8. Fluvial landforms Long profile and graded rivers Degradation, aggradation and stream power Floods and floodplains Deltas and alluvial fans Fluvial terraces 8. Fluvial landforms Emphasis on interactions adjustments between variables in relation to the transport of eroded material Rivers are open-systems, in a steady-state equilibrium. self-regulation within the system, i.e. mutual adjustments of variables determined by the nature of the independent variables. Long profile A longitudinal profile is a diagram of surface elevation (meters above sea level) of the river bed as a function of distance downstream Concave shape frequently observed; the local slope gradient being the result of mutual adjustments among variables Both long profile and meanders are examples of graded forms. The slope of the channel is adjusted to provide the velocity needed to transport material provided. The end result is usually a gradual decrease in channel slope as you go downstream Stream power Stream power controls degradation/agraddation (erosion/deposition) in alluvial channels: = g Q S where is stream power (W m -2 ) is water density Q is discharge S is channel slope Stream power: rate of potential energy expenditure (or rate of doing work) Stream Power usually expressed as power per unit area (Wm -2 ). May range in natural conditions from 5 to 300 Wm -2 for instance for active meandering rivers Stream power (cont.) If the available power is greater than what the river bed sediment can withstand, erosion (degradation) will occur. If too much sediment is provided in relation to the available power, then aggradation (deposition) will occur Graded river Def: a graded river is one in which, over a number of years, slope and channel characteristics are delicately adjusted to provide with available discharge just the velocity required for the transportation of the load supplied from the drainage basin. The graded stream is a system in equilibrium. Any change of the controlling factors will lead to adjustments to tend to absorb the effect of the change. Slope and channel characteristics are adjusted (self-regulation). Examples of landforms created by fluvial deposition (or aggradation): Floodplains Floodplains Deltas Deltas Terraces Terraces Alluvial fans Alluvial fans Floods and Floodplains Floodplains: Low, flat ground, present on one or both sides of a stream channel, and subject to inundation by a flood. Flood frequency analysis: establish how frequently a particular discharge peak is likely to occur. This is called recurrence interval or return period. Analysis of flow records: Floods Floods are costly in terms of both life and property. Engineering structures (e.g. dams, bridges) are built in such a way to withstand flood with a recurrence interval much greater than their expected life-span. A significant return period is that corresponding to the bank full discharge (i.e. return period at which flooding actually occurs); varies from one river to anotheron average 1.6 years.(T=1.6) Floodplain formation When over bank flow occurs, average velocity decreases (because of a suddenly much wider river flow) Stream ability or capacity to transport sediment significantly reduced above bank full level deposition on floodplain (alluvium) Floodplain built by two different depositional processes: Lateral accretion (deposition) Lateral accretion (deposition) Vertical accretion Vertical accretion Floodplain formation. Vertical accretion: over bank flow deposits. Right at the edge of the channel, formation of natural levees. Zone where more deposition and coarser deposition occurs. Lateral accretion: lateral deposition of sediment associated with meander migration across the valley. Sediment being deposited (formation of bars) along the inner bank of meander bends. Human response to flooding Modification of channel banks or bed to enable the river to carry greater discharges e.g. height of levees raised along channel e.g. height of levees raised along channel Dams built to control flooding Flow rate regulated Flow rate regulated River diversion Diverting rivers away from vulnerable areas Diverting rivers away from vulnerable areas Raising level of floodplain by dumping material on it Fraser River (B.C.) Fraser River: drains 250,000 km 2 of rugged terrain on west coast of Canada. Minimum flows: of the order of 1000 m 3 s -1 Flood flows: of the order of 10,000 m 3 s -1 Fraser river poses significant flood hazard within Lower Mainland of B.C. Fraser River (B.C.) under high flow condition (June 2002) Geomorphic impacts of flooding Example: Saguenay area (Qc), 1996 Flooding in the Saguenay area was caused by a major storm system that stalled over the mouth of the St. Lawrence River between July 18-21, 1996, and which dropped record amounts of rainfall. In excess of 200 mm of rain fell, most within a 36-hour period beginning at about 08:00 on July 19 and continuing until approximately 20:00 July 20. Widened river channel Channel lateral erosion of terrace deposits Extreme example of concave bank erosion Breach in wall of dam New channel eroded beside a dam Valley bottom buried with sediment Deep channel erosion Fluvial terraces: Fluvial Terraces A terrace is a former floodplain located at a level that is never reached today by river flows. The incision (down cutting) of the river forms a new floodplain at a lower level, leaving the old floodplain as an abandoned remnant or terrace. Terraces are fossil features, formed in conditions that no longer prevail (due to variations in sea level, glaciations) River Terraces SW Montana. Earthquake prone area. Land repeatedly uplifted Snake River, Wyoming. Land uplift and stream down cutting Deltas and alluvial fans Deltas: Deltas and alluvial fans Both result from same cause: very sudden loss of capacity to transport sediment in a stream Deltas: sub aqueous forms (although upper surface may be above water level) Formed when river enters standing body of water (lake or sea); stream velocity considerably reduced Formed when river enters standing body of water (lake or sea); stream velocity considerably reduced Deltas and alluvial fans. In lakes, delta formation involves: Coarsest particles settle first form horizontal layers called top-sets. Coarsest particles settle first form horizontal layers called top-sets. Moving seawards are the inclined fore-sets beds; eventually grade into nearly horizontal bottom-set beds on the floor. Moving seawards are the inclined fore-sets beds; eventually grade into nearly horizontal bottom-set beds on the floor. In the sea, pattern may be different because of (i) differences in density and (ii) tidal effects. River flow spreads forward over the surface of standing sea water (density differences due to salinity primarily) With tidal currents flowing into and out of estuaries, scouring action which may prevent material from being deposited until further out to sea. Delta formed in Gulf of Mexico Satellite view of delta. Yukon River, AK. Satellite view of delta Colorado River, Mexico. Alluvial fans: Alluvial fans Sub aerial fluvial landforms. Formed when a stream channel widens suddenly, for instance. Abrupt decrease in downstream gradient; widening and shoaling of the channels when high-gradient stream reaches a floodplain. Alluvial fan. Death Valley, California Coalescing alluvial fans Alluvial fans. Bedload deposition Stratification may be present, parallel to the fan surface Often measure up to 10 km across with surface slope angles of less than 10; gently concave Surface morphology characterized by radiating, distributary channels Cross-section of alluvial fan Central Utah Alluvial fans (cont.) Surface slope of alluvial fans increases with: Increased grain size Increased grain size concentration of sediment load concentration of sediment load Relation between alluvial fan surface area (A f ) and corresponding area of drainage basin (A d ): A f = c A d b Where c is an empirical constant. Average observed exponent b is 0.9. Alluvial fans. Alluvial fans do not increase in size quite as quickly as their contributing drainage areas, A d. Reasons: Propensity for progressively larger basins to store part of their sediment yield along channels upstream from the basin outlet Propensity for progressively larger basins to store part of their sediment yield along channels upstream from the basin outlet Possibility for larger basins to yield sufficient flow to transport a greater proportion of sediment across the fan. Possibility for larger basins to yield sufficient flow to transport a greater proportion of sediment across the fan. In either event, proportionally less sediment delivered to the larger fans. Fans and terraces