Narrowing creeks widen flooding woes

An increase in mangrove cover and built-up area from 1972 to 2016 has reduced the width of creeks in the Mumbai Metropolitan Region (MMR), a new study by the National Environmental Engineering Research Institute (NEERI) has revealed. This could eventually slow down the flow of rainwater, exacerbating waterlogging during monsoon, clogging drainage systems and hampering intertidal flow.

Using remote sensing analysis and 1972 as the base year, the four-member NEERI team tracked the growth of mangroves and urbanisation vis-à-vis changes in the width of the Thane, Malad and Manori creeks in 1994 and 2016. They found in the 44-year-period, the built-up area increased by 7.1%, while the area of mangroves increased by 1.75%, leading to a simultaneous decrease in the size of the three tidal creeks by 1.4% and mudflats by 3.5%. Researchers said that examining changes in the width of creeks is important as they flush out the city’s domestic sewage and industrial effluents into rivers and ultimately into the sea, and play a key role in recharging groundwater, capturing nutrients, controlling floods and filtering sediments.

WHAT THE STUDY FOUND

Researchers divided the creeks into three sections – lower creek (point where the creek meets the ocean), upper creek (point where the creek meets the river) and middle creek (portion between upper and lower creek) — and found a reduction in the width across all stretches with a corresponding increase in mangrove cover around them, barring Malad creek. The results — published in the latest issue of Journal of Coastal Conservation Planning and Management — showed that while mangrove cover around Thane creek increased to 57.6sqkm in 2016 from 50.7sqkm in 1972, the highest reduction in average width of the creek — 46% — took place in the upper portion from 594m to 320.7m during the same period, owing to siltation.

The highest decline in the average width of Manori creek — 36% — was also in the upper portion from 110m in 1972 to 70m in 2016 owing to availability of less water for dilution even during a high tide, thereby increasing chances of siltation. Mangrove area at the creek increased to 25.2sqkm from 8.4sqkm.

The Malad creek, however, witnessed a decrease in mangroves — from 13.44sqkm in 1972 to 8.97sqkm in 1994, and 9.7sqkms in 2016, and the maximum decrease in the creek’s average width — 33% — is in the lower portion, from 490m in 1972 to 328m in 2016. The study attributes developmental activities during the first 22 years of the study period to the decline in mangroves around the creek, and their subsequent increase (in 2016) to growth along the mudflats of the creek. “Malad creek receives treated wastewater from Malad and Versova treatment plants along with the untreated sewage, which degrade the water quality and cause siltation. This siltation leads to formation of mudflats and growth of mangroves,” stated the study.

MANGROVE GROWTH

Ritesh Vijay, principal investigator, Centre for Strategic Urban Management, Mumbai zonal centre, NEERI, said, “While an increase in mangrove cover sounds positive, their haphazard growth towards the creek, is problematic. Creeks can’t get choked owing to mangroves,” said Vijay. “Further growth of mangroves towards the creek-ward side will limit flow of tidal waters in the creek,” he said.

“Increase in mangroves creek wards can seem like a good sign but it can result in loss of mudflat habitat which will adversely affect faunal groups,” said Goldin Quadros, principal scientist, Salim Ali Centre for Ornithology and Natural History, who was not involved in the study and has undertaken habitat studies in Mumbai creeks. “If a balance has to be maintained between mangrove growth and protecting the creek ecosystem, there has to a proper flow of water form the riverine and seaward sides.”

Researchers said that one reason for the increase in mangrove cover is the formation of mudflats in the inner periphery of the creeks owing to tidal and ocean currents that bring along silt. Dumping of unauthorised solid waste and construction debris leads to further expansion of mudflats. “Disposal of domestic sewage from drains and release of industrial waste into these water bodies provides nutrients for mangroves,” said Vijay. For instance, mudflats around Manori creek measured around 20sqkm in 1972, which reduced to around 5sqkm in 2016 owing to an increase in mangrove cover.

Rakesh Kumar, director, NEERI, said the government should designate mangrove areas that need to be protected, and restore and maintain areas such as the creek ecosystem.

RECOMMENDATIONS

Kumar said that declining creek width raises the risk of flooding as the holding capacity of these water bodies will decrease. “With limited space, they will not be able to deal with the total volume of water during high tide or in monsoon and it will spill out inundating nearby areas.”

If untreated domestic sewage and industrial effluents continue to be released into these creeks owing to urbanisation, based on a life expectancy analysis, researchers found there would a further reduction in their width in 2025 and 2050, using 2016 as the cut-off year. Life expectancy was defined as a condition when the creek gets completely choked, leaving no space for tidal phenomenon that occurs otherwise.

The study recommended immediate measures in terms of cleaning and dredging so that upper stretches get tidal water to maintain the creek ecosystem, and stringent bye-laws for sewage disposal. “Creeks such as Manori or Gorai were always dredged. With a halt to navigation and a decline in fishing, dredging is not taking place,” said Kumar. “The importance of the depth of the creek has vanished.”