Storm Drainage Design Project - Case Study Example

6, April 2009 Storm Drainage Design Project Introduction To evaluate the effects of different land management practices on water economy of drainageareas. It is necessary that the stream-hydrograph that is obtained from drainage areas be separated into ground-water flow and the components of storm flow as accurately as the data permit. The difficulty of drawing a ground –water hydrograph for separation of complex storm-hydrograph has been specifically pointed out by a number of workers. For example, W.G. Hoyt stated that the peaks occurring in such rapid succession that rarely does surface-runoff have a chance to drain out “make the task of drawing a ground water hydrograph difficult and the results is uncertain”. The improvement of the present methods and the development of new methods for the differentiating ground-water run-off and the surface run-off has been specifically pointed out as one of the deficiencies in hydrologic precipitation and other measurements that are essential to water shed studies are obtained from drainage areas that are supporting a continuous stream flow and ranging in size. The flood hydrograph is a plot of river discharge, usually a line graph over time. Rainfall is also plotted over time in the form of a bar graph. There are factors that control the shape of hydrographs. The typical shapes are shown and the main components are labeled (Weyman, 1975). A hydrograph tells the difference between the peak rainfall from the peak discharge. The difference is termed as lag time. When the lag time is greater, there is a lesser chance of flooding. A short lag time indicates that water had reached the river channel quickly. The rising limb is the rise in discharge shown on the graph, while the falling limb is the decrease in the discharge. There are factors that affect a flood hydrograph. Large areas of basins receive more precipitation than small ones and therefore have larger runoff . Larger size would mean longer lag time as water has a longer distance to travel to reach the trunk river. The shape of the basin, (according to Gillesania, 2006), typically the elongated basin produces a lower peak flow and longer lag time than a circular one with the same size. Elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size. The slope has a very important effect. Channel flow will be faster down a steep slope, producing a steeper rising limb and shorter lag time. Permeable type of rocks would mean rapid infiltration and little overland flow thus producing a shallow rising limb. Since the soil is one important factor, infiltration is generally greater on thick soil, although clay acts as impermeable layer. If there is more infiltration, longer lag time occurs and shallower rising limb. Concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag. Afforestation intercepts the precipitation, therefore creating a shallow rising limb and making the time lag longer. A higher drainage density allows a rapid overland flow. Precipitation and temperature is also taken into account. Short intense rainstorm produces rapid overland and steep rising limb, while short intense rainstorm produces rapid overland flow and steep rising limb. The drainage density is a higher density that allows rapid overland flow. In precipitation and temperature, short intense rainstorms produces rapid overland flow and steep rising limb. The tidal condition is that the high spring tides can block the normal exit for the water, hence, extending the length of time the river basin takes to return to its base flow. Overland flow is the volume of water reaching the river from the surface run off and the through flow is the volume of water reaching the river through the soil and its underlying rock layers. Hydrograph of Cynon River With respect to the amount of Rainfall Hydrograph of Cynon River with respect to water discharge The above graphs illustrate the change in the river height after a rainfall. The study took 96 hours continuously taking the river height reading every hour. Each of the readings is different from the other. In the analysis, there was an almost steady flow of water from the start of the study up to 42 hours. After 42 hours, the river height began to rise. Thus it is now in the process called the rising limb. The time between the rise up to the time water reaches its peak is called the basin lag time. It reached the peak flow at the 57th hour of the study. This means that water had reached its maximum discharge and water is starting to fall down. We call this stage the recession limb. After the recession limb the water would start to normalize its flow discharge. The storm flow is the total of the overland flow and the through flow. The overland flow is the flown that rises above the through flow, and the through flow is the water that rises above the base flow. Channel Design Given the discharge in the channel is to be Q m3/s. Apply the Manning formula to design a suitable breadth b, of channel with depth d v = R2/3S1/ 2 where: v = velocity, m/s n R = hydraulic radius, m S = channel bed slope, m/m n = Manning’s coefficient of roughness A = db where: A = area, m2 b = breadth, m d = depth, m Q = Av Given Data Q = 1.3 m3/s n = 0.017 S = 1 in 3000 = 0.0003 d = 0.4 m Required = width of base b of the open channel Discharge Q of the river into the open channel Design of water pump to discharge water from the river to the open channel Computations: A = db = (0.4)(b) A = 0.4b m2 Wetted Perimeter = 2d + b = 2(0.4) + b = 0.8 + b = 0.8 + b. Hydraulic radius  = =  v =  n Q = Av 1.3 = 0.4b 1.3(0.017) = 0.4b 0.0221 = 0.4b   = b  3.1899 = b  =  32.4575 =  32.4575 =  32.4575 =  32.4575(0.8 + b)2 = 0.16 b5 32.4575(0.64 + 1.6 b + b2) = 0.16 b5 20.7728 + 51.932 b + 32.4575 b2 = 0.16 b5 20.7728 + 51.932 b + 32.4575 b2 – 0.16 b5 = 0 b = 6.3588 m. = width of base b of the open channel In the computation of the river flow volume(discharge), we can get the lowest reading of water or river height from the Cynon River hydrograph. Use the lowest reading in the river height to get the discharge of water per unit time. It would be very safe to get the minimum reading in order to have a safe value for the design of the pump to be used in extracting water from the river to the open channel. Computations; Q = Av where: A = cross-sectional area v = velocity = 4.0 m/s A = bd b = 15 m. d = 0.249 m = minimum river height A = 15 x 0.249 A = 3.735 m Q = Av = 3.735 x 4.0 Q = 14.94m3/s In comparing the minimum discharge of water in the river which is 14.94 m3/s, to the discharge of the channel which is 1.3 m3/s, the potential of extracting water from the river is great. Since the channel is located anywhere between 3 meters to 12 meters, a pump will be needed to lift water from the river. In order for the channel to have a steady supply of water coming from the river, a design for a pump to be used is necessary. (Young and Freedmen, 2000), stated that The design of the pump will depend on the discharge of water in the open channel. In the computations done regarding the discharge of water in the channel with respect to the discharge of water in the river, the pump to be used must have a discharge equal to the discharge of water in the channel to avoid overflowing or shortage of water supply in the canal since the water from the river that will pass through the channel will go to a reservoir storage. Computations for the design of a water pump: HP =  where Q = discharge H = total head 3960 = constant H =  Q = Av A = bd A = 6.3588 x 0.4 A = 2.5435 m2 Q = Av 1.3 = 2.5435 x v v =  v = 0.511 m/s H =  + d where H = total head v = velocity H =  + 0.4 g = 9.81 = gravitational constant d = depth =  + 0.4 HP = design load 3960 = constant value H = 0.0133 + 0.4 1.3m3/s = 20605.420083936 gallons/min 0.4133m = 1.3559711286089 feet H = 0.413 3 HP =  HP =  =  HP = 7.0556 horsepower The results of the computations for the design of the pump going to the open channel is responsible for what water pump will be the best choice(King, Wisler and Woodburn, 1988) . Disregarding the brands of the pump to be used, the pump must perform to have a discharge equal to 7.0556 horsepower. However, the we cannot buy a pump which has an exact output of 7.0556 HP. Therefore the most conventional pump design would be a pump with a 7.5 HP. The design of the pump will have to satisfy the conditions that the channel needs. It must be able to fill the channel according to its cross-sectional area design. More over, the flow of water to pass thru the channel going to the reservoir will be enough to supply water to the end consumers who are dependent on the supply coming from the reservoir. In relation to the consumption of the end users of water coming from the reservoir, there will be a constant supply of water from the reservoir. The water in the reservoir is assured of constant supply of water from the open channel and the open channel will not dry up because the water volume of water in the river is sufficient enough the supply the end consumers. Conclusion While it may be necessary to consider stream-flow as such, the division of rainfall into surface-runoff and ground water runoff to take place on both on top and below the soil surface instead of entirely above the surface. IN other words, in so far as direct flood run-off in contrast to surface run-off is concerned, many of the phenomena may not be relating to surface detention. Net rainfall-excess and infiltration relate not only to the surface of the ground but also to an indeterminate zone lying at and extending a considerable distance below the ground surface. In the development of the concept, I don’t believe we would be in any way retrogressing in our way of thinking, but rather we believe that we would be taking a step forward with respect to the knowledge that we earned about stream flow from drainage-basins of significant sizes. The rise and fall of hydrograph analyses in relation with the results of experimental data indicate that there is a dynamic phase of ground water flow associated with floods. This dynamic phase is of considerable significance in present problems and it is a phenomenon about which we have a lot to learn. It is in the study of this study of ground water can be very helpful. References 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