Hydrology formations Flashcards
Explain the formation of waterfalls.
Geology, long profile, harder/soft rock, overhang, plunge pool.
-The formation of a waterfall is predominately as a result of geology.
-Throughout the long profile of the river horizontal bands of soft rock are passed over.
-The harder rock is generally on top of the underlying softer rock thus is it called cap rock. This leads to a differing rate of erosion as the underlying soft rock is eroded at a faster rate the the hard rock above it. This is due to abrasion (def) and hydraulic action (def)
-The soft rock will over time be gauged out to leave an overhang as the profile of the river is steepened.
-Eventually the overhanging rock will be unable to support its weight and will collapse in the plunge pool at the base of the waterfall and will provide further material for erosion. This deepens the waterfall.
-For gorge: this process continues and is repeated over hundred of years causing the waterfall to cut back into the landscape, leaving a steep sided valley know as a gorge.
Explain the formation of a meander.
Banks of sediment, weaves, riffles and pools, riffle, corkscrew, asymmetric, meander migration
-Due to banks of sediment at the bottom of the river (which are deposited at times of low flow, that is, a low velocity and low discharge, meaning deposition increases), the river weaves around these alternating shallow and deeper sections (riffles and pools) on what was an initially straight channel.
-This movement targets on section of the bank after the riffle. This part of the bank is eroded by processes such as abrasion and hydraulic action, leading to formation of the outside bend of the meander. This leads to the formation of the river cliff on the outside bend.
-The material eroded further upstream is deposited on the opposite bank (inside bend) on the slip off slope. This is due to a corkscrew movement (helicoidal flow) which results in water levels on the outside bend to be elevated, giving it a faster velocity (the Thalweg line is located here).
-The deposition on the inside bend gives an asymmetrical cross section of the channel.
-Erosion and deposition continues which leads to the formation of meander. As theses processes continue, meander migration occurs (i.e. these processes shift downstream and are not static in one particular location).
Explain the formation of an oxbow lake.
middle course, slip off slope, pronounced, neck, flood, scar
-Oxbow lakes are most commonly found in middle course of the river, where the river has more energy and higher discharge.
-A meander is first formed as a result of deposition and erosion - velocity is higher on the outside of the bend as a result a slower velocity, forming a slip off slope.
-The meander is gradually made more pronounced as result erosional processes (abrasion and hydraulic action) and deposition change the depth, and subsequently the levels of friction, which again determines velocity and erosive power.
-The neck of the meander will gradually narrow.
-In the event, of a flood, when the river has a high discharge and energy, it will take the fastest course, which cuts off the meander.
-Deposition will occurs in the channels, and an oxbow lake will be formed. Water in the oxbow lake will be
evaporated and the area is likely to become vegetated, leaving a scar of the oxbow lake.
Explain the formation of levees.
natural embankments, floodplain, coarse, finer, tapered, well-formed, natural flood defences.
-Levees are natural embankments along a river that build up due to repeated flooding. They create raised edges on the river bank, making it less likely for the river to break its banks.
-When a river floods, the sediment being transported in the river floods onto the floodplain.
-The heavy, coarse material is deposited first (closest to the river channel), because it requires the most energy to be transported.
-The finer, light sediment (such as sand and silt), required less energy to be transported, so it can be transported and deposited further away from the channel. This creates a tapered shape.
-Over time, the levee becomes larger and more well-formed. This means that there is taller riverbank. so, river levels can be higher without breaking the bank and flooding; the levees act as natural flood defences.
What are riffle pool sequences and how are they formed?
Riffle-pool sequences are alternating patterns of shallow and deeper water in a river channel. They are common in rivers with gentle slopes.
These are formed because a river tends to adjust its course of flow to efficiently transport its load downstream (using as little energy as possible).
Explain the formation of deltas.
Sediment loads, lateral, flocculation, small islands, splits, fine or coarse.
-Rivers typically contain their highest sediment loads near the their mouth, where they meet seas and oceans. As the river enters a body of water its velocity decreases.
-At the mouth of the river, the flow of water becomes increasingly lateral. This lateral movement of water reduces the hydraulic radius (so there is more friction) and increases the wetted perimeter.
-This causes sediment to be deposited, as does flocculation where clay sediments join together, gain in mass and sink.
-Over time sediment builds up. This can create small islands that split the channel. This continues until the river splits as more islands form.
-If sediment is fine-grained bird’s foot deltas are created. If sediment is coarse-grained arcuate deltas form.
