The interception drainage capabilities of slotted drains as pavement surface drainage systems

Abstract

One of the most common hydraulic risks on a pavement surface is the accumulation of surface runoff caused by insufficient surface drainage. As the water depth on roadway surface increases, the risk for hydroplaning and splash and spray to initiate also increases. Several pavement design considerations can be implemented to minimise the flow depth on a pavement surface such as the geometry design, the variation of pavement surface textures and the focus of this study, the implementation of drainage systems to enhance surface drainage. After reviewing the various literature of surface drainage on pavement structures, it is evident that minimum research is available for the interception capabilities of the slotted inlets in South Africa. A research study was thus performed at the University of Pretoria (UP) to estimate the interception capabilities of proprietary slotted drains operating under safe driving conditions. The slotted drains evaluated in the study are manufactured and distributed by the company registered as Salberg Concrete Products (Pty), and the drains are not tested for sufficient verification on the hydraulic interception capabilities of the slotted drains. During the research, a model study was conducted on different drainage systems installed within a pavement structure to simulate the slotted drains operating under various field conditions. This was completed by building pavement models with replica slotted inlets from the actual slotted drains, with adjustable features such as pavement slopes, slotted drain inlet widths and different volumes of sheet flows. The slotted inlets were tested for operating individually and with a median barrier installed adjacent to its length. All parameters and conditions tested during the experiment are in accordance with a South African National Road Agency Limited (SANRAL) project case study. A total number of 120 tests were performed during the experiment where slotted inlets were tested performing under the following conditions for maximum sheet flows of approximately 3l/s: pavement cross slopes and longitudinal slopes up to six percent, slotted inlet widths of 20mm, 40mm and 60mm, and slotted drains operating with or without a median barrier. For all parameters tested, nearly all (98-100%) of the sheet flows were intercepted by the slot inlets of the pavement models tested while operating without the median barrier. An additional test was completed on the pavement model operating with a median barrier. It was found that the barrier did not affect the drainage capability of the tested inlet for all the conditions tested, as all the sheet flow was nearly 100% intercepted before the barrier could function. Calculations were done on the sheet flow test parameter to determine the flow depths and rainfall intensities simulated during the experiment that will typically occur on different pavement widths in practice. Rainfall intensities up to 3000mm/hr can occur on different pavement widths of 3.6m, 7.2m, 10.8m and 14.4m in practice that were simulated during the experiment. By utilising various flow depth prediction models, calculations have shown that sheet flows with water depths higher than 6mm were simulated during the experiment, which can occur on the respective pavement widths in practice. SANRAL (2013) recommended a maximum flow depth of 6mm on pavement surfaces in a 1:5 year storm to prevent any hydroplaning risks. This means that the tested slotted inlets will intercept almost 100% of these sheet flows with the predicted flow depths and rainfall intensities, as calculated per meter flow width, occurring on pavement surfaces during wet pavement conditions. It is, therefore, safe to conclude that slotted drainage systems operating in practice with the same pavement geometric design, drainage design and pavement texture as tested during the experiment, will have the capability to sufficiently remove almost all the surface water and promote safe driving conditions. For further studies on this topic, actual field observations during rainstorms in practice can be conducted to analyse the effect of splash and spray caused by vehicles travelling during wet pavement conditions which were not consider during the experiment of this study. The investigation to determine the minimum slotted width to intercept 100% sheet flow is also a topic to consider for future research. Additional experimenting on slotted drains can be useful to confirm the results and findings obtained during this study and to develop comprehensive mathematical formulae by describing the interception drainage capabilities of slotted drains as pavement surface drainage systems.