*Using Rainfall Simulators to Design and Assess the Post-Mining Erosional Stability DOI: http://dx.doi.org/10.5772/intechopen.112240*

The earliest versions of the rainfall simulators belong to the non-pressurized category, where drop-forming mechanisms completely rely on passing the water through a perforated pipe, hanging yarns, or an array of syringe needles which form the droplets, then the droplets are left free to fall under the impact of gravity from a height of not less than 9.1 m to ensure the droplets reach the required terminal velocity, which should be almost equal to the one of the natural rainfall droplets. This type of rainfall simulator has always suffered, due to the long distance between the raindrop generator and the flume surface, from the effect of wind on the falling droplets which make it a necessity to install a huge wind shield (**Figure 2**). The huge structure (the frame and wind shield) makes it impossible to use this type of rainfall simulator in field experiments (lack of portability). Later, the second type of rainfall simulator appeared, which depended on nozzles and pressurized water flow system, and it achieved a widespread popularity—at the expense of the first type—due to it being more portable and usable in field experiments as it is smaller in size (no more than 2–3 m high, **Figure 3**) and less expensive to build and run. Nevertheless, due to the pressurized nature of the simulator which produced a high-intensity spray, rainfall intensity is usually controlled by applying the water intermittently.

In this section, we will describe the features, structure, and calibration processes of a modified version of Queensland Department of Primary Industries (QDPI) rainfall simulator (as depicted in **Figure 3**), The device has undergone testing by numerous researchers [7, 55]. Queensland Department of Primary Industries (QDPI) rainfall simulator has been constructed multiple times in various locations throughout Queensland and beyond, with minor variations between each iteration. As a result, it can be replicated with ease by others. The authors of this chapter constructed the Griffith University version of the model, known as the Port-RFS, which will be presented and discussed here. This rainfall simulator is characterized by the following specifications:


#### **Figure 2.**

*The Griffith University tilting flume simulated rainfall (GUTSR). The left picture shows the rainfall generator above the flume. The right picture shows a side view of the sloping flume and the wind shield.*

**Figure 3.** *The Griffith University portable rainfall simulator (port-RFS) installation and operation in the field.*


As a pressurized rainfall simulator (nozzle type), the Port-RFS consists of a structural frame, the rain drop generator system, the water supply unit (tank), and the flume/soil container. The structural frame was constructed from aluminum tubing of 38 mm outside diameter (O.D.) and a 3 mm wall thickness. The bottom sections of some of the upright tubes are bent to form detachable legs to make it possible for the frame to be mounted on a hydraulic-tipper trailer when used inside a closed place/ laboratory, so that it can use the slope mechanism of the trailer; as well as make it possible to be pegged to the ground, if the Port-RFS is used in the field (**Figure 3**). In order for the RFS to be transportable, the frame consists of a number of separate parts (12 pieces of metal tubes) that are connected together by nylon joiners that fit inside the end of aluminum tubes. The joiners are machined from solid nylon material and are about 180 mm in length. Locking pins ensure that the vital parts are securely interconnected.

The nozzle boom is made up of a 4 m long aluminum tubing with an outer diameter of 38 mm and a wall thickness of 1.6 mm. It can be shortened to 3 m to allow for mounting on a hydraulic-tipper trailer of the same length. The boom rotates in two plastic bushes (graphite-impregnated) that also prevent lateral movement of the boom. Four male threaded (1/2" BSPT (British Standard Pipe Taper)) unions are welded onto the boom 1 m apart. Check valves are threaded onto the bushes to prevent nozzle flow or dripping when the unit is not in use, and nozzles are fitted to the check valves. The water supply inlet is via a 1¼" BSPT tee fitting attached to the boom, opposite to the row of nozzles. A tapping from this fitting provides a connection for a pressure gauge, so that the operating pressure of the unit can be monitored. The rain drop generator system of Port-RFS consists of three to four VeeJet 80,100 nozzles (depending on the length of the boom) that are installed on the boom, 1 m apart, to cover (overlapped) the flume area (3 m 1 m) underneath. A McLennan Unipolar Permanent Magnet Stepper Motor 1.8°, 3.8 Nm, 120 V dc, 4.3 A, eight wires with a programmable digital controller to drive and control the oscillating action of the nozzles' boom. The water supply unit consists of Matrix 10-5 VFD: The Ward 10-5304 stainless steel horizontal multistage pump coupled to a 2.2 kW single-phase motor drawing 13 Amp full load current, with 40 mm female BSP inlet and 32 mm

*Using Rainfall Simulators to Design and Assess the Post-Mining Erosional Stability DOI: http://dx.doi.org/10.5772/intechopen.112240*

**Figure 4.** *Excess water collecting trays.*

female BSP outlet. Controlled via the SteadyPress variable speed drive unit capable of a maximum flow rate of 200 L/min; the pump was used to feed the system with water from a 200-liter water tank. A solenoid valve and pressure gauge are attached to the water tank unit to help control the water flow rate.

The decision to use VeeJet 80,100 nozzles was made based on previous personal experience of the authors as well as the previous research of Bubenzer [56], Boulange, Malhat [15], and Loch, Robotham [7]. In pressurized rainfall simulators (nozzle type), it is important that the water flow rate provide enough pressure to allow the rain drops to have the capability to reach the required terminal velocity [46], which ensures that the kinetic energies of the generated storm satisfactorily resemble those of an intense natural rainstorm as well as the drop size distribution of erosive storm patterns. The operational sequence of this RFS relies on a continuous flow of water through the nozzles. Excess water that falls outside the soil is recycled via catch trays manufactured from galvanized aluminum plates that are arranged to collect excess water from either side of the individual oscillating nozzle (**Figure 4**).

During operation, the nozzles oscillate through "108°. Of this trek, the middle ≈ 68° applies raindrops onto the flume area below, and 20° is used at either end of the travel for overlapping, then the excess water gathered at the catch trays will go back to the water tank. Changing the frequency of nozzle oscillation using the stepper motor controller board, we can control and adjust the rainfall intensity coming by changing the waiting time and the consecutive sweeps (**Figure 4**). The stepper motor's control system is composed of a DVP-14SS211T2 Programmable Logic Controller circuit board, EM806 Stepper Driver, a 4-button digital switch, and transistor switching circuits. The microcontroller runs in single chip mode using only internal randomaccess memory (RAM) and electrically erasable, programmable read-only memory (EEPROM) for data and program storage. The 4-button digital switch set the waiting time between consecutive sweeps of the spray manifold in 0.1-s increments. The transistor switching circuit is required to provide the correct voltage and current levels to the stepper motor driver module.
