The river Yamuna is the primary source of water supply in India’s capital, Delhi. This river has become increasingly polluted in recent years because of the discharge of city effluents and wastes from the burgeoning urban population and from the industrial establishments located along its banks.
Out of the total volume of water supplied to Delhi, about 20 per cent is consumed and 80 per cent flows back into the Yamuna through 15-20 open drains. In some of these drains, e.g., Najafgarh, Tughlaqabad, Maharani Bagh, Kalkaji, etc., the pollution load is particularly high.
The Najafgarh drain alone contributes some 50 per cent of the drain flow and 50 per cent of the total B.O.D. load to the river. Thermal power stations and municipal sewerage systems add to the pollution levels. The present sewage treatment capacity of Delhi is about 120 mgd, but the actual volume of Delhi’s sewage is just double of this. This means that some 120 mgd of sewage simply goes untreated into the Yamuna.
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Some of the industries that add to the pollution of the Yamuna include printing, electroplating, soap manufacture, food processing, rubber, plastics, chemicals, and leather tanning.
Going downstream, most natural rivers become finer grained, higher in discharge, gentler in slope, and more sinuous. These changes have important effects on vegetation, flood characteristics, ecological habitats, and so forth. There has long been debate about the relative importance of abrasion versus selective deposition of the coarsest clasts in causing downstream fining of sediment in river systems.
Although high fining rates observed in many natural rivers seem to require strong selective deposition, the ability of selective deposition to produce downstream size sorting has never been measured under controlled conditions. In an experiment using a long flume and a poorly sorted, bimodal gravel feed, Paola et al. (1992) produced downstream fining by a factor of 1.3 in median size and 1.8 in 90th percentile size, over a distance of 21 meters.
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The experimental conditions ruled out abrasion effects. Selective deposition appeared to be a natural consequence of the transport and deposition of sufficiently poorly sorted or bimodal gravels and could account for fining rates observed in natural gravel rivers.