Drainage Facility Design for Car Park - Essay Example

ESSAY Topic: A car park for a large shopping complex has to be constructed. Discuss the methodology to be adopted to estimate the rainwater flowing into the sewers from such a catchment (car park). The report should include the following issues: 1. Construction and the use of IDF curves. 2. Probable maximum precipitation and its estimation. 3. Rational method to estimate rainwater runoff including proper selection of Runoff coefficient. 4. Suitability of rational method to estimate runoff 5. Recommendations which methods are suitable for the above topic Car parks are the most needed place near any shopping complex. It observed that the customers do not like to visit a shopping complex without car parking facility. Car parks usually take large space and there are set standards to follow while constructing a car park. Designers follow minimum standards on planning dimensions, bay width, aisle width, ramp dimensions, planning grid etc. to provide sufficient facilities to the user and accommodate maximum vehicle varieties. As car park covers a very large area, it is necessary to have sufficient facilities to drain the runoff from the area. A proper drainage facility should be designed to avoid any water logging situation in the park. The runoff from the car park will depend on the rainfall. Rainfall at a point varies significantly with change in season and occurance. Hence, it is utmost important to have sufficient information on the maximum rainfall in the area. By knowing the amount of water coming into the catchment of car park will determine the quantity of water flowing out from it. The water flowing out of the car park will also depend on the type of parking surface, water traps in the parking area and time of concentration. This will greatly influence the water flowing out to the sewers. The water coming into the parking area could be determined by use of various hydrological modelling and it is also possible to develop different scenarios of the same. World is facing serious problems due to changing climate scenario. This change is due to unplanned use of resources and pollution caused due to the same. Changing carbon concentration in atmosphere has affected the climate system. This has also resulted in ever increasing natural disasters throughout the world in the form of earthquakes, cyclones, flood, drought, tsunami etc. The frequency of less probable extreme events has increased and the world is facing serious problems to meet MDG. With globalization and technological advancement people are looking for coping mechanism to live with the shrinking resource base. A major impact of this climate change has been on the rainfall pattern. The frequency, intensity and duration of the rainfall in many parts of the world have changed significantly in the recent past. Flash floods, acid rains and high intensity rainfall is a unique feature. Surface hydrology depends largely on the precipitation pattern, it is important to estimate forecast to reduce damage. Water harvesting is an effective way to conserve water resources. Runoff from the parking area could be used in many ways. Normally, there is provision to collect this water and divert it into sewer for safe disposal. This water could also be used for storage and recharge purposes in the areas with less rainfall availability. Recharging of the aquifer by use of this water is a good idea. The major task while planning for these systems is to calculate the maximum rainfall for the region. Various methods are used in estimating the probable maximum rainfall for an area. 1. Construction and the use of IDF curves: Intensity-duration-frequency curves are used in determining the intensity of rainfall. It is commonly used to calculate the value of (I) in rational method. These curves are the graphical representation of the probability of occurance of a rainfall of average intensity. The rational method illustrates a proper application of IDF curve. IDF curves should not be used to obtain storm runoff volumes. The Intensity-Duration-Frequency curve is plotted with duration and intensity on the horizontal and vertical axis. When more than one curves are plotted on the graph, it represents the average frequency between the occurance of a rainfall of given intensity. These curves are generated by using the rainfall data and frequency analysis using Gumbel distribution. There are many other methods available to generate IDF curves. Historical series of maximum rainfall intensities for different durations should be available for developing IDF curves. IDF curves are location specific and the expression varies between regions. These curves are usually available with the local environment agencies. The following figure is an example of IDF curve. Fig 1. IDF curve 2. Probable maximum precipitation and its estimation. Probable Maximum Precipitation (PMP) is defined by the Manual for Estimation of Probable Maximum Precipitation (WMO, 1986) as : "...the greatest depth of precipitation for a given duration meteorologically possible for a given size storm area at a particular location at a particular time of the year, with no allowance made for long-term climatic trends." Statistical estimation and meteorological methods are used in the calculation of the probable maximum precipitation. Meteorological methods are normally preferred by many countries which has the following steps. Maximisation of storm data Storm transportation Relation among Depth-Area-Duration The following equation could be used in the estimation of PMP: Pactual = Actual measured rainfall. Wmax = Humidity of maximum dew point temperature. Wactual = Humidity of dew point temperature at the rainstorm area. The probable maximum precipitation is estimated by using the above equation. The change in rainfall due to changing climate conditions are not considered in the calculation. 3. Rational method to estimate rainwater runoff including proper selection of Runoff coefficient. Rational method is one of the oldest methods best suited for the estimation of peak runoff from a storm event. It is based on the following assumptions: Frequency of peak discharge is the same as the frequency of the average rainfall intensity. Time of concentration is the time required for the runoff from the remotest point of the drainage area to flow to the point of concentration. Peak runoff at any point is a function of the average rainfall intensity during the time of concentration to that point. The Rational Method formula is an empirical equation where the units as assigned are not consistent. The peak runoff is calculated by use of the following formula: Q = CiA Q - Peak runoff rate (cfs) C - runoff coefficient i - Intensity of rainfall (in/hr) (determined using IDF curve) A - Drainage area (acres) If the peak runoff exceeds the drainage capacity of the existing site, a storm water management plan could be developed to divert the runoff to a temporary storage area to reduce the rate of runoff. The runoff coefficient is the constant representing the fraction of rainfall which will result in the peak runoff rate from land surface to a drainage system. It is denoted by C and is dimensionless. The value changes with the type of land surface. The value of C increases with decreased water retention capacity of the land surface. The designer should select the C value based on the type of land surface, slope and vegetation cover etc. This value of C is used in calculation of runoff using rational method. The following table shows a set of values for C. Land Use C Land Use C Business: Downtown areas Neighborhood areas 0.70 - 0.95 0.50 - 0.70 Lawns: Sandy soil, flat, 2% Sandy soil, avg., 2-7% Sandy soil, steep, 7% Heavy soil, flat, 2% Heavy soil, avg., 2-7% Heavy soil, steep, 7% 0.05 - 0.10 0.10 - 0.15 0.15 - 0.20 0.13 - 0.17 0.18 - 0.22 0.25 - 0.35 Residential: Single-family areas Multi units, detached Munti units, attached Suburban 0.30 - 0.50 0.40 - 0.60 0.60 - 0.75 0.25 - 0.40 Agricultural land: Bare packed soil *Smooth *Rough Cultivated rows *Heavy soil, no crop *Heavy soil, with crop *Sandy soil, no crop *Sandy soil, with crop Pasture *Heavy soil *Sandy soil Woodlands 0.30 - 0.60 0.20 - 0.50 0.30 - 0.60 0.20 - 0.50 0.20 - 0.40 0.10 - 0.25 0.15 - 0.45 0.05 - 0.25 0.05 - 0.25 Industrial: Light areas Heavy areas 0.50 - 0.80 0.60 - 0.90 Streets: Asphaltic Concrete Brick 0.70 - 0.95 0.80 - 0.95 0.70 - 0.85 Parks, cemeteries 0.10 - 0.25 Unimproved areas 0.10 - 0.30 Playgrounds 0.20 - 0.35 Drives and walks 0.75 - 0.85 Railroad yard areas 0.20 - 0.40 Roofs 0.75 - 0.95 Table 1. Values of Runoff Coefficient (C) for Rational Formula 4. Suitability of rational method to estimate runoff Rational method is well suitable for estimation of peak discharge from an area. It is an empirical calculation based on rainfall and loss coefficient. Rainfall intensity, drainage area, time of concentration and runoff coefficient are the independent factors in the calculation. This method is widely used in UK and other parts of the world in sewer design. The disadvantage of this method is that the estimation do not provide peak volume while it provide peak flow. The estimation is based on limited variable, which makes it difficult in calibrating to the local conditions. It has an external dependency of selecting the runoff coefficient. It is not suitable for very large catchment areas but is obviously suited for small catchments like car parks. As the rational method is designed to estimate peak discharge, it is difficult to obtain the runoff volume. Use of this method is sensitive to the coefficient selected for the calculation. It is best suited for the estimation of value for a small area where an additional time used by other more accurate methods is not justified. Considering the smaller size of the parking area, this method could be used to estimate the runoff from the area. 5. Recommendations which methods are suitable for the above topic Many models are suitable to estimate the runoff from an area. Rational method is sufficient for calculation of the runoff from a car park. There are many methods which could be used for the estimation of runoff and its characteristics like, peak flow, runoff volume, return period. Some of them being: 1. Rational Method 2. Peak Discharge Method 3. Tabular Method 4. Unit Hydrograph Method There are many other revised methods and drainage models which could be used for developing drainage models for the estimation of the runoff. Rainfall runoff methods are best suited for Wallingford procedure (modified rational method) (DoE, 1981) is one of the suitable among the existing methods for the estimation of runoff volume. This method uses rearranged runoff volume algorithm for the estimation. This method is found more suitable in the runoff catchments in UK. This method used peak runoff discharge based on volume estimate. Drainage area, precipitation, runoff volume index, impermeability and antecedent catchment wetness are the independent variables used in this method. This method is specifically designed for the urban areas of UK and is rigorously field tested. Inorder to address the variable accuracy of the basic method in the urban catchments in UK, the rainfall - runoff equation is modified in the modified rational method as follows: Qp = 2.78 (Cv Cr i A) Where, Qp - peak runoff rate Cv - volumetric runoff coefficient Cr - routing coefficient i - peak runoff rate A - Drainage area (acres) The value of routing coefficient is derived from the field calibration. Hence, this method is more suitable for calculating the runoff from the car park near the shopping complex. The limitation of the modified rational method is that the equation has limited usage outside the targeted area. This method provides better value of runoff from the car parks and the local pavements and hence will eventually help in designing an appropriate drainage facility. Reference: Manual for Estimation of Probable Maximum Precipitation, Operational hydrology, Rep.1, WMO-No.332. WMO, 1986. The Quality of Urban Stormwater in Britain and Europe. Available from: http://www.geog.leeds.ac.uk/projects/nps/reports/npsch4.pdf [Cited on 07/04/2009] Values of Runoff Coefficient (C) for Rational Formula Available from: http://water.me.vccs.edu/courses/CIV246/table2b.htm [Cited on 07/04/2009]. V. T. Chow, David R. Maidment and L.W. Mays., Applied Hydrology, McGraw-Hill, New York, 1988. Read More