Several days ago when we met with the men’s community of Alisathna here in the Dry Zone of Sri Lanka, the first problem they conveyed to us was the buildup of silt in their tank. The ‘tanks’ (named thus by Portuguese cartographers who depicted them in 16th century maps) are human-built lakes that have irrigated and nourished this agricultural landscape of Sri Lanka for centuries. Because the tanks hold and control water they are also susceptible to the buildup of silt. Rural Sri Lankan society evolved around the village tank. By extension we may posit that it’s a society preoccupied with silt.
Silt was traditionally used to reinforce the bund (dam), as well as to build smaller, secondary protective structures that control the circulation of water in the tank. Silt was used in the manufacture of bricks that we see all over the ancient city of Anuradhapura, including the gigantic stupas here
Silt may enter the tank in suspension or solution, and thanks to the tank comprising its own micro-ecosystem there are methods for reducing the buildup of silt. The Alisthana villagers, in cooperation with SAP SRI, an NGO funded by the UNDP and GEF, have planted some 8000 saplings that are native to tank ecosystems. The saplings, which were planted close together, will form a revitalized gasgommana, the protective girdle of trees that line a healthy tank, shading it, purifying its water, and reducing the impact of rushing water. Part of, and just in from the gasgommana grows the perahana, an area of grasses and reeds, which filters the water further.
But the villagers’ work with SAP SRI is only 14 years old. Their tank was collecting silt for decades before they began these interventions. I’ve been told that some tanks may lie under two feet or more of silt, which affects seriously their capacity to hold water. This affects how much water is available for irrigation and it follows that the amount of rice that can be grown is impacted by the buildup of silt.
Thus our conversation of yesterday. Desilting tanks by heavy machinery, which is what the men’s community wants to do, is a destructive practice that disturbs the anatomy and morphology of the tank. As tank function is impacted so is its ability to maintain itself, and the once complex functions of the tank, which involved filtering and distributing water differentially, are simplified and ultimately lost.
Yet people have been desilting tanks for as long as tanks have been in existence. Silt was traditionally used to reinforce the bund (dam), as well as to build smaller, secondary protective structures that control the circulation of water in the tank. Silt was used in the manufacture of bricks that we see all over the ancient city of Anuradhapura, including the gigantic stupas here. I’m told that in the dry season villagers still set up kilns for brickmaking in the dry upper portions of the tank.
We don’t know that much about how silt was removed in ancient times. Perhaps some of the heavy work was done with elephants. One report I heard is that the muddy water was churned up and its silt emptied through a special “silt” sluice that is not in evidence today.
So rather than bemoan the buildup of silt in the tanks Janet has encouraged me to look at silt in a different way. Like the gasgommana and the perahana, like the bund, the sluice gates, and the spillways, silt is part of the tank ecosystem. It’s something to be used and benefitted from, not something we should try to eradicate.
I grabbed some handfuls of muddy silt from different tank bottoms. In some tanks it’s dark with organic matter, elsewhere it’s lighter. It’s has a distinct odor, different in different tanks, and its clayey consistency is full of promise. It’s a kind of plastic substance, something we may be able to shape into still-unknown uses. Like the world of rural Sri Lanka and like the many Sri Lankan people I’ve met, tank silt is tough yet pliable, unique in its character and composition. Like the tanks in which it’s deposited, silt holds lessons for us we still haven’t learned.