Storm Drainage Design Project and Flooding - Assignment Example

April 24, 2009 Storm Drainage design Project Introduction The study of storm events in rivers are used to gather information. Actual rainfall data and data from flow gauging stations are made available for the recording of actual flooding events. Data gathered are often sparse, however theoretical calculations of a runoff into the rivers are usually limited in accuracy. In real storm events, flow of water is constantly changing as floodwater is stored out of the river and return to its original level as water subsides. The prediction of the behavior of water in the environment is more complex. The run-off generated by rainstorm will depend on the soil type. Surface streams and groundwater flows and the wetness of soil before a the storm are the factors to be considered. According to Dr Tim Stott, rainfall prediction is a matter of statistics. No one can predict what the rainfall and catchment conditions will be in the future. With climate change, past records are not reliable for future references. Engineering design will be based on specified probability storm and the runoff generated by catchments Cynon River Hydrograph, Time with respect to rainfall Cynon River Hydrograph Time with respect to river height In the hydrographs of Cynon River, there was an almost steady flow of water in the river for the first 40 hours. As the height of water start to rise on the 42nd hour, water had a abrupt rise and on the 44th hour the water temporarily had a constant flow and began rising again on the 48th hour. The rise was constantly rising until it reached the peak flow and it was recorded to be 0.65m above the river bed and the rainfall recorded at 1.2mm. In the analyses of the river hydrograph, the rise of water is faster than when it starts to subside. It would take more time to subside, and having a steady downward motion. There are factors that control the shape of hydrographs. The typical shape are shown and the main components are labeled according to Weyman (1975). A hydrograph would show the difference between the peak rainfall from the peak discharge. This is known as the lag time. Then a lag time is greater, there is less chance of flooding whereas a short lag time will show that water had reached the river channel quickly. The rise in discharge or rainfall as shown in the graph is called the rising limb and the decrease in rainfall is the falling limb. There are several factors that affect a flood hydrograph. Areas with large basins receive more precipitation that the small ones and they have a larger run-off(Hoyt, 1936) Larger basins will have a longer lag time as water has a longer distance to travel before it can reach the river trunk. According to Gillesania, 2006, the shape of the basin which is typically elongated, would produce a lower peak flow and longer lag time than a circular one. The effect of the slope is also very important. The flow will be faster down a steep slope, thus producing a steeper rising limb and shorter lag time. Channel design Given Data Note: Given the discharge in the channel, apply the the Manning Q = 1.5 m3/s formula to get a suitable breadth b, of a channel with n = 0.019 depth d S = 1/2500 = 0.0004 d = 0.6 Formula to be used V = where: v = velocity Q = Av R = Hydraulic Radius Q = A S = slope A = bd n = Manning's coefficient R = Q = discharge Computations: A = db = 0.6(b) Q = A R = 1.5 = 0.6b 1.5(0.019) = 0.6b 0.0285 = 0.6b = 0.6 1.425 = 0.6 = 2.375 = (2.375)3 = b3 13.3964 = 13.3964 = 13.3964(1.44 + 2.4b + b2) = 0.36b5 19.2908 + 32.1513b + 13.3964b2 = 0.36b5 19.2908 + 32.1513b + 13.3964b2 - 0.36b5 = 0 b = 3.9798 m. The discharge flow of the river is also to be computed. The computation is necessary because a comparison will have to be made in order to determine whether the water flowing in the river would be sufficient to supply the open channel which in turn will be directed to a reservoir that would further supply water to the end users(King,1988) Computations for the depth of the river where water is to be drawn from Q = Av where: A = cross-sectional area A = bd v = velocity = 4.0 m/s A = 15d b = 15 m. We can compute for the value of d using the Manning Formula for velocity v = 4.0 = 4.0(0.019) = 0.076 = = 3.8 = (3.8)3 = 54.872 = 54.872 = 225d2 12,346.2 + 3,292.32d + 219.488d2 = 225d2 12,346.2 + 3,292.32d + 219.488d2 - 225d2 = 0 12,346.2 + 3,292.32d - 5.51d2 = 0 Using the quadratic equation, solve for the value of d d = where: a = -5.5 b = 3,292.32 c = 12,346.2 d = d = d = d = d = d = 3.4357m. Solve for the discharge Q: Q = Av A = bd A = 15(3.4357) A = 51.5355 m2 Q = 51.5355 (4.0) Q = 206.142m3/s Now that the depth of water in the river is obtained, and the value for the discharge was computed, we can try to compare the discharge of water in the open channel and the discharge in the river. The water flowing in the river is sufficient to supply the required amount of water in the open channel. Since the open channel is located somewhere between 3 meters and 12 meters above the river bed, a pump is needed to push the water upward to the open channel. Collard,1988 stated that a design load for the pump is important to ensure that if water is pumped from the river to the open channel, there will be no overflowing, in the same manner that an underflow will not happen. In the computation of the design load of the pump, the discharge of the pump must be equal to the discharge of water coming from the pump. Computation of the design load of the water pump: HP = where Q = discharge H = H = total head Q = Av 3960 = constant A = 3.9798(0.6) A = 2.3878 m2 Q = Av 1.5 = 2.3878 x v v = v = 1.5918 m/s H = + d where H = total head v = velocity H = + 0.6 g = 9.81 = gravitational constant d = depth = + 0.6 HP = design load 3960 = constant value H = 0.1291 + 0.6 1.5m3/s = 23775.484712233 gallons/min 0.7291m = 2.3920603674541 feet H = 0.7291m HP = HP = = HP = 14.361715832612 horsepower The computation of the design load of the water pump will determine the what kind of pump will be used in drawing water from the river to the open channel. The computed discharge rate of water that will pass thru the pump is 14.3617 horsepower. Therefore we need to install a 14.50 - 15.00 horsepower pump in order to complement the design data. It is necessary to use a pump with a higher design load than the result in the computation. The pump must perform to have a water discharge equal to 14.3361. However, since we cannot buy a pump with the exact computed design load, we can either use a 14.50 or a 15.00 horsepower pump. In the pumping operation, water is drawn from the river. The pump will draw water from the river, and will discharge the water to an open channel. The channel being designed to have a discharge equal to the discharge of water flowing from the pump, will flow to a reservoir nearby. Conclusion Drainage design had been used for water impounding for so many years now. The design of any storm drainage system involves the accumulation of basic data, the familiarity with the site and the basic understanding of the hydrologic and the hydraulic principles and the drainage policy that is associated with the design. The design of a storm drain system is a process from which a project will evolve. The processes are in a general sequence from which they are carried out. The design should have a land use pattern and the physical development of the areas to be served by a storm drainage system, a storm water management plan for the area and the pattern for drainage in both overland and storm drains to existing outfall locations. There must be an understanding of the nature of the outfall since it has an influence on the storm drainage system, and there maybe water quality requirements for consideration. References Collard, R. 1988. The physical Geography of Landscape, London, Unwin Hyman. 93 -111 Gillesania, Diego Inocencio T. 2006, Engineering Formula Series, Civil Engineering, Diego Inocencio T. Gillesania, Manila, Philippines. Hoyt, W. C. and others, U.S. Geological Surveys. W - S pages 772, 1936. King, Wisler, and Woodburn, 1988, Hydraulics John Wiley and Sons, Inc. New York. Weyman DR. 1975. Runoff processes and streamflow modelling, London, Oxford University Press, 54 pp. Young and Freedman, 2000, University Physics, Addison-Wesley Publishing Company, Inc. Singapore Dr. Tim Stott, Flood Hydrographs, Fluvial Geomorphology, Learning and Research Technology University of Bristol, April 14, 2009, . Flooding, BBC - GCSE BITESIZE - Flooding, BBC April 12, 2009,. Pump Equation and Formula Calculation, 2007, AJ Designs, April 10,2009, . Read More