Hydraulics of large diameter gated flexible fluming

Gated flexible or “layflat” fluming is used in furrow irrigation to supply water to individual furrows. Laboratory experiments were conducted to establish the flow characteristics of these pipes under the low heads and high flow rates typical of furrow irrigation of cotton in Australia. Two different diameters of layflat were used in the study, 222.8 and 425 mm. Proprietary 50 mm diameter outlets (gates) were installed in the layflat at 1 m spacings. Tests were conducted at various pressures and flow rates to determine; the geometric characteristics of the fluming and their relationship to pressure, the friction loss coefficient for the fluming with and without the outlets installed, and the head-discharge characteristic for the outlets. It was shown that the fluming is capable of supplying the desired flow rates (about 6 l s−1 per furrow) at pressure heads <1 m. The head-discharge characteristic for the proprietary outlets followed a similar form to the standard orifice equation. No evidence was found to suggest that the velocity head played any significant role in determining the outlet discharge. The Hazen–Williams coefficient (CHW) of the fluming with the outlets installed was a low 80 compared to a value of about 140 for the fluming without gates. This clearly illustrates the increase in energy losses caused by the protrusion of the gates into the flow and that these losses far exceed the losses due simply to pipe friction. These experimentally derived coefficients and characteristic equations were used to validate the simulation program GPIPE.

► Pressure heads increased slightly towards the downstream end of the flexible pipes. ► Outlet discharges of up to 6 l s−1 were measured with valves removed. ► With the inserts installed (at fully open position) the maximum outflow was 2.5 l s−1 ► Velocity head had no significant impact in predicting the outlet discharge. ► Hazen–Williams friction coefficient with outlets 80,145 recommended value without gates.