4.0 - Technical Specifications
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Riffle 1
30m long x 11m wide x 1m deep x 1.8 (tonnes / density) / 2 = 297 tonnes of gravel, 165m3 volume. A channel will be left approx. 1m deep at the LHB side of the riffle to allow passage of EA weed cutting boat.
Note: The density figure of 1.8 tonnes is an industry standard. The total is divided by 2 to take into account the overall shape of the riffle.
Spoil heap 1
30m long x 7m wide x 1m high x 1.8 (tonnes/density) = 378 tonnes
Calculation of available gravel from spoil heap 1
The dredging out of the original gravel riffles took place several decades ago. During this time the clean gravel has accumulated fine, silts sands and organic matter (soil etc.…) post deposition on the bank.
With this in mind a methodology has been attempted to work out the available gravel from the total volume of material in the spoil heap. This will enable a more accurate way of ordering any extra gravel required to complete each riffle.
It must be stated that this is ‘rough science’ and will give an approximate figure of the quantities and weights of materials involved. However, it will enable a more accurate bill of quantities to be produced, one that is less reliant on total ‘guesstimates’.
This is an innovative technique, and thus a certain amount of experimental development will be required. An appropriate safety margin will be built in to the final quantity needed to provide a contingency if available gravel from the spoil heap has been under calculated. If this figure over calculated the less gravel will need to be delivered to site thus saving on the overall projected costs.
5 random test digs were taken from the spoil heap. These were weighed in each case to a 10 litre capacity. Weights were then taken (A). The spoil was then sieved in the river to remove fines and organic matter, then re-weighed (B). From this a percentage gravel to original spoil was calculated (C), and averaged out over the number of test digs.
Gravel in the spoil heaps is of good mixed sizes ranging from 10mm – 70mm+. This material is suitable for barbell spawning requirements.
100 / A x B = C - e.g. 100 / 29.5 x 9.5 = 32%
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Test Dig
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Original Weight (lbs)
(A)
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Sieved Weight (lbs)
(B)
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% of Original Weight
(C)
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1
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29.5
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9.5
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32%
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2
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31.5
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9.5
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30%
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3
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36.0
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18.0
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50%
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4
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37.5
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21.0
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56%
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5
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32.5
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15.5
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48%
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Average % / 5
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43%
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Therefore assuming an approx. availability of 43% gravel from the total weight of the spoil heap of 378 tonnes, we are left with 163t of useable gravel.
This leaves a deficit of 134 tonnes of gravel needing to be imported to site to complete the riffle. To this a margin of error of around +20% should be added, bringing the figure to 160 tonnes.
As the gravel in the spoil heaps is good spawning substrate it is recommended that imported gravel should be used as a central core on which to place the finer material over. This will also reduce costs.
This material should be whole stone reject 100-175mm and costs in the region of £10.30 per tonne delivered +VAT (as of Sept 2002)
Therefore the quantity & cost of gravel for Riffle 1 is 160 tonnes x £10.30 = £1648.00
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Riffle 1 - Two Stage Channel
As per guidance in EA memo dated 29th May 2002, and subsequent site meeting on the 26th March 2003, a two stage channel will be excavated to compensate for any loss of channel capacity.
Dimensions of the 2-stage channel are: 50m long (10m u/s &d/s of riffle) and to a width of 2.5m and height of 1.5m. This adequately takes into account recommendations made by the EA to widen the 11m channel by 115% (1.65m)
This removes a volume of 187.50m3
The volume of gravel required for riffle 1 is 165m3
All excavated material will be deposited along the field hedge boundary.
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Riffle 2
22m long x 10m wide x 1m deep x 1.8 (tonnes / density) / 2 = 198 tonnes, 110m3 volume.
A channel will be left approx. 1m deep at the LHB side of the riffle to allow passage of EA weed cutting boat.
Spoil heap dimensions.
29m long x 4.5m wide x 1m high x 1.8(tonnes/density) = 234 tonnes
Calculation of available gravel from spoil heap 2.
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Test Dig
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Original Weight
(A)
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Sieved Weight
(B)
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% of Original Weight
(C)
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1
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36.5
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21.0
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58%
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2
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34.0
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18.0
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53%
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3
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36.0
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18.0
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50%
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4
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34.0
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15.5
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45%
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Average % / 4
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52%
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Diagram 1 – Riffle X & Long section drawings
(See appendix 1 for schematic diagram of riffles)
Cattle Drink Riffle and Faggot Specification
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Length: 10m
Width: 7m
Depth: 0.5m
Length of Post & Rail Fencing: 21m
Gravel for Drink: 35m3 x 1.8t per m3 = 63.3t
Material to be excavated from drink: 35m3 x 1.8t per m3 = 63.3t
Gravel for Top-dressing riffle: 25.5m3 x 1.8t per m3 = 45.9t
Material to be excavated as 2-stage channel to a minimum 115% of channel width and 5m u/s & d/s.
Diagram 2 - Cattle drink. (not to scale)
Brushwood Faggots
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Faggots are approx. 2m long x 50cm wide. They will be placed in a line following that of the active deposition of sands along a length of 15m by 2m wide. Depending on the nature of the river bed faggots will be staked onto the ground using pointed alder poles or angle iron, every 1m.
(See appendix1 for schematic diagram of cattle drink and narrowing)
Off-River Refuge Specifications
Two dykes to be slubbed-out to a width of 4m to a depth of approx. 1m. Slubbings will be spread thinly adjacent to the dykes.
At 10m dyke to be gradually widened out to approx. 10m wide to create shallow pool area. Pools are to be approx. 25m long. Spoil to be taken from one bank to allow excavator to work back on itself. At finish of pool continue slubbing out dyke for a further 10m.
Both slubbed out dykes and pools to be fenced and planted with clumps of native trees and bushes, such as alder, willow and blackthorn. All plantings will be over 10m from the river bank, thus not requiring Agency LD Consent. Material to be spread thinly adjacent to dykes.
Voles are not present in either dyke, as at present habitat quality is poor.
Estimated material to be slubbed out from dykes.
Two Refuges = 440m3
Two Dykes:72m3
(See Appendix 1 for volume calculation sketches)
top of page Diagram 3 - Off-River refuge profile (not to scale)
Diagram 4 – Off-river refuges plan. (see drawing for more detail & fencing spec)
Diagram5 – X-Section of Off-River refuge Culvert River bank fencing specification
Fencing needs to be 4 strand galvanised high tensile, interspaced at regular intervals with straining posts, set 3m back from the bank edge.
The river bank fencing component of the project is 436m of single bank fencing.
The fencing requirements for the dykes & refuges is approx. 85m per feature.
Therefore the total stock fencing requirement is 436m+85m+85m = 670m
There will be a need for 5 stiles placed at strategic locations to aid angler access
NB: The above is available in Acrobat PDF:
  4. Technical Specifications.pdf [197k]
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