, failure mechanisms, stability, and placement level effects for riprap at bridge piers are considered experimentally. Un der clear-water conditions, riprap is subject to sheer, winnowing, and edge failure. Under live-bed conditions, a fourth failure mechanism, destabiliz ation by bed-form progression, becomes important. Destabilization by bed-fo rm progression is dependent on the destabilizing influence of bed-form trou ghs as they pass the pier. Experiments were used to assess the ability of r iprap stones to protect bridge piers under a wide range of flow conditions. The effects of placing the riprap layer at depth within the sediment bed, rather than level with the bed surface, were investigated also. The study s howed that, as the flow velocity increases, the ability of riprap stones to protect a pier decreases asymptotically until the scour depth in the ripra p layer reaches that of an equivalent unprotected pier. In addition, it was found that the deeper the placement level the less exposed the riprap was to destabilizing bed forms and the better the protection against local scou r. Lowering the placement level also meant that the riprap performed better than for surface-placed layers as the flow velocity increased. The mode of riprap failure is also changed as the placement level below the bed surfac e is lowered. A pier riprap size-prediction equation is proposed, including a parameter to account for placement level.