Effects of Urbanization on the Hydrology - Essay Example

Hydraulics Hydraulics Q1 a) Effects of urbanization on the hydrology of catchment and SuDS mitigation The general effectwhich has become evident due to urbanization is reduction in the infiltration of water into the ground. Reduced infiltration is vital in groundwater recharge and increase in the speed of runoff above normal conditions (JEFFERIES, 2011 p 102-134). Urban areas have most of the surfaces covered with concrete or tarmac ways. The covered surfaces contribute significantly to the hydrological cycle characteristics of the urbanized catchment areas. Consequently lead to the deterioration of water quality at the water sources and catchment areas being prone to flooding. Most of the ancient towns and cities in United Kingdom are close to the river always experiencing damages from flooding over time. SuDS are small scale source control measures which are useful in draining surface water in a more natural way through infiltration, retention and storage devices in urban areas. They employ methods that mimic natural drainage systems by passively removing contaminants in surface water and naturalizing flow rate (JEFFERIES, 2011 p 189-221). After reducing the water volumes and pollutants near the water source, a corresponding reduction in the rate of flow downstream and flood discharges are evident. Reducing rate of downstream flow and flood discharges contribute in improving the quality of water. Further improvements are realizable due to reduced storm water into sewers because little surface water spills over into the sewer system, this spillage if often allowed force raw sewer discharge into the water courses. Certain infiltration based systems such as permeable pavements, swale and detention ponds are always important in controlling the effect of storm water in built-up areas. Permeable pavements are alternative grounds that allow storm water to filter through the voids into the underlying stone reservoirs for temporarily storage (KING, & WISLER, 2008 p 117-131). Permeable pavements consist of permeable surface layer, bedding a layer, underlying stone aggregate reservoir and a filtration layer laid at the bottom. Swales are shallow channels for collecting, and removing pollutants from water (KING, & WISLER, 2008 p 207-231). Swales have shallow side slopes and a flat bottom. They usually have grass covers and mostly water flows in a thin layer through the grass. Detention ponds are for improving the quality of urban runoff and reducing the rates at which the peak storm flows this is possible by providing temporary storage during large storms. b) Peak Flow T(N) TUH(m^3/s) 25mm 33mm 6mm storm(m^3/s) 0 0 ---- ---- 0 1 3 7.5 0 ---- 7.5 2 13 32.5 9.9 0 42.4 3 21 52.5 42.9 1.8 97.2 4 19 47.5 69.3 7.8 124.6 5 9 22.5 62.7 12.6 97.8 6 2 5 29.7 11.4 46.1 7 1 2.5 6.6 5.4 14.5 8 0 0 3.3 1.2 4.5 0 0.6 0.6 0 0 Q2 a) Differences between Slow Sand Filters and Rapid Gravity Filters Design Slow sand Filters have a filtration rate of 0.15 m3/m2•h, the area per filter bed is less than 200m2, and an effective grain size of 0.15m to 0.35mm. Slow sand filters system consist of two filter beds and the depth of each filter bed is at most 1m. Supernatant water depth in rapid sand filters range between 0.7m to1m (MAYS, 2011 p 139-142). Rapid sand filters have a filtration rate of between 5 and 15 m3/m2.h. The effective grain size is ranging between 0.4m to 1.2 mm. Depth of filter material has allowance between 1.5m to 2m. Supernatant water depth for rapid sand filters is 0.6-2.0m. The slow sand filters are larger in size almost 2000 m2 while the rapid sand filters are comparatively just about 100 m2 Figure1: Diagram showing a typical rapid sand filter system Figure 2: diagram showing slow sand filters system Operation Slow sand Filters maintenance cleaning is always when the fine sand clog. The cleaning is by scraping off the top layer of the filter bed. Routine clog clearing is either after some weeks or one-year period (MAYS, 2011 p 140-152). In this slow sand filter cleaning technique, the technician can eliminate a film of filtering fabric occasionally, this help in reducing frequent replacement of the upper layer. Rapid Sand Filters consists of regular back washing and cleaning between 24 to72 hours. Cleaning process usually requires an interruption of the purification process for a period of 5 to10 minutes per filter bed to achieve this successfully there should be a number of parallel filter units to ensure constant water supply. The maintenance team must ensure careful back washing process to guarantee that the rate of flow is under control to avoid the filter medium erosion. Experts must be available to operate the rapid sand filters while in slow sand filters experts must not be available Treatment For Slow sand filters after treatment, the turbidity level should be less than 1.0 NTU while Rapid sand filters turbidity level being 5.0 NTU. Slow sand filters use is in regions with large areas of land are available since they occupy vast areas of land (MAYS, 2011 p 130-156). Rapid sand Filters are usually useful in the treatment of large quantities of water when the area available for the water treatment plant is limited. Pretreatment by coagulation is necessary for the rapid sand filters but not in slow sand filters. The slow sand filters remove almost 99 percent of bacteria compared to around 90 percent in rapid. Q2 b) Impacts of Raw Sewage into Rivers Organic matter present in sewage uses up the dissolved oxygen in the river during decomposition depleting oxygen which is useful for the survival of aquatic life. Raw sewage has the effect of raising the turbidity level of the water, increasing the amount of suspended particles that reduce the intensity of light present for plant growth (MAYS, 2011 p 120-146). The suspended particles may also choke the fish gills. Raw sewage releases warmer or cooler water into the receiving water body which alter the normal temperature ranges within which the aquatic life survive favorably. The unfavorable temperatures affect the survival of eggs, larvae fish and ecosystem productivity. Raw sewage also introduces pesticides, chemicals and heavy metals into receiving water that find their way into animal tissues and have long term effects on them. The three stage conventional treatment helps in ensuring that human and industrial wastes are disposed off to water bodies in the required standard. It consists of a combination of physical, chemical and biological processes (MAYS, 2011 p 98-115). The three stages in conventional treatment include primary; secondary and tertiary processes. Primary process is mainly to remove suspended particles which mainly occur in at the screen and the grit chamber. Secondary process is useful in removing dissolved and suspended biological matter. Tertiary process is the final treatment stage that improves the effluent quality further before discharge to receiving waters by removing residual suspended matter and toxins. Q3 a) calculate the pressure at the hinge Pressure at hinge= 1x 9.81x 7.2= 70.632 =70.632KN/m2 b) Force on the gate Area= 0.65 x 0.45= 0.2925m2 Location of CG (y) = 7.2+ 0.68Sin 15/2 = 7.288m F= 9810x 0.2925x 7.288= 20912.36N =20.912kN c) Location of the Point of Action yp IG= 0.65x0.453 = 4.935m4 12 yp= 7.288+ 4.935sin215 = 7.44m 0.2925x7.288 yp= 7.44m Q4 a) Volume of water displaced by pontoon Volume of water displaced = volume of the pontoon 18m x 11m x 5.5m= 1089m3 b) Depth of immersion Weight of pontoon= weight of water displaced 7553kN= 9.81x18x11xd Depth of immersion= 3.89m c) The distance BM BM= I/V 18X113/12 = 2.59m 18X11X3.89 d) The value of GM Position of center of buoyancy= 3.89/2= 1.95m Position of C.O.G= 5.5/2= 2.75m BG= 2.75-1.95= 0.8m GM= MB-BG= 2.59-0.8=1.79m e) Pontoon stability Yes Bibliography JEFFERIES, C. (2011). The SWITCH Transition Manual: Managing Water for the City of the Future. Dundee, University of Abertay Press KING, H. W., & WISLER, C. O. (2008). Hydraulics. Charleston, SC, BiblioBazaar. MAYS, L. W. (2011). Water Resources Engineering. Hoboken, NJ, John Wiley. Read More