Lok Sabha Question No. 3298 (UNSTARRED), answered on 2023-08-09
Subject: Subsidence of Land in Mundrothuruthu
Ministry: EARTH SCIENCES
Asked by: Suresh Kodikunnil

## GOVERNMENT OF INDIA

## MINISTRY OF EARTH SCIENCES LOKSABHA

## UNSTARRED QUESTION NO. 3298 TO BE ANSWERED ON WEDNESDAY, 9 TH  AUGUST, 2023

## SUBSIDENCE OF LAND IN MUNDROTHURUTHU

## 3298.       SHRI KODIKUNNIL SURESH:

Will the Minister of EARTH SCIENCES be pleased to state:

- (a) whether  the  Government  has  received  any  reports  from  the  Government  of  Kerala about  the  subsidence  of  land  in  Mundrothuruthu  in  Kerala  State  causing  houses  to submerge;
- (b) if so, the details thereof;
- (c) whether the Government intends to constitute an expert committee for ascertaining the facts in this regard, if so, the details in this regard;
- (d) whether it further assessed that there is a rapid shrinkage of Mundrothuruthu due to soil erosion and sea level rise and associated tidal incursion; and
- (e) if so, the details thereof?

## ANSWER

## THE MINISTER OF EARTH SCIENCES (SHRI KIREN RIJIJU)

- (a) to (e)  Yes Sir. The National Centre for Earth Science Studies (NCESS), Thiruvananthapuram, an autonomous institution under the Ministry of Earth Sciences (MoES) had received communications on the Munroe Island water level rise from the Disaster Management (Revenue-K) Department, Govt. of Kerala in 2015.

The  Kerala  government  had  constituted  a  Committee  of  Experts  with  the  National Centre for Earth Science Studies as a member to study this phenomenon in Munroe Island  (Kerala).  Reconnaissance  surveys  had  been  carried  out  and  studies  were conducted including the problem faced by the people living on and around the island.

NCESS, Thiruvananthapuram had taken up investigations specifically to address the concern  of  the  people  in  the  area  pertaining  to  geological/geophysical  aspects. Research on Munroe Island was taken up in 2017.

The Munroe Island, in Kollam district, Kerala State is a cluster of eight islands located on  the  eastern  end  of  the  Ashtamudi  Lake  in  southwest  Kerala.  It  is  subjected  to flooding, saltwater intrusion and subsidence affecting housing, livelihood, agriculture, potable  water,  etc.,  due  to  the  vulnerable  natural  setting  and  typical  environmental conditions.

NCESS had  investigated  the  area  to  understand  the  geological/geophysical  aspects. Based  on  continuous  measurements  of  Global  Positioning  System  (GPS)  at  certain identified locations, satellite image-based interferometry, in situ water quality measurements, detailed bathymetric surveys of Ashtamudi Lake, tidal measurements, electrical resistivity surveys and core  sampling at identified locations, certain inferences were drawn. The key inferences are:

1. The island area with ground elevation less than 2 m (w.r.t. MSL), for the most part, was artificially built  on a  reclaimed  region,  in  the  beginning  of  19th  century.  In fact,  these  low-lying  parts  of  the  islands  are  the  problematic  zones,  where submergence and flooding are occurring. Persistent Scatterer Interferometry have yielded stable signatures within the island negating the presumption that the entire island is subsiding, which was also supported by the long static GPS measurements. However, the scatterers have shown certain unstable pixels along the railway line as well as at certain ground points, denoting the slumping into the clayey substratum due to natural consolidation and compaction of the organic-rich modern sediments, filled to build the island. The geomorphological settings of the island  also  support  the  view  of  self-weight  consolidation  of  the  buildings  and sinking of rail track built on earth bunds, due to continuous ground acceleration of trains.
2. The  entire low-lying parts of the island have wells with brackish water. Measurements  revealed  that  the  phreatic  aquifer  is  in  hydraulic  continuum  with tidal waters having  considerable salinity.  During  monsoon  flood  when  the Thenmala Dam on the Kallada River was open, considerable reduction in salinity has been observed in the wells due to freshwater influx. Before the dam was built, the Kallada River was charging the aquifers of the island with freshwater. This has almost ceased because of the dam. Moreover, the continuous semidiurnal forcing of the flood tides getting into the unmaintained small channels within the islands make the situation conducive for the aquifer salination, which seldom drains even during the ebb tides due to clogging.
3. The  island  system,  being  an  integral  part  of  the  Ashtamudi  Lake,  responds  to variations in the morpho-hydrodynamics of the lake. Comparison of the detailed bathymetry  of  the  lake  conducted  by  CESS/NCESS  in  2000  and  2017  shows considerable decrease in its water-holding capacity. A reduction of ~15% within a matter of 17 years is due to shallowing of the lake, mainly from sediment influx due to tidal exchange. Therefore, tidal excursion, especially during the ebb tides, becomes sluggish leading to accumulation and slow draining of tidal water.
4. The bed level of Kallada River has been going down because of unauthorized sand mining. This and the sea level rise have caused the tidal regime to reach far inland, more than 31 km along the course of the river from the confluence with Arabian Sea.  This  process  has made  floodplains/areas  adjacent to  the  river  susceptible  to salinity incursion.
5. Sediment accumulation in the islands, especially along with the swift monsoonal flow  of the  Kallada  River,  has  stopped  because  of  the  damming.  Annual accumulation of fresh sediment has been naturally compensating the consolidation of the sub-surface organic-rich layers, thus maintaining the elevation of the island along  with  fertility  of  the  soil.  Changes  in  this  input  mechanism  might  have affected the sustenance of the agriculture and coconut plantation of this island as well. Periodic flushing of Thenmala Dam may partly overcome this issue.
6. In the recent years, intermittent flooding has been recorded, from October through April.  Processing  of  tide  data  shows  erratic  results  pointing  to  the  fact  that  the flooding in the island is not always concurrent with the spring tide events. It has been  noticed  in  other  similar  systems  that  while  the  wind  forcing  frequency matches  with  one  of  the  natural  frequencies  of  the  basin-scale  internal  seiches (standing waves), resonant amplification of the seiches can occur. Apart from the above,  sea  level  rise  due  to  remote  forcing  is  also  considered  as  one  of  the supplementing  factors  in  the  increased  frequency  of  such  events.  The  possible 'remedial measures' require (i) engineering solutions along the railway track zone within the Munroe island are, (ii) biological and ecological solutions in the wetland areas namely mangrove and tidal flat, and (iii) adaptability, social awareness and sociological options in the settlement areas.

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