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The River Lyn, North Devon
To investigate how river channel characteristics change from source to mouth.
1. Velocity increases with increasing stream order.
2. Hydraulic radius increases with increasing stream order.
3. Bedload size decreases with increasing stream order.
The Lyn drainage basin is located in North Devon. It rises in Exmoor and flows northwards to the
Bristol Channel at Lynmouth, a small fishing and tourist resort.
Relief Narrow, steep-sided valleys, maximum height
400mm asl, dropping to base level in less than
Land Use Sheep and pony grazing on open moorland
(upper catchment) small settlement at
Geology Sandstones and slates (baked, so impervious).
Soil Type Shallow peat soils on moorland.
The river was sampled in a stratified way, on streams of increasing stream order in the drainage
basin, following the channel from Farley Water, a source tributary, downstream of its confluence
with Hoar Oak Water above Watersmeet, downstream of its confluence with the East Lyn to the river
Site 1 (Farley Water, GR SS755455) 1st Order stream, source tributary on Exmoor,
50m downstream of the source.
Site 2 (Farley Water, GR SS753453) 2nd Order stream, 350m downstream.
Site 3 (Farley Water, GR SS748453) 3rd Order stream, 1050m downstream.
Site 4 (Hoar Oak Water, GR SS 742479) 4th Order stream, 3700m downstream.
Site 5 (East Lyn, GR SS 726493) 5th Order stream, 7300 m downstream.
Each site was chosen for reasons of sampling, accessibility and safety. All of the land is in private
ownership but at Exmoor it is accessible to the public for free.
A range of both primary and secondary data was obtained to investigate the changes of
characteristics downstream on the River Lyn.
At each site, 3 locations were randomly chosen to measure the following characteristics:
1. Width On the upper reaches of the river a measuring stick was laid across the channel
from bankfull to bankfull. It was levelled by using a spirit level and was measured in metres.
The problem with the technique is determining where the bankfull is, the river responds to
extreme events by widening the channel to take the excess flow. A flood in 1952, a 1 in
3000 year event created a level which was too wide to be measure by the equipment we
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Depth The distance across baseline was divided into 10. Measurements were taken
using a metal ruler, channel depth readings were taken. Also we recorded the beginning
and ends of a river.
3. Gradient The angle of slope of the long profile of the river was measured using a sighting
clinometer. This is a crude method but it allows the researcher to assess how the long
profile changes downstream.
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Width x average depth = cross sectional area (m2)
Cross sectional area x velocity = discharge (m3/sec)
Discharge is a measure of the volume of the water that flows past a given point in the river channel.
Discharge increases with distance downstream because the channel is getting deeper and wider,
with faster flow, so more water passes through.
Cross sectional area / wetted perimeter = hydraulic radius
Hydraulic radius is a measure of the efficiency of the river.…read more
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N=11 0.623 0.736 0.780
Therefore, we can be 98% confident that there is a correlation between velocity and distance
downstream. We therefore can reject the null hypotheses and conclude it does increase with
Statically, hydraulic radius does increase down the river Lyn. There is a significant positive
correlation. It can be explained as the river becomes more efficient downstream.
Bedload size does not decrease like we expected but increases in a positive correlation. This is a
response to a short drainage basin 8km.…read more