Staying in circulation

Posted on September 26, 2009

Groynes are rigid structures that extend from a river bank into the channel, often at right angles to the stream.  While they have been traditionally used by engineers to reduce bank erosion, their role in habitat improvement (e.g., by helping to increase pool-riffle diversity) has also been recognised.  By affecting the amount of time that water is retained in small-scale eddies, groynes can influence ecological processes like nutrient dynamics and plankton growth.  However, to date the use of groyne fields in stream rehabilitation projects has been mainly a trial-and-error affair because we haven’t had a very clear idea of how groynes affect flow dynamics.  Austrian researchers have gone a long way toward remedying this situation by using computational fluid dynamics to simulate 3-D water movement in groyne fields, and then checking their model against observations in the River Danube.  Their findings contradicted a previous assumption that groynes delay the passage of water most when river flows are low: retention times were actually highest when flows were just high enough to submerge the groynes.  The flow model is transferable to other groyne systems, and is being refined to include the effects of wind and bank vegetation. 

Reference:  Tritthart, M., Liedermann, M. & Habersack, H.  2009.  Modelling spatio-temporal flow characteristics in groyne fields.  River Research & Applications 25, 62–81.