DELTA PROCESSES
Deltaic Environments
Deltas form where a river enters a standing body of water (ocean, sea,
lake) and forms a thick deposit.
The morphology of delta is divided into delta plain (delta top) and delta
front.
The delta plain is the subaerial part of a delta (gradational upstream to a
floodplain); the delta front (delta slope and prodelta) is the subaqueous
component.
Delta plains are commonly characterized by distributaries and flood basins
(upper delta plain) or interdistributary bays (lower delta plain), as well as
numerous crevasse splays.
Upper delta plains contain facies assemblages that are very similar to
fluvial settings.
Delta Front
Mouth bars form at the upper edge of the delta front, at the mouth of
distributaries; they are mostly sandy and tend to coarsen upwards.
The delta slope is commonly 1-2° and consists of finer (usually silty)
facies; the most distal prodelta is dominated by even finer sediment where
deposition is mainly from suspension.
Progradation (basinward building) of deltas leads to coarsening-upward
successions, and progradation rates depend on sediment supply and basin
bathymetry (water depth).
The forms of modern deltas: (a) the Nile delta, the original delta; (b) the
Mississippi delta, a river dominated delta; (c) the Rhone delta, a wave
dominated delta; (d) the Ganges-Brahmaputra delta, a tide dominated
delta.
Ganges-Brahmaputra Delta, Bangladesh
Rhône Delta, France
Mississippi Delta, USA
Wolga Delta – Caspian Sea
Controls on Delta
Galloway’s delta classification (1975):
a triangular plot on which the relative importance of waves, tides and river
processes factors is considered; allows any modern delta to be classified
into one of :
(1)
(2)
(3)
River-dominated delta (Mississippi delta)
Wave dominated delta (Rhone delta)
Tide dominated delta (Ganges delta)
(1)
(2)
(3)
The drawback of this classification is that some modern deltas plot in the
same part of the triangle but have very different morphologies and
characteristics.
ESTUARINE PROCESSES
o Part of the river system in which the river widens under tidal action.
o Estuary is a meeting place of:
– Salt water and fresh water
– Salt water and fresh water flora and fauna
– Sea-borne and land-borne sediments
o Estuaries are governed by two main independent variables:
(1) tidal action at the sea face
(2) river flow
o The overall boundary shape of the system changes gradually (due to
accumulation and redistribution of sediments) although there may be
short-term (local) changes (e.g. tectonic action).
o Tide plays a major role in estuarine processes.
– Tidal prism: volume of water entering estuary during rising tide/one
tidal cycle.
– Typical tidal current speed is 2.5 m/sec (max. 5.0 m/sec)
– Salinity increases during spring tide and decreases during neaps.
– Sediment (and salinity) mixing occurs due to large scale horizontal
circulations during ‘flood’ and ‘ebb’ tides.
Salinity
o Salinity is one of the key variables determining habitat.
o Salinity affects agriculture.
o Salinity is an important indicator of hydrodynamic processes
(turbulence, density currents, etc.) affected or controlled by density.
 Salinity and Density Structure
– Nearshore salinity has both horizontal and vertical structure, both
resulting from freshwater inflows.
– A salinity gradient (represented by isohalines) is usually present
along with circulation and mixing processes.
– Salinity front moves up and downstream with tide.
– Startification of saline and freshwater may occur if freshwater inflow is
unable to hold back the saline water.
– Density-driven circulations (in an estuary) may produce density
currents.
 Impact of Salinity on Sediment and Ecosystem
– Salinity has generally a strong influence on flocculation of finer
particles; flocculation and thus settling velocity increases with
increase in salinity.
– Increase in salinity is believed by some researchers to be the primary
cause of adverse impacts on the Sundarbans mangrove ecosystem.
Meghna Estuary
 Area: Approximately 11,000 km2 (see figure)
 Very dynamic estuarine coastal system connected to some of the
largest rivers of the world
 Vulnerable to cyclones and storm surges
 Complex interaction among the forces of the rivers, tide and waves
 Relatively high erosion/accretion rates
 The Lower Meghna transports more than a billion tons of sediment per
year
 River discharge varies between 20,000 and 100,000 m3/sec
 Special feature: Charlands
Meghna Estuary Study (MES)
Objectives:
 Integrated Coastal Zone Management (ICZM)
 Improve physical safety and security of the people
 Reduce effects of cyclones and storm surges
 Adopt measures against bank erosion
 Strengthen economic livelihood
Priority interventions:
 Char Montaz-Kukri Mukri Integrated Development Project
 Nijhum Dwip Integrated Development Project
 Haimchar Erosion Control Project
 Muhuri Accelerated Area Integrated Development Project
Impacts:
 Acceleration of accretion
 Erosion control
Other considerations:
 Rural development
 Agriculture and farming systems
 Marine fisheries
 Aquaculture
 Forestry
Trends:
 Silting up of the Sandwip channel and the shallow areas between Hatia
and Sandwip
 Dominant erosion around North Hatia and North Bhola
 Alternate erosion and sedimentation pattern along the Lower Meghna
between Chandpur and North Bhola (channel is mobile)
 East Shahbazpur channel is relatively stable
SEA LEVEL RISE
Climate Change: Causes and Effects
Scenarios from
population, energy,
economics models
EMISSIONS
CONCENTRATIONS
Carbon cycle and
chemistry models
HEATING EFFECT
‘Climate Forcing’.
CLIMATE CHANGE
feedbacks
CO2, methane, etc.
Gas properties
Temp, rain, sea level, etc.
Coupled climate
models
IMPACTS
Impacts models
Flooding, food supply, etc.
© Crown copyright 2004
Possible future emissions of CO2
A1FI High Emissions
A2 Medium-High
B2 Medium-Low
B1 Low
© Crown copyright 2004
Projected surface warming in IPCC 4AR analysis
© Crown copyright 2004
Source: Intergovernmental Panel on Climate Change (IPCC)
IPCC TAR range of global sea-level rise
Include uncertainty
in ice parameters
AR4
All models
all SRES
© Crown copyright 2004
IPCC : Third and Fourth Assessment Reports
EQUILIBRIUM PROFILE
Dean’s profile model
Beach profile attains an equilibrium (stable) shape under constant wave
condition. (‘Quasi-equilibrium’ in nature because of tide and other
variations)
Smaller sand grain -> milder slope
Coarser grains -> steeper slope
Bimodal sand distribution -> steeper foreshore, flatter offshore
Bruun’s model:
Dean’s model:
h = ax2/3
h = Axm
m = exponent, dependent on how energy is dissipated,
varies from 0.4 to 0.67 (m<1 indicates that the profile is
concave upwards or bowl-shaped)
A = shape factor, dependent on stability characteristics of
bed materials
 24 D3 D  
A

 5  g  2 
 = specific (unit) weight
D = sediment diameter
 = breaker index, H/d = 0.78
D3(D) = energy dissipation rate, proportional to sediment
fall velocity
Based on 502 profile data (empirically derived),
0.6 < m <0.7, and 0.0025 < A < 6.31
23
 Response of Shoreline to Sea Level Rise
(Application of Bruun rule – long-term average conditions)
Assumptions:
(1) Wave climate remains essentially the same
(2) Beach profile exists in equilibrium with wave climate
(3) There is a ‘closure depth’ beyond which sediment motion is negligible
(4) There is an ‘average’ profile which translates landward in response to
rise in water level
(5) Sand is conserved
(6) Profile is composed of all unconsolidated sand
The ‘equilibrium profile’ shape remains the same with change in sea
level. The shoreline shift is given by,
x 
AB
h  d
Δx = shoreward translation of profile
d = ‘closure depth’, (27 ft for Atlantic coast, 60 ft for Pacific
coast, USA)
A = change in mean water level
h = height of dune
SEDIMENT BUDGET