Journal of Irrigation and Drainage Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

In this paper, simple methods for validating the emitter characteristics of driplines and pump performance curves for dripline network design and simulation are presented. Pressure compensating driplines were tested and used to aid grass establishment to control erosion on the steep slopes of a newly constructed flood levee. The dripline networks consisted of several bays that had different lengths and numbers of lateral rows as dictated by fence lines of property boundaries and access road ramps. Single emitters as well as thousands in dripline networks were tested. The average emitter discharge was observed as able to exceed the supplier’s value ($2.1\mathrm{l}/\mathrm{h}$) by as much as 10%. Moreover, the emitter discharge value tended to be higher between the opening threshold pressure head of the pressure compensating emitters of 4 and 10 m compared with those for pressure heads higher than 10 m. The emitter insertion head loss coefficient exhibited considerable variability (0.117–0.753), although the average value of 0.336 was close to the supplier’s value of 0.36. The supplier’s pump performance curve overestimated the pressure head by 11 m for a flowrate of $50\mathrm{l}/\min$, but the overestimation decreased to 6 m at the higher flowrate of $300\mathrm{l}/\min$. A comparison of observed and simulated pressures at strategic locations within the dripline networks were used to identify the emitter characteristics without the need for flow measurements. Simulation results were used to determine excessive bay inlet pressures and the corresponding ball valve opening fractions to drop the inlet pressure to an acceptable limit for each bay.

The Hong Kong West Drainage Tunnel (HKWDT) system consists of 34 stormwater intake structures designed to intercept stormwater runoff in upland catchments. During stormwater events, each of these intake structures intercepts and diverts the upstream supercritical flow into a bottom rack chamber connected to a supercritical vortex drop. Small surface flows are conveyed to the downstream drainage system through a narrow low-flow channel (LFC) along one side of the intake. The LFC is designed for a capacity of $20\mathrm{L}/\mathrm{s}$. However, when the intakes are in operation, observations suggest that the stormwater flows conveyed via the LFC might be significantly more than the design values. We report an investigative physical and numerical modeling study of the hydraulic performance of a representative existing intake structure. A 1:12 undistorted physical model based on Froude similitude was designed to study the complex hydraulic interaction of the incoming supercritical flow with the transverse barrier, bottom racks on top of the rack chamber, and a low-flow channel in supercritical flow regime. The interception capacity and flow characteristics of the bottom rack intake is predicted via a two-dimensional (2D) numerical solution of the shallow water equations using a shock-capturing finite-volume method. Physical model tests and numerical experiments were carried out for representative design flow events including those for storms with 2-, 10-, 50-, and 200-year return periods with climate change. The predicted flow features—including the discharge intercepted by the bottom rack chamber and the low-flow channel—were compared with experimental observations. The results indicate that the unintercepted flow in typical rainstorm event was $0.5\mathrm{L}/\mathrm{s}$ in the physical model, corresponding to $0.25{\mathrm{m}}^3/\mathrm{s}$ in field conditions. Under a 200-year rainstorm with climate change, the unintercepted flow was $2.3\mathrm{L}/\mathrm{s}$ in the physical model, corresponding to $1.15{\mathrm{m}}^3/\mathrm{s}$ in field conditions. Overall the model results show that around 10% of the inflow enters the LFC at high inflows—around $2\mathrm{L}/\mathrm{s}$ or $1{\mathrm{m}}^3/\mathrm{s}$ of prototype flow—which is much higher than the $20\text{-}\mathrm{L}/\mathrm{s}$ design target; this confirms the observation that the stormwater runoff conveyed to the downstream drainage system is more than originally designed. Finally, through a combination of heuristic reasoning and physical model studies, the existing intake structure design was modified to improve interception performance. The improved interception was verified through hydraulic model simulation. Furthermore, the applicability of spatially varying rack discharge coefficient for supercritical flow above bottom racks in a 2D high-resolution hydraulic numerical model was validated.

The curve-number (CN) infiltration model is widely used in translating rainfall hyetographs into runoff hydrographs. The model is anomalous in that it does not place any limit on the infiltration rate within a catchment, and therefore can be inconsistent with the infiltration characteristics of real soils. An alternative methodology is proposed that generates infiltration rates that are more consistent with conventional soil infiltration-capacity characteristics. It is shown that significant underprediction of peak runoff rates can occur as a result of using the anomalous CN infiltration model.