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