How Do River Characteristics Vary Downstream?

## Introduction

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 in Fig 1.1.

Fig 1.1

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:

- Discharge will increase further downstream.
- Cross-Sectional Area will increase further downstream.
- 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:

- Width –simply the width of the actual stream using a tape measure. (Fig 2.1)
- 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.
- 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)
- 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.
- 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.
- Discharge – volume of water passing a given point in the channel, in a given time i.e. most commonly m²/s (cumecs, denoted as Q). It is calculated by the following formula: Cross Sectional Area x velocity.
- 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.
- Hydraulic Radius – this is best described as the shape of the channel, it is calculated by the following formula: Cross Sectional Area / Wetted Perimeter.