Assessing Sediment Pollution off Deltaic Region using Sediment Budget as a

Tool - A simple geospatial approach using Satellite Data

Pravin D. Kuntea,*, Kotha Mahenderb

aNational Institute of Oceanography, Council of Scientific & Industrial Research

Dona Paula, Goa – 403 004, India

bDepartment of Earth Science, Goa University, Goa, 403004, India

Abstract

There is increasing acceptance that suspended sediments represents an important diffuse

source pollutant in coastal waters, due to their role in governing the transport and fate of many

substances viz., nutrients, heavy metals, pesticides and other organic contaminants and because

of their impacts on benthic plants and animals. Sediment pollution arresting strategies therefore

frequently need to include provision for the control of mobilization and delivery of excess

sediments. The sediment budget concept provides appropriate framework for managing and

controlling of diffuse source sediment pollution by identifying the key sources, intermediate

stores and the likely sinks and help to assess impact of upstream mitigation strategies on

downstream suspended sediment and associated contaminant fluxes. Geospatial technologies and

free availability of satellite data provide solutions with simple and better understanding of such

issues with greater environmental and economic impacts. The understanding of the

sedimentological functioning of these units as sinks and sources of terrestrial matter helped in

understanding the propagation of pollutants in the marine system. The present paper discusses

the utility of the sediment budget for assessing sediment pollution explaining methodology and

results specific to the deltaic region from India. Finally, it suggests the concept the sediment

budget as a practical framework to support the design and implementation of sediment control

programmes aimed at reducing pollution by fine sediment for understanding the propagation and

thereby arresting pollutants in the marine system.  Key word: Sediment budget, East coast of India, Remote sensing, Sediment pollution, River delta      

Introduction

Rivers are the major carriers of large amounts land-derived freshwater, sediment, and

natural elements to the global ocean. Collectively, the world's rivers annually discharge about

35,000 km3 of freshwater and 20-22 x 109 tons of solid and dissolved sediment to the ocean

(Milliman and Meade, 1983; Milliman and Syvitski, 1992). As a result the large rivers play an

important role in controlling the physical and biogeochemical features of estuaries and ocean

margins (McKee et al., 2004; Meybeck et al., 2006; Bianchi and Allison, 2009). New estimates

based on historical gauging data from thousands of rivers (Milliman and Farnsworth, 2010) show

that this number could be closer to 19 x 109 tons of suspended sediments per year. Of this total

sediment flux, ~70% or ~13 x 109 tons is believed to discharge from the eastern and southern

Asian Pacific and oceanic margins alone (Milliman and Meade, 1983; Milliman and Syvitski,

1992; Ludwig et al., 1996; Milliman, 1995). In eastern and southern Asia, about one-third to one-

half of river-derived sediments is trapped in the river's low reaches and contributes to extensive

floodplain and delta plain development. Since the flux and fate of river-derived material to the

oceans play a key role in global environmental change (Bianchi and Allison, 2009), with up to

80% of global organic carbon being preserved in such marine deltaic deposits (Berner, 1982).

The Himalayas are among the youngest and most active mountain ranges on the surface

of the Earth, with high relief, steep gradients, frequent tectonic activity, intensive Monsoon

rainfall, and highly erodable rocks (Clift et al., 2008). Coupled with the seasonal melting of its

~15,000 glaciers and abundant monsoonal rainfall, the Himalaya and surrounding plateaus give

rise to seven of the world's largest river systems and account for ~ 30% of the global fluvial

sediment flux to the sea. The Ganges-Brahmaputra (G-B) river annually discharges ~1200 x 106

tons of fluvial sediments to its delta plain and the Bay of Bengal. Different from the South and

Southeast Asian river systems, the Bengal shelf is incised by a major canyon, the Swatch of No

Ground, which directly connects the Ganges- Brahmaputra Rivers to the Bengal Fan. It is

believed that this canyon behaves like a conduit in transporting a large portion of the G-B

sediment load to the deep ocean (Hubscher and Spiess, 2005; Kuehl et al., 1997; Kottke et al.,

2003).

The peninsular part of the Indian subcontinent is traversed by a number of rivers most of

which flow from west to east and in the process build large deltas at their mouths along the east

coast of India bordering the Bay of Bengal (Fig.1). These monsoon driven river systems with the

sediments embedded in their deltas are considered as excellent repositories of palaeo-monsoon

records. Of the many deltas along the 2,300 km long east coast of India, the Mahanadi, Godavari,

Krishna and Cauvery are the major ones. The Godavari and Krishna deltas, these twin deltas

including the inter-delta plain cover about 12,000km2. The Mahanadi delta, the northernmost of

the four major deltas is, in fact, a composite delta of the Mahanadi and two more small but

independent rivers, namely Brahmani and Baitarani which interlace together in their lower

reaches and build their delta at the northern end of the Mahanadi delta. The present Mahanadi

delta with its apex at Cuttack has two major distributaries, namely Mahanadi and Devi apart

from several other minor ones. Several abandoned distributary channels and beach ridges have

been recognized from the Mahanadi delta plain which spreads over about 9,500 km2 area

(Sambasiva Rao et al, 1978). Cauvery river delta situated in southernmost, is one of the four

major deltas along east coast of India. With its apex at about 30 km inland west of Tanjavur, the

Cauvery river flows eastward along its 5 distributaries. The present shorelines of the east coast

deltas exhibit more or less similar landforms such as sand spits, barrier islands, lagoons,

mangrove swamps and tidal mud/sand flats. Apparently, progradation of these deltas is mainly

by the growth of elongated spits and barriers and infilling of the lagoons that are enclosed by

these shore parallel linear sand bodies. Mangroves colonize the emerging lagoon floors and in

turn promote further deposition.

Sediment is a natural component of several aquatic systems derived from physical,

chemical and biological components of watersheds. It is considered as a form of pollution and

harmful when there is excess. Surplus sediment damages environments by smothering benthic

(bottom-dwelling) plants and animals. Suspended sediment clouds the water, prevent the sun-

light penetrating to reach the leaves and stems of underwater grasses, or submerged aquatic

vegetation and also triggers the morphology changes of an area. Sediment carries excess

nutrients, and accumulations of sediment can clog waterways and ports. The presence of high

concentrations of toxic materials in sediment contaminates waterways. A pollutant may be

defined as any substance that reduces the water quality and it may be dissolved in the water, be

attached to particles, exist as particles, float, or be mainly in benthic sediments or mud (James,

2002).

A significant but largely unknown portion of the total contaminant are eventually brought

to the river surface waters and ground water, finally reach the estuarine region and affects the

coastal waters, often beyond the limits of territorial waters. On entering coastal waters, sediment

flux (and associated contaminant) is governed by coastal processes like alongshore sediment

transport, onshore-offshore transport etc. The physical conditions, which include currents, tides,

waves, turbulence, light, temperature, salinity, bed materials and suspended particles, determine

the transport and dispersion of all suspended and dissolved material in the sea, along with

contaminants, nutrients, and pollutants (James, 2002). The pollutants like polycyclic aromatic

hydrocarbons (PAHs), and mercury readily attach to sediment particles in water. Pollutants may

suspend in a body of water and eventually settle to the bottom with the particles or be taken up

by marine organisms, which pass the contaminants into the marine food chain.

Lead pollution is reported from the shelf and slope regions of the East China Sea (Huh

and Chen, 1999), deep north-east Atlantic sediments (Veron et al., 1987), fossil carbon recovered

in coastal sediments (Baxter, 1980); mercury concentrations observed in marine sediments

collected off Southern California (Young et al., 1973), and Chernobyl nuclide is recorded from a

North Sea sediment trap (Kempe and Nies, 1987). The enrichment of anthropogenic inputs of Pb,

Zn, and Cu in the surface sediments of the Godavari estuary is observed especially in the western

shallow region (Krupadam et al., 2003, 2007). On the basis of the chemometric approach for the

water quality studies, it was found that some locations of Godavari R. at Rajahmundry were

under high influence of municipal contamination and industrial effluents, whilst other areas are

under the influence of agriculture (Krishna et al., 2009). Sediment and suspended particulate

matter (SPM) transport study is important not just for its own sake but because of associated

pollutants may exist in a particulate phase or adhere to or be adsorbed on to particles and as

particulates are an important part of all ecological environments. For example, nutrients and

detritus can exist as particles, and SPM in turbid waters reduces light levels (James, 2002).

The important role of fine sediment in the transfer and fate of nutrients and contaminants

through aquatic systems (e.g. Owens et al., 2005; Carter et al., 2006; Horowitz et al., 2007), and

in the degradation of aquatic habitats, including fish spawning gravels (e.g. Newcombe and

Jensen, 1996; Acornley and Sear, 1999; Suttle et al., 2004), is widely known and it has

emphasized its wider environmental and ecological significance as a pollutant. In order to reduce

the associated problems, effective sediment control strategies are required in catchment

management plans. The precise link between upstream erosion and sediment mobilization and

downstream sediment yield and contaminant transfer involves many uncertainties, due to

sediment retention and both short- and longer-term storage at intermediate locations, such as

delta head, the River channels, its floodplains etc. The proportion of the sediment mobilized

within a catchment that is intercepted and stored during transfer or delivery through the

catchment will frequently exceed the proportion exported. Better management point of view, it is

therefore essential to consider the sediment system in its entirety, instead focusing only on the

downstream fluxes. The sediment budget concept provides an effective basis for representing the

key components of the sediment delivery system within a catchment and for assembling the

necessary data to elucidate, understand and predict catchment sediment delivery (Reid and

Dunne, 1996; Owens, 2005; Rommens et al., 2006).

Sediment budget concept

A sediment budget is a volumetric accounting of the material eroded and deposited in a

given stretch of coast (Stapor, 1973). It is based on quantification of sediment transport, erosion,

and deposition for a given coastal segment. The sediment usually discussed is sand, and the

controlling processes are either alongshore drift or those caused by humans. Any process that in-

creases the quantity of sediments available downdrift in a given coastal segment is a source,

whereas any process that decreases the quantity of sediments available downdrift is a sink. The

coastal sector, for which Sediment budget is to be calculated, would have shore-parallel

boundaries landward of the line of expected erosion and at or beyond the seaward limit of

significant transport (CERC, 1977). It is also applicable at the catchment scale, which is now

widely adopted as the most appropriate spatial unit for characterizing and managing diffuse

source sediment problems. Based on amass balance of sources, sinks and outputs, the sediment

budget of a catchment provides an effective means to understand the interaction and linkages

between sediment mobilization, transport, storage and yield (Slaymaker, 2003). The utility of the

concept in relation to catchment management lies in the identification of the key sources, stores

and transfer pathways. The sediment delivery ratio, which expresses the ratio of the sediment

output or sediment yield from the catchment to the total sediment mobilization within the

catchment, provides a valuable measure of the importance of storage and thus of the overall

catchment response.

The primary source of the sediments deposited on the beaches is the weathering of land;

the sediments are then transported through rivers to the ocean. Rivers are the major source for

the littoral drift and the annual discharge of sediments to sea along the Indian coast is about 1.2 ×

1012 kg which accounts roughly 10 per cent of the global sediment flux to the world ocean

(Subramanian, 1993). There are 14 major rivers, 44 medium rivers and more than 200 minor

rivers along the Indian coast, which are acting as predominant sources for the littoral drift. The

average annual runoff from the major, medium and minor rivers of India is 1406 × 109 m3, 112 ×

109 m3 and 127 × 109 m3, respectively (Chandramohan et al., 2001).

Next to rivers, the headlands and beach erosion contribute significantly as sources along

the Indian coast. The quantities of materials contributed by headland erosion and aeolian

transport are both less than 2% of river transport. In addition to this, direct runoff and rainfall

contribute to the loss of sediments as rainwash from sub-aerial portion of the beach. Another

main source of sand for a particular region can be of an eroding up coast cliff and/or beach.

Beaches supply sand when the wave and longshore current transport capacity at a point exceeds

the supply of sand from updrift sources to the point. Beach erosion occurs at an increased rate

during storms. The contribution of shelf erosion to suspended sediments in the ocean is unknown

and appears to be of a very low order.

Many coastal sinks are ephemeral in nature and store sediment for a short geological time

span before it moves further downslope. The time span for which the sediment remains in a

coastal sink varies from a few minutes or hours in the case of some tidal beaches, to several

million years in the case of coastal geological rock formations. In many areas, sediments are

transported short or for a distance alongshore from their source or sources before being deposited

at one or more semi-permanent locations known as sinks (Sorensen, 1978). Submarine canyons

along the coast play important role as a sink. Harbors, bay and estuary with tide generated

reversing flow can trap large volumes of the sediment transported alongshore. The flood tide

drives the sediment through the inlet, where it is deposited in quiet water. The ebb tide may carry

sand far enough offshore to be effectively removed from the littoral zone. Sand may also be

trapped adjacent to jetties constructed to stabilize the entrance channel. Lagoons and estuaries act

as long term sediment sinks for marine sand. Wind might cause a net seaward transport of sand

from the dunes to the littoral zone but at most locations; sediments are blown predominantly to

the dune field from the beach. Another minor loss is due to the mining of beaches for sand and

placer deposits. Although tidal marshes are dominantly composed of silt and clay, sand may be

common in the channels draining the marshes and hence, the marsh acts as a sink. The deposition

over the beach face and the subsequent Aeolian inland transport forming as large, high dunes are

the major sink phenomena observed along the Indian coast, particularly along the coasts of South

Tamil Nadu and Orissa. Also the lagoons, estuaries, beach storage, sand spits, siltation at harbor

channels, delta heads, the River channels, its floodplains and formation of marshy lands act as

main sinks for the sediments. The construction of inland dams, irrigation barrages, has drastically

reduced the sediment load brought to the sea. Many coastal segments experience erosion

regularly due to the fall in influx of sediments and the increased wave energy. Further, Rock

fractures, which are parallel to the coast, accelerate the erosive activity of waves.

Sediment budget estimation

Sediment mobilization, transport and storage are characterized by spatial and temporal

variability (Walling, 1998) and it is necessary to take account of this variability while

constructing a sediment budget. There is no well-defined single procedure for establishing a

comprehensive sediment budget for an area. It has proved difficult to adapt traditional

measurement techniques to address the spatial and temporal variability associated with the

operation of sediment mobilization and transfer processes at the catchment scale. Traditional

techniques, including the use of erosion pins, profilometers and photogrammetry to document

erosion rates, and the use of sediment traps or post-event surveys to document sediment storage,

possess many logistical and operational limitations as well cost constraints (Collins and Walling,

2004). The potential for coupling recent advances in sediment tracing technique along with

traditional monitoring techniques has, however, provided new opportunities to assemble the

information required for sediment budget construction (Walling, 2003, 2004, 2006; Walling et

al., 2001, 2006).

The process of transport of sand is presented in Fig. 2. From these, an estimate of

sediment balance can be made qualitatively. Wave erosion of shores and cliffs, dune and

backshore erosion by waves, winds, and streams, landward transfer from offshore by storm

waves, and carbonate production by organisms are identified as sources. Sediment trapped in

inlets, estuaries, bays, and dunes, or transferred to offshore slopes, plus carbonate loss, and

mining and dredging are identified as sinks. Within the study area, the contributions of sources

and losses due to sinks are assessed qualitatively as significant, moderate, marginal, and

unknown (Fig. 2).

These techniques include the use of fallout radionuclides to estimate soil redistribution

and floodplain deposition rates, sediment fingerprinting to establish sediment sources, more

traditional sampling techniques to document storage of fine sediment on the channel bed and

continuous monitoring using turbidity sensors to quantify the suspended sediment flux at the

catchment outlet (Walling and Collins, 2000; Walling et al., 2001, 2002 & 2006).

New approaches to assembling the data required to construct reliable sediment budgets

has been the use of fallout radionuclides as sediment tracers (Walling, 2004). Radionuclides are

commonly rapidly and strongly adsorbed by soil particles and their subsequent redistribution

proves a means of tracing sediment mobilization, transfer and deposition. Assessment of the post

redistribution of the radionuclides offers a basis for documenting time-integrated rates and

patterns of sediment redistribution and storage within the system. The majority of studies

employing fallout radionuclides to trace sediment mobilisation and delivery have been based

upon measurements of caesium-137 (137Cs) activities and inventories. Remote sensing is extremely valuable in detecting various coastal features and to analyze

them in the integrated manner. Availability of repetitive synoptic and multi-spectral data from various satellite platforms viz. Indian Remote sensing Satellite (IRS), LANDSAT, SPOT are helpful to generate information on varied aspects of the coastal and near shore environment, including sources, sinks and transport path. Ocean color data from OCANSAT (OCM I II), SeaWiFS, MODIS provide information on sediments transport path, sources of sediment erosion and deposition and also other aspects useful for studying coastal ecosystems. In India, satellite based information has been used for generating inventory on coastal habitats, landforms, land use and shoreline assessment for determining vulnerability index and understanding sediment dynamics. The transport direction and amount of long-term average shore drift are of vital

importance while estimating sediment budget. Transportation of clay-size suspended sediment

particles along with the fluxes of organic matter, nutrients, and pollutants along with rivers

causes turbidity in coastal waters. The first band of TM imagery (Fig. 3) provides a synoptic

view of turbid water masses which helps in understanding their distribution variation and

dispersion of total suspended matter (TSM). On satellite images the sharp contrast between

various sediment laden waters is noticeable. Tonal variation is considered as a measure of

turbidity concentration. Texture and pattern help in monitoring distribution and movement of

turbid water masses. Current directions are indicated by the sediment laden plumes as they

become elongated and pointed in the direction of flow. Remote sensing images aided with G.I.S.

software help in detecting shoreline changes accurately. Shore line change study exactly

indicates areas and trends of erosion and deposition which are nothing but sources and sinks for a

budget along the coast. Sediment budget can be deduced from this information.

Based on tonal, textural variation, direction of propagation of waves and dispersion

pattern of the sediment plumes, local current direction, shore drift direction and net shore drift

direction are determined and marked on an offshore turbidity distribution map prepared by

overlaying on the TM imagery (Figure 4). Turbidity pattern distribution study provides sound

base for determining sediment budget of the region. Analysis of the TM images acquired during

successive months for two or three consecutive years, determines the accurate net drift direction,

the rate of accretion or the erosion, erodes volume, deposited volume and thus suggest

quantitative budget. Beach profile monitoring at selected locations helps in confirming sediment

budget in deltaic region.

Conclusion

The above discussion emphasizes that the design of sediment control strategies should be

founded on a holistic understanding of the sediment dynamics of the catchment concerned. A

sediment budget fulfils that need, by providing key information on the sources, sinks and

transfers involved. Focusing attention on an individual component of the sediment delivery

system, without appropriate understanding of the overall sediment budget, may result in an

incorrect assessment of the potential benefits of sediment mitigation programmes. It is suggested

that the sediment budget concept should be more widely adopted and utilized as a practical

framework to support the design and implementation of sediment control programmes aimed at

reducing pollution by fine sediment and thus the understanding of the sedimentological

functioning of these units as sinks and sources of terrestrial matter helps in understanding the

propagation and thereby arresting pollutants in the marine system.

Acknowledgements The authors express their sincere thanks to Director, National Institute of Oceanography. Source

for TM & ETM dataset is the Global Land Cover Facility, http://www.landcover.org. The

authors are thankful to GSFC DAAC, NASA, USA for SeaWiFS and MODIS data. NIO

contribution Number is ………

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Fig. 1. East Coast Rivers and their deltas in the study area.

Fig. 2. Quantitative estimation (in mg/l) of Suspended sediment using SEADAS software and

SeaWiFS (post-monsoon season) and MODIS data (of pre-monsoon season) have been presented as Fig. 2a & 2b respectively. Suspended sediment movement is shown by arrows. Black color within ocean indicates no-data region. Side color bar provides quantitative values.

Fig. 3. Thematic mapper (TM) imageries of Landsat 5 (dated 17 March 1985) overlying

direction of propagation of waves, dispersion pattern of the sediment plumes, local current direction, shore drift direction and net shore drift direction.

Fig. 4.Schematic diagram showing elements and their contribution to sediment budget study along the coast.

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