Storm Drainage Design Project - Assignment Example

STORM DRAINAGE DESIGN PROJECT I. Introduction The concept of studying and controlling the behaviour of rivers figured way back in the ancient times.The importance of rivers is realized by considering its life-giving benefits to man. It is the source of fresh water and habitat of fisheries that gave sustenance to people dwelling near it. It is fascinating to note that the birth of civilizations came about in riversides. From the Nile River in Egypt to the Yangtze River in China, settlements began to be established by reason of sustainability. Rivers brought forth the creation of floodplains through the erosion of sediments from higher elevations and their deposition in the river's vicinity forming a fertile area suitable for agriculture. Subsequently, irrigation was developed as the need to channel the river's flow arose in order to sustain the crops and livestock. The ancient nomadic nature of man changed to a domesticated one: no longer was there a need to hunt for food. In these modern times, much headway has been made in harnessing the full potential of rivers. For instance, there is now a storage and systematic distribution of potable water from rivers through dams, reservoirs, pumps, and piping systems. Further, irrigation systems are now utilized to nourish plantations and livestock. The destructive nature of rivers in the form of floods is likewise much more minimized through the understanding man has gained in predicting their behaviour. The approach to understanding is both scientific and empirical and uses tools in recording and interpreting pertinent data from observations. This important tool is called the hydrograph. Hydrograph is defined as "..a time record of the discharge of a stream, river or watershed outleta hydrograph is a representation of how a watershed responds to rainfall."1 Hereunder is an example. Figure 1. Actual Hydrograph of the Minnesota River Basin showing the river's discharge (in cubic feet per second) from Oct. 1999 to September 20002 II. Main Body and Storm Drainage Design A storm hydrograph in particular "..shows the change in the water level from before to after the passage of the storm water through a given location."3 The factors that define the storm hydrograph basically depend on how the river reacts to rainfall. These factors can be summarized under the following: the characteristics of the river basin, the condition of meteorologic events, and land use. First, the characteristics of the basin refer to its physical attributes such as area, topography (in particular, drainage density directly proportionate to the number of tributaries), geometry, geology (of its underlying soil and bedrock), presence of vegetation, and its initial conditions. The trait that will cause a large runoff and a longer lag time (time interval from peak rainfall and peak river flow) is a large basin area while a short lag time can be expected for circular basins, steep slopes, less permeable soils and rocks, less vegetation and saturated underlying aquifers. The lag time is directly related to velocity of the water travelling thru the river: the faster the water travels, the quicker it will reach peak flow. Flash floods provides a case in this point. An actual regular occurrence is at Cvennes-Vivarais Region in Southern France which is venue of severe flash flood events. The river is characterised by steep slopes in the head tributaries of the Cvennes Mountains.4 Secondly, metereological events pertain to rainfall, weather, and tidal conditions. Short intense rainfall and extreme weather swings bring about rapid surface runoff, short lag times, and graphically correspond to a steep rising limb in the hydrograph. On the other hand, the effect of tides is to block or stifle the egress of flooding water leading to a gradual slope of the recession limb in the hydrograph, thus a longer period for the river to return to its base (normal) flow. Lastly, the type of land use directly affects hydrograph parameters. Urban areas are likely to be less permeable due to presence of concreted areas and more zones with industrial establishments rather than agricultural. Deforested areas are prone to flooding and which graphically would be described as having a steep rising limb and short lag time. The significance of hydrograph becomes evident in its use as an aid in the design of hydraulic structures like channels, culverts, bridges, storm drains, pump stations, and reservoirs. In the specific case of routing the river flow to a channel, the river's hydrograph is used to predict the largest volume of flow and flood levels, etc. These serve as the parameters for design of the proposed structures to be put in place (channels, dams, weirs, sluice, and possibly river training works to prevent the meandering of the river). For instance, the following data are to be considered in the selection of the most suitable pump: a river with a maximum volume of 7,000,000 cubic meters is to be routed to a rectangular channel with dimensions 4 meters by 0.6 meters. It may be presumed that the maximum volume of the river was taken from an annual record of the hydrograph for a number of years. "To measure river flow, we first measure the river's width, depth and velocity. This is called "gauging," and there are different ways to do this, depending on the river. When we do a gauging we measure the velocity of the water from at least 20 points across the river cross-section. These points are called "verticals." Current meters are used to measure velocity and they have buckets or propellers that rotate at a speed in proportion to the water velocity."5 The volume can then be derived by multiplying the cross sectional area measured and the river's mean average velocity based on the data taken from the current meters. Usually, the derivation of the channel dimension is done by the following method: "The depth and velocity of flow are necessary for the design and analysis of channel linings and highway drainage structures. The depth and velocity at which a given discharge flows in a channel of known geometry, roughness, and slope can be determined through hydraulic analysis."6 "The selection of the pump depends on the following guidelines: the source of water (well, river, pond, etc.), the required pumping flow rate, the total suction head, and the total dynamic head. There usually is no choice when it comes to the source of the water; it is either surface water or well water and availability will be determined by the local geology and hydrologic conditions. However, the flow rate and total dynamic head will be determined by the type of irrigation system, the distance from the water source and the size of the piping system"7 The total dynamic head defines the total distance that the pump can be able to pump the water including the friction contributed by the conveying system. The suction head refers to the suction pressure imparted to be able to convey a body of water vertically upward. Based on Figure 2 above, the selection of the pump can be made by virtue of the discharge and the total dynamic head required. Table 2. Factors to Consider in Selecting an Irrigation Pump9 ------------------------------------------------------------------- Pump Type Advantages Disadvantages ------------------------------------------------------------------- Centrifugal 1. High efficiency over 1. Suction lift is limited. a range of operating It needs to be within conditions. 20 vertical feet of the 2. Easy to install. water surface. 3. Simple, economical and 2. Priming required. adaptable to many 3. Loss of prime can damage situations. pump. 4. Electric, internal 4. If the TDH is much lower combustion engines or than design value, the tractor power can be motor may overload. used. 5. Does not overload with increased TDH. 6. Vertical centrifugal may be submerged and not need priming. ------------------------------------------------------------------- Vertical 1. Adapted for use in 1. Difficult to install, Turbine wells. inspect, and repair. 2. Provides high TDH and 2. Higher initial cost flow rates with high than a centrifugal pump. efficiency. 3. To maintain high 3. Electric or internal efficiency, the impellers combustion power can must be adjusted be used. periodically. 4. Priming not needed. 4. Repair and maintenance 5. Can be used where is more expensive water surface than centrifugals. fluctuates. ------------------------------------------------------------------- Submersible 1. Can be used in deep 1. More expensive in larger wells. sizes than deep well 2. Priming not needed vertical turbines. 3. Can be used in crooked 2. Only electric power wells. can be used. 4. Easy to install. 3. More susceptible to 5. Smaller diameters are lightning. less expensive than 4. Water movement past comparable sized motor is required. vertical turbines. ------------------------------------------------------------------- Propeller 1. Simple construction. 1. Not suitable for suction 2. Can pump some sand. lift. 3. Priming not needed. 2. Cannot be valved back 4. Efficient at pumping to reduce flow rate. very large flow rates 3. Intake submergence depth at low TDH. is very critical. 5. Electric, internal 4. Limited to low (less combustion engine and than 75 feet) TDH. tractor power can be used. 6. Suitable for portable operation. ------------------------------------------------------------------- III. Conclusion Based on the given data, it would be most suitable to select a pump that can withstand the very large flow rates and has the capability to pump sediments. In this case, a propeller, centrifugal vertical turbine propeller submersible, and a cylindrical vertical turbine are to be considered. Narrowing down these selections will require determining the total dynamic head from design conditions. This would involve knowing the elevation of the river and the channel system that would convey the pumped water to its final destination. These factors contribute to the total dynamic head that the pump can impart to be able to push the water to flow against gravity resistance (due to elevation difference), the pumping medium (the type of fluid) and friction (due to the surface resistance of the conduit carrying the pumped water). Assuming that our case involves the pumping of water from a river to a plantation through a open concrete channel, the most suitable pump would be a propeller pump. "Propeller pumps are installed in vertical discharge columns the superior choice for pumping very large volumes of water at relatively low heads."10 In effect, the pump can induce very large flow rates sufficient to serve the watering needs of large tracts of land. The total dynamic head (TDH) required would not be that high since the channel can be designed wherein the flow would be assisted by gravity. "Propeller pumps are used to pump neutral or aggressive liquids, pure or contaminated liquids, cold or hot liquids, and even pumped liquids with suspended solids."11 In the case provided, the value of hydrographs is apparent in the selection of a pump. It is interesting to note that hydrographs are also used in other modern applications such as in river forecasting where timely flood warning are issued to residents in riversides.12 IV. References "Flash Flood Basins." 2009. FLOODsite Consortium. 20 April 2009. Hill, M.H., PhD. "Storm Hydrographs." Jackson State University. 20 April 2009. . "Hydrograph." 30 March 2009. Wikimedia Foundation, Inc. 20 April 2009. . "Hydrograph." 01 January 2007. Brown Nicollet Cottonwood Water Quality Board. 20 April 2009. . Marek, Mark A. "Channel Analysis Methods." 01 March 2009. Texas Department of Transportation. 20 April 2009. "Measuring River Flows." 2009. Environment Canterbury. 20 April 2009. NWS Internet Services Team "A Brief Overview of the NWS Precipitation and River Forecasting and the River Forecast on the AHPS Hydrograph." 26 January, 2009. National Weather Service. 22 April 2009. "Propeller Pumps." VertMarkets, Inc. 22 April 2009. "Propeller Pumps." 22 April 2009. Allweiler AG. 22 April 2009. Scherer, Thomas F. "Irrigation Water Pumps." April 1993. North Dakota State University. 20 April 2009. "USGS Surface-Water Data for the Nation." 22 April 2009. United States Geological Survey. 22 April 2009. "What is a Hydrograph" Geology.com. 22 April 2009. Read More