Roof Top Rain Water Harvesting Prospects for Shimla

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International Journal of Scientific & Engineering Research, Volume 5, Issue 5, May-2014


ISSN 2229-5518

ABHINAV BANSAL, IV Year, Civil Engineering Department, JUIT, Waknaghat, [email protected] MUDIT MISHRA, Lecturer, Civil Engineering Department, JUIT, Waknaghat, [email protected]

ABSTRACT: This paper talks about the possibilities of rain water harvesting for a city like Shimla where rainfall has seen a drastic change in the past 109 years. Now it is a well known fact that during summers there is scarcity of water in this region so the efforts have been made to design a roof top rain water harvesting system to meet the demand in this period. Two dimensionless less quantities storage fraction and demand fraction have been taken to find some alternative. The results clearly show that if we can store the rain water not only in the summers but round the year then a huge quantity of water will be stored without any significant loses. The demand of water for toilet flush for four persons living in a family can be fulfilled for two months with the storage of rain water of even one month only. The design of the houses will not be altered much because the roofs are already sloped and we have to find ways for the efficient storage. The non-dimensional design can be applied for the metropolitan cities and those places also where scarcity of water is always there.


system involved clearing hill sides to smooth the soil and

The world faces escalating demands for good quality water as increase runoff and then building contour ditches to collect

current usage from surface and ground is outstripping supply. the water and carry it to low lying fields where the water was

Even in those areas of the world that appear to have adequate used to irrigate crops. By the time of the Roman Empire,

water supplies, there are constant needs to balance existing these runoff farms had evolved into relatively sophisticated

supplies with ever growing demands. Cycles of droughts systems. The next significant development was the

IJSER bring into sharp contrast the need to conserve, protect and
supplement existing water supplies. The collection and storage of rainwater to supplement existing water supplies could alleviate some of these problems. Rainwater utilization may be one of the best available methods for recovering natural hydrological cycles and aiding in sustainable urban development. (Kim R-H, Lee S, Et al., 2005)

construction of roaded catchments as described by the public works Department of Western Australia in 1956. They are so called because the soil is graded into ditches. These ditches convey the collected water to a storage reservoir. Lauritzan, USA has done pioneering work in evaluating plastic and artificial rubber membranes for the construction of catchments and reservoirs during 1950’s. In 1959,Mayer of

water conservation laboratory, USA began to investigate

Water scarcity demands the maximum use of every drop of materials that caused soil to become hydrophobic or water

rainfall. (M. Abu-Zreig et al, 2000). Rainwater harvesting repellent. Then gradually expanded to include spray-able

system has been regarded as a sound strategy of alternative asphalt compounds, plastic and metal films bounded to the

water sources for increasing water supply capacities. (Hatibu soil compaction and dispersion and asphalt fiber glass

N, Et al. ,1999). Rainwater harvesting systems intercept membranes. Early 1960, research programmes in water

rainwater in hydrologic cycle through either natural harvesting were also initiated in Israel by Hillal and at the

landforms or artificial facilities. The small scale RHS does University of Arizana by Gluff. Hillal’s work related

not involve the existing water right. And it has become one of primarily to soil smoothing and runoff farming. Cluff has

the economical and practical measures for providing done a considerable amount of work on the use of soil sealing

supplementary water supplies with its easy system with sodium salt and on ground covered with plastic

installation. It can be a supplementary water source in membranes. Water harvesting was practiced more than 1000

urbanized regions for miscellaneous household uses such as years back in South India, by way of construction of

toilet flushing, lawn watering, landscape and ecological irrigation tank, ooranis, temple tanks, farm ponds etc, but the

pools, and cooling for air conditioning (Handia L et al, 2003). research in India on this subject is of recent one. Work is

taken up at ICRISAT, Hyderabad, Central arid Zone

Research Institute, Jodhpur, Central Research Institute for


dryland Agriculture (CRIDA), Hyderabad, State Agricultural

Water harvesting like many techniques in use today is not Universities and other dry land research centers throughout

new. It is practiced as early as 4500 B.C. by the people of Ur India.

and also latest by the Nabateans and other people of the In Pakistan, in the mountainous and dry province of

Middle east. While the early water harvesting techniques Balukhistan, bunds are constructed across the slopes to force

used natural materials, 20th century technology has made

the runoff to infiltrate. In China, with its vast population is

it possible to use artificial means for increasing runoff from actively promoting rain and stream water harvesting. One

precipitation. Evenari and his colleagues of Israel have very old but still common flood diversion technique is called

described water harvesting system in the Negve desert. The ‘Warping’ (harvesting water as well as sediment). When

IJSER © 2014

International Journal of Scientific & Engineering Research, Volume 5, Issue 5, May-2014


ISSN 2229-5518

water harvesting technique are used for runoff farming, the generally include simplified approaches based on user-

storage reservoir will be soil itself, but when the water is to defined relationships (e.g. Ward et al., 2010), continuous

be used for livestock, supplementary irrigation or human mass balance simulations , non-parametric approaches based

consumption, a storage facility of some kind will have to be on probability matrix methods and statistical methods . The

produced. In countries where land is abundant, water most common methodology is the behavioral analysis that

harvesting involves; harvesting or reaping the entire uses continuous simulation to assess the inflow, outflow and

rainwater, store it and utilize it for various purposes.

change in storage volume of the rainwater harvesting system

according to a mass balance equation.

In India, it is not possible to use the land area only to harvest

water and hence water harvesting means use the rain water at Same has been done here. The study has been carried out on

the place where it falls to the maximum and the excess water the recorded data on average monthly rainfall for the time

is collected and again reused in the same area. Therefore the period of 109 years i.e. from 1900 to 2009 at Shimla,

meaning of water harvesting is different in different area/ Himachal Pradesh, India.(10).

countries. The methods explained above are used for both

agriculture and to increase the ground water availability.


The water harvesting for household and for recharging The geography of Shimla is most diverse and multifaceted as

purposes are also in existence for long years in the world. the city is located on the verge of subtropical regions and

During rainy days, the people in the villages used to collect higher Himalayas. The pleasant weather, sometimes steep,

the roof water in the vessels and use the same for household sometimes perpendicular landscape of most of the

purposes including drinking. In South East Asian countries

geographical locations of Shimla India is a sure proof of that.

people used to collect the roof water ( thatched roof by The average elevation of the city of Shimla is 2397 meter or

providing gutters) by placing 4 big earthen drums in 4corners 7866 ft. above the sea level and Shimla is located on the ridge

of their houses. They use this water for all household and in the north western portion of Himalayas.

purposes and if it is exhausted only they will go for well

water. The main building of the Agricultural College at METHODOLOGY ADOPTED

Coimbatore was constructed 100 years ago and they have

IJSER collected all the roof water by pipes and stored in a big
underground masonry storage tanks by the sides of the building. These rainwater are used for all labs, which require pure and good quality of water. In the same way the rainwater falling on the terrace in all the building constructed subsequently are collected and stored in the underground masonry tanks Even the surface water flowing in the Nalla’s

Schematic illustration of the rainwater used in this work is reported in Fig. 1



in the campus are also diverted by providing obstructions, to

the open wells to recharge ground water. Hence Rainwater

harvesting is as old as civilization and practiced continuously

in different ways for different purposes in the world The only

thing is that it has not been done systematically in all places.

Need has come to harvest the rainwater including roof water

to solve the water problems everywhere not only in the arid

but also in the humid region. (Dr. R. K. Sivanappan, 2006)

Collecting rainwater as it falls from the sky seems immensely sensible in areas struggling to cope with potable water needs. Rainwater is one of the purest sources of water available as it contains very low impurities. Rain water harvesting systems can be adopted where conventional water supply systems have failed to meet people’s needs. (Dr. K. A. Patil, Et al. 2006)
OUR AREA OF DISCUSSION Rainwater harvesting (RWH) is recognized as one of the tools of Sustainable Urban Drainage Systems (SUDS) which aim at restoring the natural hydrologic cycle in the urban environment. RWH limits the demand for potable water and, at the same time, rainwater storage controls storm water runoff at the source (Elliott and Trowsdale, 2007). Various methodologies for the design of rainwater harvesting systems are documented in the literature (Mitchell, 2007 etal.,) and

Fig. 1. Configuration of the rainwater harvesting system
This figure clearly explains the following equation:
Equation 1. Max Supply (or) outflow = Water stored in tank + Inflow ; where inflow(Q) depends on the precipitation (R), Area of roof (A) and Runoff Coefficient(K).
Equation 2. Q = K * R * A

IJSER © 2014

International Journal of Scientific & Engineering Research, Volume 5, Issue 5, May-2014


ISSN 2229-5518

The rainfall–runoff process is therefore interpreted by assuming a constant runoff coefficient and no quality aspects are taken into account thus neglecting the occurrence of the first flush phenomenon. As widely documented in the literature (Gnecco et al., 2006) the impact of pollutant load associated with urban paved surfaces is significant thus requiring at least to divert the first flush volume. As examples, in order to account the first flush effect, Khastagir and Jayasuriya (2010) subtracted the first 0.33 mm of daily rainfall while Basinger et al. (2010) assumed 0.4 mm of first flush occurring after 3 dry days. However, in the present configuration it is assumed that rainwater is only collected from rooftops since the pollutant load washed-off from such surfaces is limited compared to road runoff (Gnecco et al., 2005).
The runoff coefficient is taken as the average of the two limits for the runoff coefficient as stated in table 1, i.e. 0.85 (16).

Lawns, heavy soil Flat, 2 percent Average, 2-7 percent Steep, 7 percent

0.13-0.17 0.18-0.22 0.25-0.35

The water demand to be supplied by rainwater is limited in this study to toilet flushing and is assumed to occur at a constant rate. This assumption is reasonable because the demand time series generated by WC usage does not exhibit excessive daily variances. (Fewkes, 2000).
The average monthly rainfall for different time intervals is stated in table 2 & combined average monthly rainfall for time duration of 109 years i.e. from 1900 to 2009 is stated in table 3.(10)

TABLE 2: Average monthly rainfall for different time

TABLE 1: Runoff Coefficient Table

Area Description

Runoff Coefficient K

interval s. TIME INTERVAL







IJSER Downtown
Neighborhood Residential
Single-Family Multiunits, detached Multiunits, attached

0.70-0.95 0.50-0.70
0.30-0.50 0.40-0.60 0.60-0.75


41.04 39.03 31.79 28.39 36.61 129.01

41.39 42.49 33.94 29.95 34.57 130.59

35.84 41.9
45.67 32.71 40.96 122.54

37.49 52.21 36.83 35.47 42.27 108.9

Residential (suburban) 0.25-0.40


246.38 285.33 282.79 177.22




246.6 227.57 240.1


Industrial Light Heavy Parks, cemeteries

0.50-0.80 0.60-0.90 0.10-0.25


148.76 15.55 12.88 16.05

140 23.74 9.75 15.84

140.21 21.86 10.62 17.66

113.73 10.04 11.97 12.49

Playgrounds Railroad yard Unimproved

0.20-0.35 0.20-0.35 0.10-0.30

TABLE 3: Combined average monthly rainfall for time duration of 109 years.

Character of surface Pavement
Asphaltic and concrete Brick Roofs Lawns, sandy soil Flat, 2 percent

Runoff Coefficient K
0.70-0.95 0.70-0.85 0.75-0.95


1900-2009 39.19 42.86 36.73 31.5 37.55 125.13 253.99

Average, 2-7 percent




Steep, 7 percent




IJSER © 2014

International Journal of Scientific & Engineering Research, Volume 5, Issue 5, May-2014


ISSN 2229-5518


18.4 11.14 15.58

Figure 2: Pictorial representation of percentage of population using different ways for toilet purposes as stated in table 4.

Percentage of population using different ways for toilet purposes in Shimla is stated in Table 4 as per the data set available on (18).

TABLE 4: Percentage of population using different ways for toilet purposes in Shimla.



Individual Toilets


Open Defecation


Public Toilets


Indivisual Toilets Open Defecation
Public Toilets

Table 5: showing the percentage of population using different

Optimum design of the roof top rainwater harvesting system types of toilets.

may vary with the local specific constraints & conditions

which would directly or indirectly influence the analysis of

performance and the conclusions drawn on the reliability of Type of WC


the system. Thus, the design of RHS under different environmental



conditions such as amount of rainfall, water demand etc. and HETS


IJSER system characteristics like water storage capacity, is
examined as a function of two non-dimensional parameters: 1. Demand fraction 2. Storage fraction. The demand fraction is defined as the ratio D/Q between the average monthly water demand D [L3] and the average monthly inflow Q [L3] while the storage fraction is defined

ULF Figure 3: Pie diagram on the basis of table 5.


as the ratio S/Q between the storage capacity of the storage


tank S [L3] and the average monthly inflow Q [L3].


Demand depends solely upon the type of water closet being used. In a home with older toilets, an average flush uses about 3.6 gallons (13.6 liters), and the daily use is 18.8 gallons (71.2 liters) per person per day. In a home with ultralow-flow (ULF) toilets, with an average flush volume of 1.6 gallons (6 liters), the daily use is 9.1 gallons (34.4 liters) per person per day. A family of four using an older toilet will use approximately 26,000 gallons (98.4 m3) per year in toilet flushes, while a family with a ULF toilet will use approximately 11,000 gallons (41.6 m3) per year in toilet flushes, achieving a savings of 15,000 gallons (56.7 m3) per year. New, High Efficiency Toilets (HETs) use 1.3 gallons (5 liters) per flush (gpf). With an HET, a family of four will use approximately 9,000 gallons (34 m3) per year in total toilet water use. (19).

Since 53% population is using old toilets, 28% using ULF & remaining 17% using HETs (Assumption 2), total demand per year for a family of four is = 69.58 m3
Thus, Average water demand per month [D] = 69.58/12 =5.798 m3 Runoff Coefficient = (0.75 + 0.95)/2 = 0.85 [from table 1].
Area of Roof = 150 m2 (Assumption 1)

Assumptions: 1. Area of roof : 150 m2
2. 53% population in Shimla is using old toilets, 28%
is using ULF & remaining 17% using HETs. As
stated in table 5. 3. Storage tank capacity is 12 m3 be [ 3m * 2m * 2m]

IJSER © 2014

International Journal of Scientific & Engineering Research, Volume 5, Issue 5, May-2014


ISSN 2229-5518

Fig 3: Pictorial representation of table 2






[Df]Demand fraction= D/Q [Sf] Storage Fraction= S/Q
Values are accordingly calculated and stated in table 8 & 9.

JULY OCTOBER 300 250 200



Table 8: Demand fraction table.

Time interval 1900-1930 1930-1960 1960-1990 1990-2009 1900-2009

Df 0.55 0.537 0.528 0.671 0.559

150 Table 9: Storage fraction table.

100 Time interval






1900-1930 1930-1960 1960-1990 1990-2009 1990-2009

Sf 1.138 1.112 1.093 1.388

IJSER 1900-2009


Average monthly rainfall [R] calculated from table 2 & table 3 is stated in table 6.

1.6 1.4


Table 6:



Time Interval Average Monthly Rainfall 00..46 Df














Using Equation 2 ‘Q’ is calculated and stated in table 7


Table 7: Inflow value chart

Time interval 1900-1930

Q (m3) 10.54

Volume stored in tank at beginning of the month = V
Rain water supplied from storage tank = Y
Now, V = Q – D = 10.38- 5.789 = 4.591 m3 Y = min ( V, D) = 4.591 m3

1930-1960 1960-1990 1990-2009 1900-2009

10.79 10.97 8.64 10.38

Performance assessment of the rainwater harvesting system is performed by means of a non-dimensional index called water efficiency [E]
E = Y / D


E= 4.591 / 5.789 = 0.793

IJSER © 2014

International Journal of Scientific & Engineering Research, Volume 5, Issue 5, May-2014


ISSN 2229-5518

The value of efficiency is very high even under the current circumstances when the rainfall data is taken for a mean value. Though the design parameters taken are random and capacities assumed are random but as per the demand the calculation show that supply of one month will be more than sufficient to meet the demand of one month. There can be variations in all the parameters taken but one thing is certain that it can meet the demand of present and future generations
CONCLUSIONS  As per the observations the ground water level is depleting and annual rainfall is going down.  Fresh water is everybody’s need which will not be fulfilled if the current trend continues so we need a system which can meet the demand upto some extent.  Rain water harvesting is a very good alternative for upcoming crises.  The design shown here clearly suggests that roof rain water harvesting with the calculated parameters are very compatible in the current scenario.  The design done is based on storage fraction and demand fraction which may vary and optimization will have to be done to make it more applicable.

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15. Basinger, M., Montalto, F., Upmanu, L., 2010. A rainwater harvesting system reliability model based on nonparametric stochastic rainfall generator. J. Hydrol. 392 (3–4), 105–118.
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Roof Top Rain Water Harvesting Prospects for Shimla