Q. Describe types of Spurs?

Spurs can be of varied types. They may be either permeable or impermeable. Permeable spurs are open structures constructed by driving wooden balties filled in with brush wood and weighed down by stone. When concentration of suspended sediment load is heavy, permeable spurs cause quick siltation due to damping of velocities. Such spurs are thus helpful in protecting the bank by forming sediment berms along the toe. Impermeable spurs are made of solid core with exposed faces protected by pitching. Such spurs can withstand severe river attack better than permeable spurs. Deposition occurs on the downstream side of the spur due to material scoured from the nose and the high velocity current gets deflected away. Solid spurs are accordingly provided where attack of flood flow is to be diverted. According to height, the spurs are classified as full height spurs when their top level is designed to remain above the highest flood level. On the other band, when top level is lower than the high flood level, spurs get occasionally submerged. Such spurs are called part height spurs. Short submerged spurs with height not exceeding 3 m or so when suitably aligned with respect to flow direction are effective in protecting the bank and are termed as bed bars. Another scheme of classification is according to orientation of the spur. Spurs may be aligned with respect to the flow direction, facing either upstream, normal or downstream. Spurs facing upstream are termed deflecting or repelling whereas spurs facing downstream are called attracting spurs. The term attracting is used in a restrictive sense. Attracting action of the spur can only be limited and that too under favourable conditions.

Spurs aligned normal to direction of flow are called normal spurs. According to the shape of the spur head, the spur is termed a T-headed spur, a hockey stick spur, a curved headed spur or a round nosed spur. Material of construction is still another criterion used in differentiation of spurs. Spurs may be either stone spurs, brick spurs, bally pile spurs, spurs constructed of stone crate, concrete blocks or trees.