Floods and obsolete warning system

By Justice P. H. K. Kulatilaka

A flood is defined as a body of water that rises to overflow into land settlements which are not normally under water. When there is heavy rainfall in the catchment areas it is natural that flood situation will arise. Floods invariably create problems of a serious nature. Hence it is considered a natural disaster.

Riverine floods develop slowly when there is rainfall for a continuous period, especially during monsoons. It may come in the form of a flash floods with excessive rainfall falling within a period of time duration. This happens when a tropical cyclone or a depression strikes. Flooding can also occur from a dam breach episode producing effects similar to a flash flood. The flood effect can be of a local impact in the neighbourhood or community or it can be very large or it could be floods affecting a large area of the river basin.

Riverine floods

Sri Lanka is blessed with 103 rivers. Of these 17 rivers are associated with flood problems. These 17 rivers have a catchments area of about 1,600 square kilometres. It is a known fact that with the population explosion since independence, land area has become scarce and as a result low income families were forced to occupy flood plains of rivers which have been demarcated as reservations. A case in point is the unprotected areas between the bunds in the Kelani basin where people are now living. When the water level reaches, 5 feet mean sea level (msl) some of these areas within the unprotected area in the Kelani basin get submerged, when flood water rises up to 7 feet msl. all unprotected areas go under water. Flood situations would arise due to unscrupulous actions of man. Putting up structures and land use without considering flood risks have often invited flood situations. "Uncontrolled/unplanned" land use in the Kelani Ganga catchment area is an example. It has posed problems in carrying out dynamic flood control plans.

Floods of Kelani Ganga

In the year 1925, the British who were the rulers then, realised the danger of floods and built two flood bunds for protection of the surrounding area of the two banks of Kelani Ganga from floods. Since then floods in 1930 reached a level of 10.9 feet msl. The bunds overtopped and breached. In 1940 floods reached a level of 11.06 feet msl. Since the duration of the flood was short the bunds did not breach but during the floods of 1947, flood level reached 12.05 feet msl, and the bunds overtopped. The experts warn that if the flood level of Kelani Ganga reaches 13 feet msl, the floods would inundate areas right upto Thimbirigasyaya.

Mechanism

The measuring of flood level is being done at the Nagalagam Street Gauge. When the water level rises to 4 feet msl, the river is at minor flood level, when it reaches 7 feet msl, river is at major flood level and when it reaches 9 feet msl, river is at critical flood level.

Upper reaches of Kelani Ganga is steep, hence it is unlikely that the river overflows its banks. From Hanwella the river flows through flat land. Therefore, the first flood waves start from Hanwella. For this purpose a flood gauge has been installed at Hanwella. Readings at Hanwella gauge would enable the head office to predict the flood level to which the river would rise at Nagalagam Street. Information from Hanwella comes to the head office through a telephone dial-up link.

Height of the water level is measured through visual observations. This is highly unsatisfactory. If modern automated gauges are installed data will instantly be transmitted to the head office, which would enable the head office to monitor for the process of potentiality of flood situation and inform people early.

Apart from the Flood Monitoring Station at Hanwella there are stations at Deraniyagala, Kitulgala and Olonboowa. The data is transmitted through radio transmitters. Engineers in the Irrigation Department underscore the need to have automated gauges and other modern instruments in order to carry out its functions effectively.

The flood office at Nagalagam Street is manned 24 hours. The water level of the river is recorded hourly and the office has a telephone link with the head office.

There is a system of flood committees which come into operation with the recording of flood situation. When the Nagalagam Gauge records flood level of 4 feet msl, the Flood Committee comes into operation. It is chaired by a regional irrigation officer. The Divisional Secretary is also involved in this committee. When the flood level reaches 6 feet msl a flood Committee which will be chaired by the Director General of Department of Irrigation comes into operation. At this level, Superintendent of Police, Divisional Secretaries are also involved. When the flood level reaches 9 feet msl, Government Agent's Committee takes charge. This committee includes, apart from the officers of the Irrigation Department, Police and Navy personnel as well.

There are Standing Orders coming into operation during flood situations.

When the aforesaid mechanism is in operation, in case of minor floods, warning is given 12 hours ahead. In the case of major flood (7 feet msl) warning is given 18 hours ahead and in the case of critical flood level, warning is given 30 hours ahead to the people.

Anyway, there is a possibility of bund breaching when the river reaches major flood level but before it reaches the critical level. In order to avoid a calamity, continuous patrolling of the bunds is done by the Irrigation Department officials.

The need to have a proper inundation mapping project is badly felt. Inundation mapping helps to demarcate the flood prone areas and also identify the safety zone where people can be evacuated during flood situations.

Modern equipment

The present equipment installed at the Nagalagam Gauge station and the Hanwella Gauge station need upgrading with installation of modern equipment such as automated rain gauges so that real time data relating to arrival time of flood waves could be accurately ascertained.

In order to measure the rainfall in catchment areas and its intensity automated rain gauges have to be installed in the catchment areas. Then the head office in Colombo would be able to provide real time data in making proper assessment and give warnings to the people.

Kaluganga Basin

Floods in Ratnapura are more or less annual phenomenon. Kaluganga catchment are a receives the heaviest rainfall during the South West Monsoon period. Kalu Ganga in its upper reaches flows through a fairly steep slope and as it flows down, the speed and the volume of water increase.

The reaches of the river beyond Ratnapura are at a low gradient and hence the river flows at a slow speed. The volume of water gushing down the upper reaches of the river cannot immediately be accommodated by the middle reaches of the river that flow down slowly beyond Ratnapura, this resulting a flood situation submerging Ratnapura town.

As far back as 1960 there had been a proposal made by an engineering firm commissioned by the Sri Lankan government relating to flood control plan on Kaluganga. This did not see the light a day.

Apart from the geomorphologic factors referred to above, there appears to be certain human factors that accelerated flood situations in Ratnapura, namely, cutting into the banks of streams around Ratnapura for sand mining. It appears that the ulterior motive is mining for gemming. In this process heavy equipments like backhoes are being used to dig the river banks. These unscrupulous and uncontrolled digging lead to landslides as well.

Till very recently the licences for gemming and sand mining were issued by the Geological Survey & Mines Bureau (GSMB) with the approval of the Divisional Secretary. Now the GSMB directly issues licences. Approval of the Divisional Secretary is not a requirement. As a result, licences are being given to dig streams and banks which are not at all suitable for mining.

Therefore, in issuing licences for gemming and sand mining, the authorities, will have to be very cautious and the approval of the Divisional Secretaries who have first-hand knowledge of the area should be made a prerequisite in issuing such licences.

Apparently a disaster preparedness plan has been evolved by the Divisional Secretariat. Steps have been taken to increase effectiveness of emergency services with the assistance of the Irrigation Department, Police and the Navy and to implement people's awareness programs. There is a special unit in the Navy to assist the Government Agent, Ratnapura during flood situation.

Dam-breaching

Serious consequences could arise out of dam breaching. A case in point is the disaster brought about by Kantale Reservoir Dam failure in 1986. There was heavy loss of life, 68 dead bodies were recovered and 59 people went missing, 592 houses were completely destroyed, 628 houses were partly damaged, 2035 families were displaced and 2500 acres of planted paddy were destroyed. Kantale dam was an ancient dam built in the 7th century and its sluice gates have been constructed out of stone masonry and clay mortar. In this episode there was no prior observation of signs of warning.

Dangerous consequences that would result in the event of a major dam breach were highlighted by a rough inundation mapping conducted by a joint committee representing the Irrigation Department and the Mahaweli Authority in respect of Kotmale Reservoir Dam.

It appears that if the dam breaches flood waters will inundate the entire Kotmale town, part of Kandy town and Peradeniya.

The Irrigation Department has 315 major projects. Of them 54 are large dam category projects. An inspection and assessment done by the World Bank Project in 2003 had assessed 34 major dams and identified them as having some risk factors.

Some of these dams are "ageing" and as such a vulnerability assessment of each of the major dams need to be undertaken.

The factors that are responsible for dam-breaching are enumerated as follows:

i. Overtopping due to flood situations caused by heavy rainfall.

ii. Foundation failure. Even though when a hole is drilled at 100 meter intervals on the dam and if weak soil is found it is treated by injecting grout (a mixture of cement and water) there still can be weak spots through which seepage could take place leading to collapsing of the dam.

iii. Slope failure in the upstream side. This may result due to rapid lowering down of the water level to stop overtopping. It would lead to landslides in the upper stream region.

iv. Slope failure in the downstream due to seepage. Downstream slope will get saturated and slide down.

v. Erosion e.g. Kantale Reservoir Dam referred to above.

It is interesting to note that these inspections are based on visual observations.

If dam danger signs (i.e. weak spots) are detected drilling is done and injected with grout, and in some cases water level in the reservoir is brought down. If there is a danger of overtopping, the divisional officer will immediately inform the regional office. For that purpose they are supplied with residence telephone facilities and there are Standing Orders to cover up such situations.

The Irrigation Department is equipped with the expertise, they have 275 Engineers. When the reports come to Head Office the Dam Safety Unit which is chaired by the Deputy Director and a few Engineers study the repot and if the Director's intervention is necessary for the matter, it is referred to the Director himself. The annual budget for dam safety is around Rs. 15 million.

Unfortunately there is no mechanism to give early warning to people even if a dam breaches, what the Department does is to get the assistance of the Police, Army and with loud speakers mounted on vehicles tell the people.

Kantale Tank Dam episode is a case in point.

This kind of situation is highly unsatisfactory. Therefore in case of dam breach situation the Irrigation Department has no mechanism in place to give prior warning to people.

Suggested preventive steps

i. Install micro-seismometers at the dam sites which should be connected to Peradeniya Seismic Network.

ii. Install altimeters, strain gauges, piezometers whereby pressure builds up and seepage could be observed. It is heartening to note that the Irrigation Department is making negotiations with the World Bank in this regard.

iii. Implement and Inundation Mapping project. This could be undertaken by the Survey Department. If Inundation maps are available the flood prone areas in the event of a dam breach and the level to which flood water would reach can be ascertained. At the same time high level areas to which people could be evacuated or can go for safety also can be identified.

iv. Have all instrumentation necessary for testing the stability of soil in the dam site.

v. Undertake vulnerability assessment of each of the major dams that pose hazards.

vi. Set up Flood Watch so that warnings could be issued to the people in advance of possible dam break. This would help to reduce the casualties and death toll in a flash flood.

vii. Conduct awareness programmes for the people.

The writer who is an honours graduate in Geography is the Deputy Director Sri Lanka Judges' Institute, a member of the Tsunami Commission which inquired into natural disasters in Sri Lanka and a former judge of the Court of Appeal.