Storm Drainage Design - Coursework Example

Storm Drainage Design Project Introduction Hydrology and hydraulic modeling systems are extensively used by infrastructure engineers and crisis management departments for estimation of availability of water, design of dams, diversions, and flood control through reservoirs or channels, etc. River statistics such as catchment area, river flow measurements, base flow and lag times is gathered through various sources such as satellite imagery and a network of stream gauges. Hydrograph analysis involves the study of runoff records at a stream gauge, river discharge after a rain and drainage rate of the river. Hydrograph analysis is also used to explain various river behaviors such as how a watershed responds to rainfall and how lag time is related with other parameters such as soil conditions and geology. It also helps in finding out discharge patterns of a particular drainage basin and helps engineers and planners to assess the potential of a river to flood and predict flooding events, therefore influence implementation of various flood prevention measures by means of water supply pipelines or channels, dams etc. This paper critically analyses hydrograph for Thaw River and assess various factors that determine a river’s potential to flood. It investigates the chances of taking water supply from the river by means of a pipeline or channel. The paper also discusses different design considerations for such pipeline or channel. Relevance of Appropriate Data Hydrologic mathematical models for flood prediction and management involve complex mathematical calculations. They represent a system’s physical behavior in terms of mathematical equations. In this sense, any hydrological analysis heavily relies on the data related to river and river basin. Though evapo-transpiration, effect of the stream-aquifer interaction, effects of other hydrologic processes such as interception and depression storage are altogether ignored for analysis (Ramírez, 2000), other parameters should be appropriately measured for a relevant hydrological analysis. Appropriate topographical data for watercourse such as information on structures, water drainage routes, potential blockages / obstructions to drainage is necessary for considering the effects of various factors on river flow. Hydrograph for River Thaw Shape of hydrograph is controlled by various tangible and intangible factors: Catchment shape and gradient: It determines the time taken by rain water to route in to river. A clear ‘V’ shaped catchment with sharp gradient gives flash floods and a steeper rising limb of the graph. Precipitation type and rate: It directly affects flow rate of the river and influence the drainage rate. A slow precipitation rate results in higher flow rate and lesser lag time. Soil saturation: It is remaining water in soil from previous rainfalls. A saturated soil completely drains rain waters in to river which means rainfall is directly reflected in river level. Land use and vegetation: Forestation and urbanization gives hindrance to rain water. Drainage capacity: Number of tributaries and side channels also affect lag time of stream. Lesser number of tributaries gives rise to a longer lag time. Soil type: Such as clay or sand affects drainage of rain water and absorption by the earth. There are several other factors such as weather, seasons and geographical location that affect the shape of hydrograph. For example a wet weather slows down the evaporation of water and increase water discharge in cold regions. Several of these factors are often difficult to measure and generally assumed for the hydrological calculations. The relationship between these factors with the shape of hydrograph can also not be ascertained in definite terms due to several inconsistencies in measurements of these factors. Hydrograph of Thaw River shows the change in river level caused by a period of rainfall. Though it is drawn against a very limited time period of 24 hours, it shows nature of Thaw and gives a clear understanding of different parameters that may help in designing a channel or pipeline for water supply. The hydrograph shows that the rainfall is almost simultaneously reflected in river level which indicates that Thaw River has a clear catchments area that provides a faster drainage for rain water. A consistent peak flow level in the hydrograph shows that the river do not discharge enough water which gives a possibility of water supply channel. It also indicates that if river level does not go down by some means of discharges further rain fall may result in flood situations. However, exact prediction of flooding situations is difficult to make as data referred for this paper is limited to a very little span of time. The current hydrograph does not answer several questions such as if there is any discharge by the river or if this rainfall is regular, which are very important to assess the river’s potential to flood. Further investigation and data on several factors such as precipitation type and rate, land use and vegetation, number of existing tributaries and rain fall statistics for a longer period of time is necessary in order to predict Thaw’s nature with respect to flooding. Information of existing flood control measures such as dams and canals also help in assessing probability of future floods. Effect of Rainfall Patterns and Other Conditions on River Level River level at any particular time is affected by several factors such as rainfall pattern of the region, direct runoff, base flow etc. rainfall, runoff and base flow are basic considerations for any hydrograph. The rainfall-runoff relationship describes the time distribution of direct runoff as a function of excess rainfall (gross rainfall minus losses). In periods without rain, the baseflow in a stream will slowly decline due to seepage, evaporation and leakage to the groundwater aquifers. This phenomenon known as baseflow recession is often assumed that baseflow declines exponentially (Ratnayake, 2004). Baseflow separation involves dividing the hydrograph into a direct runoff component and a baseflow component. Basin lag time, the time required by rain water to reach river, which determines the slope of rising limb of hydrograph is affected by several geological factors such as soil type and catchment region. A longer basin lag time may result in loss of rain water and may affect the river level. Critical duration storm, floodplain and backwater effect (the effect where a dam or other restriction raises the surface of the water upstream from it above the normal water level) affect the river flow level. River Flow Volume: Q = Q1 + Q2 +……………………………………. + Q24; Q = (l1 * w * v) + (l2 * w * v) + (l3 * w * v) + …………………….. + (l24 * w * v); Where li is river level at each hour, w is width of the river and v is the velocity of river flow. It is assumed that width and velocity remains constant at 20 m and 3 m/s respectively. Time Rainfall (mm) River Level (m) Qi 0:00:00 0 0.33 19.8 1:00:00 0 0.33 19.8 2:00:00 0 0.33 19.8 3:00:00 0 0.326 19.56 4:00:00 0 0.326 19.56 5:00:00 0.2 0.33 19.8 6:00:00 0.6 0.326 19.56 7:00:00 4.2 0.345 20.7 8:00:00 6 0.393 23.58 9:00:00 3.4 0.412 24.72 10:00:00 2.4 0.431 25.86 11:00:00 3 0.472 28.32 12:00:00 3.2 0.532 31.92 13:00:00 2.4 0.577 34.62 14:00:00 0.2 0.6 36 15:00:00 0.2 0.626 37.56 16:00:00 0.2 0.633 37.98 17:00:00 0.2 0.637 38.22 18:00:00 0 0.637 38.22 19:00:00 0 0.641 38.46 20:00:00 0 0.641 38.46 21:00:00 0 0.637 38.22 22:00:00 0 0.637 38.22 23:00:00 0 0.633 37.98 Total 706.92 Total River Flow Volume Q = 706.92 Channelling Water from Thaw Base flow line, as shown by the following hydrograph, is an important determinant of whether water can be routed to a reservoir by the means of water supply lines or channels. It can be inferred from following diagram that water above base flow line (i.e. peak flow – base flow) can be drawn out from river. Design Considerations for Pipeline or Channels Design of a pipeline or channel to draw excess water from a river requires a though analysis of different factors that ensures a continuous water supply in long run such as availability of water, height of pipeline and cross-sectional area. Water routing pipeline or channel should be ideally above the base flow line to not further deplete river level while drawing water. It can be observed that the total volume of water that can be drawn from river, ideally, should be equal to the difference between total flow volume of Thaw during observer time period and volume if the river flows at base flow level for the same time. This means capacity of pipelines should be sufficient to draw excess water distributed over the observed period of time. The above discussion on hydrograph is still not a conclusive affirmation for constructing a pipeline as there are other factors such as financial issues, environmental and ecological issues that need to be analyzed before building such infrastructures. Conclusion: It can be concluded from above discussion that though building a perfect flood model is nearly impossible due to various intangible factors, hydrographs based on appropriate data can be effectively used for flood predictions and planning of flood control measures such as water reservoirs and water supply pipelines. The above discussion also highlights the importance of appropriate data in hydrological analysis. The hydrograph for Thaw River which shows a relatively smaller basin lag time and a consistent peak flow level gives a clear idea that excess Thaw water can be routed to a reservoir by means of pipelines or cannels. References: 1. Bergman, H. & Sackl B. (1985), Two dimensional frequency analysis of flood water flow and its application in water management; Proc. 21th Congress of IAHR, Melbourne. 2. Kay, M. (1998), Practical Hydraulics, Taylor & Francis. 3. Pilotti, M. & Rosso, R. (1990). Shell: A general framework for modeling the distributed response of a drainage basin, in: Computational Methods in Surface Hydrology, edited by G. Gambolati, A. Rinaldo, C. Brebbia, W.G. Gray & G.F. Pinder, Springer Verlag, Berlin, pp.517-522. 4. Ramírez, J. A., (2000). Prediction and Modeling of Flood Hydrology and Hydraulics. Inland Flood Hazards: Human,Riparian and Aquatic Communities Eds. Ellen Wohl; Cambridge University Press. 5. Ratnayake, U., Lecture Note by Dr. Uditha Ratnayake, Engineering Hydrology. Read More