Received on September 2015 Published on November 2015 825
Hydrologic design of rain water harvesting system at Anna University,
Chennai Krishnaveni M1 Vignesh Rajkumar L2
1- Professor, Centre for Water Resources, Department of Civil Engineering, Anna University, Chennai
2- Research Scholar, Centre for Water Resources, Department of Civil Engineering, Anna University, Chennai
doi: 10.6088/ijes.6077
ABSTRACT
Rainwater harvesting is an olden practice which is sustainable. There has been a growing
interest, particularly in developing nations, in rooftop rainwater harvesting as potable water. Domestic rainwater harvesting (DRWH) has regained its importance and is extensively
recognized in India as an alternative source of water because of seasonal water shortages. In cities like Chennai, where annual rainfall is high, rainwater could meet a significant amount of total demand. This paper ensures a sustainable system by designing a potential rain water
harvesting system in Anna University an imperative spot of the city where a few thousand populations move in and out every day. In the scenario of the university’s water supply
appears to be in peril of seasonal drying up, increased water conservation will help to maintain a plentiful water supply in the future.
The study begins with the dependability analysis of rainfall for the past 30 years. This ensures the quantification of dependable rainfall every month. Assuming 10 litres per head every day,
the water demand is of the campus is obtained. Overall water usage at the University is balanced with the amount of rainfall that could be harvested by the catchment area (i.e.) the roof area of the buildings in the campus, to calculate the deficit or surplus rainfall of each
month. The study reveals that the overall demand of the campus is served by harvested rain water for three months with 75% dependable rainfall. Considering 50% dependable rainfall
the university attains self-sufficiency in water resource only with the water being harvested. The performance of rainwater harvesting at Anna University is revealed with the results of this study. The prime objective of the paper is this method can also be applied to other
campus of the state and country to illustrate the water savings and reliability of rainwater harvesting system.
Rainwater harvesting is a practice of detaining, conveying, and storing rainwater to meet future needs. In scientific term, rainwater harvesting refers to collection and storage of rainwater and also other activities aimed at harvesting surface and groundwater, prevention of
looses through evaporation and seepage and all other hydrological studies and engineering interventions, aimed at conservation and efficient utilization of the limited water endowment
of physiographic unit as a watershed (Agrawal and Narain, 1999). Rainwater harvesting is an olden practice followed in ancient societies in different forms like check dams to reduce runoff and improve ground water recharge, agricultural dams and reservoirs. Evidences
suggest use of simple stone structure in Baluchistan for irrigation purposes around third
Hydrologic design of rain water harvesting system at Anna University, Chennai
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International Journal of Environmental Sciences Volume 6 No.2 2015
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century B.C. (Agarwal and Narain 1997) Rainwater harvesting cisterns were used in ancient Greek (Phoca & valantis, 1999) cited in Basinger et.,al 2010.
Water harvested by this practice can be a major source of potable water, complementary
source of potable water and also an additional source of non potable water. Water harvested through RWH is widely used in non potable water supplies but in recent years it has become an unavoidable source of water resource to attend the water shortage in urbanised and sub-
urbanised areas of developed and developing countries. Rainwater harvesting (RWH) primarily consists of the collection, storage and subsequent use of captured rainwater as
either the principal or as a supplementary source of water. Both potable and non-potable applications are possible (Fewkes, 2006). Rainwater harvesting structures can be constructed and maintained at all levels, residential homes, commercial plazas, community levels etc.
Chennai sometimes referred to as the "Gateway to South India," is located on the south–
eastern coast of India in the north–eastern part of Tamil Nadu on a flat coastal plain known as the Eastern Coastal Plains. Its average elevation is around 6.7 metres (22 ft), and its highest point is 60 m (200 ft). Two major rivers flow through Chennai, the Cooum
River (or Koovam) through the centre and the Adyar River to the south. The city gets most of its seasonal rainfall from the north–east monsoon winds, from mid–October to mid–
December. Cyclones in the Bay of Bengal sometimes hit the city. The highest annual rainfall recorded is 257 cm (101 in) in 2005. Prevailing winds in Chennai are usually south-westerly between April and October and north-easterly during the rest of the year.
Being a metropolitan city and the industrial hub of South India there is a rapid growth in
urban population. People from different parts of South India migrate into the city seeking education and employment. With only two major rivers, one being entirely polluted its extremely difficult for the city to balance the water demand of the population. Historically,
Chennai has relied on the annual rains of the monsoon season to replenish water reservoirs, as the major rivers have got polluted flow through the area. Chennai has a water table at 2
metres for 60 percent of the year. Water resources are limited and highly variable. The judicious use of these resources is essential (Choudhary and Aneja, 1991). Rain Water Harvesting (rainfall which is directly collected as roof run-off from buildings) can be a useful
practice in Chennai as the city is urbanised with rapidly growing infrastructures and the city receives the highest precipitation of the state. RWH is an effective method of developing
sustainable water resources in urban landscapes. 2. Study area
The study area chosen is Anna University located at Chennai, Tamil Nadu, India (Longitude
31° E and Latitude 18° S). The College of Engineering, Guindy, the main campus of the University is one of the oldest technical institutions in India. Its origin dates back to 1794, which was established as Survey School by East India Company. Situated in the Southern
part of the city of Madras (Chennai), the University's main campus extends over 100 hectares abutting the Adyar River on the north and Raj Bhavan on the South. The campus has a
variety of buildings serving the various needs of the University community. The University offers 18 UG Courses, 35 PG Courses (Regular) and 34 PG Courses Self Supporting courses. College of Engineering has 16 Departments, 12 Research Centres. Most students at CEG
reside in hostels. The campus has 16 hostels (Blocks 1-11, Kurinji, Tulip, Lavender and other blocks), of which five are exclusively for women. There is a separate hostel for NRIs and
Foreign Nationals. Hence the water requirement in the campus is plenty and rainwater
Hydrologic design of rain water harvesting system at Anna University, Chennai
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harvesting can be a useful practice. The terrain is generally flat. Mean elevation is about 15m to 10m above mean sea level. Rain gauges are available where daily rainfall is measured.
Highest rainfall is recorded between December and February whereas the dry season is from March to June.
Figure 1: Anna University main campus map
3. Methodology
3.1 Quantification of possible water harvest
In theory, approximately entire water can be harvested for any amount of rainfall. But while
practicing, however, some rainwater is lost to evaporation, conveyance loss, splash-out or overshoot from the gutters in high intensity rains, and leaks. Rough surfaces are not efficient
Hydrologic design of rain water harvesting system at Anna University, Chennai
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International Journal of Environmental Sciences Volume 6 No.2 2015
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at conveying water, as water captured in pore spaces tends to be lost to evaporation. Furthermore, after storage tanks are full, rainwater can be lost as overflow. The total amount
of rainfall that could be harvested is found by using the formula given below.
Rain water that could be harvested = Roof area x Dependable rainfall x 0.9 (1) The coefficient 0.9 is multiplied to take care of losses. 0.9 is used as the coefficient assuming
90% efficiency in harvesting. Determination of the Collection Surface (i.e.) the size of the catchment is an important step before designing the sizes of storage structures. The roof areas
of the buildings in which the RWH structures are to be installed is calculated. Hence the volume of rainfall that can be harvested at the campus is quantified at the early stage so as to ascertain the size of sumps or storage structures required.
3.1.2 Dependable rainfall
The estimation of parameters and the selection of the distribution become unreliable when the observed data happens to be very small. In such cases, the probabilities can be obtained from
the frequency analysis without bothering about which distribution the random variable follows.
Let x'1, x'2… x'n, be the original independent observations that are recorded in the same sequence of their occurrence. The same observations are arranged in the descending order of
magnitude such that x1 ≥ x2 ≥ x3 ≥ …≥ xm ≥ …xn. Sequence of observations is called the rank or the order of the observation.
Let P (xm) denotes the probability with which the value xm is equaled or exceeded. That is P (xm) = P [X ≥ xm] (2)
In other words, P (xm) denotes the exceedence probability. In ordered sequence if we consider the observation with rank m, this value xm is equaled or exceeded m times out of n
times, and therefore from the frequency interpretation of probability, we can write P [X ≥xm] = = P (xm) (3)
If the variable under consideration is an annual event such as the annual peak discharge, or annual runoff or annual precipitation etc., we can say that random variable takes a value
equal to or more than xm on the average once in years, which are variously known as the recurrence interval, return period or simply the frequency. Tr denotes the return period. Thus we find that the exceedence probability and the return period are reciprocals to each other.
Tr = = (4)
The return period, therefore, indicates the average number of years within which a given event will be equaled or exceeded. The Weibul's formula which avoids any complexities and
which is most commonly used in Hydrology is as given below.
Tr = n (n+1) m (5) Equation (3.5) will be used to determine the return periods in the dependability analysis of
rainfall data. From the probability plot we read the magnitude of the variable corresponding to an exceedence probability of p. Then this magnitude is called the (100p) percent
dependable value of the variable. The 75% dependable value of rainfall and 50% dependable
n
m
n
m
n
m
P )Xm(
1
Hydrologic design of rain water harvesting system at Anna University, Chennai
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value for each month are found out and that value is used in the calculation of rainfall that could be harvested.
3.2 Demand Calculation
The sequent peak algorithm method is used to estimate the capacity of reservoir the methodology is improvised here to find out the storage capacity required. The amount of
rainwater harvested each month and the respective demand are worked out and the net inflow volumes are calculated which is given by.
Net inflow = Volume of rain water harvested – Demand The difference between the maximum surplus and maximum deficit is the volume of water
storage structure required. The storage sumps are designed consider ing the above aspect in order to fulfil the future demands of the campus.
Sump capacity = Surplus (max) – deficit (max) (6)
4. Results and discussions
4.1. Dependability analysis
The results of dependability analysis show the exceedance probability of rainfall for each month. The probability plots of all the monsoon months are presented in the following
Figures 2 - 8 Figure 2 shows the exceedance probability of rainfall during the month of June. The exceedance probability value is high for minimum rainfall and decreases with the
increase in rainfall. This indicates the dependability on higher precipitation values is less whereas the dependability increases with the decrease in rainfall. These minimum rainfall values are obtained since month of June is the beginning of monsoon season.
Figure 2: Frequency analysis for the month of June
The exceedance probability of rainfall during the month of July is shown in Figure 3. The exceedance probability value has increased for higher rainfall values when compared with the
month of June. Nevertheless it decreases with the increase in rainfall indicating the dependability on higher precipitation values is still unreliable. The precipitation has slightly
increased due to the onset of monsoon season.
Hydrologic design of rain water harvesting system at Anna University, Chennai
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International Journal of Environmental Sciences Volume 6 No.2 2015
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0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250
Rainfall in mm
Exceed
an
ce P
rob
ab
ilit
y
Figure 3: Frequency analysis for the month of July
The dependability of rainfall during the months of August and September are shown in Figure 4 and figure 5respectively. The distribution of exceedance probability corresponding
to the rainfall has increased when compared to the previous two months. The figures show many values more than 100mm rainfall lies between 0.6 and 0.7 exceedance probabilities. There are some values of precipitation higher than 250mm with probabilities 0.2 and 0.1. The
frequency analysis for these two months infers that the months of August and September are dependable months for rainfall.
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500
Rainfall in mm
Exceed
an
ce P
rob
ab
ilit
y
Figure 4: Frequency analysis for the month of August
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300 350 400
Rainfall in mm
Exceed
an
ce P
rob
ab
ilit
y
Figure 5: Frequency analysis for the month of September
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The dependability of rainfall during the months of October and November are shown in Figure 6 and figure 7 respectively. The amount of dependable rainfall has clearly increased
and this is inferred from the graph
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600 700 800
Rainfall in mm
Exceed
an
ce P
rob
ab
ilit
y
Figure 6: Frequency analysis for the month of October
0
0.2
0.4
0.6
0.8
1
1.2
0 200 400 600 800 1000 1200
Rainfall in mm
Exceed
an
ce P
rob
ab
ilit
y
Figure 7: Frequency analysis for the month of November
. All the rainfall values are greater than 100mm and the probability of exceedance is high as these two months are the prime months of monsoon. The figures show all values more than
100mm rainfall lies between 1 and 0.8 exceedance probabilities and the rainfall values more than 200mm lies on 0.6 exceedance probabilities. There are many values of precipitation in
the month of October higher than 250mm and 300mm with probabilities 0.5 and 0.4 respectively. The frequency analysis of the month November has many values higher than 400mm rainfall with 0.5 exceedance probability. The two months influences the most of
monsoon precipitation and hence these are the most dependable months for rainfall.
The result of frequency analysis for the month of December is shown in the figure 8. The exceedance probability values are high for minimum precipitation. Most of the points with probabilities more than 0.7 have precipitation 50mm or lesser. The rainfall has clearly
decreased when compared with the last two months as it is the end of monsoon. In spite of the end of monsoon, December has significant precipitation at 50% dependability. This
month marginally influences the monsoon precipitation and hence 50% dependable for rainfall.
Hydrologic design of rain water harvesting system at Anna University, Chennai
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0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600 700 800
Rainfall in mm
Exceed
an
ce P
rob
ab
ilit
y
Figure 8: Frequency analysis for the month of December
With the results of the frequency analysis 75% and 50% dependability were considered for further analysis of quantifying rainfall harvest. The amount of 75% dependable rainfall and
50% dependable rainfall is calculated for each month. The amount rainfall that could be harvest for both 75% and 50% dependable rainfall is calculated using the equation 1. The total amount of rainfall that could be harvested while using 75% dependable rainfall and 50%
dependable rainfall is calculated. The demand for water in the campus is calculated with the details of number of persons, assuming 10 litres per head. The amount of rainwater that could
be harvested and the monthly demand is balanced for each month for the monthly balance calculation. Hence the monthly surplus and deficit is calculated, the surplus can be useful for the following month while the deficit is meet out from the previous months.
4.2. Estimation of sump specifications
Sequent peak algorithm is employed in this estimation. Based on the amount of rainfall and the amount of rainwater that can be harvested the surplus deficit is calculated. The surplus -
deficit is balanced with the demand for each month to obtain the required sump specifications. Table 1 shows the results of the amount of rainfall harvested and the monthly surplus or
deficit using 75% dependable rainfall.
Figure 1: Surplus / Deficit at 75% dependable rainfall
Month
75%
Dependable rainfall in mm
Rain water
harvested in m3
Monthly demand in m3
Surplus/deficit in m3
June 41.6 1821.042 4701 -2880
July 73.6 3221.843 4701 -1479
August 83.1 3637.706 4701 -1063
September 59.55 2606.803 4701 -2094
October 162.25 7102.5 4701 2401
November 192.1 8409.185 4701 3708
December 43.2 1891.082 4701 -2810
January 0.425 18.60439 4701 -4682
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February 0 0 4701 -4701
March 0 0 4701 -4701
April 0 0 4701 -4701
May 1 43.77504 4701 -4657
Inferences from the Table 1 illustrate that at 75% dependability the amount of rainfall harvested is surplus only for the two months October and November that can be useful to
meet the demand of the following month December and a part of demand of January. The results suggest that the water demand of the university is completely convened by rain water harvesting for three months. Table 2 shows the results of the amount of rainfall harvested and
the monthly surplus or deficit using 50% dependable rainfall.
Table 2: Surplus / Deficit at 50% dependable rainfall
Month
50%
dependable rainfall in mm
Rain water
harvested in m3
Monthly
demand in m3
Surplus/deficit in m3
June 67.2 2941.682 4701 -1759.32
July 97.3 4259.311 4701 -441.689
August 113.5 4966.278 4701 265.2779
September 134.5 5887.742 4701 1186.742
October 261.4 11442.79 4701 6741.795
November 392.5 17181.7 4701 12480.7
December 132.3 5791.437 4701 1090.437
January 5.15 225.4414 4701 -4475.56
February 0 0 4701 -4701
March 0 0 4701 -4701
April 1.05 45.96379 4701 -4655.04
May 19.15 838.292 4701 -3862.71
Inferences from the analysis using 50% dependability the amount of rainfall harvested is surplus only for five months from August to December. The total amount of surplus rainwater harvested from these five months is 21765m3. The surplus water can be useful to
meet the demand of five months. The results suggest that the water demand of the university is completely convened by rain water harvesting for ten months. Rainwater harvesting can
support the water demand of the university if the storage structures are properly designed and well maintained. Considering 50% dependability rainwater harvesting can bring a massive change in the water availability of the university.
4.3. Demand – harvest - storage
The total annual domestic demand in the campus is 56412 m3. The total catchment area (i.e.) the roof area of the various buildings in the campus is 48638.93 m2.With the amount of
Hydrologic design of rain water harvesting system at Anna University, Chennai
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International Journal of Environmental Sciences Volume 6 No.2 2015
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annual dependable rainfall and the catchment area, the total amount of rain water that can be harvested at 75 % dependable rainfall is 28752.54 m3. Whereas the total amount of rain water
harvested at 50 % dependable rainfall is 53580.65 m3. The required storage is obtained by balancing the demand and the amount of water harvested. The capacity of storage required
when using 75% dependability is 8409 m3and the capacity of storage required when using 50% dependability is 17181.7 m3.
4.4. Sump numbers and specifications
The harvested water is stored in new sumps that will be constructed in various parts of the campus. The newly constructed sumps are interlinked with each other so that water collected over any roof could be used at any building in the campus. The required number of sumps
and the sump specifications for 75% dependable rainfall and 50% dependable rainfall are presented in the following tables 3 and 4 respectively.
Table 3: Sump design for 75% dependability analysis
S.no Details Specifications
1 Number of new Sumps 25
2 Capacity of each Sump 400 m3
3 Depth of each Sump 6.5 m
4 Free Board 0.15 m
5 Radius of each Sump 4.5 m
Table 4: Sump Design for 50% Dependability Analysis
s.no Details Specifications
1 Number of new Sumps 50
2 Capacity of each Sump 400 m3
3 Depth of each Sump 6.5 m
4 Free Board 0.15 m
5 Radius of each Sump 4.5 m
5. Conclusion
Rainwater harvesting technique has a wide range of potential applica tions in the developing
world. It proves to be valuable method for minimizing water scarcity in developing countries. Rainwater harvesting system can be adapted to different ranges of climatic zones; it is
adaptable even to the hardest hit area by water crisis. Anna University receiving the highest annual precipitation of the state has ample opportunities with rain water harvesting. The roof top harvesting technique could serve the university for three months at the worst case of 75%
dependable rainfall. With the analysis from 50% dependable rainfall the entire campus attains self sufficiency and serves the water demand itself throughout the year. The sump designs are
specific to the amount of rainfall and the amount water harvested hence it is made sure that there is no loss of water by overflow seepage. With the results of the study new sumps were designed and the selection of site for construction was based on the demand. The buildings
are grouped and sumps will be placed at a requisite distance from each group.
It’s very clear that rainwater has a major role to play in substituting and/or supplementing urban water supply from metro water supply facilities. The results of this study provide useful information for further development of the rainwater harvesting practice in Chennai as
well as for other arid and semi-arid regions of the world. However, this is one among many
Hydrologic design of rain water harvesting system at Anna University, Chennai
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successful examples from India having illustrated methodological rainwater harvesting, not only it gains the trust of local communities, but, with proper training and incentives,
community members will be curious to help and educate neighbours, design systems and maintain existing infrastructures.
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13. Nicholas L. Cain., (2014), A Different Path: The Global Water Crisis and Rainwater Harvesting. Consilience, The Journal of Sustainable Development, 12(1), pp147–157.
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