| Aiming at the weir-flume combination facility suitable for seasonal stream flow measurement in mountainous areas,the flow thresholds of the weir flow and in-groove flow were determined by testing the influence of the outlet width of the flume on the hydraulic performance of the facility and the scour of the downstream riverbed.The distribution law of water depth,Fr and velocity along the facility under different conditions and the local scour and silting characteristics downstream of the facility were analyzed,and the flow measurement formula was established.Through the numerical simulation of the flow characteristics of the weir-flume combination facility under different bottom slope conditions,the appropriate flow measuring conditions are determined.The main conclusions are as follows:（1）There are two flow mechanisms of in-groove flow and weir flow in the weir-flume combination facility,and the flow threshold increases with the increase of the flume width.When the flume outlet width is 25,35 and 45 cm,the flow threshold is 32.75,42.28 and50.82 L/s,respectively,and the corresponding relative water depth is h4/d were 0.885,0.901and 0.877,respectively.The flow formulas of the weir-flume combination measuring facility with three flume outlet widths are established respectively.The calculation formulas of the flow threshold including the shrinkage ratio and the formula of the comprehensive flow coefficient of the in-groove flow are given.The relative error between the flow formula and the measured value is within 2%,and meeting the requirements of flow measurement.（2）The water depth in the upstream and inside the flume decreases with the increase of the width of the flume,and the downstream water depth has little influence on the width of the flume.With the increase of the outlet width of the flume,the Fr above the rectangular section of the flume gradually increases,and the Fr in the trapezoidal narrow section gradually decreases.The critical flow section gradually moves downstream,but it is still in the trapezoidal narrow section.With the increase of the flume outlet width,the upstream flow velocity increases,and the downstream flow velocity decreases in the thin water layer area.The overall velocity behind the weir decreases gradually.（3）With the increase of the bottom slope of the river bed,the water depth in the upstream and the flume decreases gradually,and the upstream return water distance decreases gradually.The depth of thin water layer in the downstream has almost no effect,the height of backwater in the downstream decreases gradually,and the position of backwater develops to the downstream gradually.The flow threshold increases gradually;The overall Fr and flow rate increase gradually.The greater the discharge,the less sensitive the response of hydraulic characteristics such as flow pattern and water surface line to the change of bottom slope.According to the variation law of hydraulic characteristics under different bottom slope conditions,it is suggested that the suitable bottom slope range of weir-flume combination facilities is less than 0.01.The relative error of flow measurement formulas with bottom slope parameters established within the appropriate range of bottom slope is within 1%.（4）When there is no water in the downstream,the scour depth on both sides of the outlet of the flume is larger than that in the middle,and the scour area is inverted M shape,and gradually develops into a circular shape,and the scour shape presents a more regular ripple to the downstream.When there is water in the downstream,the scour shape at the outlet of the flume presents a narrow horseshoe shape and gradually develops to the downstream,eventually developing into a circle.With the increase of the width of the flume,the horseshoe shaped scour pit in the middle becomes weaker and becomes M shaped gradually.Obviously bed sand coarsening occurs in the downstream scouring and silting area.The coarsening area mainly appears in the facing water surface of the scouring pit（that is,the elevation of the scouring pit is less than that of the downstream sand thickness area）,and the larger the discharge,the more obvious the phenomenon.（5）According to the downstream scour and siltation results,when the bottom slope of the river is less than 0.01,the height of the water metering facility is 1/10 of the width of the river channel,and the sediment of the downstream river bed is similar to the median particle size of 1 mm in this study after conversion through the law of similarity and the similar particle size scale of sediment initiation,it is recommended to carry out energy dissipation and anti-impact measures within the range of twice the length of weir-flume combination facility behind the weir,and the spacing between weir groups should be greater than 4 times the length of weir-flume combination facility. |
