"FISHING IN DEVON" -- Crediton Fly Fishing Club

FISHERY PROJECTS PAGE 1

Topics covered on this page:-

• Gravel cleaning on the R. Culvery
• Reducing channel width downstream of Codshead Bridge

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GRAVEL CLEANING

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These are notes that I made at the time of the first gravel cleaning. (Autumn 1997)
The photos show Andy Locke and Nigel Guy who are local Assistant Fisheries Officers of the EA, working the gravels of the River Culvery. They are using equipment that Andy put together for similar work in the East Devon catchments. If you have any queries he can be contacted through the Environment Agency offices on 01392 444000.

• Equipment:
  • Pump: Honda 2" Trash Pump, Throughput approx 300L/min -- (Picture 1) (30Kb)
    This has an Aluminium sump that can be opened and cleaned out..
    Andy says it will only very rarely become blocked.
  • Intake tube 15ft long, 2" (5cm) diameter, with filter.
  • Outlet pipe>20 ft long, flexible + 10ft stiffer section attached to stainless steel pipe by a 12" connecting section. Each section joined by male to female snap-lock couplings. These couplings can be either aluminium or plastic.
  • 6-7ft Stainless Steel exhaust pipe, flattened at one end. to provide increased penetrative power.
    Andy is currently developing a T-Bar handle on the stainles steel pipe so that it can be controlled at hip height rather than over the shoulder.
  • An extra length of stiff outlet pipe is necessary for reaching those stretches where access is difficult. The pump can be easily carried by two persons, and the whole kit can be transported in the back of an estate vehicle. -- (Picture 2) (35Kb)

• Time Window:
  • Needs to be at end of fishing season, but before spawning.
  • Our first effort was on 9th-10th October 1997.
  • The second effort was on 26th October 1997.
  • In 1998, the gravels were cleaned again in the first week in October
    After this the river was too high to see much activity, but there was evidence of trout spawning on the gravels in December

• Flow Conditions:
  • Low flows are best: easier to work, easier to see gravels, silt settles out quickly in pools downstream rather than colouring the river for long distances. This is particularly true of small winding streams.

• Method:
  • Start d/s and work upstream for 3-4 sites.
  • If cleaning more sites than this, it would be best to clean a small block of sites working u/s, then start again on another block lower down.
  • Inlet pipe sited d/s of gravels. -- (Picture 1) (30Kb)
  • Trout tend to use gravels near the tail of a riffle, but it varies with flow conditons.
  • Leave a sill at the tail to prevent gravels being washed away.
  • Work sideways across riffles from bottom to top, loosening the impacted pavement, and eating into the exposed gravels on the inside curve. Then wash gravels vigorously several times, keeping the nose of the jet just above the stream. -- (Picture 3) (28Kb)
  • Rake gravels level, or create a run when flows are very low. -- (Picture 4) (25Kb)
  • Rinse again, until clear water flows.

Since the gravels were cleaned, there have been heavy autumn and winter rains which have maintained the streams in a swollen state for much of the time. In consequence we have been unable to assess the affectiveness of our actions in terms of increased redds on these gravels. Only time will tell.

As the window of time when this operation can take place is very short, and because Nigel and Andy are likely to be needed elsewhere next autumn, the club has recently purchased its own pump and attachments to do the work ourselves this year (1999).

If you are considering doing something like this yourselves, it is essential that you first seek advice from the EA or a reputable fisheries consultant, as there is quite a lot of legislation relating to such activities.
The Environment Agency is keen to work in partnership with local angling clubs, where their resources permit. So it is worth consulting them first.

Before you start any work, you should also notify both the local Fisheries Officer and the Pollution Hotline. Otherwise the EA Pollution unit might be called out by a well-meaning passer by. This would be a serious waste of the EA's resources.

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REDUCTION OF CHANNEL WIDTH

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SITE:-

Downstream of Codshead Bridge from STW outfall downstream to Y5.
(Map of existing course of River as at 30/01/98) (69Kb)

DESCRIPTION:-

This is a very straight section with a wide flood channel, consequently it is very shallow during summer flows and has virtually no riffle/pool sequences.
There is plenty of tree cover but there are no natural lies for trout and the channel width is too great to maintain an adequate depth of water to hold fish during the fishing season. There are extensive gravels here, but they have become compacted due to lack of scouring activity.
Large fluctuations in the freshwater invertebrate populations from year to year reflect that the water quality below the STW outfall is a problem during prolonged low flows, but trout seem unaffected 300m further downstream.
The only way to improve this section on a permanent basis, is to provide a stronger current by reducing the channel width. However, this has to be done without interfering with:

• a.) the potential volume of the flood channel (as it is immediately downstream of a road bridge) and
• b.) the existing flow pattern during times of flood ( or there could conceivably be bank erosion, similar to damage further downstream.)

PROPOSED OPTIONS:-

• 1) A chisel harrow could be employed to loosen the impacted gravel bed. This would release large deposits of fine silt which have accumulated over past decades. This would have the effect of improving the habitat for the desirable species of freshwater invertebrates. The harrow could also cut a narrow meandering channel within the existng channel, which would provide a stronger current at times of low flow, without seriously altering the main force of currents during spates. This would help keep the gravel clean and provide a better depth for trout lies, It could also produce much needed spawning areas.
  Machinery used recently by the EA for similar gravel cleaning on the Rivers Axe and Yarty could be suitable for this work, provided that suitable access can be arranged.
  
• 2) An alternative to cutting a channel in the stream bed is the deployment of large 3 tonne boulders embedded within the stream to direct the current to produce a meander that would eventually produce a riffle/pool complex. These stones would also provide lies for trout, and the resulting riffles could improve oxygenation.
• 3) When the channel width has been reduced, the introduction of instream submerged half logs, or log slabs, placed longitudinally to could also provide safe lies for trout, without obstructing flow until more natural lies develop.

DEVELOPMENT OF A MEANDER PATTERN:-

• Meander wavelength and amplitude, and consequent riffle/pool pattern, are directly attributable to channel width, nature of substrate, the gradient, and volume of flow.
  
• Unless the correct meander pattern is chosen, the natural geophysics of the stream will work to change it.
  
• There are signs that the river is already trying to achieve a pattern, but it is happening too slowly. Furthermore, this pattern is developing based upon a weak flow in an 11 metre width channel.
  If the channel width is reduced to a more desirable 4 metres (cf the channel width downstream), the pattern will be different.
  The problem we are faced with is how to predict what will be the desired pattern for a given width.

Dr Andrew Nicholas and his colleagues in the Dept of Geography, University of Exeter, have suggested two options.

• 1) Measure the meander wavelength and riffle/pool pattern downstream where the river has been free to deveop normally, and use this as a template (assuming the gradient of the stream bed is the same). This has been carried out by FishWatch:(Map of course of River Yeo below Dunscombe at 03/03/98) (63Kb)
• 2) Measure the flow and its variations over a period of time. Measure the gradient of the stream bed. Then set up a mathematical model to predict the eventual stabilised meander pattern.

At the current time (14/3/98) we are assessing the feasibility of the second option.

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PHASE 2 (30/05/1999):-

After two years of waiting for a response from the Environment Agency to our plans, we have decided to move on our own.   We have established that the gradient of the stream bed downstream of Dunscome Bridge is very similar to that downstream of Codshead.   Consequently, we have selected the first of the options namely to use the section of river downstream as a template for our intended riffle/pool sequence in the section to be improved.   To avoid complications of gaining consent from Flood Defence for the deployment of instream obstructions as current deflectors, we have decided to produce our own meander and riffle/pool pattern by merely loosening the gravel in a strategic pattern with a trash pump, and letting the river do the rest of the work.   It may take a few years to achieve the desired effects, but time is not so important, especially as we already have been attempting to get this particular project on the agenda for about fifteen years now.

The following diagram illustrates:

• the existing river
• the superimposition of the meander pattern upon existing river
• the anticipated final riffle/pool pattern

Work is beginning in August 1999, and an update will appear here in the spring of 2000, with subsequent annual updates on the natural development of the stream bed.

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Back to Top of this Page   -   or return to Fishery Projects: Introduction

or move on to Projects Page 2
for:-

Trash Dam and Bank Clearance

Alder Root Disease and Bank Stability

Tucker's Weir