“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:
- Does the size of the bed load change from upstream to downstream?
- Does the calibre of the bed load change from upstream to downstream?
- Does the velocity of the river affect the size of the bed load?
- Does the cross sectional area affect the calibre of the bed load?
- 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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It is possible to see a direct correlation between the cross sectional area and wetted perimeter for each site. This shows that as the river flows towards its mouth, the cross sectional area and wetted perimeter increase, meaning the river gets wider and deeper on average as it flows downstream. This is already something illustrated in the Bradshaw model, and I hope to prove that it also influences the bed load quality.
1. Does the size of the bed load change from upstream to downstream?
From my results, it is possible to see that there does not seem to be a correlation between the site, and therefore the placement from source to mouth of the sediment, and the size of the bed load. The width of the bed load peaks at Affpuddle B, and then becomes gradually smaller as it moves close to the mouth of the river. Likewise, the length of the sediment remains under 3cm long, until Throop B where it peaks at over 6cm, before descending again towards the mouth.
This does not fit in with the theory given by the Bradshaw model, which states that the average bed load size becomes smaller as it moves from source to mouth. This could be due to a number of things: the Bradshaw model may not be accurate, the readings I took were not from a large enough sample, there are anomalies in my results or the river Piddle does not correlate with the Bradshaw Model. Either way, in my results my hypothesis for this aim has been proved, and in my investigation the size of the bed load does not change from upstream to downstream.
2. Does the calibre of the bed load change from upstream to downstream?
From these results, it is possible to see that the bed load calibre does change greatly from source to mouth. The number of angular samples is highest at Athelhampton, the site closest to the mouth, and that number then decreases as they move further downstream towards the mouth of the river.
The sub-angular samples peak closer to the mouth, at Warren Road and Wareham A, where before they were constantly quite low, not increasing past 30%. The sub-rounded and rounded samples gradually increase as they move closer to the mouth of the river.
As the bed load usually found at the source of the river has not been subjected to much erosion, it is suggested on the Bradshaw Model that it will be more angular, and will become smoother and rounder as it moves towards the mouth of the river. This means the Bradshaw Model predicts that there are more angular samples at the source, and more rounded samples at the mouth. This has been roughly proved in my results, and so I come to the conclusion that the calibre of the bed load does change from upstream to downstream.
3. Does the velocity of the river affect the size of the bed load?
When comparing the length of the bed load and the river velocity, it is possible to see a rough correlation between the two. The length and velocity remain relatively low until Throop B, where the length peaks at 6cm. The width does not have any correlation with the velocity however, as it peaks at Affpuddle B whereas the velocity peaks at Warren Road.
From my results, it is possible to see a very slight correlation between the length and the velocity, but it is doubtful to see whether this is by chance or whether they have affected one another. My conclusion would be that the velocity of the river does not affect the size of the bed load.