( 1 ) CV313 HYDROLOGY & COASTAL ENGINEERING LABORATORY EXERCISE: 4 20 MARKS (2%) Title: Investigation of Rainfall & Run - off Required Components: Basic Hydrologic System Sand VDAS setup Stop - watch Introduction: Rainfall - runoff processes plays an important role in hydrological cycle. The movement of water has mainly arisen from the need to evaluate the amount of available runoff water at a particular location to meet local demand as well as risk of flooding due t o excess water. Rainfall - runoff estimation from a watershed is important for safe and economical design of hydrological projects for the purpose of water resources planning, watershed management, water resources economics, flood forecasting etc. The hydraulics of surface runoff are very important in determining flow depths and velocities and, hence, the capacity of the flow to entrain and transport sediment and chemicals (Tayfur et al. 1993). Overland flow can be defined as a thin sheet of flow occurring before irregularities cause a gathering of the runoff into discrete stream channels (Motha and Wigham 1995). It is generated as a result of rainfall in excess of the saturated hydraulic conductivity of the soil or by saturation of the soil surfac es. ( 2 ) The percentage of rainfall resulting in run - off tends to vary during the period of rainfall, increasing with time (as the catchment becomes wetter and infiltration reduces) approaching a constant value. The factors in this dynamic process are the amount of rainfall going to soil moisture, interflow, storage in depressions and interception by vegetation. These factors in turn depend on topograp hy, geology, land use, vegetative cover and degree of urbanisation. (Figure 1) shows the relative amounts of run - off and infiltration during a storm. Figure 1 Rainfall Run - off Relationship This pattern is reflected in the resulting plot of river flow against time, known as a hydrograph and shown in (Figure 2). There is, of course, a base flow component to the hydrograph, which is the dry weather flow and results from water which has infiltrated the soil and percolated down to the groundwater to discharge at a much slower and more uniform rate to the river. Similarly, there is an interflow component, which is water infiltrating the surface layers of the soil but then moving horizontally to discharge to the river. This interflow takes longer to reach the river than the surface run - off, and its influence is reflected in the recession part of the hydrograph, which represents the withdrawal of water from storage in the catchment. ( 3 ) Figure 2 Hydrograph Another term used is the time of concentration, this is the time taken before all parts of a catchment are contributing some run - off at a given point after rainfall. It is often equal to the lag time for small, particularly urban, catchments. The area under the hydrograph gives the volume of run - off, which can be divided by the catchment area to give a value in, say, millimetres, this can be compared to the total rainfal l. In attempting to predict the likely quantity to run - off and hydrograph shape from a given rainfall, there is no purely theoretical means based on physical parameters which can be measured in the field. All methods are semi - empirical in nature, and many complex (mathematical) models are used, with rainfall as input and run - off as the output. Another approach is the unit hydrograph which involves establishing a typical hydrograph for a storm of given durat ion and 10 mm effective rainfall using records of rainfall and run - off. This unit hydrograph is then adjusted to give the hydrograph for any other storm of the same duration by multiplying the ordinates by the ratio: Where little or no records exist, a sim ple empirical formula may be used: (1) where: Q = Peak discharge C = Coefficient of run - off (0 – 1.0) i = (mean) rainfall intensity A = Catchment area Typical values of C range from 0.1 in thick forest to 0.8 for rocky terrain. However, all methods for predicting run - off from rainfall have drawbacks and reference should be made to a relevant hydrology test for a full description and discussion. ( 4 ) Advanced Hydrology and Rainfall Apparatus Components of Basic Hydrology System ( 5 ) Procedure: 1. Set the apparatus tank up with a suitable permeable medium, such as coarse sand, (16/30 meshsand recommended) forming the model catchment, filling the main body of the apparatus up to alevel just below the weir crest at each end. Do not put san d into the reservoir. 2. Operate the tilt mechanism to give the catchment a 50 mm slope. The two jacks should be used together, and care taken to ensure that the tank is not twisted. See Adjusting the Slope on page 8 (Appendix). 3. Make a table similar to Table 1 on page 6 for the results. 4. To simulate the behaviour of natural catchment run - off to a river or watercourse, remove the four porthole caps ( leave the filters screwed in place ) in the downstream weir to permit interflow. NOTE: If studies of purely surface run - off are being carried out, leave the porthole capsin place. 5. Close valves F, G, H and I (Figure 6) (as the wells are not being used) the apparatus will functionas indicated in (Figure 4). 6. Open value A, set the four switches on the front pannel to “I” to open the solenoid valves, closevalves C and D. 7. The adjustable overflow (item 6, (Figure 5)) is raised so that no water enters it, thus all water‘running - off’ the catchment will pass to the flow meter for measurement. 8. Start the pump, adjust valve A to give a flowrate of approximately 8.0 l.m - 1 Turn off the pump untilready to start. 9. Set up VDAS ® (see the VDAS ® User Guide for instructions). Start a timed data capture, set the interval for continuous readings. The VDAS ® socket is located on the right - hand end of the controlbox. 10. Press start on VDAS ® and start the pump. NOTE: If a computer and VDAS® are not available, record the results manually with astopwatch and record the values from the LCD display. ( 6 ) 11. Run for 10 minutes (if soil condition is dry) or 3 minutes (if the soil is under saturated conditions) taking readings then turn off the pump. 12. Continue taking readings for a further 5 minutes or until the outflow drops below readable. 13. On completion of all experiments, it is advisable to ensure that the solenoid valves are switched off “O” and valve A is opened. Results & Analysis The hydrology of the flow recorded through the outlet flow meter against time may now be plotted. Time (min) Flow (l.m - 1 ) Table 1: Results Table Experiment 1 ( 7 ) Adjusting the Slope The apparatus can be tilted by turning the two hand wheels below the left end of the tank. To assist with controlling the angle of the tank, there is a scale rule beneath the tank (Figure 3 ). CAUTION: Both hand wheels must be turned at the same time to avoid over flexing the tank. Figure 3: Turn Both Wheels Together to Raise and Lower Tank Operating the Flow Valves Figure 4 Apparatus Schematic Layout Excluding Wells and Piezometers A diagrammatic representation of the apparatus is shown in two stages for clarity in Figures 4 and 6 .A source of mains electricity is required to drive the electric ( 8 ) pump. A suitable permeable medium for all the experiments described is a medium to coarse sand, for example with particle sizes in the range 0.5 – 1.5 mm. TecQuipment do not recommend finer particle sizes as the filters and tappings will not trap the permeable medium correctly. CAUTION: TecQuipment do not recommend finer particle sizes as the filters and tappings willnot trap the permeable medium correctly. Only put sand in the tank. Never add sand to the reservoir. TecQuipment can supply 800kg of suitably graded sand (H313a) ( 9 ) Control of Rainfall Figure 5 Control Panel Refer to Figure 5 for valve and switch identification. With valve A open, but all other valves shut, the pump will simply circulate water from the storage reservoir back into the storage reservoir. If the solenoid valves are opened, and with valves C and D closed, valve A can be slowly closed until rainfall occurs up to 10 l.m - 1 Adjustment of valve A and the solenoid valve switches E1, E2, E3 and E4 controls the amount and position of the rainfall. See Table 2 for a summary of valve settings. Setup A C D E1 E2 E3 E4 Control Rainfall Circulate water from storage reservoir and back Open Closed Closed Off Off Off Off - Rainfall on upper catchment only Open Closed Closed On On Off Off A Rainfall distributed uniformly over catchment Open Closed Closed On On On On A Moving Rainfall Open Closed Closed On or Off On or Off On or Off On or Off A and E1, E2, E3, E4 Table 2 Valve Settings for Rainfall The main body of the tank would normally contain a suitable graded permeable medium, which formsthe model catchment, and this is constrained between two end weirs. Six perspex screens fit around the apparatus to limit the spray. Control of Groundwater and Wells Refer to Figure 5 for valve identification. Figure 6 Apparatus Showing Wells and Piezometers Only