Investigation of the effect of preventing sedimentation of reservoirs in a thick stream

Turbidity current is a flow formed by application of gravital forces on the differential density of two fluids. In case this flow enters a fluid with a less dense fluid forms a turbidity sub-flow. Turbidity current, in general, includes stable turbidity currents [such as saline flows] and unstable turbidity currents [such as sedimentary flow or density flow]. Density flows are known to be responsible for suspension of particles, disturbance of flow and volumetric density of sediments in currents with less than 10% density. The occurrence of such flow in a dam reservoir transfers sediments to near body dam posing substantial threats to dam facilities. The control of such currents has been always a challenge to dam operation. Construction of obstacles in reservoirs is one of the common methods in accomplishing control or diversion of such currents. The height of the obstacle can significantly impact their effectiveness and reduction of construction cost. On the other hand, the height of obstacle depends on the flow to the reservoir and one can define the objective of this research as the study of the obstacle height which causes a turbidity current to stop in different conditions (in terms of density, flow discharge and slope). In doing so, a sloping flume was built in the Faculty of Water Engineering Science of Shahid Chamran University, Ahwaz, and 131 experiments were conducted with saline and sedimentary turbidity flows under different inflow conditions [including 3 flow discharges, 3 densities and 3 slopes] and with 8 obstacle heights. The experiments with saline and sedimentary currents were carried out to determine the initial conditions of flow without using obstacle, then by patterning the obtained ranges resulted from theoretical equations, the initial heights required to stop each flow were determined. For the speed measurements an acoustic speedometer (DOP 2000), for measuring characteristics of front flow (height and speed) observed measurements recorded by a camera, and for determination of current density direct sampling were used. The measured speed profiles in wall area (near flume bottom) were obtained from an exponential relation and in jet area (the area over maximum speed height) from Gaussian and agreed well with the findings of literature. The dimensionless parameters including density Froude number, relative height of obstacle, the relation of flow depth to reservoir depth, slope and Reynolds number were obtained by dimensional analysis taking into consideration Froude number simulations of current. The results of all experiments without using obstacle showed that with the increase of density, the depth of turbidity current decreased while density Froude number of current increased. The increase of slope also reduced current depth while increased density Froude number of current. With the determination of conditions for experiments without obstacle and regarding the purpose of the present research which has been the blockage of turbidity currents using an obstacle, varying heights of the obstacle were determined under the initial conditions similar to conditions where no obstacle was used and all dimensionless parameters were calculated. A multi linear relation was drawn for the results of experiments with accuracy of 97.2% as below:

The relation shows that with the increase of lope, density Froude number and relation of current height to reservoir depth (r) and relative height of obstacle which blocks the turbidity current increases. Also, the sensitivity analysis showed that the density Froude number was the most influential parameter on the relative height of obstacle required for stoppage of the current. The results of laboratory experiments were graphically depicted showing variations of relative height of obstacle against density Froude number and a new curve was fitted using theoretical results. This curve at Froude numbers beyond 0.8 distances from that by theoretical results while the increase of Froude number increases this distance which shows the effects of entering fluid. The results of this study were in a good agreement with those obtained by other researches in literature. The curve results and the suggested relation of this study can be used as a basis for design of such obstacles. The second set of experiments was conducted using sedimentary turbidity current. The results showed that the declining trend of density in the flume where no obstacle was used was exponential which agrees with the results obtained in literature. Also, the results of current against obstacle shows that the sediments flowing over obstacle decreased between 50 to 100% as compared with the case where obstacle was not used. This value is between 80 to 100% in subcritical flows and 52 to 62% in supercritical flows. The sediments flow discharge with and without obstacle shows that the use of obstacle influences both sub- and super-critical flows, although the use of obstacle more affect