A FLUME EXPERIMENT ON THE EFFECT OF OBSTRUCTION SHAPE ON THE DEVELOPMENT OF FORCED POOLS

A series of six flume experiments were conducted in a 6-m long, 0.5-m wide recirculating flume with a bed gradient of 0.8 degrees to determine the influence of obstruction shape on the formation of forced pools. Six different shaped obstructions were added to the flume with the maximum width of the obstruction held constant at 20 cm, a forty percent constriction of flow. The shapes used included a square obstruction, a rectangle, a right triangle with the hypotenuse facing upstream, a right triangle with the hypotenuse facing downstream, a combination of a square and triangle with the hypotenuse facing upstream, and a rectangle and semi-circle shape. A profile of the flume bed was taken after each experiment and grid measurements were conducted to create a topographic map to visually compare the pool morphology. An ANOVA analysis was conducted which showed that pool depth, pool location, and the distance between the pool and the riffle all very with the obstruction shape. Some of the difference in pool shape may highlight variations in helical flow development and vortex shedding location that result from different approach conditions of the flow.