Fluvial Processes and Landforms - Geography 121 with Cote
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6.7 Hydraulics, Erosion and Sediment Transport in Rivers The activities if a river system is a accomplished through erosion, sediment transport and deposition. The amount of energy used by the river to carryout these activities is dependant on the altitude, slope of the landscape and the volume of water. The river system is summarized by figure 6.17. As water moves down slope, potential energy is converted to kinetic energy which is used to carry out the river's tasks. In the transfer from potential to kinetic most of the energy is lost as heat owing to friction encountered as the water moves. Some 95% of a river's energy is used in this way, leaving just 5% to carry out erosion and transport of debris. You student should know the types of water flow; turbulent and laminar flow. The flood hydrograph is basically a dual plot of river discharge (line) and rainfall (bars) over time. The typical shape is shown here and the main components are labelled. A large range of factors control the shape of hydrographs. These include: precipitation type and intensity, catchment shape, catchment gradient, land use and vegetation (e.g. a forestation, urbanization), soil type, geology among others. Figure 6.18: Flood hydrograph 6.7.2 Erosion activities of the river Excess energy, usually obtained when river discharge rises, is used to erode the river's bed and banks with the ultimate goal of achieving a more efficient channel shape for conveying water and sediment from its source to mouth. River's carry out vertical erosion (also called downcutting) in order to reach their base level (usually sea level, or temporarily a lake). Lateral erosion (of the banks) takes place in response to a range of factors relating to river discharge as well as sub-aerial processes (eg. frost action). Processes of erosion include corrosion, attrition, hydraulic action and solution Figure 6.20: An actively eroding river bank showing cohesive sediment slumping to the bank toe. Figure 6.21: On this meander bend, the river flows through peat. Fluvial undercutting on this bend has resulted in large scale cantilever failures. Figure 6.22: Undercutting of this bank has caused blocks of turf to topple into the river. The tensile strength of the grass roots holds the turf layer in place. 6.6.3 Stream sediment Transport Stream transport is accomplished in three ways: Solution or dissolved load (in chemical form derived from dissolved minerals and soluble salts. Suspended load consists of fine grained clastic particles, held aloft in the stream and they are deposited when the stream velocity goes down. Bedload refer to coarser materials that are drugged along the stream bed by traction or are rolled and bounced along by saltation (leaps or jumps).