Design of a Mobile, Field-Scale Rainfall Generator for Urban Runoff Water Quality Studies

Abstract

Design principles and construction of a low-cost, portable rainfall generator (RFG) as a tool for field-scale urban runoff water quality studies are presented. The novel RFG (i.e., rainfall simulator) is an adaptation of a classic Norton-Ladder-type RFG that produces and uniformly applies near-natural rainfall to a movable, field-scale footprint. The simulated rainfall mimics the kinetic energy and peak intensity of a significant natural rainfall event in the Mediterranean climate of southern California. Kinetic energy is considered the most important characteristic for mobilizing pollutants. Air-induction nozzles are used to produce a relatively coarse droplet such that the calculated kinetic energy reasonably agrees with kinetic energy modeled by intensity-energy relationships. The 32 mm/h intensity of the present RFG matches a 25 year, 60 min return period storm for the Los Angeles region. The design of the RFG system overcomes shortfalls of previous RFGs that underestimated the kinetic energy of low-intensity events, a symptom of small droplets having reduced terminal velocity. The overall system design generates a constant rainfall intensity through eight oscillating nozzles over a uniform area of 6.5 m2. Pilot testing indicated that pollutant concentrations in runoff created by the RFG are representative of urban runoff. An initial application suggests that the apparatus is useful for measuring the impact of street sweeping on runoff water quality, among other potential comparative assessments. Rainfall does not usually occur according to researcher preferences. Variability in storm characteristics, along with the inability to control contributing run-on areas, can limit the scope of experiments seeking to quantify runoff water quality from specific land uses. One potential work around is to produce simulated rainfall from a rainfall generator. However, commercially available rainfall generators are laboratory-scale models, and bespoke generators can be very costly to design and build. In this article, a novel design is presented for a low-cost, mobile, field-scale rainfall generator built from commercially available parts. The design described herein mimics the kinetic energy (the most important characteristic for pollutant mobilization) of natural rainfall representative of the Mediterranean climate of southern California, the peak intensity of a 25 year return period, 1 h design storm for the Los Angeles region, and generates runoff water quality comparable to published sources for urban runoff. The design principles and decision framework described here removes much of the barrier to entry for other research agencies to implement simulated rainfall as part of future field studies. A list of parts, sources, and assembly instructions are provided.