Grain Size Variations and ItsGrain Size Variations and Its Environmental Implications –

A Case Study

DR PURANDARA BEKALDR PURANDARA BEKAL

SCIENTIST , NATIONAL INSTITUTE OF HYDROLOGY

HARD ROCK REGIONAL CENTER

HANUMAN NAGAR

BELGAUM, KARNATAKA

Particle Transport and Movement

T f di i l• Transport of sediment involves 2 steps:

Erosion and entrainment of– Erosion and entrainment of sediment from the bed

– Downcurrent movement• Followed by:

– Settle out and deposition

Sedimentary Environments

S di t i t• Sedimentary environment– physical, biological and chemical

setting with a set of processes g pthat operate there

– It is a geomorphic unit in which deposition takes placedeposition takes place

– Parameters operating within environment lead to deposit of particular size and geometryparticular size and geometry

– Processes operate at rate and intensity that generate h t i ti t t dcharacteristic textures and

structures

Facies ConceptFacies Concept

D iti l i t• Depositional environments generate sedimentary facies

• Interpret conditions of theInterpret conditions of the environment from the deposits and their organization within th i tthe environments

• Hopefully there are unique characteristics preserved in thecharacteristics preserved in the deposit to provide clues to formational conditions

Sediment DepositionSediment Deposition

Utili l f fl id• Utilize lessons from fluid dynamics to understand when sediment deposited and in pwhat type of packages

• Utilize sediment depositional tt t h lpatterns to help answer

questions – When did sediment moveWhen did sediment move– Flow conditions causing

movementR di diti f d iti– Reading conditions of deposition (since that is actually what is preserved)

Study Area

Water way through estuary to Periyar river

Environmental Issues of the Lake System

Yellowstone River

The present study is an attempt to understand the distribution pattern of sediments inpattern of sediments in

RIVERS : (i) Pamba (including Manimala and Achankovil)(ii) Muvattupuzha

(iii) Minachil( )

LAKE: Vembanad Lake/Estuary: Pre-monsoon & Post-monsoon

Adjoining Shelf: Azhikode in the North to Purakkad in the south

: Monthly Variation of sediment concentration

B h C t l K l C tBeach : Central Kerala Coast

Sampling Locations

Formation of MUD BANKSFormation of MUD BANKS

Mud banks are natural smooth water anchorages formed at particular locations along the Kerala coast during the southwest monsoon season. It extends outwards up to a distance of 3-4 km from the shore. These are semicircular in shape, with their northern and southern edges defined by two crescentic lines of breakers

i t d t th Th f ti f drunning outwards to the sea. The formation of mud banks play a major role in moulding the socio-economic set up of the coastal people by providing a stable fishing ground during the monsoon season. Mud banks affect the coastal processes by damping the waves in thethe coastal processes by damping the waves in the following ways.

(i) traps the littoral material transported from the updrift side thereby preventing its downcoast movementside thereby preventing its downcoast movement

(ii) causes refraction of waves on its sides,

(iii) protects the beach in particular from erosion.( ) p p

Locations of formation of Mud banks of Central Kerala Coast

MethodologMethodology

Sediment samples were collected using van Veen grab from the selected rivers, lake, beach and adjoining shelf including Mud banks

Sediment samples were subjected to combined sieving and pipette analysis given by Krumbein and Pettijohn(1938). From the grain size analyses the various grain size parameters were determined by employing the formulae of Folk and Ward (1957).

Th CM di d t d t d th l ti hiThe CM diagram were drawn to understand the relationship between the transportation and sedimentation pattern of the lake sediments.

X RAY DIFFRACTOMETRYX-RAY DIFFRACTOMETRY

CHEMICAL ANALYSIS – X-RAY FLUORASCENSEORGANIC MATTER – EL-Wakeel & Riley

RIVER SEDIMENT CHARCTERISTICS

Based on River distance versus sediment size graphs it is noted that

sediments showed a decrease in size with distance. This i di t th b i d i ti Th t findicates the abrasion and progressive sorting. The arte of abrasion depended on the resistance of minerals. Progressive sorting also indicates progressive decline in the competency of the river.

However, in the downstream, wide fluctuations are observed due to mixing of sediments from different environments.

Sudden change in grain size in major rivers at some of the locations Field observations indicated that this change is duelocations. Field observations indicated that this change is due to removal of sand by local people ?

Sediments in the upstream and downstream are poorly sorted as compared to the intermediate stretch. High energy

diti d i it f th ibl fcondition and proximity of the source area are responsible for the coarseness of the sediments.

CM pattern of the river sediment denoted a transportation mechanism as graded and bottom suspensiong p

Grain Size Analysis Vembanad LakeTextural analysis of the bed sediments showed thatTextural analysis of the bed sediments showed that

phi mean size of the lake sediments vary between 2.5 and 10.55 during the study period.

The average value is 8.21 in the pre-monsoon &

6.4 during the post-monsoon season.

Majority of the sediments are very poorly sorted both during the pre-monsoon and post-monsoon seasons.

The standard deviation varies between 1.39 phi andThe standard deviation varies between 1.39 phi and 4.54 phi

The value of skewness varies between -0.38 to 0.48.

From the above results it is found that the sedimentsFrom the above results it is found that the sediments are coarser during post-monsoon and finer during pre-monsoon season.

Zone I : Starts from the north of the estuarine mouth to the northernmost part of the Vembanad lake where river Periyarnorthernmost part of the Vembanad lake where river Periyar, joins the sea. In this zone, the phi mean size of the sediments varies between 6.8 and 9.8 with an average phi mean size of 8.77. Here, both during pre-monsoon and post-monsoon season there is no marked variation in grain-size parametersseason, there is no marked variation in grain size parameters. This is attributed to the continuous influx of sediments from the adjoining sea and from the river Periyar which supplies large quantities of sediments from the northern part of the estuary.y

Zone II: This extends from the estuarine mouth to the Perumbalam area, where the phi mean size of the sediments varies between 6.15 and 10.55 phi. This is mainly a clayey zone with clayey silts and clayey silts. The existence of very fine sediments may be due to the incursion of sediments from the sea during tidal actions and estuarine circulation process. Further, the dredging operations taking place in the area also give rise to the accumulation of finer sediments.

Zone III: The central part of the lake extending from Perumbalam to Thanneermukkam. This is a region of

di t h th hi i icoarser sediments where the phi mean size varies between 2 and 9.9, however, the percentage of finer particles are significantly less in comparison to zone I and zone II. The average phi mean size is 6.

Zone IV: Southern part of the lake is chiefly composed of finer sediments of silty clays/clayey silts p y y y yextending from the southern part of the Thanneermukkam in the north to Alleppey in the south region with a grain-size 5.1- 7.8 phi. The average phi mean size of the sediment is 6 8 Theaverage phi mean size of the sediment is 6.8. The accumulation of fine sediments in this region is mainly due to the deposition of sediments that are transported and deposited by major rivers like M tt h Mi hil d P bMuvattupuzha, Minachil and Pamba.

Mud bank Sediments are characterized by very fine sizeDominated by Silty clays followed clays

80

100

y %

20

40

60

nd-S

ilt-C

lay

Sand %

Silt %

Clay %

0

20

May June July Aug Sept Dec

San

Months

Monthly Variation of Sand-Silt-clay per cent during Southwest Monsoon

GRAIN SIZE IMPLICATIONS

The Grain-size variation studied in different sedimentary environment indicated an interplay of

d h d li t l i t tisource and hydraulic control in concentrating sediments of particular grain size.

This is evident from the variation of sediment size while moving from one environment to anotherwhile moving from one environment to another. Finest sediments are found in the nearshore area during the monsoon season. Further, moving offshore from the mud bank area showed a an i i i iincrease in grain-size.

Clay Mineral Investigations

Clay mineral investigations showed that there is a marked variation in the content of Kaolinite while moving from terrestrial environment to Marine condition.

It is also noted that the percentage of Montmorillonite showedIt is also noted that the percentage of Montmorillonite showed steady increase from river to Mud bank which is attributed to the increase in suspended sediment concentration

The increase of illite towards marine environment is due to the l f illi f ff h d li hsupply of illite from offshore as reported earlier researchers.

Gibbsite showed steady decline towards marine environment indicating that the only source of gibbsite is terrestrial originindicating that the only source of gibbsite is terrestrial origin

Clay Mineral Distribution in the different environment

CHEMICAL ANALYSIS

It is important to note that the chemical concentration in sediments depends on the grain size characteristics.In this connection organic matter was estimated using El-wakeel and Riley method.

It is found that the organic matter in the rivers sediments are quite negligible in the study area

The Vembanad lake showed very high concentration fromThe Vembanad lake showed very high concentration from 3% to 11% Similar results were also obtained for mud bank sediments. It is found that coarser sediments exhibited lower percentage of organic matter and finer sediments higher concentrationconcentration.

Copper (Cu)

200

Bed SuspendedWater Cobalt (ppm)

BedSuspendedWater

100

150

200

entra

tion

ppm

15202530

tion

ppm

0

50

May June July Aug Sept DecMonths

Con

ce

05

1015

Con

cent

raMay June July Aug Sept Dec

Nickel (Ni)BedS d d

Zinc (Zn)BedSuspendedWater

8090

100

SuspendedWater

708090

pm

3040506070

30405060

ncen

trat

ion

pp

01020

M ay June Jul y Aug Sept Decr

01020

May June July Aug Sept Dec

Co

Socio-economic importance of coastal ecosystem

A socio-economic survey was conducted by contacting the farmers in the region and they expressed the view that the formation of mud banks is a boon to them because, the waters in the region is enriched with nutrients and therefore, fishes come closer to the coast during this particular season. Since, offshore movement is restricted during the monsoon season, the availability of fishes in the nearshore region h l h f h h i li lih d H h lhelps them to fetch their livelihood. However, they also opined that, in the recent years the Vembanad lake is facing threat of pollution due to various industries located in and around the lake. The pollutants entering the lake will finally reach to the open sea and may harm the production fishesreach to the open sea and may harm the production fishes due to their pollutant. Therefore, it is essential to take control measures while discharging the industrial wastes to lake.

Recommendations

The present study provides information on sediment movement pattern in a coastal environment. The study indicates that as the sediment moves away from the source area, it becomes finer and finer. The study also highlighted th i t f th f ti d b k f hi h ththe importance of the formation mud banks for which the major source of sediments are through the river systems, therefore, it is essential to maintain the continuous flow to the lake system. It is also stressed that industrial discharge should be properly treated before discharging into theshould be properly treated before discharging into the Vembanad lake.

At the outset, the main objective of this study is to highlight the conventional methods by which you can develop a datathe conventional methods by which you can develop a data monitoring system in various parts of the country as well as of the ecologically sensitive lake systems. The methods presented here are quite old techniques, however, these techniques can be used by any college or school leveltechniques can be used by any college or school level student to develop a data bank. Such data will help the engineers and scientists to go for planning and management of the lake environment.