Long-term preventative methods can reduce earth tremors around Victoria dam

By Karu Jayasundara(JMTB)
(former Senior Geologist, Geological Survey of Sri Lanka)

Minor earth tremors near the Victoria dam were predominantly due to man-made environmental changes at the Mahaweli river, the destination of which is the Trincomalee Canyon in North East Sri Lanka, where it flows along the weak and fractured zones of complex rock terrain.

For irrigation and hydro-power generation, it was planned under the Mahaweli Project to construct reservoirs across the river in Kothmale, Polgolla, Victoria and Randenigala.

My previous article, published in The Island of 07 Jan. 2021 discussed in detail how the recent earth tremors occurred around the Victoria dam. I mentioned that the weak zones developed around Gadaladeniya, Dumbara and Theldeniya Valley (Huluganga) Arenas with the highland series rocks. Furthermore, the weak zones between Arena rocks and the Highland series rocks were playing a major role in controlling the flow of the Mahaweli river around Kandy and Victoria dam region. Geologically, these two types of rocks originated at two different times and their formations and associated minerals are not compatible to blend with each other to create one type of rock formation. Thus, the weak shear zone has developed between these two rock groups.

During the activation of the shear zone, the fault can generate heat up to 200 °C (a shallow very low-grade cataclastic metamorphic event) and this can boil water surrounding the weak zone. The recent earth tremors in the South-East of Dumbara Arena, where the area was submerged by reservoir water, boiling water bubble eruptions have been observed by some of the villagers. (Fig 2)

Following the construction of the Victoria and Polgolla dams several environmental changes have affected the region.

1)

Sediment collection on the river bed (inside Polgolla and Victoria Reservoir)

2)

Groundwater saturation in the soil and weathered rocks around the reservoir

3)

Ecological change

As a geologist, born in Napana, a beautiful village in the Dumbara valley, I believe the first two topics are critical and must be discussed in this situation.

Before the construction of the dams, the floating sand, fine particles, and soluble substances had been deposited on the riverbanks and the sea. However, the majority of the sand and fine particles are now deposited on the bed of the reservoirs.

The shear zones can become active or move due to erosion (unloading effect) or from the loading effect of accumulated sediments from the filling of water on one side of the rock formation; this is the Dumbara Arena. The fault movement generates energetic stress, which can pass to other areas of the Dumbara Arena. This can be felt and seen as cracks on the ground.

The construction of the Victoria dam was completed 35 years ago, and since then, most of the Mahaweli river sediments have been deposited at the south east nose of the Arena (South of Aluthwatte), which is a weak zone.

Fortunately, the Pollgolla reservoir can retain part of the Mahaweli river’s sediments. However, some of the sediments can be released to the Victoria reservoir from the Polgolla dam upon opening the gates during heavy rains from the upper catchments of the Mahaweli river.

Further, the sediments from the catchment region below the Polgolla dam mainly from the Dumbara valley are directly deposited near the nose of the south east (weak shear zone) Dumbara Arena. (Fig 2)

The Victoria reservoir has two depositional regions, divided by a wedge-shaped antifoam land mass, running NW-SE direction. (Fig 1)

The Mahaweli river flows through the contact weak zones between the Highland rocks and the Arena rocks, and crosses the top of the wedge-shaped antifoam land mass without following the weak zone of the Dumbara Arena, to meet the weak zone of the Teldeniya Arena. (Fig 1)

The velocity of the water flow decreases at the turning point (nose of Arena) and changes to a northerly direction due to the NW-SE wedge- shaped antifoam land mass and flows between the weak zone of two rock groups.

Also, a small river, named Ma Oya flows to the NW direction and meets at the nose of the Dumbara Arena. This helps to slow down the water flow of the Mahaweli river further.

The environment of the nose region of the Arena is the best location for sediment deposition with a sediment weight, which has accumulated for 35 years, at this location. Thus, the weak shear zone can easily activate as a fault with the help of water pressure at the nose region of the reservoir, mainly towards the Dumbara Arena. (Fig 2) This is a good situation for the Victoria reservoir, as the major sediment depositional area is away from the dam site.

Therefore, we need to stop the shear zone activation around the Victoria dam.

Unfortunately, the catchments of Victoria and Polgolla reservoirs are in highly populated regions, which is not a favourable situation for such constructions. However, with the help of the local people, sediments floating through the drainage system towards the reservoirs can be reduced by implementing long-term solutions.

The Arena regions have an ancient irrigation system for paddy cultivation (Fig 3), which is different from other parts of the island.

Regarding geomorphology, most parts of the Arena region are covered by small hills and valleys, and beneath these are shear zones. The top layer of soil is mainly reddish semi-plastic clay and below is weathered rock (called Saprolite–Kudu Gal) and fresh low grade metamorphosed rocks, containing minerals of mica, amphibolite, silica and felspar. Chemical weathering of the minerals containing magnesium has helped produce fertile soil in the Arenas.

Fortunately, the saprolite weathered rocks (Kudu gal) can retain water as an aquifer. It is a unique character of the Arena rocks, unknowingly used by farmers, as it retains underground water and reduces surface soil erosion. (Fig 3) Once the surface soil erosion stops, the sediment discharged to the drainage system will be reduced. The tree like pattern of paddy fields in the Arenas follow the underground shear vertical fracture zones.

Small rainwater tanks (called in Patha in Sinhala) were constructed by ancient farmers at the upper end of each paddy field branch. (Fig 3) Most surface water was collected by the tanks when it rained and it would then gradually seep underground, by percolating through the soil and finally to the weathered rocks beneath. The percolated underground water slowly moved to lower regions by gravity. Sometimes the water can come to the surface as small springs. This wet condition can remain below the tanks throughout the year, even during the dry season. (FIG 4)

Furthermore, I like to encourage relevant landowners to dig trenches at their properties to collect rainwater, this will also stop flash flooding and surface soil erosion.

Rejuvenating the old irrigation system is strongly encouraged for the Arena zones. This will help to resolve the following two major problems for the reservoirs.

1)

Stop the discharge of extra water during rain.

2)

Reduce the sediment accumulation in the reservoir around SW nose of Dumbara.

The Teldeniya (Huluganga) Arena (the second depositional area of Victoria reservoir) also has the same shear zone condition, and the SW nose is close to the Victoria dam. Once the sedimentary load is heavy, this area could also have minor tremors.

Further, the east bank of the reservoir should be continually checked by geo-technologists for the stability of the slope.

Sedimentation in the Polgolla and Victoria reservoirs should be controlled by implementing a system of continuous dredging.

Finally, the dredged material of sand and clay can be used in the building and clay industries, respectively.

“A man-made disaster must be solved by man.”

(This article is for the general public but can be further discussed with impartial professionals)