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Fluvial Erosion
Fluvial Erosion in the Piha River Drainage Basin
| Question | Answer |
|---|---|
| What processes was the Piha River Drainage Basin formed? | Higher order processes e.g. hydrological, geomorphological, pedological, biogeographical and climatological. Lower order processes e.g. fluvial erosion, fluvial deposition and fluvial transportation. |
| What fluvial erosion proccess operate outside the river? | Rainwash, soilwash, soil creep and mass movement move debris from the land into the water system. |
| What is rainwash? | During rainfall, it occurs when there is exposed, unprotected soil. It operates when the impact of raindrop displaces soil particles. On slopes the soil particles tend to move downslope due to the force of gravity acting on the displaced soil particles |
| What is soilwash? | It is when the volume of water creates a flow of water over a bare section of soil. As the water flows over soil it finds a common path which is called a rill. The energy of water in a rill leads to deeper, wider channels resulting in gullies. |
| What is soil creep? | It occurs during times of rainfall in areas of moderate to steep slopes. Rainfall water is absrobed into pore spaces in mud and sand at times of soil saturation. As the soil strata is heavier due to water absorption downhill movement of soil occurs |
| What are landslips? | Landslips tend to be dramatic as it is a sudden movement and occurs when the weight of the soil due to soil moisture gives way under the pull of gravity. |
| What effect does fluvial processes outside the river have on the PRDB? | Outside of the river system the action of rain water (1400mm or orographic rainfall annually) displaces sediment/soil from around the river and transports it into the Piha and Glen Esk rivers |
| Give an example of how soilwash affects the PRDB? | In the PRDB rills are evident on walking tracks such as the Kitekite Track during rainfall and there are many gullies drainging down from the watershed into the Piha and Glen Esk rivers in the upper valley. |
| How does fluvial erosion operate inside the stream channel? | Fluvial erosion within the river occurs through the removal of material (sediment) from the sides and bed of the river channel. Within the river channel, fluvial erosion occurs through the processes of abrasion, hydraulic action or corrosion. |
| What is abrasion? | Abrasion is when rolling boulders and stones by the current, there is a continual collision of debris against debris. The rolling stones bounce into other stones which reduces and grinds debris into smaller particles. |
| What is hydraulic action? | It is the natural force of water flow which acts as a relentless erosive agent as it drags debris along the river bed. |
| What is corrosion? | Corrosion is the chemical process of rock weathering where acids within the water have a direct erosive influence on river material. |
| What is fluvial transportation? | Load is the debris that moves down with the river flow and originates both outside of the river channel and within it. Fluvial transportation processes include traction, saltation, suspension and solution. |
| What is traction? | Heaviest material is rolled along the river bed. |
| What is saltation? | Sand sized particles are bounced along the river bed in a leap-frog motion. |
| What is suspension? | it is when small particles (silt) are carried within the water. |
| What is solution? | It is when minerals are dissolved and carried in solution. |
| What is fluvial deposition? | It is when large debris is deposited in the upper reaches while debris in suspension is usually deposited in the floodplain and is called alluvium. |
| How does fluvial deposition operate? | it occurs in the river at any point where there is a loss of energy which happens at times where the speed of the flow reduces or slow downs. This occurs when the gradient reduces, where the river enters a floodplain or a lake. |
| In the PRDB what are the three distinct regions? | The Upper (80-280m asl), middle (20-80m asl) and lower (0-20m asl) valley/reaches. |
| Why is there high erosion in the Upper reaches | Profile gradient of 5 degrees is steep; high velocity 0.43 m/s;large rocks 121mm; hard volcanic andesite rock, low volume of water 0.38m^3/s; stream channels are narrow, 4.5m and deep 0.2m |
| What are the natural features of the PRDB? | Dendritic drainage pattern (upper-middle reaches 20-280m asl); Waterfalls (Upper-middle reaches 20-80m asl e.g. kitekite falls); Floodplain (lower reaches 0-20m asl); Meanders (middle-lower reaches 0-80m asl). |
| What is the denritic drainage pattern? | The pattern that a river tributaries make.It is an erosional, geomorphological feature. Results from the processes of rainwash, soilwash initially, then abrasion and hydraulic action. |
| What are floodplains? | They are the flat area that borders a river in the lower reaches and are geomorphological, depositional features.Their location is determinded by hydrological, climatological and geomorphological factors. |
| How are floodplains formed? | Formed through the fluvial deposition of eroded material. Floodplains gradually build up through fluvial deposition during floods.The Piha River Floodplain has formed over the past 10,000 years since the sea level rose forming a drowned valley. |
| Why are the floodplains found in the lower reaches? | It is found here because the slope of the land is gentle. Slope angle is 1 degree which means the velocity of the river reduces (0.34m/s) and energy available to carry load also reduces meaning sediment is deposited. |
| What are meander bends? | They are loop-like bend pattern in the river channel.They are geomorphological,erosional features. |
| How are meander bends formed? | They're formed through the fluvial erosion of the river banks on the floodplain where there is a greater volume of water and therefore more energy. The erosion is lateral rather than vertical so cuts into the river banks. |
| Why are meander bends are found on the Piha River Drainage Basin floodplain? | Because the relief is a gentle gradient. Lateral erosion has widened the river channel and has deepened; larger river channel means less friction so water flows and has more energy. Lateral erosion is more dominant due to greater volume of water. |
| Why are waterfalls found in the middle reaches (20-280m asl)of the PRDB on the Glen Esk Stream? | They're found here because this is where the rock is ancient, hard volcanic andesite conglomerate with softer rock beneath, where fluvial erosion by hydraulic action and abrasion are most active due to steep relief and sediment size. |
| How have humans modified the natural process of fluvial erosion? | Blocking the stream (damming)influenced fluvial erosion, transportation and deposition.Clearing vegetation modified interception, runoff, fluvial erosion, transportation, deposition, stratification and succession. |
Created by:
BlahForever
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