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Friday, September 14, 2007

Floods, rains and Dhaka city drainage

Dhaka city experiences a deluge every time it rains heavily

Every time major floods occur in the country and every time it rains, the city of Dhaka faces acute drainage problem. Parts of the city go under water. In the densely populated city, woes of people know no bounds.

In 1954, 1956, 1988, 1998 devastating floods occurred submerging 70 percent or more of the country. Along with the country most of the Dhaka city except pockets of high spots went under various depths of water. There was flood water in old city, eastern and western parts.

However in earlier years, as the eastern and western parts of the city were mostly agricultural lowlands with a few or no human habitation, flood damage remained confined to the city proper only. There was no huge and cry and suffering of people as it is today in the eastern part.

In spite of huge investment over the years, particularly after 1988 floods when almost whole city of Dhaka went under water, the 1998 floods appeared most devastating. About Dhaka city, drainage situation aggravated due to silted-up, blocked drainage channels. WASA's limited storm water drainage is too inadequate for a city of 850 sqkm (Greater Dhaka).

Causes and problems
Causes of floods and drainage problem vary in different zones (areas) of the city depending on topography (elevation, high, low), proximity to rivers, peculiar areas like housing estates etc. Two main causes of floods in the city are river floods and runoff from rainfall.

River floods: Rivers surrounding the city like the Sitalakhya, Buriganga, Turag, Tongi and Balu cause floods in the city. This year low lying areas in the eastern part of the greater Dhaka city was heavily flooded. One point is that for some reasons the proposed flood protection eastern embankment/bypass was yet to be constructed.

The western flood protection embankment protected Uttara, Airport, Nikunja and other areas from floods of Turag and Buriganga.

Rainfall: Due to bad drainage system in the city where most of the channels were encroached upon by the land grabbers or were choked up or filled up with illegal dumping of solid and construction wastes water logging occurs all around the city. Most of former flowing/living channels are now non-existent. Even the large ones like the Begunbari channel were either encroached upon or the outfall closed with consequence that runoff cannot flow as it used to in 50's, 60's and even 70's Gulshan, Banani, Baridhara and Uttara lakes are blocked at the outfall. Many other channels in the eastern Dhaka, South Badda, Khilgaon, Bashaboo, Shahjahanpur, Jatrabari etc. were partly or full occupied by unauthorised illegal structures. If channels are improved, heavy runoff will quickly flow outside the city.

Housing estates: Gulshan, Banani, Baridhara and Uttara and perhaps other satellite towns like Bashundhara, Asian City, Amin Group etc. are relatively on higher lands and runoff thus flows into the lakes which incidentally work as buffer flood control reservoirs. Except some pockets of transient water-logging in the above areas waters are quickly drained off.

Eastern part: The eastern area of the city is heavily affected by river floods and also by runoff. Flood water ... standing as river levels remain high. It strongly suggests pumping of flood/runoff waters when the eastern embankment would be constructed. Similar pumping is necessary in western embankment area.

Old city: The old city is affected by runoff form rainfall. Numerous low pockets remain water-logged. If drainage channels are built or water flows through improved channels this flood situation will improve. WASA may also be of help with storm water drains. It is necessary as open drains may be difficult to run because of congestion in the area and its unique situation/location.

Water-logging in DND Project: One of the earliest irrigation projects, the DND (Dhaka-Demra-Narayanganj) project was built by the then EPWAPDA (now BWDB). The project was meant for producing rice crops by irrigation. In recent years agricultural low lands were purchased or illegally acquired for residential use. Canals were encroached upon, both irrigation and drainage channels were encroached upon by influential people. Due to heavy downpour on June 15, 2007 the whole Dhaka city and adjoining areas were flooded. With drainage channels closed here and there, DND project area was fully water-logged.

Another reason is that due to low level of the outfall of the drainage channels, natural drainage is not possible due to higher level of water of the river Sitalakhya. As a result the water of the DND needed to be pumped out. But sometimes some of the pumps remain out of order. BWDB should keep the pumps in good working condition at all times.

Some unconventional causes: Though the unconventional causes like the slow subsidence of ground surface in general can occur over a long period of time, they deserve serious consideration. For gradual subsidence over many years might turn out to be serious in magnitude and dimension. This may happen for both natural and man-made causes.

Natural process soil erosion: Anytime heavy rainfall occurs, it erodes some soil in the city area. Ceaselessly the rainfall has caused erosion and sediment was transported away by runoff to the adjoining rivers bounding the city, through rills, small channels. Only a proper contour survey available for 50's or 60's or later years may be compared to see the magnitude of erosion, consequently lowering the ground surface level (subsidence)

Man-made causes: There are two such causes of land susbidence --

Subsidence due to pumping: Due to continuous pumping by hundreds of deep tubewells, ground water level below Dhaka city is falling. It may be mentioned that for continuous pumping in Tamil Nadu and Gujrat States in India, ground water dried up in millions of tubewells. There is no irrigation now, only low-yielding rain-fed agriculture is in practice. Drinking water is brought to the large affected area with hundreds/thousands of trucks daily! One may ascertain difference between ground level now and that some 30 or 40 years ago and estimate the evidence and extent of subsidence.

Groundwater aquifer (aquifer is water holding stratum/layer of soils) consists of trillions of water-filled voids. As pumping continues over the years, voids are depleted of water and as a result due to overhead soil pressure empty voids may get contracted which may accentuate subsidence. Though a very long term process, it may happen. It is worth investigation and studies.

High rise buildings: For the last 25 years or so thousands of high rise buildings were constructed all over Dhaka. Though developers generally take good care in foundation design considering bearing capacity versus subsidence, there are thousands of multi-storied buildings which were privately constructed. I have doubt whether they took care of foundation design adequately. In the years to come there may be a global effect of millions of tons of imposed weight on the surface which may cause subsidence in the soft alluvial sedimentary soils underlying the city. A six-storey residential building on 5 katha (3600 sft) may weigh 5000-7000 tons. Our learned geologists and foundation engineers might have answer to this. If the answer is an emphatic yes, it may be of concern to all of us.

All the above three causes particularly the man-made ones might look speculative at the moment. But after serious investigations, comprehensive relevant surveys and intensive studies, we might obtain a suitable answer.

June 15, 2007 rainfall: Let us analyse the rainfall of June 15, 2007 that occurred for a duration 6 hours (6 pm to 12 midnight) with a total rainfall of abut 100mm.

Greater Dhaka has on area of 850 km2. Preliminary estimate shows that a flow (discharge) of 3,000 m3/s was generated in the greater Dhaka and 2,000m3/s in the metropolitan area. Both the figures are designed discharge. Canals of 200m bottom width x 1.5 deep x side slope 1.5:1 x bed slope 1/1000 and other canals of 150m, 100m, 50m and 30m bottom width can be designed for the drainage of the greater Dhaka city. About 35 trapezoidal channels of various capacities are required to achieve drainage of the city.

Begunbari channel takes off from Dhanmondi lake but Mirpur road blocked it near Panthapath crossing. It is again perhaps blocked by roads notable Nazrul Islam Avenue before Sonargaon Hotel. It resurfaced behind the hotel. On its way a lot of illegal structures, some multi-storey buildings were constructed between Sonargaon Hotel and its crossing at Shahid Tajuddin Road (Tejgaon Road). The channel then veers north-eastward meeting the combined drainage channels from Gulshan-Banani lake and Mohakhali drainage channel at a point 5 km south of Gulshan Lake (E). Then it moves east-southeastwards meeting a north-south channel in low areas south of Badda and moves eastward to the proposed Eastern Bypass.

Begunbari channel is the main drainage channel of the city, both larger in dimension and longer in stretch, and moves west to east draining the crowded parts of the city: Tejgaon, Mohakhali, Kawran Bazar, Panthapath, Banani-Gulshan and open areas in the east and Green Road, Farmgate area. With 200m width and proper designed dimensions, it can carry a discharge of 400-500m3/s i.e. 1/7th to 1/6th (15%) of the total runoff generated by June 15 rainfall. Begunbari appears to have large catchments. It also drains entire Badda, Rampura and Khilgaon areas, open areas in the east on both sides of Begunbari. The feeder channels, Mohakhali-Banani-Gulshan lake, north-south canal meeting Bagunbari should be excavated and improved. Begunbari channel acts like a drainage divide between the north and south of the metropolitan area. Another divide existed in old city, now filled up Dholai Khal channel which ran west to east dividing the old city drainage to the canal from the north and south.

From contour plan of the Begunbari and catchment and characteristics of channel catchment plan, synthetic unit hydrograph may be drawn by using maximum daily rainfall data for 20 to 30 years. From unit hydrograph peak flood of 25 years, 50 years or 100 years frequency versus depths can be determined for the design of channels. Similar methods can be used for other channels in other zones/areas.

Some recommendations
Master Plan: Master plan of Dhaka City Drainage should be prepared by an experienced consulting engineering firm. A national committee composed of experts from relevant organisations may guide and oversee the activities of the firm. The whole city may be divided into drainage zones according to topography and location.

Eastern Bypass: An embankment by the east of the city (Eastern Bypass) may be constructed as quickly as possible. If the Eastern Bypass is delayed for the Master Plan, the city will continue to suffer. Western embankment was not delayed for the Master Plan.

Cleaning of canals: All silted up and choked up channels large, medium or small, should be excavated and made efficient for drainage of rain water. These moribund channels should be cleared and designed to carry runoff in 25 or 35 channels as outlined earlier.

Open area drainage: Drainage channels in the open areas between the city and the existing west and the proposed eastern Bypass will be designed to carry flow and evacuate in the embankment sluices. In some points pumping may be required. We are to design pumps, pump houses etc.

Embankment: Embankment along the Buriganga in the southern side should be strengthened in order to prevent river floods. Drains should carry runoff from rainfall.

Protection from future grabbing: Excavated/cleared existing closed or blocked channels should be protected from future grabbing by enacting strict laws, if necessary.

DND drainage: Drainage channels in DND project area should be cleared and made efficient to carry runoff. Pump house should be made efficient and repaired when necessary. Water-logging in DND area is man-made artificial creation.

Multi-disciplinary national committee: A multi-disciplinary national committee composed of experts from relevant organisations may be formed to find causes and effects of floods and drainage problems in the city (Greater Dhaka and Metropolitan area), and outline preventive and regulation measures. The committee will assist in the selection of an experienced consulting company. The Committee will prepare its own TOR and draft TOR for the consultants. The committee will continue to guide and oversee the activities of the consultants during its tenure.

The writer is professor of Civil Engineering in the World University of Bangladesh, Dhaka and a Water Specialist (formerly in World Bank, Washington DC and Planning Commission).

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