Roof Rainwater Harvesting - Essay Example

Water Engineering - Rain Water Harvesting Introduction Roof top rainwater harvesting refers to the process of collecting, conveying, storing and distributing the natural water for various non-potable applications. The objectives of such type of installations are to compensate for the reduction in the volume of water due to large scale building construction process. As the construction of car park requires large area of covered space it could limit the infiltration of rainwater which other wise would have infiltrated into the ground and enriched the prevailing ground water table. The various applications for which the water collected from the roof top collection systems usually used are landscape irrigation, fire fighting storage, toilet flushing, vehicle cleaning and house floor washing etc. They are also considered as a decentralized water supply units where ground water resource is becoming very scarce and the cost of the replacement of various components in the municipal supply systems are becoming very high. The entire system of rainwater harvesting starts with a siphonic roof drainage unit which could drain the water from the roof to an onsite storage tank. This water is filtered and is used for various applications either indoor or outdoor as discussed earlier. In addition to savings in the water costs it would also help to obtain the building permits very easily due to the importance given in the renewable energy systems. Methodology The car park area is a very limited space and hence the rational method would be sufficient to compute the runoff into the storm sewers. As the carpark could be expected to be covered the area of the drainage basin shall be taken as the roof area. And the volume of the water available shall be computed as the area of the car park roof multiplied by the depth of rainfall. This quantity is emptied into the storm drains placed on the ground using conveyer pipes whose cross section is decided based on the intensity of rainfall. The intensity values would be helpful to understand how quicker the rainfall reaches the roof. Thus the discharge from the roof is computed using rational formula by substituting appropriate values for intensity of rainfall, time of concentration and coefficient of runoff. Major components of rainwater harvesting unit. Irrespective of the type of rainwater harvesting unit the important components that are attached to a typical roof top units are as follows: Figure 1 Layout of rainwater harvesting system (CRIACC, 2008) (i) Collection systems: The collection region consists of various arrangements installed to collect water from the catchments. In the case of roof top unit, the catchment is referred to the roof top available for harvesting the rain water. The systems to convey water to the storage starts from the catchments. Thus the conveyance systems consists of series of gutters and down pipes that are necessary to direct the water from the roof top catchment to the desired location of storage. (ii) Vessels for storage: The tanks or holding vessel of estimated volume shall be provided for storing the water collected during the rains. The water in the storage units would be left unused until the time of lean season. (iii) Roof wash system: A small portion of initial part of the rains is diverted using cisterns for cleaning purpose and also to remove the debris. Thus volume of water of about 20 gallons is often used for this purpose which often improves the water quality. (iv) The water delivery system: This consists of series of network of pipes and other arrangements that are installed for proper delivery of water. If the water need to be lifted to higher elevation then separate pumps need to be installed to lift the water to higher elevation. Some time, if the water is used for irrigation purpose, then suitable arrangements need to be installed to facilitate the process. (v) Filtration: The process of removing any suspended debris or fine dust from the water referred as the filtration. For most of the situations the filtration unit consist of slow sand filter prepared using graded sand particles. These filter units would be able to give reasonable good quality of water. IDF Curves Intensity Duration Frequency (IDF) curves are very essential for more accurate estimation of the expected quantity of water from the recharging units. The rainfall data from the rain gauge stations are collected along with the probability for various rain event intensity for that particular place. IDF curves are prepared based on these information and curves that represent a particular rainfall intensity and frequency of occurrence or that have a certain return period is also determined. Such type of information would be of great help in understanding some of the rare event or torrential summer rains that are very remote. The summer torrential rains which are very rare often accompany high impact floods that constituting one of the rarest events. Further, it would also help in the proper planning of sewage systems, dikes and also to ensure protection to life and property in that region. The curves would be of much help to the regional administrative bodies like municipalities, governments, engineers, insurance and risk management companies etc (CRIACC, 2008). These curves would help the water engineers to forecast the quantity of available water across the season from a particular catchment. Thus a good estimation about the quantity of water is very essential for the design of storage facility, pumping assembly and other important components. Further, as the potential recovery from roof could be judged judiciously comprehensive water planning for the building units could be undertaken successfully. Probable maximum precipitation and its estimation These are some of the most effective methods of estimation of peak flow conditions is by determining the maximum probable precipitation. The time series data of the precipitation available in regions across large period of time could be used to determine the maximum precipitation values. It is a probabilistic estimation based on the occurrence levels of different magnitudes of the precipitation. The precipitation represented as PMP (Probable Maximum Precipitation) is an extreme event and hence need not be considered in the design of rain water recharge systems. As the high volume of water estimated in these type of precipitation would lead to large sized storm pipes and tanks. Thus the cost of construction increases and also leads to large volume of utilized space in the system. But at the same time, underestimation of these factors would result in the lesser volume of storage and also lead to small amount of flooding which would turn detrimental to the structure. Runoff estimation using Rational method One the conventional methods of estimation of the runoff is through the method of rational method. Though it is the oldest method it is still used as a acceptable method for the estimation of runoff in catchments. The peak runoff from catchments is determined as follows : Q = kCiA Where, Q is the peak discharge (in cu.m per second), k is a conversion factor (usually taken as 1.008 for SI units and 0.00278 for metric units), C is a catchment runoff coefficient (dimensionless), i is the rainfall intensity (mm/hr) and A is the drainage area (acres or hectare). The intensity of rainfall is determined either using hyetograph or using IDF curves. Hyetographs are bar charts that represent the rate at the rainfall is received for a particular magnitude of precipitation in an area. Rational approach usually demands the existing precipitation data of the area considered to permit a suitable estimation. The storm pipes though not designed on the basis of the maximum shall provide sufficient capacity to drain away the water reaching the surface without excessive flooding. Though slight flooding could be accommodated in the system if not very frequent, continual flooding would also cause damage to the structural components besides the inconvenience to the occupants. Factors that influence the determination of the runoff from the roof top are as follows. The most important factor is the surface area of the roof. Larger the area of the roof higher would be the volume of runoff due to the larger area of catchment. The type of material used for the surface also determines the rate of runoff. Highly absorptive material would result in lesser volume of runoff than the expected quantity. Time of concentration, depression storage and detention effect are the factors that influences the runoff. Suitability of Rational Method Rational method is one of the oldest method, introduced in the year 1889, to determine the runoff from small drainage areas where no significant flood storage occurs. The basic assumption in the rational method is the rate of runoff from a constant intensity of rainfall is considered maximum when the duration of the rainfall matches with the time of concentration. Hence, according to this if the rainfall intensity is constant the total area of the drainage basin contributes to the peak discharge within the time of concentration. But this assumption would hold good as long as the drainage area is limited and might turn invalid as the drainage area increases. In the case of large drainage basins, the time of concentration for these places would be also be large and the rainfall intensities being considered equal across the area might not exist. Thus shorter and intense rainfall are capable to produce larger peak flows. Also, it is observed that the rainfall intensities varies with storm and especially in the semi-arid and arid regions the storms are characterised by short duration and high intensity. The modern drainage construction practices might result in the detention of urban runoff which reduces the peak rate of runoff downstream. This causes serious limitation to the applicability of rational method in these circumstances. The rational method seriously limit's the application of various design alternatives for drainage which needed runoff computation in most of the urban and in a few rural situations due to its inability to accommodate the presence of storage in the drainage area. Also, the type of surface is also determines the acceptability of rational method to a particular situation. The assumption of uniform intensity of rainfall would be acceptable for impervious area like streets, rooftops and parking spaces. The factors that influences the selection of rational method consist of type of storm, soil and land use factors also. Thus considering the above mentioned reasons, for the present case the rational method would be sufficient to compute the runoff into the storm sewers provided in the car park area. Recommendations. Roof top rain water harvesting is an excellent method of water conservation measure in the building units. It shall be incorporated at the time of the construction of the building thus making the installation process very effective and cost efficient. Further, besides the construction aspects that need to be observed in the fixing of rain water harvesting units various details like size of the pipes and their configuration need to be appropriately considered by analyzing the precipitation details in each area. This is very essential considering the volume of water that would be received over the roof top for the specified intensity of rain fall in that region. This would result in proportioning the systems properly and also making them function as per the expectations. For the carpark area considered in this report the roof top rainwater harvesting would be very effective method for ensuring the sustainability. References Pfister energy (n.d.) [Online] Availabe at [12 April 2009] CRIACC (2008), Precipitation Intensity - IDF Curves, [Online] Available at [12 April 2009] Read More