Drag Force and Resistance around Single Groyne in Submerged Flow Condition.

Akihiro Kadota, Wim Uijttewaal

Tuesday 30 june 2015

16:00 - 16:15h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 7F. Engineering - Instrumentation

Groynes have been used for the protection of banks, stabilization of bed morphology and also for establishing navigation channels. A typical Japanese river is characterized by large and sudden variations in discharge due to big amounts of rainfalls. As a result the groynes are often in submerged condition and increase the flow depth neighboring of groyne. The increased depth is related to the extra resistance due to the groyne. The present study therefore focuses on the effects of submergence on the mean flow pattern and resistance around a single groyne. Experiments were conducted with a scaled and schematized model of an existing Japanese groyne by means of surface flow visualization technique and Particle Tracking Velocimetry (PTV). This method is preferred because there are large-scale flow structures and velocity differences in horizontal plane between the outside and inside of the dead zone behind the groyne. The study site represented by the experiment, is the Hiji River, located at Ozu City, in Ehime prefecture at north-west side of Shikoku Island, Japan. The Hiji River is famous for its many tributaries and narrow river-mouth resulting in frequent flooding. The groynes in Hiji River were constructed in 17th century. They were used for creating a navigation channel and for protecting the bank. Due to the frequent flooding, the groynes are often in submerged condition so that it is needed to investigate the detailed structure of flow pattern around the groyne and to clarify the submergence effect caused by increased flow depths. There are several studies on flow pattern around groynes as described in the above. However, only few studies focus on variations of water level, especially the case of submerged groynes. The present study, focuses on flow pattern around the submerged groyne and the relations between flow resistance and submerged flow depth. The PTV method is adopted and implemented in order to calculate velocities accurately. The flow pattern and flow resistance are estimated by comparing velocities at several points between upstream and downstream of groyne. The relations between flow resistance and submerged flow depth are discussed. As a result, the roughness of groyne is reason-ably estimated from groyne height and flow depth. The reasonable estimation of Manning roughness coefficient of groyne as a flow resistance can be pro-posed. The result is also applicable for weirs and other river construction.