Storm Drainage Design Project: Scientific Methods - Assignment Example

STORM DRAINAGE DESIGN PROJECT I. Introduction Scientific methods are very important in predicting the behaviour of the earth's natural processes. There are lots of natural destructive phenomena that would have been averted should there have been ways of accurate prediction of their occurrences. Earthquakes, hurricanes, typhoons, and many other natural calamities have wreaked havoc and destroyed lives and property. As a matter of fact, the majority of disasters are somehow related to water. Floods, storms, hails, tsunamis, rise in the water of oceans and a host of other global events are brought about through the destructive action of water. This project composes the discussion on the studies that have been made in the conduct of rivers, particularly on the use of storm hydrographs. To understand more on the subject at hand, we first define what is a storm and a hydrograph. According to Wikipedia, a storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather. It may be marked by strong wind, thunder and lightning (a thunderstorm), heavy precipitation, such as ice or wind transporting some substance through the atmosphere.1 Meanwhile a hydrograph is defined as "a representation of flow for a particular stream and is used to evaluate the watershed runoff volumes and the peak flow rates from a single storm or set of storms. This information is then used in the design of flood control facilities."2 A storm obviously has a fundamental effect on the course of rivers and other bodies of water and is part of the process known as the water cycle. The water cycle "..consists of four distinct stages: storage, evaporation, precipitation, and runoff. Water may be stored temporarily in the ground; in oceans, lakes, and rivers; and in ice caps and glaciers. It evaporates from the earth's surface, condenses in clouds, falls back to the earth as precipitation, and eventually either runs into the seas or reevaporates into the atmosphere."3 Thus, we see the relationship between storm and runoff (which is flooding) in this cycle. Storm hydrographs are therefore "..graphs that show how a drainage basin responds to a period of rainfall. They are useful in planning for flood situations and times of drought as they show the discharge (amount of water reaching channel via surface run-off, through flow, and base flow) that originated as precipitation."4 The hydrograph in other words is simply a record of the flooding events in a river as an effect of rain and storm. II. Factors Influencing Hydrographs What are the factors that influence the appearance of storm hydrographs Since the hydrograph is a graphical representation of the flow versus time, any aspect that will affect flow will likewise affect the graph. Flow, being a unit of volume over time or velocity and area, implies that the parameters of area and velocity contribute to how the hydrograph is presented. The following are the factors as stated in Wikipedia : Soil Saturation is dependant on previous rainfall, or otherwise known as antecedent rainfall. The surroundings; Rural or Urban (Could be less impermeable surface, or the surface type could vary) Vegetation type (Deforestation and amount of interception) Steepness of surrounding land, or 'relief' land Drainage density (Number of tributaries) Geology (Rock Type; Impermeable=flashier hydrographs. Or Permeable) Season dependent; Very dry weather creates a crust on the river bed. Wet winters create increase in discharge. Soil Type (Clay, sand etc.) Clay would create a flashy hydrograph, but there could be a continuum between the two.5 As can be observed from the above factors, when the factors contribute to a faster flow, the resulting hydrograph is a "flashy" one. It means that peak flow is high and the time from the heaviest rainfall and the largest flow of the river is very small. It is noted that the term "flash" floods are similarly coined. Other factors mentioned above such as impermeable surface and strata, steep surrounding land cause a faster velocity of the flow. "To say that a hydrograph is flashy means that the graph depicts sharp vertical jumps and equally steep vertical declines. What this means for the actual stream represented by the graph is that a flashy stream is one that exhibits significantly increased flows immediately following the onset of a precipitation event and a rapid return to pre-rain conditions shortly after the end of the precipitation."6 7 The table above depicts the effect of urbanization to the hydrograph. Due to urbanization, there are more surfaces that are impervious to seepage, as such, a larger runoff is expected and a larger and more rapid peak flow. The vegetation of the area affects the graph. "Trees in a river catchment area delays water by intercepting it. This reduces run-off and therefore time lag and river discharge. Where land has been built on by humans, time lag is shortened due to the covering of the ground by impermeable surfaces (tarmac and concrete for example). This will decrease the time lag and increase discharge."8 Since the hydrograph is intrinsically a historical data, this can be used in the design of structures related in the river water conveyance and control. Some scientific methods have even been developed from hydrographs to cater to the specialized work. "These methods are simple, requiring only an easy determination of watershed and land use characteristics. Therefore, these methods serve as useful tools to simulate runoff from ungauged watersheds, design, rainfall events and watersheds undergoing land use change."9 III. Design of Pump for a River In designing a pump for a river, a hydrograph can significantly help. We look at the requirements to be able to consider the relevant factors in the design. In this particular example, the river has a max volume of 7 million cubic meters and the water shall be diverted to a rectangular channel. If the flow of the channel shall be from a high to a lower elevation, the pump to be designed shall consider the effect of gravity on the flow of the water. If the channel were conveying the water to a lower elevation than the river, then the water from the river would flow freely without much need of a pump. However, if we consider the length of the channel and the loss of momentum due to the friction of the medium travelled upon, then a pump is needed to overcome this. In the case that the channel will convey water to a higher elevation, a high capacity pump must be utilized. "There are two main categories of pumps and they are dynamic pumps and positive displacement. These two categories also have many subcategories of pumps. The selection of the pump class and type for a certain application is influenced by system requirements, system layout, fluid characteristics, intended life, energy cost, code requirements, and materials of construction. "10 Dynamic pumps most often have lower efficiencies than positive displacement pumps, have lower maintenance requirements, and are able to operate at high speeds and high flow rates.11 Dynamic pumps are also able to operate at fairly high speeds and high fluid flow rates. Examples of these are centrifugal and axial flow pumps. Positive displacement pumps impart far more power than dynamic pumps in the sense that the high-power density that it can achieve. This means that it can push the fluid even though it is dense. It is used for viscous liquids. IV. Conclusion A dynamic pump would be most applicable in pumping a tremendous amount of water from the river. Specifically, a centrifugal pump would be opted provided the following conditions are met during its service life: river water coursing thru the channel shall be screened first of debris and sand so the viscosity of the water is not affected and "..should not be used at a flow rate less than 50% of the B.E.P. (best efficiency point) flow rate B.E.P. is the point on a pump's performance curve that corresponds to the highest efficiency."12 V. List of References AbsoluteAstronomy.com (2009) Hydrograph [online] available from [23 April 2009] Beilby I., DeMay J., Sheehy J. and Woodworth A. (1995) Hydrographs [online] available from [23 April 2009] Chaurette, J. (2009) Pump and Pump System Glossary [online] available from [23 April 2009] Kwon, H.Y. (2000) PPT Slide [online] available from [23 April 2009] McAllister D., Slade B. (1994) Pumps [online] available from [23 April 2009] McAllister D., Slade B. (1994) Dynamic Pumps [online] available from [23 April 2009] McAllister D., Slade B. (1994) Types of Pump [online] available from [23 April 2009] Microsoft Encarta Online Encyclopedia (2009) Water Cycle [online] available from [23 April 2009] Royal Geography Society (2009) Glossary [online] available from [23 April 2009] S-Cool Limited (2008) River Profiles [online] available from [23 April 2009] Wikimedia Foundation, Inc. (2009) Storm [online] available from [23 April 2009] Read More