تعيين دبي جريان همزمان درون و روگذر غير مستغرق در سدهاي تاخيري پاره سنگي

Estimation of Discharge for Unsubmerged Detention Rockflll Dams With Simultaneous Through and Over Flows

Introduction Abundant flood events occur in Iran mainly due to variations in spatial and temporal patterns of rainfall along with the mis-management of watersheds. Because of the large economical and environmental effects of floods, it is necessary to control and reduce their damages. There are several methods for flood control. Rockfill dams arc one of the best methods to store flood volume and deplete the flood peak. Flood control by these structures requires a hydraulic design which is dependent on the behavior of the flow in coarse porous media. Many studies have considered the flow through coarse porous media, but the effects of the over-flow on the flow through porous media has rarely been mentioned. In this paper simultaneous through and over-flow in rockfill dams 1S investigated experimentally. Objectives Understanding the flood flow mechanisms in rockfill dams and investigating the effects of over-flow on through-flow was the main objective of this study. The stage-discharge relation in overtopping rockfill dams was obtained and an experimental model for flood discharge in rockfill dams were developed. In this study an experimental setup was developed to investigate simultaneous through and over-flow in rockfill darns. Using dimensional analysis, the governing functional equation of simultaneous flow was obtained and finally the constant and coefficients were derived. Methodology Because of fully turbulent flow in rockfill dams and severe interaction between over and through-flow in rockfill dams. the flow pattern and mechanism are fully complicated. There are three major types of flow in rockfill dams; A) Through-flow in rockfill dams. where flood flows only in the porous body of the dam and there is non-darcian free flow in rocks (Samani et aI., 2003; Samani and Shaiannejad, 2004; and Michiuka et al., 2005). B) Submerged flow in rockfill dams, where rockfill dam or rock mass IS submerged and downstream level of water is higher than the dam height which consists of two types of flow, one pressurized non-Darcy flow in rockfill and the other open channel flow over the rockfill dam (Li and Graga, 1998). C) Unsubmerged Simultaneous through and over-flow, where rockfill dam overtops and flood flows freely through and over the rockfill dam which consists of two types of flow; one free non-Darcy flow in the body of the rockfill dam and the other spatially varied flow over the rockfill dam. There is no special research on Unsubmerged Simultaneous through and over-flow in rockfill dams. so in this paper this type of flow in rockfill dams is investigated experimentally. Dimensional analysis and experiments Based on the physical characteristics of simultaneous over and through-flow in rockfill dams, the previous studies, and the results ofthc initial experimental tests, the non-dimensional functional relation of simultaneous flow discharge using dimensional analysis technique is derived inventionally: q - d g J6 pʹ pʹ , pʹ H - d d g d where Cd is the discharge coefficient, H, is the head of water upstream of the dam, g is the acceleration due to the gravity, f is the function symbol, P is the dam height, L is the thickness of the dam, S is the slope of the up-downstream slopes, DH is the difference between the upstream and the downstream depth of water, and q is the discharge per unit width of the dam. The main objective of this paper is to derive the relation between Cd and the five dimensionless parameters of HiP, UP, S, diP, and DH/P. Results and Discussion The effects of dimensionless parameters of equation (I) on Cd were investigated through 240 runs. The functional relation of C, was odtained. Based on the experimental results an increase in the upstream depth of water caused an increase in the flow discharge. In low flow rates, the dominant flow is the through-flow and the variation in upstream depth of water was highly affected by the discharge variations. In large flow rates, the variation in upstream depth is less affected by discharge variations. This was mainly because of the domination of the through-flow in low flows. In this condition the dam behaves as an underflow structure while in large flows it behaves as an overflow hydraulic structure. The variation of the discharge coefficient with HiP, UP, S, dIP, and DHiP is presented in fig. I. Finally the mathematical relation of Cd in rockfill dams was derived by a nonlinear regression analysis; P DII { J II. 11361 /1-) [ J-0 Oil, II) c; =0.29 ~ .1 DH ,I n, H d -UO\i7(!::..) -U83r!!.-) [~] r [~] P [sin(PJ]-683,,,nIPIl with Rʹ~0.936 and RMSE~O.OO I. This equauon can be used to estimate the simultaneous over and thorough flow rates in rockfill dams and the design of detention rockfill dams to reduce the output flood peak. - HU 0.5, [Hd ~ S ~ DHJ- C H" 0.5 ( I ) q - d g J6 pʹ pʹ , pʹ H - d d g d where Cd is the discharge coefficient, H, is the head of water upstream of the dam, g is the acceleration due to the gravity, f is the function symbol, P is the dam height, L is the thickness of the dam, S is the slope of the up-downstream slopes, DH is the difference between the upstream and the downstream depth of water, and q is the discharge per unit width of the dam. The main objective of this paper is to derive the relation between Cd and the five dimensionless parameters of HiP, UP, S, diP, and DH/P. Conclusion In this paper unsubmerged simultaneous through and over-flow in flood detention rockfill dams has been investigated experimentally. Based on the experimental results, a non-linear regressive equation for discharge estimation with a correlation coefficient of 0.936 and root-mean-square-errors of 0.001 has been proposed. Results also showed that the flow rate has been directly affected by the ratio of the dam height to the upstream head, the ratio of material size to the dam height, and the submerged ratio. The flow rate has been inversely affected by the ratio of the dam length to the dam height, and the upstream and downstream side slopes.