Civil Engineering

RAINWATER HARVESTING IN BANGLADESH

By

MOHAMMAD ASIF IDRIS

SABBIR AHMMED

DEPARTMENT OF CIVIL ENGINEERING

STAMFORD UNIVERSITY

BANGLADESH

Introduction

Millions of people throughout the world do not have access to clean water for domestic purposes.

In many parts of the world conventional piped water is either absent, unreliable or too expensive.

One of the biggest challenges of the 21st century is to overcome the growing water shortage.

Rainwater harvesting (RWH) has thus regained its importance as a valuable alternative or

supplementary water resource, along with more conventional water supply technologies. Much

actual or potential water shortages can be relieved if rainwater harvesting is practiced more

widely.

People collect and store rainwater in buckets, tanks, ponds and wells. This is commonly referred

to as rainwater harvesting and has been practiced for centuries. Rainwater can be used for

multiple purposes ranging from irrigating crops to washing, cooking and drinking.

Rainwater harvesting is a simple low-cost technique that requires minimum specific expertise or

knowledge and offers many benefits. Collected rainwater can supplement other water sources

when they become scarce or are of low quality like brackish groundwater or polluted surface water

in the rainy season. It also provides a good alternative and replacement in times of drought or when

the water table drops and wells go dry. One should, however, realize that rainfall itself cannot be

managed. Particularly in arid or semi-arid areas, the prevailing climatic conditions make it of

crucial importance to use the limited amount of rainfall as efficiently as possible. The collected

rainwater is a valuable supplement that would otherwise be lost by surface run-off or evaporation.

During the past decade, RWH has been actively reintroduced by local organizations as an option

for increasing access to water in currently underserved areas (rural or urban). Unfortunately

decision-makers, planners, engineers and builders often overlook this action. The reason that RWH

is rarely considered is often simply due to lack of information on feasibility both technical and

otherwise. During the past decade the technology has, however, quickly regained popularity as

users realize the benefits of a relatively clean, reliable and affordable water source at home.

In many areas RWH has now been introduced as part of an integrated water supply, where the

town water supply is unreliable, or where local water sources dry up for a part of the year. But

RWH can also be introduced as the sole water source for communities or households. The

technology is flexible and adaptable to a very wide variety of conditions. It is used in the

richest and the poorest societies, as well as in the wettest and the driest regions on our planet.

Designing a rainwater harvesting system The main consideration in designing a rainwater harvesting system is to size the volume of the

storage tank correctly. The tank should give adequate storage capacity at minimum construction

costs.

Four steps to be followed in designing a RWH system:

Step 1: Determine the total amount of required and available rainwater

Step 2: Design catchment area

Step 3: Design delivery system

Step 4: Select suitable design of storage reservoir

Step 1: Determine the total amount of require and available rainwater

On our work basis at first we collected the rainfall data from “BANGLADESH

METEOROLOGICAL DEPARTMENT”. Then we divide our country into four zones

according to similar rainfall data. Next we assume three types of family based on family

member. 4, 6 & 8 person but you can assume many types of family based on person. Then we

find out the required water (liter/month) for each family. In Bangladesh, people need 7 to 8 liter

water per day for their domestic use. So, require water for a family in a month (4 persons)

Daily require water × no. of person × Month

i.e. Require water for January.

7.5 × 4 × 31= 930 liter/month

For 6 person

7.5 × 6 × 31= 1395 liter/month

For 8 person

7.5 × 8 × 31= 1860 liter/month

Then cumulate the require water one by one. Now assume many types of Roof tops or house

area. Here we take five types of roof tops. There are 10 m2, 20 m

2, 30 m

2, 40 m

2, 50 m

2. Now

find out the quantity of water. We can find it from multiplied by monthly rainfall data with

house area. We take the average value for each zone. For example –

For 40 m2 house, water quantity of January,

Rainfall data of January × Area

1.5 × 40 = 60 liter/month

Now cumulate the Quantity one by one. Then subtract all cumulative of quantity with the all

cumulative of require water. Now take the highest negative value from each calculation table.

The negative values are main require water and the positive values are surplus water.

Figure 1: Horizontal plan area of the roof for calculating the catchment surface

Step 2: Design Catchment Area Now draw a summary table from calculation. Then plot area against negative value in a normal

graph paper. From the graph we will get different reservoir area for different house area and

different family.

Step 3: Design Delivery System The collected water from a roof needs to be transported to the storage reservoir or tank through

a system of gutters and pipes, the delivery system is called guttering. Several other types of

delivery systems exist but gutters are by far the most common. Commonly used materials for

gutters and downpipes are galvanized metal and plastic (PVC) pipes, which are readily available

in local shops. There is a wide variety of guttering available from prefabricated plastics to

simple gutters made on-site from sheet metal. In some countries bamboo, wood stems and

banana leaves have been used. Gutters made from extruded plastic are durable but expensive.

For the guttering, aluminum or galvanized metals are recommended because of their strength,

while plastic gutters may suffice beneath small roof areas. Almost all plastics, certainly PVC,

must be protected from direct sunlight. Generally, the cost of gutters is low compared to that of

storage reservoirs or tanks, which tend to make up the greatest portion of the total cost of a

RWH system.

Gutters are readily available in different shapes. They can be rounded, square, V-shaped, and

have open or closed ends with attached downpipe connectors. They can be made in small

workshops in sections that are later joined together or they can even be made on-site by

plumbers. Workshop-made gutters usually have a square shape and tend to be two to three times

more expensive than similar gutters made on-site. On-site gutters are usually V-shaped. These

are quite efficient but they tend to get more easily blocked with debris and leaves. V-shaped

gutters are usually tied directly under the roof or onto a so-called splash guard. V-shape gutters

often continue all the way to the tank without addition of the usual rounded downpipe section.

Wooden planks and bamboo gutters are usually cheap (or even free of charge). These gutters do,

however, suffer from problems of durability as the organic material will eventually rot away and

leak. Their porous surfaces also form an ideal environment for accumulation of bacteria that may

be subsequently washed into the storage tank.

Aluminum is naturally resistant to corrosion, which makes it last indefinitely. The cost of an

aluminum sheet is over 1.5 times the cost of steel of the same thickness and the material is less

stiff so for a similar strength of gutter a larger thickness of material is required, resulting in gutters

that are up to three times more expensive. Nevertheless, there is a growing market for aluminum

sheets in developing countries so the price will almost certainly come down over time. Half

pipes have been proposed as an inexpensive form of guttering and are used in many areas.

The production is relatively simple, and the semi-circular shape is extremely efficient for RWH.

The cost of these gutters depends on the local cost of piping, which may be more expensive than

an equivalent sheet metal gutter.

Figure 2: Different types of gutters: square, rounded, V-shaped

Proper construction of gutters is essential to avoid water losses. Gutters must slope evenly

towards the tank to ensure a slow flow. Gutters are often the weak link in a RWH system and

installations can be found with gutters leaking at joints or even sloping the wrong way.

Figure 3: Proper construction of the gutter is important

Gutters must be properly sized and correctly connected around the whole roof area. When high

intensity rainfall occurs, gutters need to be fitted with so-called splash guards to prevent

overshooting water losses. A properly fitted and maintained gutter-downpipe system is capable of

diverting more than 90% of all rainwater run-offs into the storage tank. Although gutter size may

reduce the overflow losses, additional splash guards should be incorporated on corrugated iron

roofs. Splash guards consist of a long strip of sheet metal 30 cm wide, bent at an angle and hung

over the edge of the roof about 2-3 cm to ensure all run-off for the roof enters the gutter. The

splashguard is connected to the roof and the lower half is hung vertically down from the edge of the

roof.

During intensive rainfall, large quantities of run-off can be lost due to gutter overflow and

spillage if gutters are too small. To avoid over-flow during heavy rains, it makes sense to create

a greater gutter capacity. A useful rule of thumb is to make sure that there is at least 1 cm2 of

gutter cross section for every 1 m2 of roof surface. The usual 10 cm wide rounded (e.g. 38 cm

2)

gutters are generally not big enough for roofs larger than about 40 m2. A square-shaped gutter of

10 cm2

can be used for roof areas measuring up to 100 m2 under most rainfall regimes. For large

roofs, such as on community buildings and schools, the 14 × 14 cm V-shaped design with a cross-

sectional area of 98 cm2

is suitable for roof sections up to 50 m long and 8 m wide (400 m2).

When gutters are installed with a steeper gradient than 1:100 (1 cm vertical drop over 100 cm

horizontal distance) and used together with splash guards, V-shaped gutters can cope with heavy

rains without large amounts of loss. A gradient of 1:100 ensures steady water flow and less

chance of gutter blockage from leaves or other debris. Downpipes, which connect the gutters to the

storage reservoir, should have similar dimensions to the gutters.

The following tables give some examples of guttering systems. The guttering requirement for

a typical household roof of 60 m2 is shown in table 1. Typical gutter widths for such roofs are

presented in table 2.

Section Roof size Slope Cross sectional

area

Gutter sizes

Square 40-100 m2 0.3-0.5% 70 cm

2 7 × 10 cm

Rounded 40-60 m2 0.3-0.5% 63 cm

2 125 mm bore

45° V- shaped Not specified 1.0% 113 cm2 15 cm on each side

Table 1: Examples for guttering systems

Square 0.5%

slope

Square 1.0% slope Rounded 0.5%

slope

45° V-shape

1.0% slope

Gutter width (at top) 71 mm 63 mm 96 mm 124 mm

Cross sectional area 47 cm2 39 cm

2 36 cm

2 38 cm

2

Table 2: Gutter sizes quoted in literature

Step 4: Selection of a suitable storage reservoir design Suitable design of storage reservoirs depends on local conditions, available materials and

budget, etc. It is often best when constructing tanks to choose a design based on the use of local

materials. This is usually the cheapest option. A tank is referred to as an above ground storage

reservoir and a cistern as a below ground storage reservoir. Most storage tanks have a round or

cylindrical shape, which is much stronger and uses less material than square or rectangular

tanks. Both types of reservoirs (tank or cistern) can vary in size from a cubic meter or 1,000

litres up to hundreds of cubic meters for a tank at community level, but they typically range

from 10 to 30 m3 for a domestic system at household level.

ANALYSIS

Data Analysis and Calculation

At first we divide our country (Bangladesh) into four zones according to similar rainfall data.

Figure 5: zones of Bangladesh divided according to similar rainfall data

Zone 1: Western Area – Low Rainfall

Dinajpur, Rangpur, Bagra, Rjshahi, Ishurdi, Jessore, Nilphamari, Kurigram, Thakurgaon,

Naogaon,Pabna, Meherpur, Sirajganj, Gaibandha, Tetulia, Burimari, Jhenaidah.

Zone 2: Central and Eastern Area – Average Rainfall

Mymensing, Dhaka, Comilla, Chandpur, Faridpur, Madaripur, Feni, Sitakunda, Sandwip,

Kutubdia, Cox’s bazar, Rangamati, Tangail, Patenga, Ambagan, Tangail, Narshingdi, Sherpur,

Kishoreganj,Brahmanbaria, Comilla.

Zone 3: North – Eastern Area – High Rainfall

Sylhet, Srimongal, Moulvi bazar, Habiganj.

Zone 4: Coastal Area – Medium to high Rainfall

Khulna, Satkhira, Barisal, Bhola, Hatiya, Teknaf, Patuakhali, Khepupara, Mongla, Bagerhat,

Narail, Benapol, Gopalganj, Tekhnaf.

All the calculations are same for each zone. We have taken the five years (2007 - 2011) rainfall

data and take average value for each zone. We have also taken 5 types of roof tops which are 10

m2, 20 m

2, 30 m2, 40 m

2, 50 m2 and three types of family which is based on family member. 4

persons, 6 persons, 8 persons.

The difference between cumulative available rainwater from the calculation area for the rainfall

of the months and the cumulative water requirements of the family are presented in the tables.

The highest negative value is the volume of storage tank required for uninterrupted water supply

for the family.

Zone 1 calculation table is shown in below. The calculations of other zones are similar to Zone

1. A summary table for each zone from the calculation tables which are based on the maximum

negative value from each table. From these summary tables graphs showing Catchment area vs.

storage capacity are produced. From those graph one can easily get storage capacity of the tank

he requires for his family.

Here we have taken 10000 m3

the maximum range of the storage capacity. Because if we take

more than 10000 m3 tank, then it will be much bigger and more costly. We have removed the

value of 10 m2 for 8 person family of the graph. Because in this fact, the storage capacity of the

tank will be too large. To solve this problem, minimum 20 m2

roof top area should provide for 8

persons family. We get four different summary table and graph for four zone.

ZONE 1

Area 10 m2 and family member 4

Month Rainfall

mm

Area

m2

Quantity

Liter/month

C2*C3

Cumulative

Liter/month

Require water

Liter/month

Cumulative

Liter/month

Difference

C5-C7

November 2.5 10 25 25 7.5*4*30 = 900 900 -875

December 0 10 0 25 7.5*4*31 = 930 1830 -1805

January 1.5 10 15 40 7.5*4*31 = 930 2760 -2720

February 6.5 10 65 105 7.5*4*28 = 840 3600 -3495

March 15.5 10 155 260 7.5*4*31 = 930 4530 -4270

April 57.5 10 575 835 7.5*4*30 = 900 5430 -4595

May 147.5 10 1475 2310 7.5*4*31 = 930 6360 -4050

June 219.5 10 2195 4505 7.5*4*30 = 900 7260 -2755

July 197.5 10 1975 6480 7.5*4*31 = 930 8190 -1710

August 367 10 3670 10150 7.5*4*31 = 930 9120 1030

September 213.5 10 2135 12285 7.5*4*30 = 900 10020 2265

October 7.5 10 75 12360 7.5*4*31 = 930 10950 1410

November 2.5 10 25 12385 7.5*4*30 = 900 11850 535

December 19.5 10 195 12580 7.5*4*31 = 930 12780 -200

January 0 10 0 12580 7.5*4*31 = 930 13710 -1130

February 1.5 10 15 12595 7.5*4*28 = 840 14550 -1955

March 2 10 20 12615 7.5*4*31 = 930 15480 -2865

April 50.5 10 505 13120 7.5*4*30 = 900 16380 -3260

May 133.5 10 1335 14455 7.5*4*31 = 930 17310 -2855

June 269 10 2690 17145 7.5*4*30 = 900 18210 -1065

July 152.5 10 1525 18670 7.5*4*31 = 930 19140 -470

August 152 10 1520 20190 7.5*4*31 = 930 20070 120

September 135 10 1350 21540 7.5*4*30 = 900 20970 570

October 93.5 10 935 22475 7.5*4*31 = 930 21900 575

November 0.25 10 2.5 22477.5 7.5*4*30 = 900 22800 -322.5

December 0.5 10 5 22482.5 7.5*4*31 = 930 23730 -1247.5

January 0.5 10 5 22487.5 7.5*4*31 = 930 24660 -2172.5

February 2.5 10 25 22512.5 7.5*4*28 = 840 25500 -2987.5

March 19.5 10 195 22707.5 7.5*4*31 = 930 26430 -3722.5

April 33 10 330 23037.5 7.5*4*30 = 900 27330 -4292.5

May 160 10 1600 24637.5 7.5*4*31 = 930 28260 -3622.5

June 161.5 10 1615 26252.5 7.5*4*30 = 900 29160 -2907.5

July 204 10 2040 28292.5 7.5*4*31 = 930 30090 -1797.5

August 347.5 10 3475 31767.5 7.5*4*31 = 930 31020 747.5

September 180.5 10 1805 33572.5 7.5*4*30 = 900 31920 1652.5

October 100 10 1000 34572.5 7.5*4*31 = 930 32850 1722.5

November 0 10 0 34572.5 7.5*4*30 = 900 33750 822.5

December 0 10 0 34572.5 7.5*4*31 = 930 34680 -107.5

January 29 10 290 34862.5 7.5*4*31 = 930 35610 -747.5

February 5 10 50 34912.5 7.5*4*28 = 840 36450 -1537.5

March 17.5 10 175 35087.5 7.5*4*31 = 930 37380 -2292.5

April 28.5 10 285 35372.5 7.5*4*30 = 900 38280 -2907.5

May 150.5 10 1505 36877.5 7.5*4*31 = 930 39210 -2332.5

June 237 10 2370 39247.5 7.5*4*30 = 900 40110 -862.5

July 284 10 2840 42087.5 7.5*4*31 = 930 41040 1047.5

August 226.5 10 2265 44352.5 7.5*4*31 = 930 41970 2382.5

September 172.5 10 1725 46077.5 7.5*4*30 = 900 42870 3207.5

October 98.5 10 985 47062.5 7.5*4*31 = 930 43800 3262.5

November 19.5 10 195 47257.5 7.5*4*30 = 900 44700 2557.5

December 0 10 0 47257.5 7.5*4*31 = 930 45630 1627.5

January 0 10 0 47257.5 7.5*4*31 = 930 46560 697.5

February 28 10 280 47537.5 7.5*4*28 = 840 47400 137.5

March 18.5 10 185 47722.5 7.5*4*31 = 930 48330 -607.5

April 26 10 260 47982.5 7.5*4*30 = 900 49230 -1247.5

May 114.5 10 1145 49127.5 7.5*4*31 = 930 50160 -1032.5

June 330.5 10 3305 52432.5 7.5*4*30 = 900 51060 1372.5

July 362 10 3620 56052.5 7.5*4*31 = 930 51990 4062.5

August 182.5 10 1825 57877.5 7.5*4*31 = 930 52920 4957.5

September 200.5 10 2005 59882.5 7.5*4*30 = 900 53820 6062.5

October 80.5 10 805 60687.5 7.5*4*31 = 930 54750 5937.5

All calculations are same for 10 m2 to 50 m2 and for person 6 to 8.

Summary table for zone 1

Person

Catchment Area

4

6

8

10

4595

26112.5

51192.5

20

4010

6475

9190

30

3750

6015

8355

40

3490

5755

8020

50

3230

5495

7760

Table 3: Volume of storage tank (litter) required in zone – 1 for different family size and catchment area.

Zone 1

Graph

Figure 6: Catchment Area (m2) Vs. Volume of storage tank (litre)

CONCLUSION

The following conclusion can be drawn from this study on rainwater harvesting system in

Bangladesh,

1. The size of storage tank required by a family is dependent on the size of the roof catchment

area available for the family.

2. The size of the tank is minimum and remains constant if family has a larger roof area. But the

tank size increase if the roof area decrease.

3. The minimum volume of rainwater storage tank is required in zone – 3 where high rainfall

occurs.

4. The outcomes (Graph) of this study can be used by the people of the country living in

different zones to select the sizes of the storage tank for uninterrupted supply of water for

domestic purpose for their family.

0

10

20

30

40

50

0 2000 4000 6000 8000 10000

4 Persons 6 Persons 8 Persons