Considerations for the design of bottom intake systems José M. Carrillo, Luis G. Castillo, Juan T. García and Álvaro Sordo-Ward ABSTRACT Knowing the scarcity of water in the southeast of Spain and how the rain occurs, we considered the design of intake systems in ephemeral riverbeds in order to try to capture part of the runoff flow. The intake systems generally consist of a rack located in the bottom of a river channel, so that the water collected passes down the rack and leads to the side channel. This behaviour has been studied in the laboratory by several researchers. However, due to the many effects that occur on the bars, it is not possible to analyse the whole problem of characterization with traditional methodologies. For instance, the wetted rack length necessary to collect a required flow presents important differences depending on what each author has considered relevant. Computational fluid dynamics simulations have been done to improve the knowledge of the hydraulic phenomenon observed in different laboratory experiences, for which we have previously calibrated the numerical models using laboratory results. The ANSYS CFX code was selected. Several two-equation turbulence models have been considered. The results show differences smaller than 1% in the wetted rack length, and discharge coefficients also present good agreement. José M. Carrillo (corresponding author) Luis G. Castillo Juan T. García Civil Engineering Department, Universidad Politécnica de Cartagena, UPCT, Paseo Alfonso XIII, 52, 30203 Cartagena, Spain E-mail: jose.carrillo@upct.es Álvaro Sordo-Ward Civil Engineering Department, Universidad Politécnica de Madrid, UPM, Campus Moncloa, 28040 Madrid, Spain Key words | ANSYS CFX, bottom intake system, bottom rack, discharge coefficient, street gully, turbulence models INTRODUCTION Intake systems generally consist of a rack located in the bottom of the channel that allows water to pass through. These structures have been adopted in small mountain rivers with steep slopes and an irregular riverbed, intense sediment transport and flash flood. Their design is intended to satisfy two primary objectives: (1) to maximize water intake; and (2) to minimize sediment intake. In designing intake systems, we need to consider geo- morphologic, hydraulic, structural and economic aspects to avoid unnecessary maintenance and functionality pro- blems during the lifetime of the project. The efficiency of racks depends on a number of factors, such as the number of bars, incoming flow conditions, longitudinal inclination, shape and spacing between bars. The hydraulic behaviour of the racks is also influenced by the bars’ disposition. In longitudinal bars, the flow collected appears as a function of the local energy flow. However, in transversal bars or circular perforations, the flow collected is related to the local flow level (Mostkow ). In the analysis of clear water flows it is assumed that the flux over the rack is one-dimensional, the flow decreases progressively, and the hydrostatic pressure distribution acts over the rack in the flow direction. Two broad approaches to dealing with the energy head over the rack are typically used and these are presented in Table 1. The classical approach considers a two-dimensional per- spective. However, when analyzing the flow near the solid edges, the flow becomes extremely three-dimensional, ren- dering the two-dimensional analysis tools less useful. In numerical modelling, different approaches have been used. Some codes, such as HEC-RAS, MIKE, TELEMAC or 232 © IWA Publishing 2018 Journal of Hydroinformatics | 20.1 | 2018 doi: 10.2166/hydro.2017.008 Downloaded from http://iwaponline.com/jh/article-pdf/20/1/232/239339/jh0200232.pdf by guest on 12 July 2022