report of the committee of experts tcl

8/11/2019 Report of the Committee of Experts TCL

1/47

Report of the Committee of Expertson Ecological and Environmental

Impact of Dredging at Vaduthala Kayaland Vaikam Kayal, Kerala

Submitted to

The Government of Kerala,Thiruvananthapuram

By

Dr. K. Ravindran

Dr. K.K. Appukuttan

Dr. V.N. Sivasankara Pillai

Dr. M.R. Boopendranath

SEPTEMBER 2006


2/47

Contents

0. Executive summary1. Introduction2. Background

3. Information sources4. Area proposed for dredging5. Dredging techniques and operational practices proposed to be used6. Impact of suction dredging on water quality, benthic habitat, and biota

6.1 Suspended sediments and turbidity6.2. General effects of increased suspended solids and turbidity levels6.3 Effect of resuspension6.4 Toxicity of mobilized sediment constituents

6.4.1 pH6.4.2 Turbidity6.4.3 Nutrients

6.4.4 Heavy metals6.5 Removal of benthic species and communities6.5.1 Entrainment6.5.2 Recovery of benthic communities following dredging activities

6.6 Organic matter and nutrients6.7 Contaminated sediments6.8 Settlement of suspended sediments6.9Changes to hydrodynamic regime and geomorphology

7. Fishery resources7.1 Studies on effect of dredging done elsewhere

8. Livelihood issues9. Conservation of black clam resources of the Vembanad Lake10. Conclusions11. Recommendations12. References

ANNEXURES1. Report on Sediment analysis2. Photographs

2


3/47

0. Executive Summary1.

Shell deposits in the proposed area lies roughly between 3m to 8m fromthe estuarine bottom. Overburden is about 3m to 6 m. Deep dredgingoperations required for mining the shell deposits will have impact on

benthic community in the area of dredging. The impact of increasedsuspended solids, turbidity levels and nutrients is seen in near-field (< 1km) from the dredging location which do not constitute an hazard. Theimpact of dredging is observed to be localized. The recovery of the benthiccommunity is expected to take place within 1.5 to 3 years, aftercompletion of the dredging activity. As regards heavy metals, impact ofcontaminated sediments was observed to be minimal, as no majorindustrial establishments have been historically operating in thedredging area. This is substantiated by the present observations in theVaikam Kayal. Hydrographic studies at dredging site have revealed thatturbidity, dissolved oxygen, pH, nutrients, heavy metals etc. do not

shootup to a hazard level and in most cases the parameters are atnormal level as for a dynamic estuary. In view of this, dredging could bepermitted with a proper environment management plan.

Good environmental management practices are recommended whilepasturing on new areas: Formulation of an environment managementplan and implementation of an environment monitoring plan. Base linestatus of the diversity and population of biota of economic significance inthe proposed dredging site is to be determined. Monitoring of the areaand immediate environs spanning about 1 km upstream anddownstream may be arranged under the supervision of competent

persons, during the pre-dredging, dredging and post-dredging periods ifdredging operations continue for months.

Livelihood alternatives may be provided for fishermen dependent on thedredged area and immediate environs, affected by dredging. The majorimpact is on the livelihood of persons and families (project affectedpersons - PAP) who are depending on the site for livelihood: fishing, clamharvest etc. The proponents have to identify PAPs and formulate a planto rehabilitate them during the impact period. Affected stakeholders to bemade beneficiaries for mussel culture, cage culture, ornamental fishculture, sea weed culture and freshwater and brackishwater fish culture;

value addition to fish products, under Kerala government schemes.

Measures for conservation of clam beds are given in the Report.

3


4/47

1. Introduction

Government of Kerala constituted a 4-member Commission of Experts, as below, tostudy the impact of ecological and environmental impact of dredging at Vaduthala Kayaland Vaikam Kayal in Vembanadu backwaters, Kerala by Travancore Cements Ltd.,Nattakom, Kottayam, 686013, vide G.O.(Rt) No. 454/05/2005/ID dated 07.05.2005 andG.O.(Rt) No. 606/2006/ID dated 16.06.2006:

1. Dr. K. Ravindran

(formerly Director, Central Institute of FisheriesTechnology, Cochin and currently UGC VisitingProfessor, School of Industrial Fisheries, CUSAT)

Chairman

2. Dr. K.K. Appukuttan

(Principal Scientist & Head, Molluscan Division,Central Marine Fisheries Research Institute, Cochin)

Member

3. Dr. V.N. Sivasankara Pillai

(formerly Prof. & Head, School of EnvironmentalSciences, and currently Honorary Director, School ofRural Development and Appropriate Technology,CUSAT)

Member

4. Dr. M.R. Boopendranath(Principal Scientist, Central Institute of FisheriesTechnology, Cochin)

Member Secretary

2. Background

The Travancore Cements Limited (TCL), a state public sector undertaking locatedat Nattakom, Kottayam District is engaged in manufacture of white cement, from 1959.The company directly employs 600 people. The TCL started production in 1947 asprivate sector undertaking with technical expertise given by M/s FL Smidth & Co.,Denmark for producing grey cement. TCL became a state-owned public sectorundertaking in 1989. The main raw material used for cement production by TCL is limeshell, which is dredged out of Vembanad backwaters. The required lime shell is dredgedfrom the Vembanad Lake by using dredgers owned by the company. The yearlyproduction capacity of grey cement was 50,000 t and the total requirement of lime shellwas around 80,000 t. From 1959 onwards, TCL started production of white cement. Grey

4


5/47

cement production was completely stopped in 1975, due to escalation in cost of rawmaterial. Production capacity for white cement was 30,000 t per year. This is reportedlythe only cement manufacturing industry in the country using lime shell as raw material.The white cement made out of lime shell is considered to be highly durable and superiorin quality, due to absence of magnesium oxide. This product enjoys good reputation in

the county.

TCL is using wet process technology which has production cost higher than dryprocess cement production technology used by competitors. There are also nocompetitive alternative sources of raw material available for white cement production byTCL. The required quantity of lime shell was obtained by dredging at reasonable cost andthe company was running on profit until 2000. Subsequently, due to competition fromother white cement manufacturers and insufficient supply of lime shell, the companystarted incurring loss and diminished production.

TCL dredges lime shell from the Vembanad Lake where mining lease is given by

the Government of Kerala. It is sanctioned by government after getting clearance fromthe Departments of Revenue, Fisheries and Irrigation and on the basis of detailed miningplan approved by the Indian Bureau of Mines. Company can hold up to 1,000 hectares inthe lease hold. Due to proximity, Company was working during the last many years inKumarakaom area. As there is practically no dredgeable lime shell deposit inKumarakom, dredging has to be shifted areas like Vaduthala, where deposits are knownto be available. TCL is having mining lease for 200 hectares in Vaduthala from 1985onwards. It is estimated that there is around 5 lakh tonnes of lime shell deposit in this 200ha area. Out of the 200 has under leasehold, only 100 ha is valid now, which is in Sy. No.98/1 in Chertala Taluk. The total estimated deposit in this area is 2 lakh tonnes which willbe sufficient for 5 years working of the company. Yearly requirement of lime shell forwhite cement production by TCL is around 40,000 t.

The availability of requisite raw materials is normally the determining factor inthe location of a cement factory. The process of cement making is flexible in terms ofraw materials that can be used to achieve required chemical compositions. However, thisposes considerable obstacles while considering the quality requirements of superior gradewhite cement. The Fertilisers and Chemicals Travancore Limited (FACT),Udyogamandal has bulk quantities of gypsum, the hydrated calcium sulphate which is abyproduct of phosphoric acid production. The gypsum generation is about 3 lakh tonnesper year and the stock in yard is around 30 lakh tonnes. Chemical analysis on dry drybasis reveals: Total Calcium Sulphate 95 % wt.; Total P2O5 1.05 % wt.; Fluorine as F 0.32 % wt.; Chloride as Cl ND.; Balance Silica, iron, Alumina etc. TCL may considerutilization of gypsum in their works through an R&D Project entrusted to a suitableresearch establishment in the field.

There is uncertainty in the production of white cement by TCL because ofhindrance in the supply of the raw material, lime shell. The Company currently procureslime shell from co-operative societies and individual fishermen at open market price,

5


6/47

from the Kumarakom area. The supply is far below the requirement of the companydespite spending more money on procurement. The TCL is unable to the begin dredgingoperations in the shell bearing area under lease hold in Vaduthala area, due to oppositionfrom the local people. If this issue is solved, according to the Company sources, TCLcould resort to dredging which would not only reduce the raw material cost by 50 per

cent but ensure a consistent supply channel. Local fishermen, opposed dredgingoperations on the ground that it would destroy the delicate and vulnerable ecosystem ofVembanad Lake. It was in this context, that a four-member Commission was appointedby the State Government to study the issue and the company may wait until thesubmission of the report for deciding on their future course of action.

The Commission had five sittings in the CUSAT and had a session with Shri V.Dhinakaran, Ex-MLA and General Secretary, Dheevara Sabha along with his nineMember Team and two meetings with Shri Charles George, President, KeralaMatsyathozhilali Aikyavedu, TUCI. The Commission visited dredging areas ofVaduthalaKayal and Vaikam Kayal three times during the course of the study for field

measurements and samplings.

3. Information sources

Information sources for this study are mainly the following:

(i) Report on the investigation of lime shell deposits in Vembanad Lake , Alleppeyand Kottayam Districts. 1978. N. Janardhana Iyer, Geologist on Deputaion, TCL,Kottayam.

(ii) Report of the Technical Committee to look into the problems arising out ofdredging in the Vembanadu lake, 23 November 1985, submitted to theGovernment of Kerala

(iii) Research papers on lime shell fisheries of Vembanad Lake(iv) EIA Reports, Research and Technical papers pertaining to the impact of dredging

on ecology and environment.(v) Field data collected and analyzed.

4. Area proposed for dredging

Revenue Survey Number of the mining lease is 90/1 and the area is 100 ha of kayalpurampokku, now under lease area of the TCL. The lease area is lying NW-SE, inbetween Purumbalam Island and Arookutty, as part of Vembanad Lake lying in the NWpart of Alappuzha District. The NW boundary of the lease area is 600 m away from thetip of Arookutty, the NE boundary is 50 m away from the northern tip of PerumbalamIsland, the SW boundary is 800 m away from the northern tip of Perumbalam Island, theSW boundary is 800 m away from the Panavally Jetty and SE boundary is 500 m awayfrom the Perumbalam Jetty (Fig. 1).

The water column is very high in the western boundary of the lease area and isabout 4.6 m called as chaal, whereas it is very shallow (0.5 m) at the eastern boundary.

6


7/47

The chaalis used for transportation purpose mainly for steel barges to transport furnaceoil, etc. The clayey sand overburden is maximum at the eastern part of the shell bearingarea because of the considerable deposition from the adjacent area and erosion from thePerumbalam Island. There are boat services from Poothotta to Panavally and fromPerumbalam to Panavally run by State Water Transport Corporation. These boat chaalis

at the southern and western boundaries of the lease area. The water column will rise andfall by 0.9 m - 0.6 m due to tidal effect.

There are shell deposits on the northern and western portion of the lease hold(which is almost one-fourth of the total area) but the maximum shell deposits are ateastern and southern parts of the survey area (Fig 2). There are about 40 stake nets(owned by fishermen) lying within the lease boundary and five Chinese dip nets are inthe eastern portion of the shell bearing area.

5. Dredging techniques and operational practices deployed

TCL uses cutter suction dredger, which has a pumping capacity of 2,500gallons/minute. There will be 20 % solids in the dredged material and out this 30-50 %will be lime shell. The entire dredged material is pumped by a 200 hp engine drivenpump through a rotating filtering unit, which separated lime shell from sand, clay, andwater contained in the dredged material. The clean shell is discharged to the barge whichis transported to the factory. The clay, sand and water from the filter fall back to dredgedcanal. Company has got 6 barges of 60 t capacities for transportation of lime shell fromthe dredger. The trenches formed due to dredging get more or less leveled with the clayand discharges from the filtering unit. The canal portion gets deepened only to thevolume of lime shell removed. The operation of TCL dredger is limited to 20 ha per yearand the daily operation is proportionally limited to a few cents only.

7


8/47

8


9/47

9


10/47

6. Impact of suction dredging on water quality, benthic habitat, and biota

The potential environmental effects of suction dredging are generally two-fold,

firstly as a result of the dredging process itself and secondly as a result of the disposal ofthe dredged material. During the dredging process effects may arise due to the excavationof sediments at the bed, loss material during transport to the surface, overflow from thedredger. The extent to which dredging might effect the environment is highly varied andsite specific, depending upon a number of factors:

Magnitude and frequency of dredging activity.Method of dredging and disposal.Channel size and depth.The size, density and quality of the material.Background levels of water and sediment quality, suspended sediment and

turbidity.Tidal range.Current direction and speed.Rate of mixing.Seasonal variability and meteorological conditions, affecting wave conditions and

freshwater discharges.Presence and sensitivity of animal and plant communities (including birds,

sensitive benthic communities, fish and shellfish).

The potential impacts of dredging and disposal have been discussed in following:IADC/CEDA (1998), ICE (1995), PIANC (1996) and others. Short-term increases in thelevel of suspended sediment can give rise to changes in water quality which can effectmarine flora and fauna, both favourably and unfavourably, such as increased turbidityand the possible release of organic matter, nutrients and or contaminants depending uponthe nature of the material in the dredging area. Settlement of these suspended sedimentscan result in the smothering or blanketing of benthic communities and/or adjacentintertidal communities. The impact of dredged material disposal largely depends on thenature of the material (inorganic, organically enriched, contaminated) and thecharacteristics of the disposal area.

The evaluation of the environmental effects of dredging and disposal must takeaccount of both the short-term and long-term effects that may occur both at the site ofdredging or disposal (near field) and the surrounding area (far field). The IADC andCEDA (1998) have provided the temporal and spatial scales in which variousenvironmental effects of dredging might be realized (Table 4). Near field effects aresimply defined as phenomena occurring within the geographic bounds of the activity, orless than approximately 1 km from the activity, and far field effects as occurring morethan approximately 1 km from the activity'. However, other sources suggest that cautionshould be used when adopting an arbitrary distance to distinguish between near and far

10


11/47

field effects, due to the site-specific nature of the potential effects that arise fromdredging.

In addition to the environmental effects that may occur as a direct result ofdredging and disposal activities, impacts also may occur as a result of the physical

changes to bathymetry and hydrodynamic processes that dredging makes. These effectsare listed in Table 1 (IADC/CEDA 1998).

Table 1. Timespace matrix of potential effects associated with dredging and dredged material

disposal (IADC/CEDA 1998)

Near-field

Environmental Effects

(1km)

DredgingTurbidity

Smothering/removal of organismsReduced water quality

DredgingNone generally expected

Short-term

Environmental Effects

(1 week)

DisposalAltered substrate typeAltered community structureChronic chemical toxicityBioaccumulation

DisposalOffsite movements of chemicals byphysical transport and/or biotamigration

Potential effects from dredging and disposal may include alterations to estuarinemorphology, sediment pathways and changes to siltation patterns, which may affecthabitats and species; and alterations to water currents. Some potential effects of dredgingare discussed below:

6.1 Suspended sediments and turbidity

When dredging and disposing of non-contaminated fine materials in estuaries andcoastal waters, the main environmental effects are associated with suspended sedimentsand increases in turbidity. All methods of dredging release suspended sediments into thewater column, during the excavation itself and during the flow of sediments from hoppersand barges. In many cases, the locally increased suspended sediments and turbidityassociated with dredging and disposal is obvious from the turbidity plumes which maybe seen trailing behind dredgers or disposal sites.

11


12/47

Increases in suspended sediments and turbidity levels from dredging and disposal

operations may under certain conditions have adverse effects on marine animals andplants by reducing light penetration into the water column and by physical disturbance(Anon, 1997; IADC/CEDA 1998).

6.2. General effects of increased suspended solids and turbidity levels

Increased suspended sediments can affect filter-feeding organisms, such asshellfish, through clogging and damaging feeding and breathing equipment (Brehmer1965; Parr et al.,1998). Similarly, young fish can be damaged if suspended sedimentsbecome trapped in their gills and increased fatalities of young fish have been observed inheavily turbid water (Wilbur 1971). Adult fish are likely to move away from or avoidareas of high-suspended solids, such as dredging sites, unless food supplies are increasedas a result of increases in organic material (Anon, 1997 Research R701 1997).

Increases in turbidity results in a decrease in the depth that light is able topenetrate the water column which may affect submerged plants, by temporarily reducingproductivity and growth rates (Parr et al., 1998).

In many estuaries background turbidity levels are high (Parr et al., 1998).Organisms in these environments are able to tolerate continuous exposure to highsuspended sediment concentrations, for much longer than would occur in most dredgingoperations (IADC/CEDA 1998; Peddicord & McFarland 1978; Stern & Stickle 1978).Marine plants and animals living in areas where the waters are normally clear may beespecially vulnerable to the effects of increased suspended sediments.

The degree of resuspension of sediments and turbidity from dredging and disposaldepends on four main variables (Pennekamp & Quaak 1990), viz., (i) the sediments beingdredged (size, density and quality of the material), (ii) method of dredging (and disposal),(iii) hydrodynamic regime in the dredging and disposal area (current direction and speed,mixing rate, tidal state), and (iv) the existing water quality and characteristics(background suspended sediment and turbidity levels).

In most cases, sediment resuspension is only likely to present a potential problemif it is moved out of the immediate dredging location by tidal processes (Bray et al.,1997). In general, the effects of suspended sediments and turbidity are generally shortterm (


13/47

6.3 Effect of resuspension

During dredging process the shell is recovered by washing down the dredged material.Sand and other heavier particles settle immediately at the washing site which is only 6-

10 m from the suction head. The shell volume varies from 5- 10 % of the dredgedmaterial. This wash debris slides back into the dredged trench thus leveling it to smallextent. Particles less than 100 microns migrate in the direction of current. Depending onthe magnitude of the current, particles finer than 100 microns settle slowly while particlesless than 2 microns do not settle under natural conditions and causes a mild turbidity inthe whole water column. This usually affects the passage of light through the watercolumn only mildly as indicated by Secchi Disc measurements. In strong currentsparticles above100 microns are also mobilized due to turbulence. To delineate themigration of suspended solids and their settling characteristics and consequential effectson surrounding water body of the dredging site samples were collected with spatial andtemporal distribution upto 1.6 km. The results are presented in Table 2

Table 2. Details of Stations and Test results Date: 12.01.2006

Parameters Stn1 Stn2 Stn3 Stn4

GIS position 94219 N

762235 E

94238N

762227 E

94341 N

762233 E

94439 N

762243 E

Time of sampling,h 1155 1235 1315 1345

Temp. C Atmos 31.5 30.0 31.0 32.0

Surface C 28.5 28.5 30.5 31.5

Bottom C 29.5 28.0 30.0 31.0

Depth of water column, cm 4.50 4.50 4.35 5.0

Turbidity, cm 30.0 60.0 77 90

Water current, cm s-1and direction (degree)

Surface 10; 358 9; 285 9;250 10; 010 Middle 12; 340 11; 015 17; 050 10; 280

Bottom 4; 080 10; 015 18; 315 10; 330

Dissolved oxygen, ml l-1

Surface 6.2 5.8 6.4 -

Bottom 6.0 6.2 5.4 6.8

Salinity, ppt

Surface 0.000 0.391 0.572 0.933

13


14/47

Bottom 0.391 0.391 0.572 0.933

pH

Surface 7.15 6.99 7.14 6.92

Bottom 7.07 6.40 7.05 7.03 Sediment 7.34 6.26 6.74 6.09

Turbidity, NTU

Surface 12 10 8 7

Bottom 12 80 10 9

Nutrients

Phosphorous, ppm Surface 0 0.037 0.010 0.030

Middle

Bottom 0.100 0.045 0.140 0.165

Nitrite, ppm

Surface 0.015 0 0.03 0

Bottom 0.015 0.04 0.045 0.02 Toxic elements in water

Fe, ppm

Surface 0.395 0.087 0.067 0

Bottom 0.115 0.040 0.011 0

Cu, ppm

Surface 0.004 0.006 0.003 0.007

Bottom 0.014 0.005 0.007 0.007

Pb, ppm

Surface 0 0.00 0.00 0.00

Bottom 0.26 0.00 0.00 0.00

Zn, ppm

Surface 0.00 0.00 0.00 0.00

Bottom 0.065 0.00 0.00 0.00

Cd, ppm

Surface 0.00 0.00 0.00 0.00

Bottom 0.00 0.00 0.00 0.00

Toxic elements in sediments

Fe, ppm 48540 83422 47740 41280

Cu, ppm 61.75 60.66 48.87 26.00

Pb, ppm 21.91 37.81 17.95 16.00 Zn, ppm 87.65 127.28 76.80 48.00

Cd, ppm 0.00 0.00 0.00 0.00

Particle size analysis ofsediment (separately given)

Station 1 Dredging site, Southern end of marked area for dredging

14


15/47

Station 2, 3 and 4 approximately 1mile [1.6 km], 2 mile and 3 mile apart from thedredging site towards northern side.

Station 5: 1 mile south of the dredging area

It was observed that 95 % of the TSS settles within 60 min at the zone of dredgingand at the vicinity of dredging. This becomes very near to 100 % at 1.6 km indicating that

the sediment has only negligible concentration of particles less than 2(microns). Undermoderate conditions of water current particles from 2 200 microns may be mobilized.However, the expanding cone of dilution renders its impact negligible.

During the rainy season, the natural turbidity as well as velocity of flow are fairlyhigh and dredging has only negligible cumulative effect. During the transitional seasonand lean months tidal effect increases salinity. As salinity increases settling of suspendingsolids becomes faster as the electrostatic effect offsets the density effect on the

sedimentation of finer particles. In conclusion it is inferred that the physical effect ofdredging on the quality of adjoining water body is minor and limited to the immediatevicinity. This minor impact cannot be further amended since it is due to the naturalforces. The impact would be slightly different depending upon the chemical compositionand distribution of particle size of dredge sediment.

6.4 Toxicity of mobilized sediment constituents

6.2.1 pH. The pH in the water body in the sampled site varied from 7.0 to 7.3 which isnormal. This range does not exert any adverse impact on the biodata.6.2.2 Turbidity. [A measure of total suspended solids]

Dredging usually affects turbidity of water column by forced re-suspension of materialonly near the bottom of the dredging site where abnormal turbidity is observed. In thesurface waters it is below 12 NTU and reduces to 7 at a radial distance of 1.6 km. It isconcluded that the sediment effect is predominant at the bottom of the dredging site.6.2.3 Nutrients.Phosphorous and Nitrogen are two nutrient constituents which are critical in primaryproductivity. Both these constituents are found at very low concentrations(0 to 0.14 mgl-1for phosphate and 0 to 0.045 mg l-1 in the case of nitrogen. The water body isoligotrophic at the time of this study. Dredging does not alter the trophic status.6.2.4 Heavy metals.One of the predictable impact of dredging is mobilization of toxic heavy metals from thesediments. Five heavy metals were investigated of which iron is found in the highest ratioin the sediment (table 3). Copper and Lead have moderate presence in the sedimentwhereas Cadmium is not detected. Mobilization of heavy metals in the soluble form inwater was also investigated. The results are given in Table. It is found that all except ironare far below the risk-based concentration levels permitted even in potable water.

6.5 Removal of benthic species and communities

15


16/47

During all dredging operations, the removal of material from the seabed alsoremoves the animals living on and in the sediments (benthic animals). With the exceptionof some deep burrowing animals or mobile surface animals that may survive a dredgingevent through avoidance, dredging may initially result in the complete removal ofanimals from the excavation site.

Sorting and re-deposition of substrata moved through a dredge were expected toalter the estuarine geomorphology and create "dredge piles" downstream of the dredges.This type of physical disturbance of benthic substrata generally reduces periphytonstanding crop, and macroinvertebrate density. Thus, substrata moved through the dredgewere expected to support less periphyton than substrata in undisturbed areas of the river(Peterson 1996). Abundance and diversity of macroinvertebrates also were expected to besharply reduced in dredged areas, as physical tumbling of substrata is known to killand/or dislodge associated organisms (Resh et al., 1988), in addition to reducing theavailable food base.

6.5.1 Entrainment

Entrainment is theaccumulation or drawing in of substrate material and aquaticorganisms by current, such as at a nozzle intake. While adult fish did not show asensitivity to entrainment it is unlikely that they would be sucked into a dredge in the firstplace. They have the ability to avoid entrainment in a suction dredge by moving to a saferlocation. All of the investigators who examined the impacts of suction dredges on adultfish concluded that this life stage was not acutely affected. Molluscs could suffermortality during entrainment (Harvey et al.,1995).

6.5.2 Recovery of benthic communities following dredging activities

Certain marine species and communities are more sensitive to disturbance fromdredging than others. The recovery of disturbed habitats following dredging ultimatelydepends upon the nature of the new sediment at the dredge site, sources and types of re-colonising animals, and the extent of the disturbance (ICES 1992). In soft sedimentenvironments recovery of animal communities generally occurs relatively quickly and amore rapid recovery of communities has been observed in areas exposed to periodicdisturbances, such as maintained channels.

A review of dredging works in coastal areas world-wide showed that the rates ofrecovery of benthic communities following dredging in various habitats varied greatly(Nedwell & Elliot 1998; Newell, Seiderer & Hitchcock 1998) (Table 2)

Table 3. Recovery of benthic communities following dredging activities

Location Habitat type Recovery time

Coos Bay, Oregon Disturbed Muds 4 weeks

16


17/47

Gulf of Cagaliari,Sardinia

Channel muds 6 months

Mobile Bay, Alabama Channel muds 6 months

Goose Creek, Long

Island

Lagoon muds >11 months

Klaver Bank, NorthSea

Sands-gravels 1-2 years

Chesapeake Bay Muds-sands 18 months

Lowestoft, Norfolk Gravels >2 years

Dutch coastal waters Sands 3 years

Recovery rates were most rapid in highly disturbed sediments in estuaries that are

dominated by opportunistic species. In general, recovery times increase in stable graveland sand habitats dominated by long-lived components with complex biologicalinteractions controlling community structure.

These findings are supported by studies of Stickney & Perlmutter (1975) in theGeorgia Estuary system, USA, which suggest that maintenance dredging has only a shortterm effect on the animal communities of the silt and clay sediments. Although almostcomplete removal of organisms occurs during dredging, recovery begins within 1 monthand within 2 months the communities were reported to be similar to pre-dredgeconditions. Other studies suggest that dredging impacts are relatively short term in areasof high sediment mobility (Hall et al., 1991). Complete recovery of benthic animals in a

channel in the estuarine Dutch Wadden Sea occurred within 1 year of the removal ofsediments from this highly mobile sand environment (Van der Veer et al.,1985). Sincethe study area lies in tropical estuarine area, recovery time after dredging can bereasonably be expected to be a year or less.

Results of the field study conducted at Vaikam Kayal are presented in Table 3.

6.6 Organic matter and nutrients

The release of organic rich sediments during dredging or disposal can result in the

localized removal of oxygen from the surrounding water. Depending on the location andtiming of the dredge this may lead to the suffocation of marine animals and plants withinthe localized area or may deter migratory fish or mammals from passing through.However, it is important to stress that the removal of oxygen from the water is onlytemporary, as tidal exchange would quickly replenish the oxygen supply. Therefore, inmost cases where dredging and disposal is taking place in open coastal waters, estuaries,bays and inlets this localized removal of oxygen has little, if any, effect on marine life(Bray et al.,1997). However, despite the temporary nature of the effect, if oxygen

17


18/47

depletion were to occur during important life stages of sensitive species in the estuary andbay habitats, the effects could be adverse.

The re-suspension of sediments during dredging and disposal may also result inan increase in the levels of organic matter and nutrients available to marine organisms.

This can result in two main effects. In certain cases, such as environments adapted to lownutrient conditions or sensitive to the effects of eutrophication which can simply bedescribed as nutrient enrichment leading to the formation of algal blooms. These bloomscan reduce the surrounding water quality by causing the removal of oxygen as the bloomsbreak down or occasionally by the release of toxins which may disturb marine wildlife.The potential formation of algal blooms in coastal and estuarine areas is generally limitedby high turbidity levels and tidal flushing (Anon, 1997). In other cases, increased organicmaterial, nutrients and algal growth may provide more food for zooplankton and higherorganisms, with possible knock-on effects on the productivity of the marine ecosystem.For example, there is evidence of increased productivity of benthic communitiessurrounding a disposal site in Liverpool Bay that receives considerable amounts of

dredged silts. The beneficial effects are reported to be a result of organic enrichment fromthe dredged material and due to the stabilization of sediments through the incorporationof fine organic matter (Murray 1994).

6.7 Contaminated sediments

Although generally not heavily contaminated, much dredged material is subject tosome contamination (Murray 1994). A variety of harmful substances, including heavymetals, oil, TBT, PCBs and pesticides, can be effectively locked into the seabedsediments in ports and harbours. These contaminants can often be of historic origin andfrom distant sources. The dredging and disposal processes can release these contaminantsinto the water column, making them available to be taken up by animals and plants, withthe potential to cause contamination and/or poisoning. The likelihood of this occurringdepends upon the type and degree of sediment contamination; however, someremobilisation of very low levels of pollutants would be expected during many dredgingoperations. The highest levels of contaminants generally occur in silts dredged fromindustrialised estuaries. If low level contaminants are released into the water columnduring disposal, they may accumulate in marine animals and plants and transfer up thefood chain to fish and sea mammals.

In the present study in the dredged area of the Vaikam did not indicate hazardouslevels of copper, lead, zinc and cadmium. The levels of nutrients were also with inlimits.

6.8 Settlement of suspended sediments

Sediments dispersed during dredging may resettle over the seabed and the animalsand plants that live on and within it. It may prevent the development of stable benthiccommunities, and may result in partial or complete loss of benthic production (Murray1994). The finer the material and the greater the energy at the disposal site, the greater the

18


19/47

possibility of increased suspended sediments and of far-field effects. However, asmentioned previously, these far-field effects of turbidity and smothering are generallyonly of high concern in areas of low background levels of suspended solids.

This blanketing or smothering of benthic animals and plants, may cause stress,

reduced rates of growth or reproduction and in the worse cases the effects may be fatal(Bray et al., 1997). Generally sediments settle within the vicinity of the dredged area,where they are likely to have little effect on the recently disturbed communities.However, in some cases sediments are distributed more widely within the estuary orcoastal area and may settle over adjacent subtidal or intertidal habitats possibly somedistance from the dredged area.

The sensitivity of marine animals and plants to siltation varies greatly. In areaswith high natural loads of suspended sediments, the relatively small increases in siltationaway from the immediate dredging area are generally considered unlikely to have adverseeffects on benthic populations. Animals with delicate feeding or breathing apparatus,

such as shellfish can be intolerant to increased siltation, resulting in reduced growth orfatality (Anon, 1997). In important spawning or nursery areas for fish and other marineanimals, dredging can result in smothering eggs and larvae. Shellfish are particularlysusceptible during spring when spatfall occurs.

6.9 Changes to hydrodynamic regime and geomorphology

The impact of dredging on the hydrodynamics and geomorphology of a site aresite-specific and very difficult to isolate from other 'forcing effects'. Although alldredging activities can cause some change to the hydrodynamic flow, the magnitude andtype of effect will be related to the overall size of the excavation compared to the overallsize of the system. The magnitude of dredging related alterations of dredging on thehydrodynamics and geomorphology, in many cases, may fall well within the range ofnaturally occurring phenomena and probably impose little or no additional stress tomarine features (IADC/CEDA 1998).

7. Fishery resources with particular reference to clam and the impact of shell

dredging

The Vembanad Lake as it is know today was formerly a part of the Arabian seaand the separating land between sea and land was not extant centuries back. It is reported(1913) that in the year 1341 as a result of deluge, part of Alleppey and ErnakulamDistricts arose, this separating a large island water body with openings to the sea atThottappally, Andhakaran Azhi and at Cochin. This conversion of originally marineenvironment into a brackishwater one is evidenced by the existence of a large quantity ofthe marine shells in the lake. (Rasalam and Sebastain, 1976).

The Vembanad Lake having water spread of 22,750 km2in three coastal districts

of Kerala, spanning between latitude 928 and 1010 N and longitude 7613 E is thebiggest backwater in the State. The lake is narrow and sinous in the north while much

19


20/47

broad to an extend of 14.5 km in South. Running to a length of 96 km parallel to the seafrom Azhikode to Alleppy the lake is connected to sea in Azhikode and Kochi. Seasonalsea water incursion occur through Thottappally spillway, situated 20 km south of Alleppyand a small spillway at Andakaranazhi in Cherthalai about 25 km north of Alleppy. It isa complex water body comprising estuary, lagoons, marshes and mangroves with number

of manmade canals in between and the Thanneermukkom barrage in the middle of thesystem, preventing sea water incursion to the southern part of the water body. (Unnithanet al., 2001)

Vembanad is a wetland ecosystem with an area of over 24,000 ha, largestbackwater of the State contributing more than 7000 t of fish, shellfish and producingannually 30,000-40,000 t of black clam. Approximately 20,000 fishermen are directly orindirectly involved in the exploitation of the aquatic resources earning Rs.100 millionannually. The Vembanad Kol wetland system and its associated 10 drainage basins aresituated in the humid tropical region on the southwest coast of Indian peninsula. TheVembanad-Kol wet land was brought under The Ramsar List of wetland on 1

st

November, 2002.Estuarine oceanography of the Vembanad Lake between Pallippuram and Thevarashowed that during pre monsoon and post monsoon periods the surface bottomdifferences in salinity is much less than those found in monsoon indicating that theholocline has become weak and by a process of mixing there is more or less isohalinewater column in the estuary. It is significant that the velocity of the surface sea waterflow during the pre and post-monsoon is much less when compared to that duringmonsoon. But the reversing of currents associated with the tides could still greatlyenhance the exchange between the surface and bottom waters. Thus during the pre andpost monsoon seasons southern arm of the Vembanad is a partially mixed type owing tothis turbulent mixing which enhance exchange process in the vertical. During the pre andpost monsoon seasons shallow regions south of Arukutty belongs to a well-mixedsectionally homogenous type because of the reversing tidal currents and greaterturbulence resulting in increased fluxes of salt in the horizontal as well as vertival,leading ultimately to homogenous conditions. The salinity distribution of the estuaryreveals that the variations in longitudinal plane are from 5-30/ down the estuaryindicative of a laterally homogenous type of estuary. (Ramamritham et al., 1986).

The Kuttanad water Balance study (1989) showed that the average rainfall inKuttanad is 3,250 mm. The wet season is from June to November. The average tidalrange in the bar mouth is 0.9 m. The maximum water temperature of more than 30Coccur during pre-monsoon and minimum of 24

C in August. Dissolved O2 is high in

monsoon. The total flood discharge of about 2500 m3/s is there to Kuttanad from

Meenachal, Manimala, Pamba and Achankoil rivers. Maximum salinity measured inVembanad Lake were 10 ppt during February 1988 in relatively deep (5 m) restrictedareas south of the barmouth and in the shallow areas 1-2 m. Salinities did not exceed 4ppt in NW of lake and 2 ppt in south coast. (South of the barrier). The zone parallel tothe southern and eastern borders of Vembanad lake remains fresh (less than 0.5 ppt) evenin the dry season. During peak monsoon, freshwater near the surface and at depth below3 metres it varies from 5-18 ppt. Salinities north of barrier is greatly affected by flowfrom the water from Muvattupuzha river. Before the construction of the barrier the

20


21/47

salinity in the entire backwater was isohaline in the dry season ranging from 18-31% .Although the barrier has contributed to improvement of crop production in the Kuttanad,the adverse effects, particularly on the aquatic environment and fisheries have beensignificant. Reclamation is high in Northern part by both public and private sector.Large scale dredging is carried out in Vaikom and Alleppy in Vembanad Lake using

cutter suction dredgers to collect subfossil of lime shells upto a depth of 7 m. Theprocedure of disposing of clay-water slurry into the lake has resulted in unevensettlement of sediment. This has resulted in the destruction of live clam beds and fishinggrounds. In southern part of lake explosives and poisons are used for fishing. Aquaticweed, water hyacinth is flushed out into the backwater during monsoon months andspread prolifically over the water surface impending fishing gear. Low fishing intensityfrom July to October is owing to this reason.

Contamination of Cochin backwaters by pesticides flushed into the waterfrom agricultural lands occur mainly in Vembanad lake area. Significant concentrationof organochlorine pesticides were detected in black clam samples from the lake. Thehighest value of Y-HCH (Lindane) in sediment samples occurred in south-eastern part of

Vembanad lake where agricultural activities are high. In 1983 pH value below 5 wererecorded resulting into mortality of fish, clam and crustaceans.Industrial wastes cause pollution in the lower reaches of Cochin estuary.

Sewage pollution from urban and semi-urban township along the banks also causespollution especially coliform bacteria was detected in fishes and clams. (Kuttanad WaterBalances Studies, 1989).

The estuarine fish fauna consists of species originating from the sea, freshwaterspecies and true estuarine species. The latter reproduce in the estuary, where as themarine fish and prawn reproduce in the saline, cooler water of sea, but migrate back toestuary after completing the pelagic development. Fish distribution in Cochin backwateris thus directly related to salinity. (Kuttanad water balance study) Final Report Vol.1,1999).

Vembanad lake was subject to a lot of studies particularly on itsenvironmental details, the estuaries part of the lakes and the impact of Thannermukkambarriers on the aquatic resources and studies on the water resources significantobservations were on Fish and fisheries of the Vembanad backwaters (Shetty, 1965)Kurup and Samuel (1985, 1987) Kurup et al (1993) Unnithan et al (2001) Padmakumar etal. (2002, 2004). Rasalam and Sebastian (1976) Achary (1988) Laxmilatha andAppukuttan (2002) have studied in detail the clam fishery of Vembanad lake and broughtto light various problems related to both white clam and black clam fishery. Variousother studies by the Central Marine Fisheries Research Institute, Cochin, Central IslandCapture Fisheries Research Institute, Barrackpore, Kuttanad water Balance study (DraftFinal Report) Govt. of Kerala (1989) and Report of the Technical Committee on theproblems Arising out of dredging in the Vembanad Lake submitted to Govt. of Kerala(Thampi (1985) throw light on the aquatic resources of Vembanad and the problem itfaces through environmental degradation, human interference, over exploitation,pollution problems, dredging and other related issues.

21


22/47

Commercially important species of fishes, crustacean and mollusks reported fromthe lake are:Fishes : Grey mullets (Mugil, Liza), Sciaenids (Daysciaena albida) sea bass or cock-up(Lates calcalifer), Milkfish (Chanos chanos) Marine catfish (Tachysurus), Half beaks(Hyporamphus), Tarpon (Meglops cyprinoids) Pearl spot (Etroplus suretensis)

Crustaceans: Penaeid prawns, palaemonid prawns (Macrobrachium) Edible crab (Scyllaserrata) Mollusc: Black clam (Villorita cyprinoides)

The Vembanad aquatic ecosystem is noted for its biodiversity. InVembanad more than 20,000 fishermen are directly dependant on aquatic resources,fishing 7000 t of fish and shellfish annually. The growth and distribution of fish in thebackwaters are greatly affected by the changes is salinity. In monsoon due to decline insalinity estuarine fish catch dwindle and in post monsoon to premonsoon period marinefishes move even upto the upper reaches where good salinity prevails. After theestablishment of Thanneermukkam barrage in 1975 the salinity range in southern part ofthe backwaters declined from 20-23 ppt to 6-11 ppt. The stagnation of water in canals

and water ways in southern part of barrage during summer resulted in serious pollutionand health hazards in the area. The fish catch in southern stretches beforecommissioning the barrage in Kottayam and Alleppy were fish 2,500t, Clams 89,000tPrawns, 1700 t Macrobrachium 429 t, others 1500 t whereas in 1990-2000 it was Fish1,084 t , clam 18,800 t, prawn 1,150 t Macrobrachium 741 t others, 200t. This clearlyindicate that the major aquatic resources of south of barrage before the commissioning ofbarrage and after showed drastic decline. The same is reflected in the studies of Kurupetal., 1990, Unnithan et al.,2001, Padmakumar et al.,2002, where they have indicated thetotal fish catches after partial commissioning of barrage as around 415 t 584 t.

FisheryThe general shallowness and protected nature of the backwater system permit the

fishermen to do fishing throughout the year Major fishing activities in the lake areduring night when more catches are obtained. Approximately 21,000 fishermen in thestudy area use stake and dip nets. Marketing is done through auction in the main landingcentres or by direct sales by fishermen where as the larger sized penaeid prawns,Macrobrachium and Sylla serrata are taken by agents of fish exporters. Shell co-operative societies play a major roll in the collection and marketing of live and subfossilshells of clams in the lake.Fish production:The historic data on catches of prawn at Cochin Harbour shows a downward trend sinceearly 1970s coinciding with the closure of Thanneermukkam barrier and commissioningof the Idukki hydro-electric Project (1976). Downward trend is also correlated withoverfishing. However 70% of the prawn catches at Cochin consists of species that spendpart of their life cycle in the Cochin backwaters. The value of this is estimated aroundRs.100 million per year.

The studies made by Fisheries College of Agricultural University of Keraladuring 1988-99 period showed a total annual production of 7,200 t worth Rs.96 million ofwhich fish catches are 3,300 t, prawn 3500 t, paleamonid 100 t, crab 300 t from north ofbarrier with a total of 6,700 t and south 500 t. Stake and dipnet accounted 70% of total

22


23/47

catch and premonsoon period contributed the maximum catch of 45%. Clam fishery wascontributed by live and dead clam deposits, where the live clams gave shell and meat fortrade. In 1976 the subfossil deposit of clam in Vembanad was estimated as 4.5 milliontones while shells are exploited at the rate of 1,35,000 t year -1while annual production ofblack clam is 18,000 t year-1. The production of black clam could be increased by

adoption fishery management measures, but at the present rate of mining, white shell willnot be present leading to the natural depletion of the deposits.The Report of the Technical Committee on Operation of Thanneermukkam

Barrage Govt. of Kerala, 2002 has highlighted the problems related to fisheries sectordue to Thanneermukkam barrage. The commissioning of Idukki hydro-electrical projectin 1976 has increased the dry season freshwater flow in the Moovattupuzha river andreduced sanities.

Over past 15 years increased fishing pressure in both marine and estuarine prawnswere observed. The sub adults were caught by stake nets and post larvae were filteredand used as seed for farming. Sub adults prawns are caught from inshore areas of coastalwaters on their way back to breeding ground in deeper part of the sea. The decline in

catch per unit effort accompanied by reduction in the size of prawns in the catches. There is marked depletion on the number of live clams, due to over fishery insome part of clam beds of Cochin backwaters. The average size of clams diminished andundersized clam in large numbers are removed from the backwaters. Sources of pollutionof the backwater system are domestic waste, fertilizer residue, pesticide residue,Industrial pollution, coconut retting.Clam Fishery Studies

Apart from the fish and crustacean resources in the lake, immense quality of liveclams and subfossil `white clam form a major resource of the lake. Ever since industriesbased on shells as raw material started establishing in this locality, the demand for liveand deadshells increased and by dredging, the white shells were exploited regularly fromthe lake badly affecting the traditional live calm fishery. The main factories which utilizeshell were M/s.Travancore Cements Ltd., Pallam and The Travancore Electro ChemicalIndustries Ltd., Chingavanam. The annual requirement of these factories was around1,30,000 t. In addition The Hindustan Paper Mills, Velloor, Gwalior Rayons, Mavoor,FACT, Alwaye and Punalur Paper Mills, Punaloor also were consuming around 80,000 tannually and 30000 t for agriculture and building sector. Unlicenced shell collectionactivities are also going on, which is beyond the control of the Government. The shelldeposit collection is controlled by licensing system and in shallow areas collection isdone manually whereas in 8-9 m deep water dredges are used for exploitation. The liveclams in the surface are regularly fished out manually both for their meat and for theshell.

The traditional fishermen feel that due to dredging considerable damage is doneto live clams and consequent deposition of silt prevent recolonisation and growth ofclams. Further, inconvenience of bottom created by dredging leaving deep furrows inone hand and mouth of discharged clay on the other affects the flow of water andoperation of fishing nets. They also feel that the fish fauna move away from the dredgingareas causing reduction in catch.

23


24/47

Villorita cyprinoids(Black clam)Black clam contribute the shell fishery of Vembanad lake. This species belong to

the family Verniridae of class Bivalvia and phylum Mollusca. Usually found in thefreshwater zone of estuaries lake tolerating salinity upto 2-10 %. The colour of the liveclam Villorita cyprinoidesis black - Length range 10 42 mm, - Peak mode are

18 22 mm & 24 28 mm, of which the earlier dominated the catch. 18 22 mm is thedominant size group for the entire year, 30-42 mm size were contributed much less, 10-14 mm were found in most of the month indicating continuous spawning, peak inNovember-December and February-April. The wet meat average percentage ranged from9.19 in December to 14.07 in April with an average of 10.95 %.

Clam fishery is carried out for 15-16 days a month involving 2,200 active fishermenrepresenting the clam societies around the lake. The average catch per day is 25-35 kgespecially from Vaikom-Pallipurathussery area registered under Kuthiathode society.The clam fishery site varies from year to year according to the spat settlement due toenvironmental variations. In Nettoor, Kumbalangi, Cheppanam and Arookutty

transplantation of clam seed is practiced. This simple method of clam culture byrelaying is practiced by several fisherman to augment the shell production. Thoughindependent exploitation of undersized clams (mallikakka) is done cladestinely, forusing it as duck feed and for industrial purpose, there is an emerging practice of relayingof seed calms for increased production by certain groups of fishermen. If scientificbacking for selection of site for relaying and optimum level of stocking is taught to thefishermen, these practice can be made more popular through societies and increase theclam production of the lake.

In the northern part of the lake where marine condition prevails throughout theyear, Meretrix casta population is available where as in the southern part Villorita ispredominant. In the south the clam beds were subject to the south-west and North-eastmonsoon rains and the flood water carry large quantities of mud and silt into the lakefrom rivers Pamba, Achancoil, Manimala, Meenachal and Moovattupuizha. This causesmortality to the live clams. This and the natural mortality of clams have contributed forcenturies to the accretion of a wealth of live-shell deposits in the lake.

The white-shells are found as sub-soil deposits to a depth of 2- 4 m below thesurface of the soil and these shell deposits accumulated through centuries as dead remainsare found in puramboke lands, coconut plantations and paddy fields adjoining the lake.Live Villorita is known as black clam and this forms a fishery, the meat is sold locallyand shell is marketed for shell based industries.

Analysis of the total annual production of live clams for the past 26 years is givenin Table 4. The annual catch fluctuated from 1,1652 t to 42,026 t, the minimum is 1985and maximum in 2004. The monthly average also fluctuated from 971-3,502 t during thisperiod and the average per year is 27,615 t.

The clam exploitation in the lake is being done in an organized manner by 7 co-operative societies located in different parts of the lake. The co-operative societies buy

24


25/47

the clam shells from these members at the rate of Rs.600-700 t -1. They are alsorequested to pay an amount of Rs.25 t

-1as royalty to the Govt. The shell is sold in bulk

to the merchants of Kerala and Tamilnadu at the rate of Rs.800-1000 t-1

. The shell issold for industrial purposes viz lime industry, Cement, Pharmaceutical and CarbideIndustry.

Clam meat as food: Out of the live black clam exploited annually, only 20-30% is usedfor extraction of meat and for consumption. The meat weight is roughly estimated as 8-11% of the total weight of the clam. Mostly the meat is heat shucked and separated andsent to the local markets @ Rs.7-9 kg-1 and they earn Rs. 50-70 per day by this trade. Thenutritional value and other parameters of the meat are given in Table 5.

The lime shell fisheries of Vembanad LakeThe lime-shell deposits and live clam shells in various estuaries are the main

source of calcium carbonate in Kerala as the occurrence of lime-stone is scanty. Rasalamand Sebastian (1976) made a thorough investigation on the lime shell resource fromVembanad which contributed the bulk for commercial utilisation. According to onereport the sub-soil shell deposit of Vembanad are concentrated in beds varying inthickness from 22 cm 50 cm under a silt burden of 20 cm 60 cm. Seven zones ofabundance of shell deposits were recorded by Associated Cement Company incollaboration with the Department of Research of the erstwhile University of Travancorein 1941 and estimated the area of deposit as 108 million m2with a deposit of 4.5 milliont. M/s. Rudinger, Engineer, Bombay estimated the deposit as 3 million t in 1946 andBijawat and Sastry 1957, Macedo, 1958 estimated the deposit as 2 million tons. KunjuPanicker (1957) stressed the need for a survey to be conducted by the FisheriesDepartment. Geology and industrial experts in consultation with the officials of the limeshell co-operative societies to formulate necessary steps for increased and sustainedfishing of lime shells. Lime shell was considered as a natural resource, though live clamswere being exploited from these areas from time immemorial. Industries using lime shellas raw material were given license for exploiting lime shell using mechanical means.

The clams which enjoy an ideal habitat for growth were subject to the south-westand North eastern monsoon yearly and during these periods flood water from Pamba,Achancoil, Manimala, Meenachal and Moovattupuzha carry large quantities of mud andsilt and freshwater with the lake leading to mortality of large quantity of live clams. Thisannual process have contributed over the centuries might have lead to the accretion of awealth of live-shell deposits in the lake.

The lime shell that contribute to the fishery are white shells and blackshells inthe lake. The so called white shells are subsoil fossil deposits of clam known to extend atplaces upto 2.1 m below the surface soil (Loganathan, 1962). In dredging conducted bythe Travancore Cement Factory shows that deposit is found more than double the abovedepth in several places. Such deposits of shell are found also in the purambok lands alsoin the adjoining areas of the lake.Clam Fishery

25


26/47

The annual average production of Villorita cyprinoidsfrom the lake from 1979 to1984 was 21,490.5 t. The subfossil deposit `white clam being ranged from 41,445 to69306t. The Travancore Cements Ltd., Nattakam, Kottayam, TravancoreElectrochemicals, Chingavanam and the Pallathura Bricks and Tiles Ltd., Sherthallaiwere the three factories utilising lime shell at annual average of 98,000 t. First two

factories collect their shell by dredging from the lake. The first factory consumes50,000 t, second 43,000 t and third 4000 t earlier.Fishery

Achari (1988) stated that the clam resources, both live and subsoil deposits arecategorized under minor minerals and as such the licensing and general policy decisionare being formulated by the Department of Mining and Geology of the respective states.Eventhough the meat of the clam is consumed in certain areas of the country, the clamsare collected as raw material for lime and related industries like cement, calcium carbide,textile, paper etc. It is observed that the production of clam shells from Vembanad lake isabout 2,00,000 t annually comprising of shell deposits and live clams. The clamresources of the country need immediate attention for conservation and replenishment to

meet future demands of the lime based industry. In addition generation of employmentopportunities and programmes for the fishermen also can be tackled by establishing co-operatives for these fishermen and activities can be coordinated by an autonomousadministration agency.

Fishing for live clam in the lakesSimple method of hand picking of clam is done often and sometimes they use

toothed iron rack (Varandi or Kolli) and collect shells from the bottom. Hand pickingis done by women and children in shallow water where depth is m in low tide. 1-2baskets are collected duty women do fishing in slightly deeper water of less than 1-5 mwithout diving, using the feet they accumulate the live clams and collect them in baskets(bamboo or aluminium unda) 3-4 baskets 6-8 kg per women are collected. Fishing inmostly done by operating hand dredges (Varandi or Kolli) operated from canoes bymen (usually two) at deeper water. The hand dredge consists of a rack with metal toothand a conical met of mesh size 15 mm and a metal or wooden handle of 4.5 metersfixed to the metal frame of the rack. The tail end of the conical bag net is attached with along nylone rope. When one fisherman push the rack into the clam bed bottom the otherperson in the canoe will drag the dredger through the bottom by pulling the foot ropesattached to the conical frame of the dredge, clams are thus collected in the net bag. Afterwashing the mud & sand the clam is deposited to the canoe. Usually a canoe can bring150-300 kg of clam, depending on the area of operation.White shell exploitation

Rasalam and Sebastian (1976) rightly reported that lime shell was continued to belooked upon as a mere mineral resource and completely overlooking the biological sourceand the renewable nature of the this resource. They have also indicated that in thebeginning mechanized dredging resulted in the exploitation of the shallower and earlyexploited beds, that too from the upper layers resulting in the serious disturbance of theenvironment of the living clams. Added to that indiscriminate fishing of undersized liveclam also became rampant. Under these conditions only a complete evaluation of the

26


27/47

lime shell fishery was done by the Department of Fisheries with financial support fromthe Indian Council of Agricultural Research, New Delhi in 1976.

For the white shell exploitation a canoe, a bench or vechukettu, a basket and aspade is used. On reaching the site in the canoe, the wooden platform with pointed

wooden legs or bench is faced down into the mud. The fishermen stands on the bench,heaps up the shells with help of the spade and collect the shells periodically in the basketand empty them in the canoe. This type of fishing is still practiced but now the mainmethod is hand dredge or drag net with a long handle attached to the semicircular metalframe to which the net is attached. Mouth portion of the net has got the toothedhorizontal portion. The net is operated by fishermen standing in a canoe forces the metalframe down into the bottom and another man in the canoe dragging the net along puttingthe rope attached to the cod end of the net. Mechanised suction type dredging are usedby Travancore Cement Ltd.

Company has got mining lease for 200 hectares in Vaduthala Kayal southof Kumarakom from 1985 onwards. So far they have not exploited this. During 2000 an

attempt was made to dredge this area for white shell and due to opposition from localfishermen and local people, this was abandoned. Another attempt made recently inOctober, 2005 have also lead to opposition from local people, forcing the company toabandon this venture. Company made an indication that around 5 lakh tonnes of shelldeposit is available in this area.

In the Report of the Technical Committee on the Problems arising out ofthe dredging in the Vembanad lake (2002) it is stated that it is difficult to contend fromthe available data that there has been significant decline in this major living resource onas a result of dredging. They also observed that eventhough dredges are employedprimarily to collect the shells occurring in the deeper layers of the mud which the manualeffort cannot exploit, the removal of the live clams from the surface of the mud isinevitable in the initial process before the suction penetrates the deeper layers. Thedredgers collect the shell-layers mud creating a deep pit extending an area ofapproximately 1000 sq. ft., strain the mud and separate the shell. This process it leavesnot only a depression in the area but also creates mounds all around and causestemporarily turbidity to the water in that particular spot. The average time taken forcomplete settlement is around 20-40 h. This may vary from place to place dependingupon the cutting area and the percentage of silt in the deposit strata. The washed outmud and sand naturally spread over the surrounding and cover the clam beds and cancause mortality to animals and plant population. Young clams cannot come out of thesurface because of their restricted movement. But clams can survive outside a radius of0.5 km from the dredging point. The penetration of light in dredged area ranges from 15-25 cm. The sound of dredger also disperses the animals around dredging point.

As regards the apprehension that breeding of these fishes is hampered because ofthe disturbances escalated by the dredgers, the committee is inclined to discount this onthe basis of scientific fact that except for few commercial species like the pearl spot(Karimeen) many of the other fishes or prawns do not breed in these surroundings(prawns, chanos, mullets, etc.) since many of them are of marine origin and breed in thesea.

27


28/47

It has also been brought to the notice of the Committee by the fishermen thatrecolonisation by clam does not take place in areas where the dredges had once operated.Such phenomenon is not unlikely as dredging and depositing of a different type of sandand mud at the surfaces could greatly alter the conditions, otherwise favourable for thesettlement and growths of clam spats. However, since clam larvae are free swimming, it

will settle in other favourable areas in the lake and form beds, may be in another area,thus not affecting the total production.The committee opined that considering the view that any harm caused to the

fisheries wealth of this southern region of the lake is not wholly due to dredging, whichis only one of the factors, but is the result of other causes of which the protracted closingof Thanneermukkam bund during the crucial periods in the lake ward movement of thefishes is more important.

A study on the effect of dredging in Vembanad made by the Committee observedthat the suspended sediment transport caused by dredging area spreads around 1.5 kmdown the direction of water flow. A detailed Report on the Sedimentation Analysis isgiven in the Annexure I.

Sedimentation characteristics of the dredged material depend on the texture of thecut material. Particles larger than 2 microns settle within 24 h if left undisturbed. Thepenetration of light ranged from 15-25 cm during bright sunny days around dredged areawhere as in undisturbed areas it was 50-80 cm which is the normally observed values in adynamic estuary. It was also observed that the clam can survive beyond 0.5 km radiusfrom the dredged area. The washed out sand and silt during the dredging make the lakebottom uneven and the superficial layer formed by the settlement of silt make the layervery smooth covering the settled larvae of clam which may induce stress. However thedredging process may not affect the total production of clam since the larvae are capableof moving to favourable areas for forming the clam bed and where the total are of the labis considered, the silted area due to dredging is negligible.

Table 6. Harvest of live Villorita cyprinoidesfrom Vembanad lake from 1979-2004

Year Average monthlyproduction (t)

Total production(t)

1979 1623.1 19,478.1

1980 1885.5 22,625.5

1981 2031.5 24,378.6

1982 2082.0 24,985.0

1983 1972.6 23,671.4

1984 1150.3 13,804.5*

1985 971 11,652.1*

1986 1628 19,533.8

1987 1716.6 20,599.01988 2292.2 27,507.0

1989 2336.8 28,041.0

1990 2753.7 33,044.7

1991 3204.6 38,455.6

1992 3105.8 37,269.1

1993 2842.7 34,112.3

1994 3006.5 36,078.1

28


29/47

1995 2450.0 29,400.0

1996 2632.5 31,589.9

1997 2661.3 31,935.5

1998 2400.5 28,805.8

1999 3272.7 39,273.0

2000 2619.2 31,430.542001 1490.0 17,879.0

2002 1712.5 20,550.5

2003 2487.3 29,848.51

2004 3502.1 42,026.71* Mass mortality and over exploitation of small sized clams in the clam beds

Table 5. Average chemical composition of shell and meat are as follows:CaO Si02 Al2

03 Mg

0 F0202 Total Ca Co2

% range 52.2 - 0.8 - 0.6 - 0.1 - 0.4 - 93.3 53.7 2.3 1.4 0.3 1.0 95.8

Clam meat Protein Fat Ash Calcium Phosphorous Iron MoisturePercent 14.4 7.8 2.9 1.01 0.48 0.06 72.0------------------------------------------------------------------------------------------------------------

Table 6. Major size groups contributing fishery ofVillorita cyprinoids2003-05 period

2003 14-22 mm in April-June20-30 mm in July-September

18-28 mm in October-December

2004 18-30 mm in January-March

14-24 mm in April-June

14-30 mm in July-September

16-24 mm in October-December

2005 14-22 mm in January-March

14-22 mm in April-June

14-24 mm in July-September

Table 7. Fish landings of Vembanad Lake (Quantity in Tonnes) 2000-2003 periodSl.

No.Fish landings 2000-2001

(t)2001-2002

(t)2002-2003

(t)

1 Prawns 2,758 2,550 2,511

2 Etroplus 2,343 3,305 2,075

3 Murrel 5,522 1,161 1,202

4 Mullets 1,493 1,048 1,286

29


30/47

5 Catfish 1,530 1,309 1,266

6 Jewfish 1,026 808 893

7 Tilapia 2,309 1,902 2,062

8 Labeo fimbriatus 586 887 445

9 Barbus 115 94 95

10 Mrigal 836 806 71611 Crabs 221 170 192

12 Common Carp 792 940 787

13 Catla 728 648 680

14 Gourami - - -

15 Chanos 143 80 162

16 Eels 17 21 14

17 Labeo rohitha 1,645 1,302 1,401

18 Miscellaneous 1,018 737 878

Total 18,992 17,768 16,671

* Landings of Alleppy and Kottayam District pooled.

Source: Inland Fisheries Statistics of Kerala 2004, Department of Fisheries, Kerala

7.1 Studies on effect of dredging done elsewhere

Parulekar et al.,1986 while studying effect of mining activities on clamfisheries and bottom fauna of Goa estuaries observed that besides the overall decrease inthe dissolved oxygen concentration, other obvious reason for decline in the clamresources are due to the immense increase in the quantity of suspended solids andstructural deformation of bottom deposits and blanketing of bottom deposits by miningrejects leading to 70% reduction in clam production, near extinction of resident fauna andthe appearance of low diversity bottom fauna, comprising of tolerant but vagrant species.

Increased water depth and reduced current velocity can act in concert to lower theflushing rate within the dredged waterway which can increase the impact of pollutantsentering the system.

Recovery from the effects of dredging can take a few weeks to a number of years(Taylor and Solomon, 1968, Kaplan et al 1974 Pfitzenmeyer 1975, 1978, Van DeDolah etal 1984). The impact of dredging may be compounded if shallow water habitat isconverted Deeper water, particularly, if submerged aquatic vegetation beds areeliminated. Current policy in Maryland discourages dredging in waters shallow than-3feet (meanlow water).

In summary, the effect of dredging upon the benthic community of tidal

waterways found to be mild and short duration when less than 2% of the bottom aredisturbed. The impact becomes severe and recovery can take more than one year when2% to 7% of bottom is disturbed. Reduction in benthic organisms can be severe andlong lasting when dredging causes a substantial shift in bottom composition from sand toclay and a reduction in current velocity sufficient to impede the return of coursesediments. The loss of shallow water habitat, particularly vegetated areas, also results inan unusually high degree of impact.

30


31/47

8. Livelihood Issues

Livelihood alternatives may be provided for fishermen dependent on the dredged areaand immediate environs, affected by dredging. The major impact is on the livelihood ofpersons and families (project affected persons - PAP) who are depending on the site forlivelihood: fishing, clam harvest etc. The proponents have to identify PAPs and formulate

a plan to rehabilitate them during the impact period. Affected stakeholders to be madebeneficiaries for mussel culture, cage culture, ornamental fish culture, seaweed cultureand freshwater under Kerala government schemes.

9. Conservation of black clam resources of the Vembanad Lake

1. The indiscriminate removal of clam by hand dredge and removal of subfossildeposit by dredger causes damage to living resources. It is suggested thatconservation measures such as, limiting the dredging to few months of theyear, avoid fishing when spawning of the aquatic animals are profuse,repopulating the dredged area by relaying of clam seed and to meaningfully

manage the lake, monitor the environmental changes of dredged area regularlyto assess the damages made during the time of dredging2. Removal of juvenile clam from natural bed in huge quantities for industrial

purpose is a real concern leading to over exploitation and depletion of stock.Being an annual renewal resource, live clam fishery has to be well managedfor increased and balanced production. The shell of the clam can ultimatelybe used for cement production by the company. Chempu, Vaikom, Vechoor,Muhamma, Kumarakom and Komalapuram are the nerve centers of live clamfishery in Vembanad lake. Incidentally in all these centers clam co-operativesocieties have licence for fishing and they have an effective control over theclam pickers. It is suggested that clam conservation programme such as clamrelaying, seed ranching etc can be effectively done through these societies.The research institution like CMFRI, Agricultural University, CUSAT, StateFisheries Department and TCL can jointly implement these programmes infuture years. It is also suggested that CIFT can help the clam pickers indeveloping value added products from clam meat, which is now discarded.

3. To increase the black clam production, new areas in the lake, especiallydeeper areas are to be fished. At present the fishers find it difficult to reachfar off places in small canoes and do the fishing operation. If a mechanizedboat is made available to groups of fishermen, as it is being done byKomalapuram. Clam co-operative society, access to clam bed is possible andproduction rate also increase. It is suggested that all co-operative societiesneed help the fishermen for easy fishing activity by providing mechanisedboat for taking clam pickers in group to clam beds & back.

4. Deep pits made by mechanized dredges tamper the bottom topography.Redeposit of silt and sand from the excavaded subfossil deredged area is to bemade mandatory by the TCL and this has to be regularly monitored aftercompletion of dredging activity. Otherwise the repopulation of this area iseither impossible or will be considerably delayed to 3 to 7 years. It issuggested that the entire black clam shells collected by the fishers in an year

31


32/47

(about 20-30 thousand tonnes) can be utilized by the TCL for cementproduction. This has to be done through the co-operative societies oncompetitive rates. The Company can also explore the possibilities of gettingbivalve shells from Ashtamudi estuary, Kayamkulam kayal, Chettuva,Korapuzha and Chandragiri. An approximate 20,000 tonnes of shells of

Meretrix casta, Paphia malabarica and Villorita are harvested every yearfrom these estuaries. Now the entire shell is transported to neighbouringstates for Calcium carbide, lime and other industrial purposes. This will helpto obtain required quantity of raw material for the Company.

5. For conservation of aquatic life of the Vembanad it is suggested to havei) regulated opening of the Thanneermukkam bund/barrier/spillway for

making the water conducive for breeding of aquatic organisms, especiallyMacrobrachium, shrimps and Etroplus

ii. mesh size regulation of 35 mm for fishing nets for avoidingdestruction of juveniles.

iii. Declaring selected areas in the lake of 100 m2

as clam sanctuaries for

replenishment of the clam stock. This has to be declared as protectedareas (atleast 5-7 areas).iv. Issue of lease right for dredging for subfossil deposits and collection of

clams may be issued jointly by the Department of Fisheries and Dept.of Mining and Geology with a Committee of experts monitoring thewhole activities. The right to control the living organisms in the lakeshould be given to the State Fisheries Department.

6. It is also suggested that value addition of clam meat is an urgent requirement.It is estimated that every year 20-30 thousand tones of live clams are exploitedfrom the take. Ultimately the meat yield is almost 2-3 thousand tons and outof which only 3-4% is utilized for human consumption. Another 3-5% is usedfor duck feed and shrimp feed. The rest is simply discarded and wasted. Ifthis meat is properly processed and made into value added products, this willgenerate addition income and employment for fisherfok. In this context,appropriate depuration techniques are to applied before extraction of meat forconsumption. Govt. has to establish common depuration facilities for thispurpose, as this is done for shrimp peeling.

7. It is also suggested to initiative integrated farming of clam and fish and clamand paddy by which production can be increased many fold for both thegroups.

8. As pollution of the water body occurs through industrial discharges cases ofillegal dredging and large-scale pesticide application, the polluter-payprinciple has to be implemented, where the agencies which cause damage tothe aquatic life may have to compensate the loss to the affected people, aswell the expenditure for restoration of the original condition.

32


33/47

10. Conclusions

1.Shell deposits in the proposed area lies roughly between 3m to 8m from the estuarinebottom. Overburden is about 3m to 6 m. Deep dredging operations required for miningthe shell deposits will have impact on benthic community in the area of dredging, due toentrainment and physical removal. The impact of dredging is observed to be localized, as

only a few cents will be covered during dredging operations per day and about 20 ha peryear. The recovery of the benthic community is expected to take place within 1.5 to 3years, after completion of the dredging activity.2. The impact of increased suspended solids, turbidity levels and nutrients is seen innear-field (< 1 km) from the dredging location which do not constitute an hazard.3. As regards heavy metals, impact of contaminated sediments was observed to beminimal, as no major industrial establishments have been historically operating in thedredging area. This is substantiated by the present observations in the Vaikam Kayal.4. Settlement of suspended sediments is expected to cause blanketing or smothering ofbenthic animals and plants may cause stress, reduced rates of growth or reproduction andin the worse cases the effects may be fatal. However, the effect is expected to be near-

field, as in estuaries background turbidity levels are high and estuarine animals aregenerally adapted to such conditions.5.Changes in geomorphology are expected to be localized in the dredging area and itsimmediate vicinity and changes in hydrodynamic regime is expected to be minimal.6.Fishing by stake net, Chinese dip nets and gill nets in the dredged area and immediateenvirons is expected to be affected temporarily by the dredging operations, due to themovement of fishes away from the area of disturbance.

11. Recommendations

1.Ecological and environmental impacts during suction dredging in the dredged areaand in the immediate environs are inevitable. However, such disturbances are foundto be minimal and localized in the dredged area and in the immediate environsspanning about 1 km up and down stream and reestablishment of the fauna isexpected within a period of 1-3 years. In view of this, dredging could be permittedwith a proper environment management plan.

2.The following good environmental management practices are recommended whilepasturing on new areas: Formulation of an environment management plan andimplementation of an environment monitoring plan. Base line status of the diversityand population of biota of economic significance in the proposed dredging site is tobe determined. Monitoring of the area and immediate environs spanning about 1km upstream and downstream may be arranged under the supervision of competentpersons, during the pre-dredging, dredging and post-dredging periods if dredgingoperations continue for months.

3.Environmental windows may be provided, during the intensive breeding periods ofestuarine organisms.

33


34/47

4.Possibility of providing silt curtains around the dredged area may be explored, if theprevailing current pattern is favorable.

5. Livelihood alternatives and welfare measures may be provided for fishermendependent on the dredged area and immediate environs, affected by dredging. The

major impact is on the livelihood of persons and families (project affected persons -PAP) who are depending on the site for livelihood: fishing, clam harvest etc. Theproponents have to identify PAPs and formulate a plan to rehabilitate them during theimpact period. Affected stakeholders to be made beneficiaries for mussel culture,cage culture, ornamental fish culture, sea weed culture and freshwater andbrackishwater fish culture; value addition to fish products, under the extensive aquaparks envisaged under schemes of the Fisheries Department of the Government ofKerala.

12. References (partial listing)

Anon, 1997. Vickers Shipbuilding & Engineering Ltd - GEC Marine (VSEL)-Reactivation ofshipbuilding facilities at the VSEL Shipyard: Impacts to Nature Conservation Interests.ABP Research Report No. R707.

Balchand, A.N., and Rasheed, K. 2000. Assessment of short term environmental impacts ondredging in a tropical estuary, Terr et Aqua,79 pp. 16-26

Bray, R.N., Bates, A.D., and Land, J.M. 1997. Dredging - A Handbook for Engineers. SecondEdition. Arnold, London.

Brehmer, M.L. 1965. Turbidity and siltation as forms of pollution. J. Soil and WaterConservation, 20, pp.132-133.

Hall, S.J., Basford, D.J., and Robertson, M.R. 1991. The impact of hydraulic dredging for razorclamsEnsissp. on an infaunal community. Neth. J. Sea Res., 27, (1), pp. 6.

Harvey, B.C. 1986. Effects of suction dredging on fish and invertebrates in two Californiastreams. North American J. Fisheries Management 6: pp. 401-409.IADC/CEDA, 1997. Environmental Aspects of Dredging, 2a - Conventions, Codes and

Conditions: Marine Disposal. International Association of Dredging Companies/CentralDredging Association.

Indo-Dutch Mission, 1989. Kuttanad Waterbalance Study Draft Final Report forGovernment of Kerala.

International Council for the Exploration of the Seas 1992. Effects of extraction of marinesediments on fisheries, ICES Report No. 182.

Iyer N. Janardhana, 1978. Report on the investigation of lime shell deposits inVembanad Lake , Alleppey and Kottayam Districts. Geologist on Deputaion, TCL,Kottayam.

Kurup B.M., Sebastian M.J., Sankaran T.M., Rabindranath P. 1990. Exploited fisheryresources of Vembanad Lake Clam fisheries. Mahasagar,23(2), pp. 127-137.

Lakxmilatha P. and Appukuttan, K.K. 2002. A review of the black clam (Villoritasuprenoides) fishery of Vembanad Lake. Indian J. Fisheries, 49(1), pp. 85-91.

Murray, L.A. 1994. Progress in England and Wales on the development of beneficial uses ofdredged material - Dredging '94, Proceedings of Second Int. Conf. Lake Buena Vista,Florida.

34


35/47

Nedwell, S., and Elliott, M. 1998. Intertidal mudflats and sandbanks and subtidal mobilesandbanks. Draft Report, Institute of Estuarine and Coastal Studies, University of Hull.

Newell, R.C., Seiderer, L.J. and Hitchcock, D.R. 1998. The impact of dredging works in coastalwaters: A review of the sensitivity to disturbance and subsequent recovery of biologicalresources on the sea bed. Oceanography and Marine Biology: an Annual Review 1998,36,127-178.

North, P.A. 1993, A Review of the Regulations and Literature Regarding the EnvironmentalImpacts of Suction Gold Dredges. U.S. Environmental Protection Agency, Region 10,Alaska Operations Office.

Parr, W., Clarke, S.J., Van Dijk, P. and Morgan, N. 1998. Turbidity in English and Welsh tidalwaters. WRc Report No. CO 4301. Report for English Nature. WRc Medmenham, Bucks.

Peddicord, R.K., and McFarland, V.A. 1978. Effects of suspended dredged material on aquaticanimals. Technical Report D-78-29. U.S. Army Engineer Waterways Experiment Station,Vicksburg, MS,USA.

Pennekamp, J.G.S., and Quaak, M.P. 1990. Impact on the environment of turbidity caused bydredging. Terra et Aqua, 42, pp.10-20.

Peterson, C.G. 1996. Response of benthic algal communities to natural physical disturbance.Pages 375-402 in (R.J. Stevenson, M.L. Bothwell, and R.L. Lowe, editors) Algal Ecology,

Academic Press, San Diego.Pfeiffer Jnr P. E. Et al (1992). Beneficial Uses of Dredged Material : A Practical Guide. Report of

Working Group 19. Permanent International Association of Navigation Congress (PIANC),Brussels. In ICE (1995). Design and Practice Guides : Dredging. Thomas Telford, London.

PIANC 1996. Handling and treatment of contaminated dredged material from ports and inlandwaterways "CDM". Volume 1. Report of Working Group No. 17 of the PermanentTechnical Committee 1. Supplement to Bulletin No. 89. PIANC, Brussels.

Rasalam E.J., and Sebastian, M.J. 1976. The limeshell fisheries of the Vembanad Lake,Kerala. J. Marine Biological Association of India, 18(2), pp. 323-355.

Report of the Technical Experts Committee on operation of Thanneermukku Barrage,2002. Submitted to the Government of Kerala.

Resh, V. H., Brown A.V., Covich A.P., Gurta M.E., Li H.W., Minshall G.W., Reice S.R.,Sheldon A.L., Wallace J.B., and Wissmar R.C.. 1988. The role of disturbance in streamecology. J. N. Am. Benthol. Soc. 7: 433-455.

Stern, E.M., and Stickle, W.B. 1978. Effects of turbidity and suspended material in aquaticenvironments: Literature review. Technical Report D-78-21. U.S. Army EngineerWaterways Experiment Station, Vicksburg, USA.

Stickney, R.R., and Permutter, D. 1975. Impact of intrac

report of the committee of experts tcl

Documents