Storm Drainage Design Project - Scientific methods Assignment

Storm Drainage Design Project - Scientific methods - Assignment Example 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; i n 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 surf