Intensive sand mining puts river systems in peril

By Eng. Thushara Dissanayake

sand is an essential aggregate material used for construction work. After 2004, Sri Lanka experienced a boom in the construction industry due to rehabilitation the tsunami-hit infrastructure in the country, creating a huge demand for sand. Even later, demand increased exponentially in tune with the expansion of construction activities such as roads, condominiums, and other mega projects during the post-war period.

While there appears to be no dearth of sand, not all sands are suitable for construction work as the properties of the sand matters a lot to maintain the accepted standards. Sand should be clean, devoid of problematic chemicals, and conform to required sizing to be used in construction activities.

In Sri Lanka, sand is mainly obtained from river beds. Being a readily available raw material, it requires no processing at all, in most cases. However, it is now well obvious that continued and indiscriminate sand mining has caused irreparable damage to the ecological and physical environments of our river systems. The Kelani Ganga, the Kalu Ganga, the Deduru Oya, and the Maha Oya have suffered the worst damage.

Attention paid to the issue of overexploitation of river sand seems to be inadequate at present. As a result, rivers have been adversely affected, and this has led to many physical, ecological, and socio-environmental impacts. River bed and bank erosion often damaging riverine structures, loss of biodiversity, lowering of groundwater table and water pollution are some of severe impacts caused by indiscriminate sand mining.

Sand budget

The main impact of sand mining is the altering of the sediment budget of the river. Besides, it affects the river hydraulics. The geological replenishment process takes a considerable time period.

Meanwhile, many reservoirs were constructed in our river basins in the recent past. This has further aggravated the situation. These reservoirs limit downstream sand availability as sand is trapped in the reservoirs as sediments.

Physical Impacts

Large-scale river bed lowering due to sand mining can increase bank height and aggravate bank collapse due to undercutting. This scenario endangers adjacent lands and structures. Further, lowered main stream bed triggers tributary stream bed erosion. Thus, lowering of riverbed sometimes goes beyond the sea level as in the case of the stretch of the Kelani Ganga beyond Hanwella. As a result, drinking water intakes, located in rivers close to the sea mouth, are affected by saline water intrusion into freshwater. The Ambatale water intake in Kelani Ganga is an example and sophisticated remedial works are imperative for controlling the salinity issue.

Groundwater

Lowering of riverbed due to sand mining drops the river water level as well. Eventually, the water table in the adjacent land area also goes down. This is a serious problem for groundwater users as the water level of their domestic wells falls in parallel with the water level of the river. Low groundwater levels can also affect vegetation in floodplains and adjacent wetlands, which are important components of riverine ecology.

Stability of structures

When sand mining is done at a certain location the river bed erosion tends to migrate upstream to cover up the supply deficiency. During high flows, the bed erosion extends to downstream as well. Thus, the erosion of bed and banks is not limited to the area of mining but affects a considerable distance to the upstream and downstream of the river. This scenario adversely affects most of the riverine structures, especially bridges. A classic example is the Badalagama Bridge across the Ma Oya; its foundation is exposed (see picture). During the recent floods, one abutment collapsed.

Flora and fauna

From an ecosystem perspective, sand is an important abiotic component, which provides habitat for many aquatic animals. Riparian and instream vegetation are integral components of the river ecosystems. Further, the woody debris in aquatic ecosystems is an important habitat and spawning site for many aquatic animals. The dragonfly preys on mosquitoes. Hence, it is a beneficial insect as far as the present dengue menace in the country is concerned. Nevertheless, the sand extraction process destroys dragonfly nymphs before they emerge from riverine habitats in adult form.

Flooding

With the lowering of river bed, river water levels go down reducing overbank flooding during high flows. This in turn reduces the supply of rich organic matter to flood plain wetlands. In addition, it aggravates downstream flooding, as water storage in the floodplain is limited.

Water quality

The content of suspended particles in the water column rises as a result of sand mining operations.

The high concentration of suspended particles in water blocks the respiratory system of fishes and other aquatic species endangering their lives. Similarly, it affects respiration and photosynthesis of instream flora, and they, in turn, lead to reduced growth rate and finally its total destruction. When the river water is used for drinking, additional purification efforts have to be made to remove the turbidity.

Coastal erosion

The conveyance of sand to the shoreline by the rivers is also very important. The sand deposited at the shoreline is gradually moved along the coast by waves and it is a nourishment for beaches. When the sediment supply to the coastal environments decreases, undernourished beaches suffer erosion. Some areas in the eastern coastal zone of the country survived the Tsunami in 2004 due to such sand dunes.

Present regulations on sand mining

River sand mining is a lucrative business activity in the country. However, it is reported that almost one-third of the total sand supply to the country is from illegal sources. According to Mines and Minerals Act No. 33 (1992) of Sri Lanka sand is a property of the state and a permit is required to mine and transport. The Geological Surveys and Mines Bureau (GSMB) is responsible for identifying locations and quantities of available sand deposits. Further, this mining and transportation process should be in line with the National Environmental Act and laws and regulations of other relevant line agencies. Unfortunately, illegal sand mining taking place at a massive scale is yet to be controlled effectively.

Alternative sources for sand

Restricting sand mining from riverbeds due to aforesaid environmental issues will adversely affect the construction industry as the present supply is hardly enough to meet the ever-increasing demand. Therefore, it is very important to focus on alternative sources.

Land-sand and manufactured sand are the best alternatives to river sand. It is the sand extracted from earth containing a higher percentage of sand by washing. Manufactured sand is obtained mainly by recycling demolished building material and by crushing rock. However, the degree to which manufactured sand can replace natural sand depends on the quality of processing and cost. Appropriate policies are to be developed for promoting these alternative sand sources.

We have thousands of ancient irrigation tanks throughout the country. Most have been silted to a considerable level limiting their water storage capacity. Often, desilting of these tanks is not promoted owing to high cost. In many tanks, the deposited materials are rich in sand. These silt deposits can be excavated and used as a source of sand under appropriate environmental regulations. Thereby, it is possible to increase the water storage capacity of these tanks as well.

Off-shore sand is another alternative source for sand though not suitable for immediate use due to high salinity. In order to prevent environmental damage to the coastal zone, sand has to be pumped from a distance of about 10 km from the shore. This sand can be used for construction works after removing the salt content. Still, artificial methods of washing out are costly. As a natural method, sand can be exposed to rain for a period of six months and this is sufficient to make it usable, according to studies carried out by the Moratuwa University.

Sand conservation as an option

Conservation is another important strategy to limit sand demand. Following actions are beneficial to minimise sand requirements and promote its optimal utilisation.

= Use river sand as a raw material only for construction works and not for landfilling or similar land improvement activities

=Adaptation of new construction technologies that minimise the use of sand

=Quantification of river sand replenishment rates and issuing mining permits accordingly.

It has to be taken into account that the global construction industry is set to grow by more than 70% by 2025, and contribution from our country to it will not be much less than that.

(The writer is a Chartered Engineer

specialized in water resources with over 20 years experience)