Indian Journal of Marin e Sciences Vol. 29, March 2000, pp. 32-36

Siltation at a capital dredged site j n coast of dia

K - as hecd &'A N Balchand ( Department of Phys ical Oceanography, Schoo l of Marin e Sciences, Cochin Uni vers ity of Scicnce and Techn ology, L-- Cochin 6R2 ~ Indi a

Received 24 JUli e 1998; revised 27 December 1999

A capital dredging site at Cochin harbo ur was closely mon ito red fo r topographic variab ilit y aed the ro le of natu ral pro­ cesses i" n rep rieving the scenario. The tex tural characteri stics poin ts out th e ab undance of sil t and clay (76-95 %) and spar­ ingly sa nd; on conduct of capital dredging, th e prom inence 01 sa nd is noteworthy. The bath ymetri c contours revealed the pe­ ri odi c changes on dredgin g and th e result ant sediment ati on lead ing to th e gradu al fi ll up of the dredged sites. Th amo lln t of dredgi ng performed to reclaim the adjacent land was quanti fll:d using a co mputer aided ana~sv I '-t V ~ "

Tropi ca l estu aries exemplify multitudinous features (Fig. I). Approximate depth of these channels are in shaping th e la nd and nearshore environments. One 10-1 2m. The study mainly concentrated on either side essenti al entity that often attracts sc ientific attenti on of the Mattancherry bridge. Tides in this region are is the large scale siltation in estu ari es having a proven mixed semidiurnal of maximum amplitude I m during relati onship to the coastal zone processes I . Often spring tide. Further deta il s on environ men tal setting are 7 12 many coastal water bodi es are ideal locati ons for de­ ava il able elsewhere - including the features of bottom velo·pment into full fl edged harbour(s) or port(s) ne­ textural characteri stics which are allu vial depos its of cessitatin g large scale dredging operati ons to counter laterite ori gin .(s ilt and clay sediments). sil tati on. The procedures adopted in thi s context and The area was surveyed by lead/echo sounding for associated envi ronmental impacts of dredgin g and depth measurements, reduced to chart datum and miti ga ti on of adverse effec ts have been well docu­ pos ition fi xing was ac hi eved again st pro minent land 2 mented -4 In thi s study, a region of the Cochin estu ary marks with the aid of di stance viewer giv ing accuracy wh ich is a major port along the south wes t coast of of 2 m. Sediment sampl es were coll ected UStng a Indi a is closely in ves ti gated. This, being a tropical estu ary, receives suspended load along with river di s­ charge whi ch find the estu arine harbour site quite s 6 favorable grounds fo r siltation . . A deve'lopmental project aimed at eas in g road traffic by effec ti ve di ver­ sion from trunk roads was implemented by rec lama­ ti on at the southern parts of Coc hin port. Conse­ quentl y, on the condu ct of capital dredgin g at the far end of the Mattancherry channel, a large area within the estu ary was subj ected to topographic alterati ons to the extent that normal siltati on processes were di s­ rupted within the harbour (Fi g. I). This paper reports on the results of an investigation aimed at studying the vari ability of siltation in a tropi cal tidal es tu ary with capital dredging processes.

Materials and Methods The Cochin Port Trust maintains three dredged channels: Approach channel (9 km), Ernakul am channel (5 km) and Mattancherry channel (3 km) Fig. I-Locati on map RASHEED & BALACHAND: SILTATION IN COCHIN HARBOUR 33 van Veen grab at five stations on days of collection. (1 March 1994) after one week of pause in dredging Station 3 is at the main dredging location with stations (Fig. 2B) showed the following features. The relative I and 2 on the southern part and two more stations (4 sand percentage showed remarkable decrease in and 5) on the northern part (Fig. I). The sediment abundance but silt concentration showed a marked samples were subjected to textural analysis". Estimates similarity on either side of the dredging si te. Clay on amount of material sedimentedJdredged were percentages were comparable to that of si lt and was in arrived at by means of computer aided techniques. far higher proportions at station 3 compared to other stations. This feature points out the tendency of fine Results and Discussion material to be selectively deposited at subsurface Dredging at the chosen site commenced by the end layers. of December 1993. The first survey was carried on On completion of one month of continuous capital 5 Jan. 1994. The analyses indicated that sand propor­ dredging, the survey conducted on 22 March 1994 tion gradually increased towards the dredging site, revealed presence of very minor quantity of sand, but further northwards, its presence decreased similar to the results obtained in the previous survey (Fig. 2A). However, the silt concentration (%) shows (Fig. 2C). The silt concentration marginally decreased a graduall y decreasing tendency towards station 3; giving way to the presence of clay material at stations thereafter it was distributed uniformly. The clay con­ 3-5 . This observation reveals that more finer material centration showed an increasing tendency towards the than silt is either sedimented or translocated on the dredged site and later showed similar distribution seaward location of the dredged site. This factor also features as that of silt. Investigations at the site excludes the possibility of river borne material to reach

100 80 (B) 60 (A) 40 20 0 : x ~

~ ~ (0) 80 (C) VI 60 z ~ 40 : 0 ....- 20 w 0 ·ct ===>

'0: 100 U- sa (E) .... 60 z lJJ 40 ~ 20 0 0 lJJ 2 3 ~ 5 VI STATION S 80 (G) -+- --tr- GO Sand Silt• Clay 40 20 0 2 3 4 5 STATIONS Fi g. 2-Percentage distribution of sand, silt and clay at stations I - 5 during - A) 5 Jan. 1994, B) I March 1994, C) 22 March 1994, 0)20 April 1994, E) 19 May 1994, F) 25 June 1994 and G) 24 August 1994. 34 INDIAN 1. MAR. SCI., VOL. 29, MARCH 2000

the dredged area under diminished river discharge rates 3.0 to 5.0 m with the presence of two deep pools of during premonsoon season (February-April). depth 13 m and 10.5 m lying adjacent towards the After a gap of one month, collection was again held northern parts of the study area. This is due to con­ on 20 April 1994 which indicated practically little tinuous dredging of material to reclaim adjacent land, presence of sand but extensive proportions of silt leaving little or no time for natural suspended or bed material. The percentage of clay at stations I to 5 material to replace the excavated quantity. The survey varied between 10 to 25% which was less than those held on 20 April 1994 indicated depth variation from values observed in the previous collection (Fig. 2D). 3.2 to 7.4 m (Fig. 3D). During the following period The pattern of percentage distribution among three (monsoon months) the topography of the region indi­ fractions during this month suggests substantial cated filling up of the pools, resulting in shallowing sedimentation of material which caused shallowing of to depths around 6.4 m. This directly coincided with dredged site and its neighborhood as observed from the the monsoonal inputs, transporting material to down .. topography map. estuarine sites. The effect of intermittent dredging are A remarkable change was noticed from the results of quite projected in Fig. 3G. During the survey held on the survey held on 19 May 1994. On commencement of 24 August 1994, the presence of a single pool was dredging after three months of stoppage, the dredged noticed to the right of the deeper area as seen from area indicated the presence of comparatively higher Fig. 3F. The influence on isobaths due to overflow proportions of sand whereas the neighborhood stations from the nearby reclaimed land did add to the features lacked the presence of this grade of the constituent in Fig. 3G. (Fig. 2E). Stations I and 2 on the upstream side The amount of excavation and later the contained more silt in comparison to stations 4 and 5 accumulation of sediments at the dredging site and in on the down estuarine locations. This change coincided the nearby area was calculated by the trapezoidal with the commencement of early monsoon bringing method supported within sur:fer software on computer. higher amounts of landborne material with the river The volume of material r~m oved between the first and discharge as suspensate (30-120 mg/I) which is second survey ( 5 Jan. 1994 to I March 1994) was l evidenced by the textural variations causing shallowing derived as 2,31 ,153 m· . The amount of material of deeper portion of the channel. removed between I March 1994 to 22 March 1994 was 1 1 The dredging operations were held in an intermittent 27,550 ill", followed by an amount of \,00,982 ill" pattern during the southwest monsoon period. The within the next period of observation from 22 March survey held on 25 June 1994 indicated that relative 1994 to 20 April 1994. The difference in volume increase in sand concentration is limited towards the between 20 Apri I 1994 to 19 May 1994 V{as. 88,053 m3 dredging site (Fig. 2F). Silt concentration showed mild and that between 19 May 1994 'and 25 kine 1994 was irregular distribution but finer material at stations I and 1,75,2 17 m 3, this being the material that gradually 5 on either sides of the dredged site was more than filled up in the dredged area, The net amount removed 40% of the clay. The last survey held on 24 August stood at 2,31,153 m' and aggregate. volume sedirnenting 3 1994 after a gap of two months exhibited very low totaled 3,91,802 m . Estimates could n& include the presence of sand, but equal shares of silt and clay, results of the last survey held on 24 August 1994 as distributed irregularly, between stations I and 5 (Fig. land overflow and runoff lead to non-point sources of 2G). The inference from the last two figures evidence material driven into the study area. Calculations also the role of monsoonal inputs of finer (clay) in point out to the daily deposition rate in Mattancherry suspension leading to siltation in the proximity of channel, between days of observation (I March 1994 to Mattancherry channel. 22 March 1994) as 1300 m'iday in March 1994, 3500 The bathymetric features were closely monitored m' /day from March end to mid April period followed apart from textural characteristics. Figure 3A repre­ by a rate of 3,000 m' /day up to mid-May 1995. During sents the isobaths at the dredged site which was in­ peak monsoon, deposition rate around 4700 m'/day vestigated on 5 Jan. 1994. The contours in thi s figure (riverine inputs wash down terrestrial material in showed that the depth ranged from 2.5 to 4.5 m in the suspension and/or bed loac movement). It may be region of study. The survey held on I March 1994 noted that within Mattancherry channel or in the (Fig. 3B) and 22 March 1994 (Fig. 3C) indicated that nearby vicinity, dredging which would excavate nearly most of the region to be at a depth varying between 2 x 106 m' of material from an area approximately RASHEED & BALACHAND: SILTATION IN COCHIN HARBOUR 35

...... § 50 0'L--_--L-.J...J...... a.:::...----I 400 650 01 wi dth (m) c

cal 550

oL..-_--'-_..L...L...Jo----I 100 200 300 400 100 200 300 400 100 200 300 400 wid t h (m)

Fig. 3-Two dimensional plots of isobaths at the study area for the dates of investigation - A) 5 Jan. 1994, B) 1 March 1994, C) 22 March 1994, D) 20 April 1994, E) 19 May 1994, F) 25 June 1994 and G) 24 August 1994.

2 12,500 m to depths 10m below estuarine bed would be with huge amounts of sediment dredged from the refilled by natural processes in about three months channel(s). However, the continuous dredging of time, given that the event occurs before monsoon. In material from a fixed site will lead to unprecedented case, the contribution from increased riverine inputs increase in channel depth, thereby changing the geo­ are considered during monsoon alone, the deepened morphologic character of the island situated in the portions of the channel are subjected to rapid Cochin backwater system. Studies on dredging at sedimentation leading to shallowing. Norfolk harbourl4 which revealed changes in the tidal The dredged spoil dump grounds on the southern prism of the estuary which lead to weak tidal cur­ part of the Willingdon island was gradually nourished rents. This is counterproductive which then will likely 36 INDI AN 1. MAR . SCI., VOL. 29, MARCH 2000

enhance sedimentation within the channels. As for the Acknowledgement Cochin estuary, such a scenario is comprehensible in One author, KR thanks University Grants the light of demands for land fill material. Commission, New Delhi for financial assistance. Sedimentation in Cochin estuary, had by and large Support from Department of Science and Technology, remai ned unquantified. The net amount of sediments New Delhi is also gratefully acknowledged. recycled during the tidal cycle(s) across the inlet is yet References to be accounted. This happens to be the first study at I Dyer K R, Estuarine hydrography and sedimentation, (Un i­ versity Press, Cambridge) 1979, pp. 145. Cochin to account for sediment budgets at a capital 2 Windom H L, in C R C Critical reviews ill environmental dredged si te. A reasonably good picture of the control, (CRC Press, In c. Boca Raton, Florida.) 1976, 9 I. sedimentation pattern within the channel has aided the 3 Canter L W, Environmental impact of wa ter resources proj­ study in depth variation after each dredging operation ects (Lewis Publishers, Inc., Mi chi gan) 1986, pp. 352. 4 Munawar M, Norwood W P, McCarthy L H & Mayfield C I, so as to arrive at the volume of material removed from Hydrobiologia , 18811 89 (1989) 60 I. the bottom; on stoppage ,of operations, studies on the 5 Rasheed K & Balc hand A N. in Proceedings of the national fill up procedure with time and space has lead to seminar on environmen tal aspects of water resources devel­ ascertain the amount of material inputs which settle opment and management. ed ited by P N Un ni (CWRDM. Cali cut) 1995, 59. and its textural distribution. 6 Gopalan U K, Vengayil D T, Udayavarma P & Kri shma M, J The observation during the study mainly indicates Mar BioL Assoc India, 25 ( 19 83) 131. that the sediment characteristics and bottom 7 Anto A F, Udayavarma P & Kri shna Iyer H, Indian J Mar topographic features will be restored after a period of Sci, 6 (1977) 76. 8 Udayavarma P, Pylee A & Raju V S R, Mahasagar- BulL intermittent dredging. The tidal flushing characteristics Na ill In st Oceanogr, 14 (198 1) 225 . and river di scharge plus material inputs helps the 9 Joseph J & Kurup P G, in Proceedings of the nationaL semi­ dredged site to return to its initial status. This aspect nar on estuarine managemelll', edited by N Balakrishnan has added significance in view of the tropical Nair (S I. Joseph Press, Trivandrum) 1987,88. environmental setting and the extent of maintenance 10 Jose ph J & Kurup P G, Mahasagar--BulL Natn In st Oceanogr, 22 (1989) 165. form of dredging that is currently practiced at Cochin I I Balchand A N, Nair S M & Nambisan P N K, Toxicol Envi­ Port channels to ensure uninhibited traffic operations ron Chem , 27 (1990) 285. around the year. The influence of seasonal monsoon(s) 12 Ajith J K & Balchand A N, in Proceedings of th e Kerala and the input of material, requisite better scientific Science Congress, edited by R Ravi Kumar (SB Press, Trivandrum) 1994, 85. management to lower recurring cost of operations, in 13 Carver R E, Procedures in sedimentary petrologv, (W il ey light of planned development of a super container Interscience, New York) 197 1, pp. 653 . berth(s) at this port. 14 Nichols M M & Strobel M M H, J Coastal Res. 7 (1991) 745 .