Field investigation around the hypothesis: The River Piddles bed load will become smaller and rounder downstream.

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Hypothesis:

“The River Piddle’s bed load will become smaller and rounder downstream.”

The aim of my study is to investigate the bed load of the River Piddle, in particular the size and texture of the rocks and gravel in the river. I will compose this from various different points along the river, from places near to the source going down to the mouth of the river.

I aim to discover:

  1. Does the size of the bed load change from upstream to downstream?
  2. Does the calibre of the bed load change from upstream to downstream?
  3. Does the velocity of the river affect the size of the bed load?
  4. Does the cross sectional area affect the calibre of the bed load?
  5. Does the cross sectional area affect the size of the bed load?

The River Piddle (See fig 1.0) is a small rural river in Dorset. It is roughly 25.8km long, and runs from its source at Alton Pancras church to its mouth in Poole Harbour. I investigated at eight different sites along the river – Athelhampton, Affpuddle A, Affpuddle B, Throop A, Throop B, Warren Road, Wareham A and Wareham B (listed in order from source to mouth).

I predict that as the river moves further from the source, the bed load will get smoother, as this has been proved in various other studies before, and has been illustrated in the Bradshaw Model (see fig 1.1). The Bradshaw model states that various factors of the river change as the river flows downstream. It shows that on average, the bed load of the river becomes less rough in relation to the distance downstream. This could be because the further the bed load travels, the more abrasion and erosion it is subjected to, and so it is far more likely to become smooth after a longer period in the river – meaning the bed load closer to the mouth of the river would almost certainly be smoother and more rounded than that found closer to the source. However, the Bradshaw Model does not mention the size of the sediment in the bed load, and so I hope my investigation will discover whether in the case of the River Piddle, the position in the river of the sediment will have an effect on its size. Likewise, the Bradshaw Model shows that channel velocity, depth and width increase as the river moves further downstream, and so I hope to show that there is a correlation between these factors, and the size and shape of the bed load. The Hjulstrom curve (Fig 1.2) also illustrates this, as it shows whether a river will erode, transport or deposit sediment, depending on the sediment size and the river velocity. This will be very useful for my investigation as it shows a correlation between velocity and sediment size, which is one of my key aims to discover.

To make the results easier to categorise, I applied the shape of the sediments to four subgroups: rounded, sub rounded, sub angular and angular (see fig 1.3). This shows that the angular sediments have sharp edges and are rough shapes, whereas the rounded sediments are much smoother and round – with sub angular and sub rounded being in between these two different sediment shapes.

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Methodology:

Athelhampton:


Affpuddle A:

        


        

Affpuddle B:

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