Design of an Open Channel with Assessment of Pollution Transport - Essay Example

CIVL 3076 Environmental Fluid Mechanics Design of an Open Channel with Assessment of Pollution Transport Introduction An open channel is defined as aconduit in which a liquid flow with a free surface. The flow normally occurs under pressure as contrasted with liquid in a pipe, the liquid conveyed by an open channel exerts no pressure other than that caused by its own weight, and pressure of the atmosphere. The general theory applies to all liquids but since there are a few test data available on open-channel flow of liquids other than water at natural temperatures, it only applies to water. Open channels may be either artificial or natural. Natural water channels vary in size from tiny side-hill rivulets through brooks, small rivers, and large rivers, to tidal estuaries. Underground streams, in caves are considered open channels as long as they have free surfaces. Natural channels are usually irregular in cross section and alignment and in character and roughness of stream bed. Streams with erodible material may frequently or continuously shift their location and cross-section. Such irregularities and changes in natural streams introduce engineering problems, for example, in navigation and flood control, which treats only of flow in fixed channels of uniform roughness. Artificial channels are those that are built for various purposes. In water power development, water is brought from streams or reservoirs to head works above water plants. In irrigation, water is brought from streams or reservoirs to storage ponds or tanks or directly to lands to be irrigated. In the city water supply, water is brought from streams or storage reservoirs to ponds supplying city distribution systems. Sewerage, city sewerage, although usually covered conduits or pipes, ordinarily are designed as open channels because they are not supposed to flow but to have a free surface under atmospheric pressure. In drainage, low-lying, swampy, or waterlogged lands are frequently made productive by draining them through open ditches or by laying and covering pipe which may or may not flow full. In flood control, protection of cities or valuable lands from floods often requires improving a natural channel by straightening, cleaning, or paving to increase its capacity, or buy building additional flood channels on new locations. Design of the open channel: From the topographic map that was taken from the Data Library, University of Edinburgh, the first thing to be done is to modify further the topographic map. The designation of elavations to 1(one) meter interval to get the precise elevation of the pathway of the proposed design of the open channel is important. From the modified topographic map, we are now able to pinpoint the exact location or track of the open channel. From the elevation of Esthwaite water which is 65 meters, it will flow down to an elevation of 39 meters which is the elevation of Windermere Lake. The analysis of the critical points of the channel is very necessary because it will determine the design of the open channel to be constructed. The length of the open channel is approximately 3.25 kilometers from Esthwaite Water to WindermereLake. Assign point 0+000 at the mouth of the channel which is at southern most tip of Esthwaite Water. Hence, cross-section of the open channel track at a 100 meters interval must be plotted in order to make the specific design or the slope and elevation at that particular place or station. Aside from the plotting of the 100 meter interval of the proposed track, critical points must also be noted and, cross sectional drawings must be done. This is to make sure that the design of the open channel is efficient, and that the estimate to be done in the construction is factual. But as we can see from the open channel layout map, the elevation from the southern tip of Estwaite Water which is station 0+000 to station 2+000 is constant. But at station 0+030, the channel will have to cross a road. The road elevation is at approximately 67 meters as shown in the map, therefore the channel has to be build under the road crossing or a short bridge so as to give way to the open channel to pass under the bridge or road. Station 0+030 will be considered a critical point in the design, because the velocity of the water passing under the bridge decrease depending on how the bridge would be build. In consideration to the design of most economical point of view, the bridge pier to be constructed must mot pass thru the channel, instead it must be designed having the channel pass thru between the two piers of the bridge to be constructed. Thus, making the horizontal flow of water in the open channel constant. As seen in the channel layout, at station1+175 or Pt. A in the map, the channel will pass between two higher elevations and that a cross section of Pt. A must be plotted in order to know the amount of soil to be excavated in that area, but the course of the channel remains horizontal until it reaches Pt. B which is another road with an elevation of approximately 67 meters. A bridge must be constructed in order to give way to the channel underneath. The design of the bridge may or may not affect the channel under it. The bridge must be designed in such a way that the channel under it must pass thru between the bridge posts or piers. At about 15 meters after station 2+000, the elevation starts to go down by 5 meters. At this point, the flowing water would now experience a gradual change in the flow rate as it reaches down the elevation of 50 meters at approximately station 2+900. From this station, the flow rate of water becomes greater because of the change of elevation at short intervals until it reaches the mouth of Lake Windermere which has an elevation of 39 meters. As it approaches the lake, a hydraulic jump occurs because of the merging of water in the lake and water coming from the open channel. After all considerations had been studied, the design of the waterway is to be finalized. Open channel can be designed using the concept of maximum permissible depth of flow. It is the fundamental premise of this approach that for a selected channel, there exist a limiting depth of flow above which scour will occur. Depths that is less than the limiting depth is non eroding. This depth which is limiting is known as the maximum permissible depth of flow. The maximum permissible depth of flow for a particular lining material is determined by channel gradient and the erodibility of the underlying soil. The design can also be based on the concept of the maximum permissible velocity. Both concepts are connected through the Manning Equation. The following are the design steps to be followed. Perform hydrologic computations and select design flows. Estimate the erodibility of the soil. Next is to define the type of channel lining materials desired. Definition of channel slope and restrictions on channel geometry. Determine the maximum permissible depth of flow or the maximum permissible velocity of flow, for lining materials Also, select channel geometry and channel lining which is suitable for the design flows that are being considered. Do not forget to consider other possible factors. Waterways are normally sized to carry the runoff from a 24 hour rainfall with a 10 year period, computations on hydrology are necessary. For the design of the temporary lining, a lower return period is considered if a vegetative lining is feasible, and a temporary lining is to be used during the establishment period. Usually, the materials for temporary lining are biodegradable. Bare soil, or straw with erosionet are used for temporary linings. Soil may be identified as erosion resistant, slightly erodible, moderately erodible, very erodible or highly erodible. The use of the design charts for maximum permissible depth of flow requires specification of the soil erodability class characteristic of the underlying soil. In general, sandy non cohesive soils tends to be highly erodible, large grained gravel-silt-clay mixtures are erosion resistant, and colloids are moderately erodible. The estimate of erodability class are usually based on the determination used in the conjunction with the Universal Soil Loss Equation. The hydraulic and scour resistance of the waterway or open channel are the factors that the lining materials determine. The initial capital outlay and the cost of labor machinery required for maintenance are the choices that should be based on economic considerations. Specification of riprap should include rock size, thickness of riprap, toe trench dimensions, but also durability, hardness, angularity, and resistance to weathering. Knowing the resistance or the constrains that are to be placed on the channel geometry is also important. The constraints may have the effect of increasing or limiting the size (width, depth, or both) of the channel. Roads, buildings, or established property lines may limit the available space. Hence, restrict the waterway width and or the side slopes can often be achieved by the use of more rigid lining materials that incorporate some 'retaining wall' features. On the other hand, for purposes of safety, erosion resistance, construction or maintenance ease, channel side slopes involving flexible ,linings should be kept relatively flat. It is suggested for erosion resistance that the side slopes must not be greater that 3:2 for medium textured soil, 1;1 for cohesive well-drained clay, and 4;1 for non cohesive sands. The side slopes should be 2;1 or flatter for erosion, resistance, and still flatter slopes may be necessary for construction or other reasons. The geometry of the channel can be determined for selected channel lining material son the basis of factors such as channel slope, water flow rate, and soil erodability. The stage of the design involves one-step for unlined channels, channels lined with straw mulch, or channels lined with rock riprap. Cross-sections can be determined by using the Manning equation. The hydraulic radius, wetted perimeter, and area are all functions of depth. The channel is sized so as to ensure that the flow depth is less than or equal to dmax. For channels lined with grass mixtures, waterway dimension should be determined for conditions when the channel is susceptible to erosion, and that is when the established vegetation is dormant. This provides a stable waterway cross section for the maximum permissible depth of flow occurring on the established vegetative lining in the most vulnerable condition. The next step is for the vegetated channel design to determine the depth of flow that is required to transmit the design flow rate when the grass mixture is mature and dense. The channel is less susceptible to erosion with this condition, but this is providing maximum retardance to the water flow. The required depth to carry the design flow over the increased conditions has been termed as the depth to design capacity. There are other design considerations. The freeboard- where most lining materials should extend to the top of the bank or at least 3 ft above the design water level. Protection in bends - these extra protection from erosion are often required at bends and corners of a channel with flexible linings. Circular curves should be used. Tile outlets - both the outlet portion of the tile and the bank surrounding the outlet pipe require special consideration. Construction and maintenance of equipment - a decision on the final size and shape of the waterway should take into consideration the type of equipment to be used. The channel design may be widened or curved, and the side slopes maybe flattened to facilitate construction, maintenance or both. Computations: With the conditions given for considerations, that the elevation of Esthwaite Water is 65 meters and that a peak of daily rainfall should not exceed 66 meters in elevation, therefore we can safely conclude that the depth of 1.0 meter, a rectangular open channel be used in the design. As expected, change in flow rate would occur at given points or critical points. There are factors that may affect the given points like road crossings and changes in elevation of the channel route. In the case of this design, the shortest possible route of the open channel with the most economical consideration for the construction can be seen in the attached map. Computations of the design are done by assuming a value for the discharge Q, determining by trial the depth of uniform flow for that Q, then testing the solution by the equation of energy at the entrance, neglecting velocity head in the reservoir. An elaborate investigation of all available records of measurements of flow in an open channel has to be performed. As shown in the computations, assumed values of Q are to be considered in the trial and error method of computation. Solving for the critical depth, and its location from the brink. Specific energy can also be computed from the assumed value of Q. Getting the Froude Number to determine whether it is subcritical or super critical. As water enters the canal from a reservoir and discharges over a fall, critical depths in this case occurs a short distance upstream from the brink of the fall. O'Brian found that, for a channel with level bed, the location of the critical depth is approximately 12 dc upstream from the brink, the distance increasing as the slope of the channel increased. Performing similar solutions for successive reaches gives us the value of lengths. As the depths of uniform flow is maximized, the difference between Sc and S approaches for even a small change in depth, the curve of the water surface is asymptotic to the line d, but practically uniform flow exists for several hundred meters downstream form the entrance. As we progress in the computations the Froude number may change from supercritical to subcritical or vice versa. The position of jump would occur after station 2 + 000. This is so because there is a change in elevation in the ground surface. The elevation becomes steep and that the hydraulic jump will be experienced. At this point Froude Number will be more than 1 (one) and the flow is super critical. At the end of the open channel, or at the entrance of Lake Windermere, there will also be a hydraulic jump due to the merging of the flowing water from the open channel and the water at the lake. Materials to be used for the walls and bed of the channel are to be studied carefully. In rectangular channels, neat concrete or cement finish is applicable. This is to avoid erosion in times of storm where in the channel is subjected to a great volume of water . If we use some other materials for the walls of the rectangular channel, the erodability of the material to be used must be taken into consideration. Such materials must be able to withstand the high pressure applied to them during a steady flow of water from Esthwaite lake during runoff. The most suitable material for the stream bed is natural vegetation. Plants that lives in water are the best materials for open channel beds. Grass is one that is suitable for the bed. Grass also acts as strainer for pollutants coming from upstream. They would act as blocks to the impurities from upstream. A flow meter is a device used to measure the flow of liquid at a certain point in an open channel. There are several kinds of flowmeter that are used. But just the same all of them are used to measure the height or HEAD of any liquid as it passes thru an obstruction. The obstruction could either be a flume or a weir. POLLUTION TRANSPORT Often times, milk is recognized to have a significant polluting effect on water, if allowed to reach rivers or streams or any body of water for that matter. It can be as much as 400 times more polluting than untreated domestic sewage. Naturally, bacteria, break down milk entering a watercourse, using up oxygen in the water more quickly than it can be replaced. As a result of falling oxygen levels, fish and other creatures can suffocate. It can make water not suitable for the downstream domestic users, and kill aquatic life and cause long-term damage to waterways. In the case of the spilling of 19,000 liters of milk in the southern most tip of Esthwaite Waters, The pollution of the water in the lake will affect the water. Considering that an open channel that was designed to flow from Esthwaite Lake to Windermere Lake which is approximately 3,250.00 meters long, the water that will flow from the lake upstream to Windermere lake will have its share of the pollutant. Milk has a specific gravity of 1.02 to 1.05. Comparing the specific gravity of milk with water which has a specific gravity of 1.0, we can say that the portions of the spilled milk would mix with water because their specific gravity are almost the same. The difference in the specific gravity of water and milk is almost negligible. So when milk and water mixes together, bacteria would set in. With the volume of the milk spilled which is 19,000.00 liters which is equal to a fractional percentage of water in the lake, pollution would be a great threat. The design of the open canal happens to be in the southern most tip of Esthwaite Lake and it was also where the truck carrying the milk accidentally spilled its contents. Considering the ratio of milk to flow down the open channel, it would only take a day or two for the spilled milk to travel down to Windermere Lake. With the length of the channel which is more than 3 kilometers, and with channel bed of grass and other plants that live in water, the pollutant spilled would already be almost zero. The grass bed or bottom lining of the open channel would block portions of the pollutant as it go down the open channel. As it reaches the mouth of the downstream lake it the milk pollutant would have already evaporated or is blocked by the natural vegetation in the open channel bed. References 1. M. P. O'Brien "Analyzing Hydraulic Models for Effects of Distortion", Engg News-Record, September, 1932 2. H. Rouse, Discharge Characteristics of the free Overfall, "Civil Engineering", 1936, p. 257 3. Horace W. King, Chester O. Wisler, James G. Woodburn, Open Channels, "Hydraulics", 1975, Pp.240 -290. 4. Diego Inocincio T. Gillesania, Fluid Mechanics and Hydraulics, Engg. Formula Series, Civil Engineering. 2006, pp.389 - 433. 5, Hugh D Young, Roger A. Freedman, University Physics "Fluid Mechanics" , 2000, pp. 427 - 455. Read More