Effective void ratio and rack length estimation in flows with gravel size sediment through bottom racks

Luis G. Castillo, Juan T. García, José M. Carrillo

Friday 3 july 2015

12:00 - 12:15h at Asia (level 0)

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

Parallel session: 15C. Engineering - Industrial

Bottom rack intake systems are designed to get the maximum quantity of water in mountain rivers with important transport of sediments. Several experimental studies have focused on the occlusion of the racks due to the deposition of debris over them. However, more studies are necessaries. An intake system has been constructed in the Hydraulic Laboratory at the Universidad Politécnica de Cartagena. It consists of a 5.00 m long and 0.50 m wide channel, a discharge channel, a water collected channel and a bottom rack with different slopes. The racks are T shape aluminium bars. Bars can be set with different spacing between them (5.7, 8.5 and 11.7 mm). An experimental campaign of 60 probe measurements with flow and sediments were done. Two different sieve curves were used: gravel with d50 = 8.3 mm, and d50 = 10.8 mm. The volumetric concentration was between 0.5 and 2%. The tests were done with racks of void ratio 0.22 and 0.28, different longitudinal rack slopes (0, 10, 20, 30, and 33%), and five inlet specific flows (114.6, 138.6, 206.2, and 279.2 l/s/m). In each test, the flow collected by the rack and the water depth over the rack were measured. At the end of each test, the total rejected materials and the sediments collected through the rack were weighted. Materials trapped in the spaces between bars and its position along the rack were measured. Due to the deposition of gravels over the rack, the effective void ratio is modified. The present study is focused in the estimation of the void ratio. Considering the sediment trapped over the rack, wetted rack lengths are proposed to derive a defined flow. Computational fluid dynamics simulations are used to estimate the effective void ratio by using the lab data. Experimental and numerical results in both, clear water and sediments transport, will allow to improve the existing design criteria of bottom rack intake systems.