Reject flint from Cemex stacked at the hard surface area was loaded by a digger into a 3t dumper which ferried it to the various areas of river bank where it was to be used, and similarly for the gravel.  The stone was placed by digger in the river bed to a depth to around 75% of the final level of stone and gravel required. The lengths of the riffles were a compromise between the ideal (15 metres or more) and what could be contained within the project costs, and the material spread by the digger bucket. The stone was followed by the upper layer of 20-40mm gravel, a compromise between what was best for spawning brown trout and the habitat for the white-clawed crayfish. The weight ratio of stone to gravel was around 3 to 1, but varied with water depth.

 

The first and last (sixth) riffles were formed to serve as both a riffle and a cattle crossing. The other four riffles were set so as leave a narrow and deep run along the side of the far bank for the benefit of larger trout. Best estimates of the amounts of stone in gravel in each are shown below.

 

                    Length  Reject Flint     Gravel     Total

 

CC1/Riffle 1.     8m           17t              5t         22t

 

Riffle 2.          13m           24t              7t         31t

 

Riffle 3.          15m           27t              8t         35t

 

Riffle 4.          10m           18t              9t         27t

 

Riffle 5.          12m           21t            10t         31t

 

CC3/Riffle 6.    4m              8t              1t           9t

 

Total.            62m           115t            40t       155t

 

The deeper the river section, the more reject flint required. The positions of the riffles are shown on map 2, which shows the location for all the finished works.

 

The licence plan submitted to the Environment Agency was for up to 10 small riffles (including the two with dual function as cattle crossings/drinks) from the screened gravel, with a length of riffle at 15m or more in length. The riffle would increase the retained head by 15-30 cm. The depth of gravel in each riffle should exceed 50cm for the benefit of macro invertebrate. For the optimisation for brown trout to spawn the water depth should be between 25cm and 60cm; and water velocity between 25cm/sec and 75cm/sec.

 

There is an option over time to extend the length of the riffles by hand labour should more stone and gravel material become available and experience indicated this would be beneficial. In fact, cattle crossing 3 was done by hand, after the other riffles were done by heavy plant work. Some local sourced reject flint was barrowed in, and all the gravel gleaned by rake and shovel from the digger spill on the bank side of the other riffles.   

 

The need to build a bridge over the side channel at cattle crossing 2 required that the stone that had been placed there was supplemented by spoil to give a firm base on which to place the sleepers. The stream here was about a metre wide with a bed of small stone and gravel bed in shallow water. After the bridge was removed, the digger then cleared the stream, and levelled the stone and spoil over the approaches. This was followed by 4 days of digging by hand to recover the large stone that had been placed there and the material that had formed the channel bed. With some additional replenishment the bed of the stream was enhanced and extended on completion of this work, with a length of small stone and gravel extended from

2.75 m to 9 m.

RIFFLES

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