Storm Drainage Design Project - Essay Example

Storm Drainage Design Project 0 Introduction Flood hydrographs are very helpful to study the flood potential of a river. The flood hydrographs are constructed using the hydrological data consisting of rainfall distribution in the basin and the corresponding rise the river flow depth. When the discharge of the river versus the time is plotted a storm hydrograph is obtained (Geobytesgcse, n.d.). Various components of a hydrograph consist of a rising limb which represents the initial portion of a hydrograph, the peak discharge which related to the maximum discharge estimated and the receding limb , which relates to the decreasing part of the discharge with time. In addition hydrograph also carries information of base flow, which is the initial flow condition of the river existing in river prior to any storm. The base lag refers to the time repose of the river with respect to a rainfall which signifies the drainage potential of the catchments. The time interval between the peak rainfall the time of occurrence of peak discharge is reported as the basin lag. The rivers with short basin lag are more prone to floods as the respond very quickly to rainfall and hence the discharge in the river increases tremendously sooner the catchment receives a rainfall. The two major component of the runoff from the catchment that contributes to the volume of water in the river are the surface flow, which includes all the flows through the surface of the river and sub surface flow component which incorporates all the ground water flow discharges into the river after the onset of a storm. The slow rise is the level of river flow depth signified the majority of volume reaching the river as ground water flow. The properly constructed hydrograph would be able to considerable amount of information of the behaviour of drainage basin with respect to a rainfall. Characteristics of drainage basin and precipitation, pattern of land use in that particular region, disturbance caused by human interventions, size and shape of drainage basin and major river management measures adopted are the factors that influence the runoff from a drainage basin into the river (Flood hydrograph, n.d.). Figure 1 - Flood Hydrograph for river Cynon 2.0 Hydrograph analysis of river Cynon. The flood hydrograph for river Cynon is prepared by plotting he discharge through the river along the Y-axis and the time along x-axis. In order to plot the rainfall distribution, the depth of rainfall is plotted along a secondary y axis with time along the x - axis. The hydrograph hence plotted is shown in figure 1 and provides the detailed information on the drainage characteristics of the basin. The reports available have said that the subsurface conditions of the river Cynon basin is low permeability soil that could result in low infiltration rates. Thus these conditions would result in high surface runoff rates to the river and as a result the depth of flow rapidly rises. Thus the hydrograph obtained have steep rising limb followed by early peak discharge levels. Further, the receding limb is less steep than the rising limb justifies the discussions presented earlier. Also, the lag time is estimated from the hydrological observations as 20 hours. The flood volume of the river is computed from the total volume of the water in river during the period of runoff, by multiplying the depth of flow in the river with the mean river width given as 15 m and the average river velocity given as 4 m/s. The discharge computation from the hydrograph is as given below Particulars of computation Equation Calculation Discharge computed for the rising limb Velocity Depth of flow in river (for the rising limb) width of flow 4 m/s 0.48 m 15 m = 29.1 m3/sec Peak discharge Velocity Depth of flow in river (for peak discharge) width of flow 4 m/s 0.658 m 15 m = 39.48 m3/sec Discharge computed for receding limb Velocity Depth of flow in river (for the rising limb) width of flow 4 m/s 0.34 m 15 m = 20.4 m3/sec Total area of the catchment for the river Cynon From records (CEH, 2005) 150 km2 Total rainfall depth received in the region From the river data given 15.8 mm Runoff coefficient for the area From records (CEH, 2005) 0.41 The total volume of water in the river 971700 m3 For basin lag of 21 hours, rate of accumulation of runoff water 971700 m3 / 21 46271.4 m3 / hour The rate of recovery for 40 hours 971700 m3 / 40 24292.1 / hour From the details of the hydrograph computation, the huge difference in the rate of accumulation and rate of receding emphasizes the potential for flood in the river. The usual methods adopted for the flood controlling the impact of river flood are through implementing different ways for retention, detention and sedimentation. Thus the most easiest and effective method to control the impact of floods caused by river Cynon is by constructing small detention facility which would help to prevent the damage caused by the flood. Once the excess water is stored in the reservoir, it would help to lower the flow volume and once the flood water recedes the stored water could be released safely. 3.0 Selection of pump If the field situations demands to lift the storm water mechanically against gravity due to various reason like uneconomical operational conditions of gravity systems or the lack of capacity of the river leading to the inundation of neighborhood systems . The pump is designed considering the parameters required for efficient lifting operation of the storm water. Even though rigorous engineering analysis is involved in the selection of the pump, the final decision on the selection is often made based on the collective opinions sought from the pump manufacturers, contractors and companies involved in the supply of necessary power for the operation (Hydraulic design manual, n.d.). The information available for the design of the pump in the given problem are: River width: 15 m; Average velocity of flow: 4 m/s; Type of river cross section: rectangular. The diameter of the pipe carrying water from the river to the pump is taken as 180 mm and the diameter of the pipe that delivers the pumped water to the open channel is taken as 140 mm. The elevation of the channel is fixed as 350 m and that of river is taken as 80 m The discharge through both the pipes (180 and 140 mm diameter pipes) is constant. The discharge to the inlet pipe (180 mm dia) is computed as velocity of flow area of cross section = 4 m/s (0.090.09) = 0.102 m3/s. This discharge is constant through the delivery pipe also. Hence, the velocity of the delivery pipe is discharge (0.102 m3/s) / Area (0.070.07) = 6.63 m3/s. The total head loss for pumping water from the river to the open channel is the sum of the head loss in the inlet and delivery pipe = 36.9 m. The difference in elevation between the river and the open channel is computed as 270 m (350 - 80 ). Vp2/2g + pp/ + zp + H = Vs2/2g + ps/ + zs + HL Or, 0 + 0 + H = 0 + 0 + zs - zp + 36.9 Or H = 270 + 36.9 m = 306.9 m. The power required in horse power is QH/746, where the discharge Q is in m3/s, is the unit weight of water taken as 9810 N/m3 and H is the head of water computed as 306.9 m. Or Power = 0.102 9810 306.9/746 = 411.6 HP 412 H.P. To make the pumping operations more efficient, it is recommended to provide the centrifugal pumps of total power rating 412 H.P . Centrifugal pumps are more efficient and easy to manage for these type of operations and hence recommended. The line diagram for details of pumps is given in figure 2. 350 M 80M Figure 2 : Line diagram of river Cynon, pump and open channel. 4.0 Open-Channel Design The design of the channel cross section is undertaken using the Manning's equation. According to Manning's equation for an open channel the velocity of flow is , V , is given as (1/n)( R2/3S1/2 ) where V is velocity (m/s); R is hydraulic radius (m); S is channel bed slope (m/m); and n is Manning's roughness coefficient. And the discharge, Q, through the channel is computed as, Q = A V , where Q is the discharge of the river in m3/s, V is velocity (m/s) and A is the area of cross section in m2. The data available for the calculation are as follows Discharge =0.9 m3/s , Mannings coefficient , n =0.020 , depth = 0.4 m, velocity =0.55 m/s and slope = 1/3000 The steps in computation are as follows 1. Compute the area of cross section (A) from known discharge (Q) and velocity (V). A = Q/V = 1.2 / 0.55 = 2.18 m2 Consider a hydraulic radius equal to 0.5d or 0.5 Velocity, V = (R2/3S1/2) / n = (0.2)2/3(1/3000)1/2 / 0.02 = 0.312 m/s R = (A/P) = (b d) / (2 d+b) d = 0.4m and R = 0.2 m or, 0.2 = 0.4 b / (0.4 + b) or, 0.8 + 0.2b = 0.4b or 0.2b = 0.8 or b= 4 m Breath of channel is 4m and depth is 0.4 m Area of channel = 4 0.4 = 3.6 m2 Maximum possible discharge through the channel is Q = VA = 0.312 3.16 = 0.99 m3/sec , hence acceptable. 5.0 Conclusions The following conclusions are drawn from the present study on the flood hydrograph for river Cynon.: (a) The hydrograph shows a very steep rising limb and less steep receding limp. This would mean accumulation of flood water in the river for a brief period of time and confirms the possibility of flood in the basin. Such a behavior might be due to the low permeable layer of soil conditions along with the land use pattern prevailing in the catchment area. (b) The pump design was undertaken based on the limited data available. The centrifugal pumps for the installed power of 412 H.P is proposed to undertake the pumping operations. (c ) The rectangular channel proposed for carrying flood water has following dimensions. Width of the channel: 4.0 m Channel height : 0.4 m References Centre for Ecology and Hydrology (CEH). 2005. 57004 - Cynon at Abercynon [online]. [Accessed 17th April 2009]. Available at [22 April 2009] Geobytesgcse (n.d.), Hydrographs and river discharge [Online] Available at [16 April 2009] Witpress.com (n.d.) Optimal unit hydrogrpah , [Online] Available at [17 April 2009] Flood hydrograph (n.d.) [Online] Available at [17 April 2009] Design Hydrographs (n.d.), [Online] Available at < http://www.egr.msu.edu/northco2/BE481/SCShydrograph.htm> [18 April 2009] Hydraulic design manual (n.d.), [Online] Available at [18 April 2009] Read More