Experiment on hydraulic characteristics of trapezoidal side weir for small channels

Abstract: Water shortage is one of the problems that limit economic development in China. However, farmers adopt flood irrigation in most irrigated areas for lacking of suitable flow-measurement devices, which leads to water waste. Thus studying convenient and adaptable flow-measurement devices is particularly important. Side weirs have simple structure and high precision. They are installed in the side of channels and directly connected with the small channels without changing the cross-section structures of channels. To explore convenient and adaptable flow-measurement devices used in small channels or water inlets in the field, a simple trapezoidal side weir combined with diversion sluices for small channels was designed. Based on previous researches of rectangular side weirs, different angles of side weir were set at -9 °, -6 °, -3 °, 0 °, 3 °, 6 ° and 9 °. Experiments were conducted in rectangular channel under different discharges up to 40 L/s to study side weirs' hydraulic characteristics in Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas in Northwest A&F University. Flow profiles, Froude number, discharge coefficient and head loss were obtained from experiments. Water profiles in different locations were compared, which reflected that the flow profiles nearby side weir fluctuated obviously. Based on the dimensionless, the relationship between discharge coefficient and its impact factors were studied. The results showed that side weirs could affect flow in main channel, and the influence range was from side weir to the center of main channel. Flow profiles function derived from this study could be applied in practice, with maximum relative error 1.85%. Moreover, discharges formula was established based on it. Discharge coefficient decreased with increasing ratio between side weir height and flow depth upstream when the crest angles of trapezoidal side weirs were greater than 0° and increased when the crest angles of trapezoidal side weirs were less than 0°. While the relationship between discharge coefficient and Froude number shows the opposite trends, namely discharge coefficient increased with Froude number when the crest angles of trapezoidal side weirs were greater than 0° and decreased when crest angles of the trapezoidal side weirs were less than 0°. Regression models developed for discharge versus water head, side weir width could meet the common requirements of flow measurement in irrigated areas, whose maximum absolute of relative error was 8.97%. The percentage of head loss was bigger when discharge was smaller under the same crest angle of side weir. The percentage of head loss changed more obviously when the side weir crest angle was bigger under the same discharge. Thus to get small head loss, discharges over side weirs shouldn't be too small and side weir crest angles shouldn't be too big in the practical application. Regression models developed for flow depths versus discharges and crest angles of side weirs had high precision with determination coefficient more than 0.9, and it was suggested to be used for calculation of the appropriate height of side weir when the geometric dimension of channels and discharge ranges are known. The study could provide useful information for application of side weir in channels.