A river becomes wider and deeper due to various processes of erosion. Erosion is split into two categories, Lateral and Vertical. Lateral erosion is further categorised into subheadings of hydraulic action, corrosion and abrasion means that the river has the capacity in order to erode both sides so as the river becomes wider. Whereas vertical erosion, means that the river has the capacity to erode the river bed hence making it deeper. However transportation may bring obstacles into a river thus causing the bed of the river to be blocked by boulders or lots of pebbles.
The Bradshaw model shows that the source of the river is shallow and narrow, as distance from the source increases it gets wider and deeper. This hypothesis is being used, to verify that the Moka River follows this pattern.
The table of results show that. Site 1 is the shallowest and shortest with a maximum depth of 20cm and maximum breadth of only 0.61m.The second site increases drastically in width (3.49m) and more than triples in depth (69cm). Containing an average depth of 17.5cm and width of 3.49m site three is a lot smaller than size two. Site four is 6.40 m wide and has an average depth of 115 cm, with a maximum depth of 165cm making it the deepest site. Site five and six are a lot smaller than site four in both width and depth, yet site five is wider and deeper than site six. Site seven is only slightly wider than site 6 by 2cm (Site6: 5.08m; site7: 5.10m) however, it is a bit shallower (Average depth: Site6=27.8cm and site7=23cm). Site eight is 6.50 meters wide, and has an average depth of 33.1cm. The ninth site is less wide than the 8th with a breadth of 6.18m yet it is a lot deeper and more than doubles in value (site 8 33.1cm; site 9 75cm).
From the above results it is evident that site one is indeed the smallest. Site four is the deepest site, and site eight the widest. Yet these results do not exactly follow the Bradshaw models. The reason for this is due to anomalies in the river, i.e. rocks and huge boulders covered the river bed and thus measuring the true depth and or width of the river bed was impossible.
t
Hypothesis 2: velocity increases downstream as gradient decreases
Velocity is the speed of the river. It is calculated by using the following formula velocity=distance over time. It can differentiate depending on how much friction there is on the river bed, or even the shape of the river. However, the main factor being studied will be how velocity and gradient coincide. This hypothesis is being used to test whether or not it is true that velocity and gradient increase downstream.
Site 1 has nil velocity and contains the lowest gradient of 2 ̊. However although there is a massive increase in gradient in site two, its velocity is still very low a mere 0.005 m/s. Site three, in comparison to the previous site, has quite a large increase in velocity. Both site 4 and site 5 has almost the exact same velocity (0.065m/s-0.064m/s) and identical gradient 5 ̊. Site six increases drastically where velocity is concerned and doubles in value for gradient, compared to that of the previous two sites. The seventh site has the highest velocity (0.54m/s) yet its gradient has lowered since the previous site from 10 ̊ to 4 ̊. Site 8 decreases in gradient and velocity yet compared to most other sites its velocity is still relatively high. The ninth site experiences an increase in both gradient and velocity.
From the above results it is evident that, the velocity does increase as the river goes downstream. However, there are a few anomalies yet these where mainly caused by obstacles momentarily pushing back or in some cases stopping the float. As for the gradient, it can be concluded that it actually increases instead of decrease downstream. This is due to the fact that the river Moka follows a convex long profile more closely than it follows a concave profile. However, for gradient, some of the readings obtained where odd, however these where due to human error, and inexperience in using the Clinometers.
Hypothesis three: Particle size decreases with distance away from the river
Particles in the river are referred to as Bed load. These are the materials within the river which get transported, Eroded or deposited to different regions of the river. In a typical concave profile the larger bed load is usually deposited at the source of the river, this is due to the fact that as a river moves downstream the average velocity decreases, hence larger materials should be found near the source of the river and smaller ones, further down. A pictograph is used in order to display the results obtained from the trip.
The results obtained from the field trip were accurate and any mistakes made were likely due to human error. Having said that, the field trip was successful in the way that it was well planned and provided enough information in order to accomplish the coursework.
The hypotheses were based on Bradshaw’s model and partially on the long profile of a concave river. However from the results obtained it can be seen that the river Moka actually does not completely follow a concave long profile but instead more closely resembles a convex profile, which was a surprising twist.
The River Moka did follow most of Bradshaw’s model closely, especially in terms of depth, width and velocity. However some anomalies did occur, these were due to, as previously mentioned, human error (Clinometers), River bed being blocked by obstacles and obstacles obstructing the velocity. Having said this it can be concluded that the characteristics of the Moka River do change to a great extent as it flows downstream.
