How holes in groynes affect holes in the streambed

Posted on March 27, 2012

Groynes are commonly used to protect riverbanks from erosion by deflecting the flow towards the middle of the stream. Flow separation and recirculation patterns downstream of a groyne tend to create scour holes. Although scour holes can provide important habitats for aquatic organisms, especially in disturbed streams and during floods, few studies have recorded how the extent and position of scour is affected by the type of groyne. Recognising that the angle and velocity of flow at the groyne tip are the main factors influencing scour, Korean engineers examined how scour patterns responded to variations in groyne permeability (0 – 60% of the surface area open to the flow) and groyne angle (60o, pointing upstream; 90o, perpendicular to the bank; 120o, pointing downstream). They used a 2 m deep, 40 m long flow channel with a sandy bottom. Impermeable groynes created deep, steep scour holes while permeable groynes created shallow, wide holes. As permeability increased there was a linear decline in maximum scour depth, but the area scoured showed a rapid initial decline (up to 20% permeability) followed by a slower decrease. With regard to groyne angle, the depth of scour holes was greatest when groynes were perpendicular to the bank, and declined symmetrically with increasing slant upstream or downstream.

Reference: Joongu Kang, Hongkoo Yeo, Sungjung Kim & Un Ji. 2011. Experimental investigation on the local scour characteristics around groynes using a hydraulic model. Water and Environment Journal 25, 181–191.