On Wednesday 2nd October 2002 a river study was undertaken, investigating how the characteristics of a typical river vary downstream. The river to be studied was the River Lyd; this small river’s source is at the junction of the A4136 and the B4234 near Upper Lybrook, Forest of Dean. The Lyd then meanders its way down a small valley towards Lydney, flowing parallel to the now disused Dean Forest Railway. The Lyd then flows into the man-made harbour of Lydney, situated on the Gloucestershire flood plains and then finally confluences with the River Severn, the course of the River Lyd is shown.
The investigation took place on mild day subsequent to a long period of dry weather, this inevitably meaning the water table was fairly low and so decreasing the volume of water within the river, the Forest of Dean is an area of dense woodland. The aim of the study is to prove or disprove the three hypotheses: 1. Discharge will increase further downstream. 2. Cross-Sectional Area will increase furth er downstream. 3. The larger the wetted perimeter, the greater the discharge.
Data Collection The aim of the study was to either prove or disprove the hypotheses concerning the varying characteristics downstream, this was achieved by collecting various measurements at twelve sites, each site being progressively further downstream, at each site a ten meter stretch was marked out to assist with the study.The aspects of the river that were to be measured are described below: 1) Width -simply the width of the actual stream using a tape measure. (Fig 2.1) 2) Depth – to accumulate both an average and a reliable measurement of the depth, three readings were taken using a meter rule as shown on Fig 2.2. 3) Angle of slope – using a range pole at each end of the ten meter stretch, a clinometer was used to calculate the angle, this is important as a steep angle may increase the velocity and may prove to be an anomaly result. (Fig 2.
3)4) Cross Sectional Area – this is best described as a two-dimensional plane through the river, measured perpendicular to the river’s banks. It is calculated by multiplying the width and the depth. 5) Velocity – a river will attempt to adopt the path of least resistance and maximize its velocity.
There are two ways in which the velocity could have been calculated (i) using a flowmeter, however during the field study, the flowmeter broke and so the velocity had to be calculated using both techniques: (ii) the time it took a ‘ping-pong’ ball to travel the set distance of ten meters.6) Discharge – volume of water passing a given point in the channel, in a given time i.e. most commonly mï¿½/s (cumecs, denoted as Q).
It is calculated by the following formula: Cross Sectional Area x velocity. 7) Wetted Perimeter – this is the total length of the bed and bank sides in contact with the stream. Calculated by (Width + Depth) x 2.
Fig 2.4 provides a clear demonstration of a river’s wetted perimeter. 8) Hydraulic Radius – this is best described as the shape of the channel, it is calculated by the following formula: Cross Sectional Area / Wetted Perimeter.Subsequent to the data collection, the statistics then had to be studied, tested for relationships/reliability. To establish any relationship between the data, the results were put through the calculations of the Spearman’s Rank Correlation Coefficient theory to determine the value of ‘r’. To decipher the extent of the data reliability, the ‘r’ value was placed upon the ‘Significance of the Spearman rank correlation coefficients and degrees of freedom’ graph. This graph then conveys the likelihood of the correlation occurring by chance.
These methods have been chosen, as they are both well-established geographical tests and have been used in prior investigations.Results Analysis Hypothesis One: Discharge will increase further downstream. At eight of the twelve sites shown in Figure 3.1, the discharge is between 500 and 1000 cumecs, with no obvious linear trend. Measurements at site numbers three, seven and twelve indicate an increase in discharge downstream. The scatterplot as a whole does not show a clear relationship between site number and discharge. This is confirmed by a Spearman’s Rank Correlation Coefficient (SRCC) of r = 0.147 (shown in Fig 4.
1), with a significance level (0.649) that far exceeds the 0.05 confidence level. i.e. there is virtually no correlation between the two variables and a high likelihood that the coefficient has occurred by chance.
On this basis, hypothesis one cannot be accepted. i.e. discharge does not increase further downstream. This is going against well-established geographical theory that discharge does increase further downstream due to the following reasons.
A greater accumulation of water via catchment area input such as (i) throughflow – water will flow laterally underground (ii) surface run-off – if the soil becomes saturated, then excess water will over the surface (iii) groundwater flow – water being transferred laterally from the water table. In addition to these transfers, the further downstream, the more confluences with tributaries occur and so increase the river’s capacity, the river then compensates for the extra water by increasing the size of the channel, i.e. a greater volume of water is able to flow downstream.