ROOFTOP RAINWATER HARVESTING in MODERN CITIES: a CASE STUDY for SANA’A CITY, YEMEN

8/12/2019 ROOFTOP RAINWATER HARVESTING in MODERN CITIES: a CASE STUDY for SANAA CITY, YEMEN

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48Journal of Science & Technology

Vol. (17) No. (2) 2012

ROOFTOP RAINWATER HARVESTING in MODERN CITIES: a CASE

STUDY for SANAA CITY YEMEN

ROOFTOP RAINWATER HARVESTING in

MODERN CITIES: a CASE STUDY for

SANAA CITY, YEMEN

Sharafaddin Abdullah Ahmed Salleh(1) and Taha Muhammed Taher(2)

Abstract

Water resources in Yemen are limited, and water is becomingscarce everyday due to ever-increasing demand due to the rapidlyincreasing population and to the drought climate the country is

characterized with. Increasing overdrawn from groundwater causes adeficit of 900 Mm3 annually leaving the country to seek alternativeresources. All major cities in Yemen facing water problems resultingin a mainly socio-economic change beside other challenges- that

produce unrest and unforeseen conflicts to acquire water when neededespecially in the capital city of Sana!a where groundwater levels dropannually by an average of 6 m. Rainwater harvesting systems have

been used since ancient times and evidence of roof systems date back

to more than 4000 years ago in the middle east as the principal watersource for drinking and domestic use. This paper summarizes thefindings of a substantial work by the authors during the past threeyears in providing a reasonable, alternative solution to the waterscarcity problem through dealing with water harvesting as analternative resource. This paper estimated the amount of water that can beharvested annually from roof tops 11.31 Mm3 for urban areas using runoffcoefficient of 0.75 and 0.172 Mm3 for rural areas using runoff coefficient

of 0.6. This indicates that there will be an annual reduction in the usage ofgroundwater in urban and rural areas by 22% and 33% respectively. Simpleand easy harvested water volume guide tables were developed for differentrun off coefficients of 0.6, 0.7, 0.75 and 0.8. It also presents a the mainfactors for the design of a complete Rooftop Rainwater Harvesting Systemfor the city of Sana!a.

Key word: Roof Tops, Water Harvesting, Design Tables, Guideline, Sana!a, Yemen

1-Assistant Professor of Hydraulics and Water resources , Civil Engineering Department,

faculty of Engineering, Water and Environment Center (WEC), Sana!a University e-mail:[email protected], [email protected].,

2- Associate Professor of Water resources, Civil Engineering Department, faculty ofEngineering, Water and Environment Center (WEC), Sana!a University P.O. Box 14636,Sana'a Yemene-mail: [email protected], [email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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1.Introduction

1.1 History of Water Harvesting

Rainwater harvesting systems have been used since ancient times and

evidence of roof catchments systems date back to early Roman times.

Roman villas and even whole cities were designed to take advantage of

rainwater as the principal water source for drinking and domestic purposes.

Rainwater harvesting is an ancient technique enjoying a revival in

popularity due to the inherent quality of rainwater and interest in reducing

consumption of treated water. Archeological evidence attests to the captureof rainwater as far back as 4,000 years ago, and the concept of rainwater

harvesting in China may date back 6,000 years. Ruins of cisterns built as

early as 2000 B.C. for storing runoff from hillsides for agricultural and

domestic purposes ]1[ .

2000 B.C. In the Negev desert in Philistine, tanks for storing runoff

from hillsides for both domestic and agricultural purposes have allowed

habitation and cultivation in areas with as little as 100mm of rain per year.

The earliest known evidence of the use of the technology in Africa comes

from northern Egypt, where tanks ranging from 200-2000 m3 have been

used for at least 2000 years many are still operational today. Thetechnology also has a long history in Asia, where rainwater collection

practices have been traced back almost 2000 years in Thailand. The small-

scale collection of rainwater from the eaves of roofs or via simple gutters

into traditional jars and pots has been practiced in Africa and Asia for

thousands of years. In many remote rural areas, this is still the method usedtoday. The world's largest rainwater tank is probably the Yerebatan Sarayi

in Istanbul, Turkey. This was constructed during the rule of Caesar Justinian

(A.D. 527-565). It measures 140m by 70m and has a capacity of 80,000

cubic meters.

According to UNESCO, arid regions are defined as areas where

potential evapo-transpiration is much greater than precipitation. Table (1)

shows the extent of aridity in the Medial East and North Africa region(MENA) as reflected in rainfall data. It also shows that arid and semi-arid

areas amount to about 96% of the North African part and 95% of the Asian

part of the MENA region.


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Table1: Extent of aridity in the Arab region ]2[

Amount of Rainfall

Less than 100 mm (A)

Arid Areas

100-400mm (B)

Semi-arid AreasSub-region Total Area(1000 km!)Area (1000

km!)%

Area (1000

km!)%

A+ B as % ofTotal

North Africa 5751 4864 85 653 11 96

Near East 3705 3033 79 589 16 95

Total MENA 9456 7897 84 1242 13 97

Many countries, therefore, in the Middle East increasingly suffer from

water shortages due to the unavailability of renewable water resources and

to the rabid increase in population (see table 2).Table2: Indicates water deficiencies in some Arab countries ]3[ .

No Country

Annual

renewable

resources

106 M3

Annual

consumption

106 M3

Deficiencies

106 M3

Deficiencies

%

Level of

deficiencies

1 Iraq 42560 47330 - 4770 10 Limited

2 Kuwait 508 640 - 132 21 Medium

3 Qatar 259 334 - 75 22 Medium4 Libya 3980 5580 - 1600 29 Medium

5 Jordan 880 1280 - 400 31 Medium

6 Bahrain 157 250 - 93 37 Medium

9 UAE 1050 2230 - 1280 57 Critical

8 Yemen 1500 3600 - 2100 58 Critical

9 Oman 345 1417 - 1072 76 V. Critical

10 Saudi 2900 23100 - 20200 87 Dangerous

Therefore, rainwater harvesting in some rural areas seen as the main

source for water supply but in other communities is the only feasible water

supply. In both cases, rainwater harvesting is an option for improving the

living conditions of many communities facing serious water supply

shortages by providing an improved water source qualitatively and

quantitatively.

Rainwater Harvesting in Yemen is a traditional practice, and in manyareas Cisterns are used to conserve rain water. The cisterns of Tawaila (rain

flood harvesting), or the Tawaila Tanks are Aden!s best historic sites.Mareb dam is an example of a water harvesting technology started 2000

years B.C in Yemen to provide agricultural and domestic waters to the left


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and right paradise as stated in the Holy Qura!an.

1.2 Water Scarcity in Yemen

Generally speaking, the total supply of water in aquifers is non-

expandable. The central challenge facing the country in general and Sana'a

Basin in particular today and in the foreseeable future is therefore how to

produce more food and enhance farmer income besides meeting the other

demands like drinking water and industrial needs. With a rapid increase of

population, it is expected that by the year 2025, the basin population will

reach 5.85 M people (recently the population is 1.75 M people ]4[ . Between

now and then, a significant amount of the additional food supply needed toserve the growing requirement will have to be produced on land served by

irrigation. There are four profound effects of the population growth and the

drought climate as a result of global climatic changes:

Rising competition by different sectors for scarce water;

Rising pressures to use water much more efficiently;

Rising socio-economic pressures to define water rights more clearly and

Look for alternative water resources such as water harvesting

1.3 Sanaa Climate and Water Characteristics

Sana!a Basin is experiencing a serious depletion of groundwaterresources with associated water quality degradation. The water resources

situation in Sana!a Basin is extremely serious as abstraction exceedsrecharge by more than five folds. Consequently, the piezometric level

declines about 4-8 meters annually. Groundwater is mainly used foragricultural activities, which have expanded several times since 1980's, and

consume about 90% of water. Mismanagement of water resources is mainly

caused by lack of data, policy and institutional framework for groundwater

abstraction and use, and inefficient irrigation practices. In addition, rainfall

is becoming much less each year due to climatic changes. There are two

rainy seasons, separated by a distinct dry interval (May-mid July). The

annual rainfall generally varies between 150 and 350 mm, with some years

having, higher rainfall amounts above 350 mm. The first rainy period startsin mid-March-beginning of April, the second rainy period begins mid-July-

beginning of August and stops abruptly end of August. The months

September through February are generally dry, although occasional


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thunderstorms may bring some rain during these months. Sixty-five to

seventy-five percent of the rain falls during the months January-June. The

number of rain days with rainfall amounts above 5 mm/day varies between

5-15 days. The average amount of rainfall per rain day is about 16-17 mm.

The potential evapotranspiration (PET) for an average year varies

depending on altitude, wind exposure and latitude. The PET varies between

3-3.5 mm/day during the dry, cold period and 5-6 mm/day during the

months May-June. The average total amount of evapotranspiration per year

is about 1700 mm.

2.RAINFALL AND RUNOFF ANALYSIS

2.1 Rainfall rate validation

For water harvesting purpose we will use the average year rainfall

from ten years data in the Sana!a city (NWRA, 2010). The average rainfallfor the years (1993-2001) is 243 mm/year (see table 3).

Table3: Rainfall data of Sanaa City (1990-2003) for 10 years

MonthsYear

1 2 3 4 5 6 7 8 9 10 11 12Annual

1990 0 2.5 40.5 19 3.5 0 31.5 2 25 0 0 0 124

Mini

Year1991 0 5.5 45 11 11.5 0 2.5 35 0.5 0 0 0.5 111.5

Max

Year1992 2.5 0.5 20 20 64.5 3 10 140 24.5 26 0 39.5 350

1993 2.5 9 13.5 83 79.5 6 3 25 30.5 1 45 19 316.5

1997 5.5 1.5 14.5 29.5 7.5 2 12.5 33.5 0 60.5 34 1 201.5

1998 0 0.5 8 19 68.5 0 63 176 0 0 6.5 3412000 0.5 8 30 57.5 9 58.5 2.5 16 2.5 146 330

Medi

anYear

2001 29 108 31 13 1 0 49 21.5 21 22.5 7 1 303

2002 0 0.5 8 1 1 0 49 21.5 21 22.5 0 0 124.5

2003 0 0 10.5 52.5 12.5 0.5 0 0 0 3 2 146 227

Ave

year4.33 12.80 19.90 27.80 30.70 1.28 22.95 51.20 12.50 15.15 9.60 39.17 243

The selection of the 2001 year to be used for the calculation of the

maximum storage requirement is based on the following:

1.It is one of ten years data which is the minimum requirement for thenumbers of years of data.


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2.The year value it has is more than the average year.3.It has 12 months of reading Arranging the rainfall in descending order:

350, 341, 330, 316.5, 303, 227, 201.5, 124.5, 124, 111.5

The 350 mm/year is equaled or exceeded only once in ten years, andthe average 243 mm/year is equaled or exceeded in five years. Use 243mm/year for the design. More accurate estimation was done throughanalysis of rainfall data from additional source i.e. NASA Tropical RainfallMeasuring Mission (TRMM) in order to validate the above selection ofrainfall rate. TRMM is a joint mission between the National Aeronauticsand Space Administration (NASA) (disc2.nascom.nasa.gov) of the UnitedStates and the Japan Aerospace Exploration Agency (JAXA). Using theTRM model, the authors have obtained table (4) for 10 years (1999-2009)

Table 4: Sanaa rainfall for the range from 1999 to 2009

latitude Longitude 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Averagerainfall

15.0000 44.000 244 144 398 306 244 263 347 464 316 241 158 284

15.0000 44.2500 184 80 368 173 288 141 347 395 200 139 106 220

15.2500 44.0000 232 101 280 192 275 182 275 426 210 168 130 225

Averagerainfall

220 109 349 224 269 195 323 428 242 183 131 243

Selected parameter: 3-hourly TRMM 3B42(V6) Accumulated Rainfall

Selected area: lat=[15N,16N], long=[44E,45E], (44"13'E, 15"28'N, Elevation: 2190m)

Selected time period: (21Z31Jan1999-21Z31Dec2009)

Unit: (mm)

The average of 243 mm/yr. coincides with the previous one obtainedfrom NWRA data. Therefore, calculations of the harvested water volumeare based on an annual average rainfall of the year!s period of 243 mm (seetable 5 above).

2.2 Water Harvested Estimation

The Rational method is probably the most popular method andpreferable in storm design systems in urban areas. It has been applied all

over the world and many refinements of the method have been produced. Ithas the following simple form:

Harvested water = C x I x A

Where:

The harvested water is the quantity of the water harvestedfrom the roofs (m3)C : The runoff coefficient (dimensionless)

I : used here annual average rainfall (mm/yr.) and

A : the roof area (m2)

The total harvested water volume is calculated based on:

Average rainfall Size of the catchments area (rooftop)


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Runoff coefficient (see table 5) For flat slopes or impermeable soils use higher values For flat slopes or permeable soils use lower values, For steep slopes or impermeable soils use the higher values.

Table5: Runoff coefficients

Area Description Runoff Coefficient C

Business

Downtown 0.70-0.95

Neighbourhood 0.50-0.70

Residential

Single-Family 0.30-0.50Multi-units, detached 0.40-0.60

Multi-units, attached 0.60-0.75

Residential (suburban) 0.25-0.40

Apartment 0.50-0.70

Industrial

Light 0.50-0.80

Heavy 0.60-0.90

Parks, cemeteries 0.10-0.25

Playgrounds 0.20-0.35

Railroad yard 0.20-0.35

Unimproved 0.10-0.30

Character of surface Runoff Coefficient C

Pavement

Asphaltic and concrete 0.70-0.95

Brick 0.70-0.85

Roofs 0.75-0.95

Simple design tables where developed applying the above simple

equation as basic guidance to estimate the water harvested volume based on

several run off coefficients of 0.6, 0.7, 0.75 and 0.8, the rainfall and the

surface area. Tables 6, 7, 8 and 9 illustrating the water volume harvested

from roof tops using rainfall average of 243 mm/year with different roof

surface areas.


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Table6: Harvested water volume guide table using run off coefficient (C=0.6)

Rainfall

(mm)100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof

areas

(m2)

Harvested Water Volume from Roof Top (m3), C= 0.6

20 1 2 2 3 4 4 5 5 6 7 8 10 11 12 18

30 2 3 4 5 5 6 7 8 9 11 13 14 16 18 27

40 2 4 5 6 7 8 10 11 12 14 17 19 22 24 36

50 3 5 6 8 9 11 12 14 15 18 21 24 27 30 45

60 4 5 7 9 11 13 14 16 18 22 25 29 32 36 54

70 4 6 8 11 13 15 17 19 21 25 29 34 38 42 63

80 5 7 10 12 14 17 19 22 24 29 34 38 43 48 72

90 5 8 11 14 16 19 22 24 27 32 38 43 49 54 81

100 6 9 12 15 18 21 24 27 30 36 42 48 54 60 90

150 9 14 18 23 27 32 36 41 45 54 63 72 81 90 135

200 12 18 24 30 36 42 48 54 60 72 84 96 108 120 180

250 15 23 30 38 45 53 60 68 75 90 105 120 135 150 225

300 18 27 36 45 54 63 72 81 90 108 126 144 162 180 270

350 21 32 42 53 63 74 84 95 105 126 147 168 189 210 315

400 24 36 48 60 72 84 96 108 120 144 168 192 216 240 360

450 27 41 54 68 81 95 108 122 135 162 189 216 243 270 405

500 30 45 60 75 90 105 120 135 150 180 210 240 270 300 450

600 36 54 72 90 108 126 144 162 180 216 252 288 324 360 540

700 42 63 84 105 126 147 168 189 210 252 294 336 378 420 630

800 48 72 96 120 144 168 192 216 240 288 336 384 432 480 720

900 54 81 108 135 162 189 216 243 270 324 378 432 486 540 810

1000 60 90 120 150 180 210 240 270 300 360 420 480 540 600 900

1500 90 135 180 225 270 315 360 405 450 540 630 720 810 900 1350

2000 120 180 240 300 360 420 480 540 600 720 840 960 1080 1200 1800

2500 150 225 300 375 450 525 600 675 750 900 1050 1200 1350 1500 2250

3000 180 270 360 450 540 630 720 810 900 1080 1260 1440 1620 1800 2700

3500 210 315 420 525 630 735 840 945105

01260 1470 1680 1890 2100 3150

4000 240 360 480 600 720 840 960 1080120

01440 1680 1920 2160 2400 3600

4500 270 405 540 675 810 945 1080 1215135

01620 1890 2160 2430 2700 4050

5000 300 450 600 750 900 1050 1200 1350150

01800 2100 2400 2700 3000 4500


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Table7: Harvested water volume guide table using run off coefficient (C=0.7)

Rainfall (mm) 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas

(m2) Harvested Water Volume from Roof Top (m3), C= 0.7

20 1 2 3 4 4 5 6 6 7 8 10 11 13 14 21

30 2 3 4 5 6 7 8 9 11 13 15 17 19 21 32

40 3 4 6 7 8 10 11 13 14 17 20 22 25 28 42

50 4 5 7 9 11 12 14 16 18 21 25 28 32 35 53

60 4 6 8 11 13 15 17 19 21 25 29 34 38 42 63

70 5 7 10 12 15 17 20 22 25 29 34 39 44 49 74

80 6 8 11 14 17 20 22 25 28 34 39 45 50 56 84

90 6 9 13 16 19 22 25 28 32 38 44 50 57 63 95

100 7 11 14 18 21 25 28 32 35 42 49 56 63 70 105

150 11 16 21 26 32 37 42 47 53 63 74 84 95 105 158

200 14 21 28 35 42 49 56 63 70 84 98 112 126 140 210

250 18 26 35 44 53 61 70 79 88 105 123 140 158 175 263

300 21 32 42 53 63 74 84 95 105 126 147 168 189 210 315

350 25 37 49 61 74 86 98 110 123 147 172 196 221 245 368

400 28 42 56 70 84 98 112 126 140 168 196 224 252 280 420

450 32 47 63 79 95 110 126 142 158 189 221 252 284 315 473

500 35 53 70 88 105 123 140 158 175 210 245 280 315 350 525

600 42 63 84 105 126 147 168 189 210 252 294 336 378 420 630

700 49 74 98 123 147 172 196 221 245 294 343 392 441 490 735

800 56 84 112 140 168 196 224 252 280 336 392 448 504 560 840

900 63 95 126 158 189 221 252 284 315 378 441 504 567 630 945

1000 70 105 140 175 210 245 280 315 350 420 490 560 630 700 1050

1500 105 158 210 263 315 368 420 473 525 630 735 840 945 1050 1575

2000 140 210 280 350 420 490 560 630 700 840 980 1120 1260 1400 2100

2500 175 263 350 438 525 613 700 788 875 1050 1225 1400 1575 1750 2625

3000 210 315 420 525 630 735 840 945105

01260 1470 1680 1890 2100 3150

3500 245 368 490 613 735 858 980110

3

122

51470 1715 1960 2205 2450 3675

4000 280 420 560 700 840 980112

0

126

0

140

01680 1960 2240 2520 2800 4200

4500 315 473 630 788 945110

3

126

0

141

8

157

51890 2205 2520 2835 3150 4725

5000 350 525 700 875105

0

122

5

140

0

157

5

175

02100 2450 2800 3150 3500 5250


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Table 8: Harvested water volume guide table using run off coefficient (C=0.75)

Rainfall (mm) 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas

(m2)


20 2 2 3 4 5 5 6 7 8 9 11 12 14 15 23

30 2 3 5 6 7 8 9 10 11 14 16 18 20 23 34

40 3 5 6 8 9 11 12 14 15 18 21 24 27 30 45

50 4 6 8 9 11 13 15 17 19 23 26 30 34 38 56

60 5 7 9 11 14 16 18 20 23 27 32 36 41 45 68

70 5 8 11 13 16 18 21 24 26 32 37 42 47 53 79

80 6 9 12 15 18 21 24 27 30 36 42 48 54 60 90

90 7 10 14 17 20 24 27 30 34 41 47 54 61 68 101

100 8 11 15 19 23 26 30 34 38 45 53 60 68 75 113

150 11 17 23 28 34 39 45 51 56 68 79 90 101 113 169

200 15 23 30 38 45 53 60 68 75 90 105 120 135 150 225

250 19 28 38 47 56 66 75 84 94 113 131 150 169 188 281

300 23 34 45 56 68 79 90 101 113 135 158 180 203 225 338

350 26 39 53 66 79 92 105 118 131 158 184 210 236 263 394

400 30 45 60 75 90 105 120 135 150 180 210 240 270 300 450

450 34 51 68 84 101 118 135 152 169 203 236 270 304 338 506

500 38 56 75 94 113 131 150 169 188 225 263 300 338 375 563

600 45 68 90 113 135 158 180 203 225 270 315 360 405 450 675

700 53 79 105 131 158 184 210 236 263 315 368 420 473 525 788

800 60 90 120 150 180 210 240 270 300 360 420 480 540 600 900

900 68 101 135 169 203 236 270 304 338 405 473 540 608 675 1013

1000 75 113 150 188 225 263 300 338 375 450 525 600 675 750 1125

1500 113 169 225 281 338 394 450 506 563 675 788 900 1013 1125 1688

2000 150 225 300 375 450 525 600 675 750 900 1050 1200 1350 1500 2250

2500 188 281 375 469 563 656 750 844 938 1125 1313 1500 1688 1875 2813

3000 225 338 450 563 675 788 900 1013 1125 1350 1575 1800 2025 2250 3375

3500 263 394 525 656 788 919 1050 1181 1313 1575 1838 2100 2363 2625 3938

4000 300 450 600 750 900 1050 1200 1350 1500 1800 2100 2400 2700 3000 4500

4500 338 506 675 844 1013 1181 1350 1519 1688 2025 2363 2700 3038 3375 5063

5000 375 563 750 938 1125 1313 1500 1688 1875 2250 2625 3000 3375 3750 5625


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Table 9: Harvested water volume guide table using run off coefficient (C=0.8)

Rainfall

(mm)

100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500

Roof areas

(m2)


20 2 2 3 4 5 6 6 7 8 10 11 13 14 16 24

30 2 4 5 6 7 8 10 11 12 14 17 19 22 24 36

40 3 5 6 8 10 11 13 14 16 19 22 26 29 32 48

50 4 6 8 10 12 14 16 18 20 24 28 32 36 40 60

60 5 7 10 12 14 17 19 22 24 29 34 38 43 48 72

70 6 8 11 14 17 20 22 25 28 34 39 45 50 56 84

80 6 10 13 16 19 22 26 29 32 38 45 51 58 64 96

90 7 11 14 18 22 25 29 32 36 43 50 58 65 72 108

100 8 12 16 20 24 28 32 36 40 48 56 64 72 80 120

150 12 18 24 30 36 42 48 54 60 72 84 96 108 120 180

200 16 24 32 40 48 56 64 72 80 96 112 128 144 160 240

250 20 30 40 50 60 70 80 90 100 120 140 160 180 200 300

300 24 36 48 60 72 84 96 108 120 144 168 192 216 240 360

350 28 42 56 70 84 98 112 126 140 168 196 224 252 280 420

400 32 48 64 80 96 112 128 144 160 192 224 256 288 320 480

450 36 54 72 90 108 126 144 162 180 216 252 288 324 360 540

500 40 60 80 100 120 140 160 180 200 240 280 320 360 400 600

600 48 72 96 120 144 168 192 216 240 288 336 384 432 480 720

700 56 84 112 140 168 196 224 252 280 336 392 448 504 560 840

800 64 96 128 160 192 224 256 288 320 384 448 512 576 640 960

900 72 108 144 180 216 252 288 324 360 432 504 576 648 720 1080

1000 80 120 160 200 240 280 320 360 400 480 560 640 720 800 1200

1500 120 180 240 300 360 420 480 540 600 720 840 960 1080 1200 1800

2000 160 240 320 400 480 560 640 720 800 960 1120 1280 1440 1600 2400

2500 200 300 400 500 600 700 800 900 1000 1200 1400 1600 1800 2000 3000

3000 240 360 480 600 720 840 960 1080 1200 1440 1680 1920 2160 2400 3600

3500 280 420 560 700 840 980 1120 1260 1400 1680 1960 2240 2520 2800 4200

4000 320 480 640 800 960 1120 1280 1440 1600 1920 2240 2560 2880 3200 4800

4500 360 540 720 900 1080 1260 1440 1620 1800 2160 2520 2880 3240 3600 5400

5000 400 600 800 1000 1200 1400 1600 1800 2000 2400 2800 3200 3600 4000 6000


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3.ATER VOLUME ESTIMATION

Average household at Sana!a city uses about 30-70 liters per personper day ]5[ . According to WHO guidelines ]6[ a minimum value of 25 l/day

is acceptable for hygiene and health care in dry regions. In this paper the

estimated consumption from harvested water is 30 liter/capita/day for rural

areas and 70 liters/capita/day for urban areas. Households served previously

by a water utility can prepare simple management plan to use both the

harvested rainwater and the utility water supply efficiently. Households

solely dependent upon rainwater should adopt efficient water use practicesboth indoors and outdoors.

Household water demand is largely affected by changes in weather,

although changes in household occupancy rates depending upon seasons

and ages of household members, more water use during the hot summer

months, and very minor changes in consumption of water due to increases

in temperature may be worth factoring in some instances.

In this paper we will deal with the computational method of theSupply Side Approach (SSA) and try to develop a systematic process for

the design of storage tanks according to the volume of water harvested.

3.1 Supply Side Approach (SSA)

In low rainfall areas or areas where the rainfall is of uneven

distribution, more care has to be taken to size the storage properly. During

some months of the year there may be an excess of water, while at other

times there will be a deficit. If there is sufficient water throughout the year

to meet the demand, then sufficient storage will be required to bridge the

periods of scarcity. As storage is expensive, this should be done carefully to

avoid unnecessary expense.

3.2 Computational Method

According to the background of the study, several types of buildings

categories with different roof surface areas have been selected to calculatethe RWH quantity. The buildings categories and areas are:

1. Hospital educational building2. Commercial building


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3. Public building with an average roof surface area of 200 m2 and4. School building

The examples of the sample calculations of the above surface areas ofthe aforementioned buildings can be applied to similar buildings with

different roof surface areas. Accordingly, the only variables in the

calculations are roof surface area and the average rainfall of that specific

location, however, rainfall can be estimated generally as an average for the

whole city leaving the surface roof area the only variable. The following

example clarifies the steps of calculations for the RWH quantity and storage

tank capacity.

Example

Site: Public building, Sana!a YemenGiven data:Roof area: 200 m2Annual average rainfall: 243 mm per yearRunoff coefficient: 0.75 (concrete roof)Required parameter to be found:

1. Harvested volume/ month2. Harvested volume/day3. Storage capacity

Solution:

Annual available water (assuming all is collected and using Rational

Method) = 45.3675.0243.0200 = m3 /yr. or from table (7) you can get

directly the same value.

1. Monthly water requirement = 038.312

45.36= m3/ month

2. Daily available water = 0.101330

038.3= m3/ day

3. The calculation of the storage tank is listed in table 10 below


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Table10: Estimation tank capacity for a public building (200 m2)

(1)

Month

(2)Median

Rainfall

for the

years

1993-

2003

(mm)

(3)

Rainfall

harvested

(m#)

(4)

Cumulative

rainfall

harvested (m#)

(5)

Demand

(based on total

requirement

(m#)

(6)

Cumulative

demand

(m#)

(7)

Difference

between

column (4)

and (6)

Jan 29 4.35 4.35 3.04 3.04 1.31

Feb 108 16.20 20.55 3.04 6.08 14.47

Mar 31 4.65 25.20 3.04 9.12 16.08

Apr 13 1.95 27.15 3.04 12.16 14.99

May 1 0.15 27.30 3.04 15.20 12.10

Jun 0 0.00 27.30 3.04 18.24 9.06

Jul 49 7.35 34.65 3.04 21.28 13.37

Aug 22 3.30 37.95 3.04 24.32 13.63

Sep 21 3.15 41.10 3.04 27.36 13.74

Oct 23 3.45 44.55 3.04 30.40 14.15

Nov 7 1.05 45.60 3.04 33.44 12.16

Dec 1 0.15 45.75 3.04 36.48 9.27

Totals 45.75 36.48

Column (2): The median year rainfall is used (refer to table 3)

Column(3): Rainfall Harvested (m") = (C # Average Rainfall#RoofArea)/1000

Column (4): Cumulative rainfall harvested (m")Column (5): Demand ( Calculated from the step 2 of the example above)

Column (6): Cumulative demand based on column (5)

Column (7): The tank storage capacity [select the max value]Table 10 explains the process taken to calculate the storage tank

capacity by taking into consideration the incoming and the outgoing

cumulative water quantity. The storage tank capacity is taken as maximum

value in column (7) as the difference between the water harvested

(incoming) column 4 and the water requirement for the building (outgoing)

in column 6 in any month. This value is shown in the month of March to be

16.08 m3. According to this value the tank size can then be designed with

an extra of 25% of the water volume in to accommodate any higher rainfallmight occur.

Graphically we can calculate the storage capacity from the rainfall

data graphically by comparing the water harvested and the amount that can


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be supplied to the building using the harvested water. It can be noted that

there are two rainy seasons with dry periods (see figure 1). The month of

January yields some quantity after the dry months of November and

December. If we therefore assume that the tank is empty at the end of

December, we can form a graph of cumulative harvested water and

cumulative demand and calculate the maximum storage requirement (figure

2) which occurs in March. All this water will have to be stored to cover the

shortfall during the dry period.

Figure1: Comparison of the harvestable water and the demand water for each month ]7[


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Figure2: Showing the predicted cumulative inflow and outflow from the tank ]7[

4.Estimation of benefits of harvested water inSanaa

It is apparent that using harvested water is essentially going to reducethe stress on groundwater. Rainwater harvesting utilization strategies andpolicies must be drawn towards the benefits of minimizing the use of fossilgroundwater. Several uses of rainwater harvesting is seen such as drinking,livestock, complementary irrigation and gardening. The following

paragraphs estimate the amounts of rainwater that can be harvested andused rather than groundwater for Sana!a city.

The annual utilization of harvested water in Sana!a will result inreducing pressure on groundwater. This means that there is substantialamount of water in the deep aquifers is being saved as the same amount wasbeing consumed from water harvesting. Such amount is represented intable 11 with a value of 11,305,952 m3/yr, and 171,519 m3/yr, as areduction in the usage of groundwater in urban and rural areas respectively.

These values are calculated with an average roof top area of 200 m2 with atotal number of buildings according to ]8[ is 310,177 and 5,882 for urbanand rural areas respectively. The percentages savings therefore are 22 % forurban areas and 33% for rural areas and the benefits of using rainwaterharvesting is about (26,204,808 US$/year); refer to table 11 below.


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In addition to the cost and saving advantages motioned above there aresome other advantages of RTRWH in Sana!a city Include:

Roof top rainwater harvesting can co$exist with and provide a goodsupplement to other water sources and utility systems, thus relievingpressure ground water as the unique water source in Sana!a city.

Rainwater harvesting provides a water supply for the city areaswhich are not cover by water supply network especially during rainyseasons.

Rainwater harvesting provides a water supply for use in times ofemergency or breakdown of the public water supply systems,

particularly during natural disasters. Water received is free of costs, so the use of this water

significantly reduces water bills for purchased water from municipalsupply.

Harvesting rainwater is not only water conserving, it is also energyconserving since the energy input required to operate a centralizedwater system designed to treat and pump water over a vastservice area is by-passed.

Rainwater harvesting can reduce storm drainage load and floodingin streets, so it reduce local soil erosion and flooding caused bythe rapid runoff of water from impervious cover such aspavements areas and roofs. Also, the RWH reduced level of stormwater requires smaller sized storm water drainage systems and helpsin reducing soil erosion into the waterways.

Rainwater Collected From Roof and Stored Underground or inStorage tanks Scarcity Period to meet Increasing Demand forWater in Urban Areas.

Rainwater Collected From Roof can be used for groundwaterrecharge through the shallow dry wells which was installed insidethe house or near of it, which will help in control decline of waterlevels (Recharge the aquifers)

Rainwater Collection in ponds through the water ways inside thecity will contribute in recharging groundwater as well as for

gardening and street trees irrigation by Tankers water for theseponds instead of watering them by groundwater.


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Table 11. Water harvesting and consumption estimation for Sanaa city

Description Unit Quantity

No. Urban Houses No. 310,177No. rural Houses No. 5,882

No. of total Houses No. 316,059

Roof area (average) m2 200

Average rainfall mm 243

Urban runoff coefficient (C) Unit less 0.75

Rural runoff coefficient (C). Unit less 0.6

Estimated quantity of harvested water in urban areas m3 11,305,952

Estimated quantity of harvested water in rural areas m3 171,519

Total Quantity of water harvested (Urban +Rural) m3 11,477,471

Estimated consumption in urban areas per capita l/capita/day 70

Estimated consumption in rural areas per capita l/capita /day 30

Estimated consumption in urban areas from GW m3/year 51,621,041

Estimated consumption in rural areas from GW m3/year 521,450

Estimated consumption in total areas from GW m3/year 52,142,491

Groundwater saving in urban areas % 22

Groundwater saving in rural areas % 33

Water value YR/m3 130

Benefits of using Harvested water (urban) YR 5,240,961,635

Benefits of using Harvested water (urban) US$ 26,204,808

5.MAIN FACTORS OF THE RTRWH SYSTEM

The main factor affects the harvested water is the rainfall availability, and

the costs that could be incurred in the construction process. Otherparameters such as water quality, hygiene and maintenance are also

important issues. The following points should be considered when thinking

to use Roof Top Rain Water Harvested (RTRWH) system:

1- Find out how much is the annual average rainfall in the city2- Calculate the rooftop area3- Select the type of storage tank4- Locate the storage tank in an area away from pollution or depending

on the space of the house compound.

a. In many cases of unavailable space install a readymade steel or


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Ferro-cement above ground tanks close to the house [ ]7 .

b. In some cases it is advisable to construct a single storage tankserving several houses.

5- Design the inflow pipes system6- Design and install first flush pipe system to flush out the first few

minutes of rains which is usually contains debris, dust leaves.. etc.

7- Design the storage tank according to the maximum storagerequirement adding 25% with the necessary openings for

maintenance. Typical tanks (above and below ground are available

with complete details for common sizes including costs). Figure 3

shows a residential building in a village utilizing the rooftop

rainwater harvesting which is stored in a tank made of masonry and

concrete.

Figure3: An existing rooftop harvesting tank used for more than 40 years (home

village of the second author)

8 Use Overflow of water from tank or from first flush for gardening,livestock or recharge.

9 Test water quality at regular basis especially at beginning season ofrain either taking samples to the lab or on site. On site water qualitytests can be done simply by:


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H2S strip test bottle check: Wash your hands thoroughly with soap.With clean hands the sealed bottle should be opened. From the tap inthe rainwater storage tank fill the bottle to the mark line. Close the

cap tightly. Bring the bottle back to a safe place in a room. Observefor 24 to 48 hours. If the water turns black in the bottle then it ismicro-biologically contaminated and requires treatment before beingused for drinking. If the water color stays brown, then the water is fitfor drinking.

10 Chlorinating the water at least once during the rainy system andwhen necessary

11 Awareness campaigns about importance of harvesting water fromroofs and about water quality with brochures could be prepared for

this purpose.12 More hints are summarized below:

a. construct rainwater tanks far away from existing cesspitsb. regular cleaning of the storage tank from sediments and before

the beginning of the rainy seasonc. Keep the roofs catchment area cleand. Boil water or use filter systems when using harvested water for

drinking or

Solar disinfection (SODIS): In this method, rainwater is kept in aglass bottle under the sun for 6 hours. One side of the bottle is paintedblack. The black surface is kept on the ground. With a combination of UVdisinfection and infra-red heat, water is sterilized and then becomes fit forconsumption. In cloudy weather the bottles need to be kept in the sun longer

]8[ . Several bottles can be used with this method.

6.CONCLUSIONS AND RECOMMENDATIONS

Rainwater harvesting is a potential parameter to be used both insaving the costs of utilizing groundwater from the water supply utility andsaving the precious non-renewable fossil groundwater. It is estimated thatan annual volume of 11,305,952 m3/yr, and 171,519 m3/yr. can beharvested in urban and rural areas respectively resulting in an annualsavings of the groundwater by 22% and 33%. Simple calculation of thecosts saved when using rainwater harvesting is 26,204,808 US$/year. Thedevelopment of guide tables present an easy and direct method to select theamount of the water harvested according to the roof area, the run off

coefficient and the annual average rainfall. Several runoff coefficients havebeen used according to the type of roof surfaces such as 0.6, 0.7, 0.75 and0.8 that corresponds to the present roof surfaces available in Sana!a. Suchguide tables can be easily modified to be used in any country by modifyingthe necessary parameters applicable to that specific country. Municipality


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and local city government should develop appropriate legislation to allowresidents to employ rainwater harvesting in their households. They shouldprovide certain technical support and financial aid if needed to residents.

Awareness and educational campaigns should be conducted to encouragepeople to use the rainwater harvesting systems. It is recommended that localgovernment starts the initiative using rooftop harvesting in their buildings.

7.REFERENCES

]1[ Centre for Science and Environment, %Rainwater Harvesting andUtilization, An Introductory Guide for Decision-Makers&.Tughlakabad Institutional area, New Delhi - 110062, India2005..

]2[ Noman, A., Taher, T., % Water Harvesting and Spate Irrigation inWadis: Yemen Case&. Wadi Hydrology Conference, Amman, Jordan,2004.

]3[ ()*+-/0 1346/0 8:

Documents

ROOFTOP RAINWATER HARVESTING in MODERN CITIES: a CASE STUDY for SANA’A CITY, YEMEN