3.0 - The Design

The following section will describe each component of the design, moving down stream, describing the benefits it will provide.

Detailed specifications follow in 4.0 Technical Specifications.

3.01 Cattle Drink Riffle & Faggot Narrowing

It is proposed to concentrate the existing flow over the riffle by narrowing the channel on the LHB using faggot bundles obtained by coppicing / pollarding of riparian tree immediately downstream. A marginal shelf area will be created over of some 15m in length with a new bank line extending 3m into the channel. The top level of the shelf will be set at 10-15cm above mean summer water level. The area behind the faggots will be in-filled with trimmings from the coppicing to create a low energy environment, that will rapidly accumulate sediment, allowing colonisation with marginal water plants such as water forget-me-not Myosotis scorpiodes, and reed sweet flag Glyceria maxima. Experience on other rivers (River Allen & Avon) has shown that this type of marginal enhancement provides excellent refuge habitat for white-clawed crayfish and feeding opportunities for water voles.
In addition, the old cattle drink on the RHB will be excavated to create a new larger fenced drink. The drink will have a layer of silt excavated to a depth of 50cm which will be infilled with graded rejects. This will stop excess silt entering the channel and promote sustainable land use practices, as featured in the Environment Agency’s ‘Best farming Practices’ guide. This approached has been trailed on the Wensum at Bintry with great success. Not only does the technique cut-off a point source of siltation, but the cattle have access to a constant supply of clean, safe drinking water.

The cattle drink will also provide a warm shallow fish fry refuge in the summer months. Spoil from the excavation will either be incorporated as backfill within the proposed LHB narrowing or disposed of outside the indicative flood plain. The slope of the drink could be constructed in such a way as to encourage winter flood flows onto the RHB ESA field, This increases storage capacity, and compensates for any reductions in channel capacity due to narrowing.

The existing riffle has been ‘concreted’ by many years off silt deposition, due to reductions in flow velocity caused by abstraction upstream. It is recommended that the riffle be jetted to purge the gravels of silt, and subsequently given a new top dressing of spawning substrate to a depth of 30cm. Experience has shown that the jetting will drop the level of the riffle by approx. 10cm, due to silts and sands being purged from the gravel matrix. To compensate further for any loss of channel capacity a two stage channel will be excavated around the cattle drink to a minimum 115% of the 8m channel width (1.2m) and 5 m u/s and d/s.

Note: All specifications for 2-stage channels have been quoted at minimum 115%. Wherever practical they will be wider to gain more shelf habitat and the desired slope angle of 30-450.

3.02 - Riffle 1

Downstream of the proposed cattle drink the fishery is severely lacking suitable gravel shallows.

It is recommended to re-instate a riffle 30m long starting at Grid Reference TG176131.
riffle 1 location

Immediately upstream of this site is a large spoil heap of gravel some 30m long which is evidence of past dredging activity. The spoil has been tested to ascertain its suitability for repatriation to form riffle 1. This adds further to the sustainability ethos of the project meaning less fresh aggregate needs to be imported onto the site. The full results of the test digs are in the technical specification section which follows. As a rough guide there appears to be useable gravel at around 43% of the total volume of the spoil heap. The remainder being made up of fine sands, silts and organic matter.

Due to the high percentage of unsuitable material in the spoil heap, there will be a need for a mobile screening unit to access the site to sort the gravel. This will be a Norberg 348 wheeled mobile three way screening plant.

Using the figures from the test diggings compared with those of the predicted gravel need for Riffle 1 there is deficit of approx. 42 tonnes of gravel. This material will need to be imported onto the site. It is recommended that whole stone reject be used. This can used as the central core of the riffle and is cheaper than spawning substrate sized gravels. There appears to be a good spread of gravel sizes in the spoil heap which are within the spawning substrate requirements of barbel, chub and trout and dace. All waste spoil will be transported out of the indicative flood plain.

In order to compensate for any loss of channel capacity, the RHB will be excavated to form a 2-stage channel, being 2.5 m wider than the existing channel (11m) This is wider than the recommended 115% (1.65m) and compensates for the volume of gravel to be placed into the channel. The channel will also be extended 10m u/s and d/s of each riffle (50m). The height of the 2-stage channel will be 10-15cm above mean summer water level. An added benefit of removing the spoil and creating the 2-stage channel is that flood flows can be encouraged to enter the RHB ESA field at a lower discharge that at present. This will increase flood storage and provide an increased degree of attenuation. The riffle will slope to a depth of 1m on the LHB to allow passage of EA weed cutting boats.

The introduction of coarse gravels back into the channel will provide habitat conditions suitable for several BAP species including, bullhead, brook lamprey, white clawed crayfish and water crowfoot.

3.03 - Riffle 2

The second riffle is to be sited at Grid Reference: TG177133, and is 22m in length.

As with Riffle 1 there is a spoil heap from previous dredging operations. Test diggings have show an average of 52 % usable gravel. When compared to the original riffle calculations there is a shortfall of 91tonnes. This figure seems a little surprising at first, however the spoil heap is not as wide or as high as the Riffle 1 dredgings. If ground conditions are wet too it may be necessary to transport the spoil for riffle 2 to the screener, and back again, rather than risk bogging down of expensive plant.

It is recommended that whole stone reject be imported to form the central core of the riffle, for the reasons outlined for Riffle 1.

Compensation for any loss of capacity will be provided by the excavation of a 2-Stage channel along the length of the riffle to a width of 2m. This is wider than the recommended 115% (1.15m) of existing channel width (10m) and compensates for the volume of gravel to be placed into the channel. The channel will also be extended 10m u/s and d/s of each riffle (42m). The riffle will slope to a depth of 1m on the LHB to allow passage of EA weed cutting boats.

3.04 - Off-River Refuges

Recent studies on the Wensum* have found that there is severe lack off channel nursery and adult habitat such as meander loops, dykes and cattle drinks. There are two dykes located in the meadow that have until recently remained un-managed.

It is proposed to re-profile to the dykes located at TG178131 & TG179131. Further to this it is strongly recommended that larger refuges are incorporated into the dykes. These features will act as important nursery areas in the summer and flood water refuges in the Winter months. Similar projects on the Waveney and Wensum have proved to be very successful being used by juvenile and adult fish alike. Connection to the main river will be using a culvert pipe, set below the bottom of the dyke. This will be back-filled to allow angler and vehicular access. The dykes and back waters will also need stock proof fencing erected. It is recommended to plant with isolated clumps of native trees and bushes for cover from predators and shade. Total length of each refuge will be 37m with the backwater being 25m long x 10m wide. Depth will be approximately 1m.
(See technical specification and Diagram 4)

Backwaters provide benefits to many other species, particularly water voles. As water voles become increasingly rare along main rivers and streams, their occurrence and survival along backwaters and ditches has a very high conservation value. Management is a key factor in realizing the potential of a dyke system for water voles and other wildlife.

The new profile of the refuges has been designed to be particularly attractive to voles. With a steep bank for burrowing on one side and a stepped berm profile at water level for latrine and foraging habitat on the other (see diagram 4 – Technical Specification). The Anglian Rivers and Otters Project have surveyed the site and voles are only present in limited numbers, due to lack of suitable river bank habitat. This component of the project would provide this much needed habitat and is fully supported by the Project Officer – Steve Henson. The fencing of the refuges will further provide a 2m buffer strip between the grazing marsh and the dyke.

The refuges will have to be periodically maintained to stop vegetation succession. When this is required, best practice should be as follows:

• De-silting without interfering with the banks
• Working from one bank only, and progress upstream, working in short stretches.
• Leaving gaps of 19-20cm as untouched refuge areas for water voles.
• At least one third of the dyke should remain untouched.

As voles are currently not present in the dykes there is no predicted impact of the re-profiling works and refuges, as habitat is being actively created.
* River Restoration Feasibility Study – Environment Agency (1999)

3.05 - Riffle 3

The third and final riffle will rely purely on the importation of aggregate as there is no available spoil.

The riffle will be 30m long requiring 297 tonnes of a mix of reject for the core and 5-40mm for the spawning substrate.

As with the preceding two riffles a 2-stage channel will be excavated to a width of 3.5m. This is wider than the recommended 115% (1.65m) of original channel width and compensates for the volume of gravel to be placed in the channel. The channel will also be extended 10m u/s and d/s of each riffle. The riffle will slope to a depth of 1m on the LHB to allow passage of EA weed cutting boats.

The creation of this 2-stage channel will encourage wetland emergent plants which are suitable habitat for water voles during foraging.
riffle 3 position

3.06 River bank Fencing

A total of 436m of 3 strand HT stock proof fencing is to be installed. This will be set back approx. 3m from the edge of the bank.

The fencing will allow the re-establishment of riparian and emergent vegetation. This will provide shade and cover for fish, and a potentially important new habitat resource for water voles and desmoulins whorl snail, both BAP species. River bank fencing is an extremely cost effective habitat restoration technique. Recent schemes on the Rivers Bure, and Wensum have been very successful. Five stiles will be erected at regular intervals along the fence line to enable ease of angler access.
riverside fencing

3.07 – Access from Costessey Mill.

At present the access from the main car park is very wet and muddy. It is suggested that a short length of board walk be lain down to allow safe access, especially in the winter months. NACA currently have a stock of 2nd hand board walk in storage at Bawburgh Pits. This could be installed at
minimal cost at a future syndicate working party.

.3.08 Future Maintenance

The Costessey Point Project ( NACA & Peter Kettringham) will be responsible for the future maintenance of the site after completion of the scheme. This should not be a major burden as the project has been designed to be sustainable. Future maintenance envisaged at this time would comprise:

• Repair of fences / styles
• Periodic re-profiling of Off-river refuges (subject to best practice)
• Tree / shrub management.

NOTE: Dimensions for all the above features follow in the next section – Technical Specifications

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%


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

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.

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.

Note: only 4 test digs as a smaller heap

Therefore available weight of gravel from spoil heap 2 is 234/100 x 52% = 122t
Gravel required for Riffle 2 is 198t leaving a deficit of 76t of gravel to be imported to complete the job.

Using the contingency figure of +20% this figure rises to 91tonnes.

Total cost of delivered gravel to be imported for Riffle 2 is:
91t x £10.30 = £937.30 + VAT

Riffle 2 - Two Stage Channel

As per guidance in EA memo dated 29th May 2002, and subsequent site meeting on the 26th March 2003, a 2-Stage channel will be excavated to compensate for any loss of channel capacity.

Dimensions of the 2-stage channel are: 42m long (10m u/s &d/s of riffle) and to a width of 2.0m and height of 1.5m. This adequately takes into account recommendations made by the EA to widen by 115% (1.5m) of the original channel width (10m).

This removes a volume of 126m3
The volume of gravel required for riffle 2 is 110m3
All excavated material will be deposited along the field hedge boundary.
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Riffle 3

30m long x 11m wide x 1m deep x 1.8 (tonnes/density) / 2 = 297 tonnes, 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.

There is no available spoil heap, and thus all gravel required will need to be imported to site.

Construction of the riffle should be split into two levels.

1) Spawning substrate

This needs to be approx. 35cm in depth and well mixed in sizes ranging from 5 -40mm+.

This size of substrate facilitates deposition and development of eggs by species such as barbel, chub, dace and brown trout.
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2) Core layer

The riffle can be ‘keyed in’ by using larger and cheaper whole stone reject gravel, as this is below the depth need by the above species of fish to successfully spawn on.

Therefore the following quantities are required:

Spawning substrate

30m long x 11m wide x 0.35m deep x 1.8 (tonnes/density) / 2 = 90 tonnes

Core layer

30m long x 11m wide x 65cm deep x 1.8 (tonnes/density) / 2 = 207 tonnes.


Costs and quantities of Riffle 3.

90t (5-40mm+) x £14 per t = £1260 + VAT

207t (reject) x £10.30 per t + £2132 + VAT

Total cost £3392 +VAT (including delivery) – including VAT £3985.60

Riffle 3 - Two Stage Channel

As per guidance in EA memo dated 29th May 2002, and subsequent site meeting on the 26th March 2003, a 2-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 3.5m and height of 1.0m. This adequately takes into account recommendations to widen the channel by 115% (1.65m)

This removes a volume of 175m3
The volume of gravel required for riffle 3 is 165m3

All excavated material will be deposited along the field hedge boundary.

Summary of quantities & costs of imported gravel for all three riffles

Riffle 1

160 tonnes (reject) @ £10.30 pt = £1648.00

Riffle 2

91tonnes (reject) @ £10.30 pt = £937.30

Riffle 3

207 tonnes (reject) @ £10.30 pt = £2132
90 tonnes (5-40mm) @ £14 pt = £1260

Totals

Reject – 340 tonnes = £3502
(5-40mm) - 90 tonnes = £1260

Total Cost £5977.30 +VAT - Including VAT = £7023.32


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

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)



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 to download below in Acrobat PDF:
4. Technical Specifications.pdf [197k]

5.0 – Bill of quantities & Project Costs

Work / Description	Unit	Quantity	Cost (ex VAT)
Plant Hire (Inc. operator & fuel)
15.5m 360 Excavator	Days	10
10t Hydrema	Days	10
JCB 3X	Days	10
Mobile Screener	Days	5
Plant Delivery Costs
Materials
Introduce Gravel / Stone	Tonnes	548
Excavate 2 Stage Berms & Off-River Refuges	Tonnes	430 (minimum)
Install 900mm Twin-Wall Culvert Pipes	6m @ £80m	2
Erect Stock Proof Fencing 3 x HT (River Bank)	£2.44 per m	436m
Erect Stock Proof fencing 3 X HT (Off River Refuges)*	£440 ea.	2
Stiles & Hand Posts x 5	£38 ea.	5
Erect Post & Rail Cattle Drink	£150 ea.	1
Gravel for Cattle Drink	Tonnes	63
Gravel for Top Dressing Cattle Drink Rifle	Tonnes	46
Faggots**	2m @ £2.5 ea	10
Stakes	1m @ £2.5ea	20
Management
Water Vole Mitigation	Strimming /Trapping, etc.
Project Manager	£ day	12
Contingency Fund***
Total Ex VAT
Total Inc VAT

Notes:
Materials purchased directly by REEF will be subject to a 10% handling charge.
Q = Quote Provided
* Off river refuge costs are more expensive per m than river bank fencing due to the number of changes in direction and strainer post required,
*** Faggots may be able to made on-site for free depending on available materials.
** If project runs-over predicted 10 days, costs for plant hire per day are £+VAT – Therefore it is wise to build into the budget a contingency fund to allow for unforeseen circumstances, events, etc.………….
All quotes are in appendix 2


NB: The above is available in Acrobat PDF form:
5-Bill-of-Quantities.pdf [65k]

The aims of the project are to increase the overall Biodiversity and sustainable conservation management of the site, and the wider Wensum cSAC. In this case the cSAC designation is confined to the river channel and does not extent to any land pockets. There is no County Wildlife Site designation and the land on the RHB has been entered into the Broads Environmentally Sensitive Area (ESA) scheme, administered by the Department for Environment Food & Rural Affairs.

A high level target of the Conservation Strategy for the River Wensum ( EA /EN 2000) is to ‘encourage self-sustaining populations of native fish species’

Issue 13 of the strategy identifies a ‘Lack of Habitat Diversity’ as a factor influencing sustainable fish populations. The subsequent recommended action arising from issue 13 is to ‘Implement rehabilitation measures in suitable stretches of river’

The project also has direct links in with seven ‘Nature Conservation Objectives’ of the Strategy:

• 1- (Physical Structure)
• 5 - (Fast Flowing Water)
• 6 - (Riparian and In-River Features)
• 7 - (Siltation)
• 9 - (Fisheries)
• 10 - (Invertebrates)
• 12 - (Mammals)

Therefore it would seem that the aims of the project closely match those of the Conservation Strategy . That said, it is of vital importance that each component of the design and restoration work proposals are considered individually to assess any impacts on SAC Features, and Biodiversity Action Plan (BAP) Species. This is essential if the overall integrity of the site is to be protected and conserved.

The SAC features are:

• Desmoulins Snail (Vertigo moulinsiana)
• Water Crowfoot (Ranunculion fluitantis)
• Bullhead (Cottus gobio)
• Brook Lamprey (Lampetra planeri)
• White Clawed Crayfish (Austropotamobius pallipes)

Further to this there are several species specific targets included in the ‘Conservation Objectives for the European interest features of the SSSI’( En 2000) that the project contributes. At the end of each target the component of the project contributing to this will be in brackets.

Bullhead

Attribute -Habitat Structure

Measure – Extent of gravel / pebble dominated structure
Target – maintain & where necessary restore. (riffles)

Measure- Extent of slack-water refuges
Target- Maintain & where necessary restore (Off-river refuges)

Measure – River form
Target – maintain & where necessary restore the characteristic physical form of the river channel (Restoration of riffle – pool sequence)

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Brook Lamprey

Attribute – Habitat Structure

Measure – Area of spawning habitat
Target – Maintain & where necessary restore (Riffles)

Measure - Area of nursery habitat
Target – Maintain and where necessary restore (Off river-refuges)

Measure – Area of emergent riparian vegetation
Target – Maintain a high extent throughout the river system (Fencing of one bank)

Measure - River Form
Target – Maintain and where necessary restore the physical form of the river channel (Restoration of riffle-pool sequence)

White-clawed crayfish

Attribute – Habitat Structure

Measure – Extent of cobbles / boulders
Target – Maintain to an extent characteristic of the river type. (riffles)

Measure – Density of bank side refuges
Target – Submerged tree root systems and/or crevices in banks ides should be available at intervals (Faggot narrowing)

Measure – Extent of submerged and marginal vegetation
Target – Maintain patchy cover where characteristic of the river type (fencing of one bank)

Measure – Extent of overhanging riparian vegetation
Target – Should be present intermittently along the bank throughout the year (Fencing of one bank)

Measure – River form
Target – Maintain and where necessary restore the physical form of the river channel. (Restoration of riffle-pool sequence)

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BAP Species are:

• Water Vole (Arvicola terrestris)

• Otter (Lutra lutra)

The Wensum is not just a tick list of SAC Features and BAPs and this design reflects this fact, by way of its holistic approach to river management . This projects design aims are to improve and restore the overall ecological quality of the Wensum, at Costessey. It will hopefully provide numerous ‘knock-on’ positive effects to a range of species not necessarily within the remit of the high level Habitats Directive.

This section will consider two main areas of potential impacts on SAC and BAP features and species.

1) Impacts of the individual components of the project
2) Impacts of the works required during completion of the project

6.1 - Potential Impacts on SAC Features

1) – Desmoulins Snail.

Potential Impacts of scheme:

• Removal of habitat during construction of 2-Stage Channel
• Accidental damage by machinery operators during construction

Distribution data for this species still remains patchy in the Wensum Valley. It is known that the snail generally favours Glyceria fringes greater than 5m wide. There is the possibility it may be present at proposed riffle sites, however most, if not all of the fringes are less than 5m wide. The major potential impact of the works would be from the creation of 2-stage channels running parallel to riffles The excavations will be taken from the ‘wet end’ which is less favourable to the snail which prefers drier habitat closer to the bank. The creation of a shallow berm will in the long term increase the overall width of Glyceria habitat available to the snail. Currently Glyceria is present on the site in isolated patches and not a continuous strip. The 2-stage channel is a condition placed on the project by EA Development Control to compensate for any loss of channel capacity via the introduction of gravel riffles. The 2-Stage Channel would be excavated to cause minimum disruption to the remaining proportion of the fringe. The extent of each feature will be staked out to guide machinery operators. All machinery operators will be fully briefed before commencement of works to avoid potential environmental damage.

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2) – Water Crowfoot

Potential Impacts of scheme:

• Smothering due to emplacement of gravel riffles

A prime consideration of many river rehabilitation projects is to change ‘Channel Dimensions’ to restore previous habitats. The majority have been carried out to redress deficiencies in some reaches due to elevated siltation, lack of discharge, or inadequate velocities associated with past channel modification, such as over-deepening and over-widening. The guiding principles of many rehabilitation projects have been to increase velocity that has invariably led, at least in the short term, to the re-appearance of Ranunculus. The exact reasons for this are uncertain, but some very significant improvements were achieved on Hampshire chalk rivers in the mid 1990s when Ranunculus was declining elsewhere. (Rannunculus and Chalk Rivers –EA 2000)

Cattle have been implicated in some studies as a major contributory cause to changing natural channel dimensions and fencing has been employed locally with good effect to help in-stream vegetation recovery and bring about channel narrowing.

Crowfoot has been identified on site, albeit in one or two isolated locations. It is hoped that by creating habitat conditions suitable for Crowfoot that it will spread form its current limited distribution when roots are pulled up and re-settle during winter flood events. Wherever possible if Ranunculus fluitantis is found to exist on proposed riffle locations, they will be shifted to avoid burial. This option would of course be subject to impacts on other SAC features. This may option may not be practicable, however the long term aim of the project is to provide a large increase in habitat suitable for colonisation by crowfoot. Studies have shown some success with the re-planting of crowfoot in areas suitable for re-colonisation. Visual observations at Bintry on the Wensum indicate an increased distribution of crowfoot on and downstream of created riffle sites.

Fencing the entire double bank of the project is not advocated in the interest of developing a patchwork of different habitats so that diversity is maintained to sustain a rich marginal invertebrate population.

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2) - Bullhead

Potential scheme impacts:

• Temporary disturbance during works

***Need distribution data from EA***

The project will benefit this species by restoring pool-riffle sequences, which are the favoured habitat. The re-introduction of gravel and stones will provide spawning substrate and daytime refuges. The improvement to the health of the river system will boost invertebrate biomass, therefore increasing food supply to the fish. The project should have a positive long term status of the species. It will assist in establishing favourable conservation status for the species as required under the Species and Habitats Directive.

Bullhead are likely to be in the reach where the restoration work will be undertaken. It is therefore possible that the population will suffer temporary disturbance during the works. Their spawning season is March to April. There are no plans for in-channel works for this site at this time. Being highly mobile, it is unlikely that restoration works will directly affect these species. Re-colonisation of the new habitat is likely to be rapid. Long term benefits to the population will greatly outweigh any short term disturbance.

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3) – Brook Lamprey

Potential Scheme Impacts

• Temporary disturbance during works
• Removal of juvenile and adult habitat

*** Distribution Data Need from EA***

As with Bullhead the restoration of a pool-riffle sequence, is the favoured spawning habitat for lampreys. Spawning typically occurs when water temperatures reach a constant 10-110C, around April time. Again there are no planned in-channel works during this period. After hatching young larvae (ammocoetes) leave the spawning nest and distribute themselves by drifting downstream and burrowing in suitable areas of silty and sandy material. Recent surveys by the local EA Fisheries Team into ammoceate and adult habitat selection found vegetated point bars containing Glyceria to be important habitat (pers comm. A. Hindes, EA Fisheries). Straight sections with Glyceria ‘hover’ were found to be poor habitat for Lamprey. The 2-stage channel excavations are planned to take place in straight areas of ‘hover’ and no works are planned on vegetated point bars. Glyceria beds only occur at the site of riffle 2’s Two-Stage Channel. During the works both Project Manager and machine operators will keep an eye out for any lampreys should they emerge from the excavated material, and place them back into the river down-stream. This mitigation technique was recently utilised by the EA on channel works at Great Ryburgh on the River Wensum.

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4) – White Clawed Crayfish

Potential Scheme Impacts:

• Temporary disturbance during works

Environment Agency records show that native crayfish have not been recorded at this Site. Although records are patchy it should be assumed likely that native crayfish are present, albeit perhaps in limited numbers. Thankfully Signal Crayfish have not been recorded on the site either. With this in mind it could be concluded, that, at present, habitat availability is the limiting factor influencing native crayfish distribution. At present the habitat suitable for crayfish at Costessey is extremely limited. The creation of chalk stream riffles will create one of the major habitat types preferred by crayfish. Submerged tree roots, which occur on the top section of the LHB, form important ‘ woody debris’ habitat. The creation of the ‘Faggot Berm’ on the LHB will increase the available habitat for crayfish, especially juveniles.

To minimise impacts of the works on Crayfish it is proposed that a hand search / netting survey is carried out immediately prior to the start of the works in August-October. This will have to be conducted by a licensed surveyor under Section 5 of the Wildlife &Countryside Act (1981) . Coincidentally this is the peak time of activity for crayfish, and hence the best time to ascertain their presence or absence. Should any crayfish be caught they will be re-located upstream away from the works. Upstream of the site is faster flowing gravel bed habitat below the Costessey Mill. If large numbers of crayfish are found by hand searching, traps may have to be deployed as a more efficient method of capture and re-location. Survey and trapping methodology from the recent publication ‘Guidance on Works Affecting White-clawed Crayfish’ (EN 2000) will be followed. As water voles are present on site it is recommended that ‘Vole-friendly’ traps are used. These are not traps but artificial refuges, comprising clusters of tubes.

The following ‘best practice’ will be adopted during the works to protect white-clawed crayfish.

• Disturbance to river banks kept to a minimum.
• Works on the channel and banks will be done in short sections.
• Works are to be conducted where current crayfish habitat is unsuitable
• Any crayfish accidentally removed during works to be re-located upstream to areas of equal habitat quality.
• Larger flints to be placed on riffles to act as refuges from high flows.

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6.2. Potential Impacts on BAP Species

1) Water Vole & Otter

The Project commissioned Norfolk Wildlife Trust (NWT) to conduct a vole and otter survey in December 2003.

The summary of the survey results, assessment and recommendations is below. A full copy is provided in Appendix 4.

Survey results

Water vole

A water vole colony occupies parts of both banks within the section identified for in-channel and bank modifications. Burrows and latrines, which indicate usage by water voles as core areas for breeding, were located.

Otter

No active otter holts or resting sites were located during the survey
There is a potential holt/resting site along the right-hand bank and areas along and adjacent to the left-hand bank could also support holts/resting sites.

Impact Assessment & Recommendations

The primary point of conflict relates to potential damage to, or obstruction of, water vole burrows which are protected under the Wildlife & Countryside Act
It is recommended that potential impacts on water voles be minimised by adopting three approaches:

Adherence to recommended limits to timing of works.
Implementation of operational best practice procedures for contractors throughout the works

Implementation of specific mitigation measures in relation to riffle installation and bank re-profiling where there is a potential impact on water vole burrows

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Water vole

Impact Mitigation Recommendation 1
Removal and damage to vegetation along the river banks during riffle installation works and any bank re-profiling should be minimised in order to ensure that water vole food sources and concealment is maintained into the winter period.

Impact Mitigation Recommendation 2
The riffle installations and any associated bank re-profiling should be completed no later than the end of November in order to avoid the important water vole over-wintering phase (December-February)

Impact Mitigation Recommendation 3
During the riffle installation process, implement an operational 3m exclusion zone for machinery, in parallel with the right-hand bank.
Mark the boundary of the exclusion zone clearly using coloured tape.
In addition to the implementation of buffer zones along the bank to prevent accidental damage or obstruction to burrows during riffle installation and bank re-profiling, general operational best practice procedural guidelines should also be adopted.

Impact Mitigation Recommendation 4
During any materials reclamation operations along the right-hand bank, implement an operational 3m exclusion zone for machinery, in parallel with the river bank.
Mark the boundary of the exclusion zone clearly using coloured tape.

Impact Mitigation Recommendation 5
Ensure that the potential impact on water vole burrows resulting from an increase in channel water levels following riffle installation is minimised.
Water levels should rise gradually, over a minimum period of 4-5 days
In relation to Riffles 1 & 3, the impact on water vole burrows of bank re-profiling requires mitigation

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There are two options:

Undertake bank re-profiling to create two-stage channel and implement measures to mitigate impact on water vole burrows that will be damaged or destroyed.

Drop two-stage channel approach to compensation of in-channel capacity lost.
The compensation for loss of in-channel capacity foregone can be achieved by widening and deepening two existing ditches along the right-hand bank, upstream and downstream of Riffle 3 respectively (where water vole burrows are not present).
This option also has the advantage that no mitigating measures are required

Preferred option is 2.

Drop proposal to construct two-stage channel along right-hand bank at Riffles 1 & 3 as this will remove water vole habitat and incur mitigating measures

Create compensatory valley-floor flood capacity by widening and deepening two existing ditches along the right-hand bank, upstream and downstream of Riffle 3 respectively (where water vole burrows are not present)
In relation to Riffle 2, bank re-profiling does not impact on water vole burrows so mitigation is not required

Impact Mitigation Recommendation 6
Drop proposal to widen the ditch mouth along the left-hand bank immediately upstream of Riffle 2.

Preferred option

Opt instead to reconnect and open out to the main river channel, two ditches along the right-hand bank located upstream and downstream of Riffle 3

Otter

The proposed works do not impact upon any otter holt or resting sites as none are known to be in current use. The timing of the works does not have any implications in terms of otters
A licence issued by DEFRA is not required provided there are no active holt or resting sites in current use prior to commencement of the works

Impact Mitigation Recommendation 7
Re-check the large bramble patch along the right-hand bank, which constitutes a potential otter holt/resting site, for signs of current usage one month prior to the commencement of works

Impact Mitigation Recommendation 8
During the installation of Riffle 2, implement an operational 10m exclusion zone for machinery around the large bramble patch along the right-hand bank, which constitutes a potential otter holt/resting site

Mark the boundary of the exclusion zone clearly using coloured tape

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Important Note:

NWT’s preferred option at riffles 1 and 3 is to drop the creation of a two-stage channel, preferring the creation of more flood plain storage.

However concerns from the Environment Agency over reductions in channel capacity, requires the creation of 2-stage channels as a prerequisite condition to be incorporated into the design prior to submitting application for Land Drainage Consent.

This has been incorporated into the Norfolk Wildlife Trust report which recommended the following mitigation measures at Riffles 1&3. This proposal is fully supported by the project.

Impacts in relation to bank re-profiling in association with riffle installation.
Re-profiling is proposed along the right-hand bank in order to create a two-stage channel. The aim is that a two-stage channel will provide volume compensation for loss of channel capacity due to riffle installation.

Direct damage to or destruction of burrows and disturbance to voles within them will ensue within Riffle sections 1 and 3 where water vole burrows are present along the right-hand bank (three and two burrows respectively).

In relation to Riffles 1 & 3, the impact on water vole burrows of bank re-profiling requires mitigation.
No water vole burrows were located along the right-hand bank corresponding to Riffle 2, so no impacts are envisaged in relation to the construction of a two-stage channel

In relation to Riffle 2, bank re-profiling does not impact on water vole burrows so mitigation is not required
Options for mitigation of impacts at Riffle 1 & 3
Once colonised by marginal aquatic vegetation, berms (shelves) created by the construction of a two-stage channel potentially yield benefits for water voles by providing additional habitat. However, the right-hand bank corresponding to Riffles 1 and 3 is a steep earth bank providing opportunities for burrowing – this steep bank profile would be lost and water vole breeding/burrow habitat removed from the river bank.

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There are two options:

Undertake bank re-profiling to create two-stage channel and implement measures to mitigate impact on water vole burrows that will be damaged or destroyed.

Drop two-stage channel approach to compensation of in-channel capacity lost.
Create compensatory river valley floor flood capacity by expanding the width and depth of two existing ditches along the right-hand bank adjacent to Riffle 3.

Option 1 –
Implement measures to mitigate the impact of bank re-profiling at Riffles 1 & 3

Displacement & exclusion technique
This involves the complete removal of bank side vegetation and emergent marginal vegetation growing in the river channel. Removal of vegetation renders the habitat unsuitable, encouraging water voles to vacate burrows, dispersing to surrounding locations.

The method requires the availability of suitable adjacent habitat for displaced water voles to move into

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Timing

Late August/September is an optimal period for undertaking the displacement technique and the technique will also still be feasible during early October.

The technique should not be attempted during the period November-February

Displacement & exclusion technique procedure

Stage 1. The locations of burrows should be marked with flagged canes before work begins.
Remove all current signs (droppings, latrines, etc.) of water vole activity.

Stage 1. Must be undertaken by a competent surveyor/ecologist.

Stage 2. Remove all vegetation within the entire displacement zone encompassing the length of bank supporting the burrows plus 10m in all directions both linearly and laterally away from the river bank.
Emergent marginal aquatic vegetation, within the channel, should also be removed to water level if present
All contractors used in the displacement procedure to be fully briefed.
A hand-held brushcutter should be used taking care not to damage banks or damage/block water vole burrows.

Stage 3. Rake the vegetation and remove off-site.
Rake to achieve bare earth or as near as possible. This is important as water voles may remain if vegetation is left in situ, as this material can still provide food and concealment from predators.
Stages 2 & 3 to be supervised by a competent surveyor/ecologist.

Stage 4. Leave displacement zone undisturbed to allow any water voles present to vacate.

Stage 5. After four days, erect water vole-proof fencing around displacement zone to form an exclosure (which prevents any water voles subsequently trapped from returning to their burrows once they have been released outside the exclosure).

Stage 5. To be supervised by a competent surveyor/ecologist.

Stage 6. Re-survey the exclusion zone for fresh signs of water vole activity four days after the vegetation has been completely removed.
Set baited live-capture traps in suitable locations (adjacent to newly located water vole field-signs or where signs were located prior to strimming, with a minimum of one trap per 5m along bank) within the exclosure, to capture any animals remaining. Traps to be checked at least three times per day.

Stage 6. Must be undertaken by a competent surveyor/ecologist.
Any animals displaced by the works will be able to locate suitable adjacent habitat along the left-hand bank. During late August/September the water vole population will be both at its highest level and most mobile, with juveniles dispersing from the colony.

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Trapping & translocation procedure

If fresh signs of water vole activity are located after four days, this indicates that the burrows may not have been completely vacated and this will be confirmed should any water voles subsequently be trapped.
However, it is expected that the displacement technique procedure should displace any water voles present and that trapping will be negative (see Bennett et al. 2001).
Traps to be set for a minimum of five days and trapping to continue for a period of five consecutive days when no water voles are captured before clearance given to commence bank re-profiling works.
Exclosure to remain in situ (to prevent water voles returning), until works commence.
Any water voles trapped inside the exclosure to be translocated to a nearby site (that has been identified as suitable prior to commencement of the works).
This procedure must be undertaken by a competent surveyor/ecologist.
The success of translocating water voles is as yet unproven (Bennett et al. 2001)

In keeping with the sustainability ethos of the project. The recommendation of NWT to create more compensatory flood storage / habitat has been retained in the design with the creation of the Off-river refuges. These features will remain, as well as the 2-stage channels. The refuges are to be profiled to a ‘vole friendly’ design. (See Technical specification 4.0). There are no mitigation measures required for this option.

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6.3 - General Impacts

Timing of Works

The project works are to be conducted in the period August/September/October. This takes into account more generalised considerations on the effects on bird breeding, and salmonid and coarse fish spawning seasons. The timing also links in with mitigation measures proposed for white-clawed crayfish and water voles. This time period is also optimal due to the fact that the banks will be at their driest and plant machinery will not get stuck and cause damage to grassland habitat.

Plant Access

Plant will arrive from the farm track leading from Costessey Mill. Specific considerations to SAC features are mentioned earlier in this report. Before work commences all personnel involved with the project will be fully briefed and maps will be provided showing access routes that will also be marked to ensure no unnecessary damage occurs on site . Machinery will track along the river bank away from marshy areas and features such as vole burrows. Recent works that REEF project managed on the Stiffkey adopted this best practice.

There maybe additional temporary impacts from tracking machinery to and from work areas. As the RHB marshes do not support grassland of high conservation value, this is unlikely to be a significant concern. There may be also be a further temporary impact arising from the disposal of slubbings from the off-river refuges. This material will be spread within reach of the excavator. The marsh is semi-improved, and does not support species rich grassland. Spreading of slubbings is an established practice that has occurred for many cycles of both Environment Agency and Internal Drainage Board maintenance cycles.

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6.4 – Impact Conclusions

Analysis of available ecological and fisheries data suggest that the stretch of river that forms the scheme to be in poor ecological condition. Data from the River Rehabilitation Feasibility Study conducted by the Environment Agency in 1998 suggests the river fishery is in a state of decline. Survey work forming Phase 2 of the Feasibility study identified poor habitat quality as a principle cause of this decline.

The long term impacts of the scheme will be positive for ecological and fisheries considerations. The impacts on protected species are also concluded to be positive in the long term.

There may be temporary negative impacts on populations of water voles in areas where contractors will be working. The project fully supports the recommendations of Norfolk Wildlife Trust to mitigate any such impacts.

 

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The SAC features are:

• Desmoulins Snail (Vertigo moulinsiana)
• Water Crowfoot (Ranunculion fluitantis)
• Bullhead (Cottus gobio)
• Brook Lamprey (Lampetra planeri)
• White Clawed Crayfish (Austropotamobius pallipes)

Further to this there are several species specific targets included in the ‘Conservation Objectives for the European interest features of the SSSI’( En 2000) that the project contributes. At the end of each target the component of the project contributing to this will be in brackets.

Bullhead

Attribute -Habitat Structure

Measure – Extent of gravel / pebble dominated structure
Target – maintain & where necessary restore. (riffles)

Measure- Extent of slack-water refuges
Target- Maintain & where necessary restore (Off-river refuges)

Measure – River form
Target – maintain & where necessary restore the characteristic physical form of the river channel (Restoration of riffle – pool sequence)

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Brook Lamprey

Attribute – Habitat Structure

Measure – Area of spawning habitat
Target – Maintain & where necessary restore (Riffles)

Measure - Area of nursery habitat
Target – Maintain and where necessary restore (Off river-refuges)

Measure – Area of emergent riparian vegetation
Target – Maintain a high extent throughout the river system (Fencing of one bank)

Measure - River Form
Target – Maintain and where necessary restore the physical form of the river channel (Restoration of riffle-pool sequence)

White-clawed crayfish

Attribute – Habitat Structure

Measure – Extent of cobbles / boulders
Target – Maintain to an extent characteristic of the river type. (riffles)

Measure – Density of bank side refuges
Target – Submerged tree root systems and/or crevices in banks ides should be available at intervals (Faggot narrowing)

Measure – Extent of submerged and marginal vegetation
Target – Maintain patchy cover where characteristic of the river type (fencing of one bank)

Measure – Extent of overhanging riparian vegetation
Target – Should be present intermittently along the bank throughout the year (Fencing of one bank)

Measure – River form
Target – Maintain and where necessary restore the physical form of the river channel. (Restoration of riffle-pool sequence)

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BAP Species are:

• Water Vole (Arvicola terrestris)

• Otter (Lutra lutra)

The Wensum is not just a tick list of SAC Features and BAPs and this design reflects this fact, by way of its holistic approach to river management . This projects design aims are to improve and restore the overall ecological quality of the Wensum, at Costessey. It will hopefully provide numerous ‘knock-on’ positive effects to a range of species not necessarily within the remit of the high level Habitats Directive.

This section will consider two main areas of potential impacts on SAC and BAP features and species.

1) Impacts of the individual components of the project
2) Impacts of the works required during completion of the project

6.1 - Potential Impacts on SAC Features

1) – Desmoulins Snail.

Potential Impacts of scheme:

• Removal of habitat during construction of 2-Stage Channel
• Accidental damage by machinery operators during construction

Distribution data for this species still remains patchy in the Wensum Valley. It is known that the snail generally favours Glyceria fringes greater than 5m wide. There is the possibility it may be present at proposed riffle sites, however most, if not all of the fringes are less than 5m wide. The major potential impact of the works would be from the creation of 2-stage channels running parallel to riffles The excavations will be taken from the ‘wet end’ which is less favourable to the snail which prefers drier habitat closer to the bank. The creation of a shallow berm will in the long term increase the overall width of Glyceria habitat available to the snail. Currently Glyceria is present on the site in isolated patches and not a continuous strip. The 2-stage channel is a condition placed on the project by EA Development Control to compensate for any loss of channel capacity via the introduction of gravel riffles. The 2-Stage Channel would be excavated to cause minimum disruption to the remaining proportion of the fringe. The extent of each feature will be staked out to guide machinery operators. All machinery operators will be fully briefed before commencement of works to avoid potential environmental damage.

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2) – Water Crowfoot

Potential Impacts of scheme:

• Smothering due to emplacement of gravel riffles

A prime consideration of many river rehabilitation projects is to change ‘Channel Dimensions’ to restore previous habitats. The majority have been carried out to redress deficiencies in some reaches due to elevated siltation, lack of discharge, or inadequate velocities associated with past channel modification, such as over-deepening and over-widening. The guiding principles of many rehabilitation projects have been to increase velocity that has invariably led, at least in the short term, to the re-appearance of Ranunculus. The exact reasons for this are uncertain, but some very significant improvements were achieved on Hampshire chalk rivers in the mid 1990s when Ranunculus was declining elsewhere. (Rannunculus and Chalk Rivers –EA 2000)

Cattle have been implicated in some studies as a major contributory cause to changing natural channel dimensions and fencing has been employed locally with good effect to help in-stream vegetation recovery and bring about channel narrowing.

Crowfoot has been identified on site, albeit in one or two isolated locations. It is hoped that by creating habitat conditions suitable for Crowfoot that it will spread form its current limited distribution when roots are pulled up and re-settle during winter flood events. Wherever possible if Ranunculus fluitantis is found to exist on proposed riffle locations, they will be shifted to avoid burial. This option would of course be subject to impacts on other SAC features. This may option may not be practicable, however the long term aim of the project is to provide a large increase in habitat suitable for colonisation by crowfoot. Studies have shown some success with the re-planting of crowfoot in areas suitable for re-colonisation. Visual observations at Bintry on the Wensum indicate an increased distribution of crowfoot on and downstream of created riffle sites.

Fencing the entire double bank of the project is not advocated in the interest of developing a patchwork of different habitats so that diversity is maintained to sustain a rich marginal invertebrate population.

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2) - Bullhead

Potential scheme impacts:

• Temporary disturbance during works

***Need distribution data from EA***

The project will benefit this species by restoring pool-riffle sequences, which are the favoured habitat. The re-introduction of gravel and stones will provide spawning substrate and daytime refuges. The improvement to the health of the river system will boost invertebrate biomass, therefore increasing food supply to the fish. The project should have a positive long term status of the species. It will assist in establishing favourable conservation status for the species as required under the Species and Habitats Directive.

Bullhead are likely to be in the reach where the restoration work will be undertaken. It is therefore possible that the population will suffer temporary disturbance during the works. Their spawning season is March to April. There are no plans for in-channel works for this site at this time. Being highly mobile, it is unlikely that restoration works will directly affect these species. Re-colonisation of the new habitat is likely to be rapid. Long term benefits to the population will greatly outweigh any short term disturbance.

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3) – Brook Lamprey

Potential Scheme Impacts

• Temporary disturbance during works
• Removal of juvenile and adult habitat

*** Distribution Data Need from EA***

As with Bullhead the restoration of a pool-riffle sequence, is the favoured spawning habitat for lampreys. Spawning typically occurs when water temperatures reach a constant 10-110C, around April time. Again there are no planned in-channel works during this period. After hatching young larvae (ammocoetes) leave the spawning nest and distribute themselves by drifting downstream and burrowing in suitable areas of silty and sandy material. Recent surveys by the local EA Fisheries Team into ammoceate and adult habitat selection found vegetated point bars containing Glyceria to be important habitat (pers comm. A. Hindes, EA Fisheries). Straight sections with Glyceria ‘hover’ were found to be poor habitat for Lamprey. The 2-stage channel excavations are planned to take place in straight areas of ‘hover’ and no works are planned on vegetated point bars. Glyceria beds only occur at the site of riffle 2’s Two-Stage Channel. During the works both Project Manager and machine operators will keep an eye out for any lampreys should they emerge from the excavated material, and place them back into the river down-stream. This mitigation technique was recently utilised by the EA on channel works at Great Ryburgh on the River Wensum.

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4) – White Clawed Crayfish

Potential Scheme Impacts:

• Temporary disturbance during works

Environment Agency records show that native crayfish have not been recorded at this Site. Although records are patchy it should be assumed likely that native crayfish are present, albeit perhaps in limited numbers. Thankfully Signal Crayfish have not been recorded on the site either. With this in mind it could be concluded, that, at present, habitat availability is the limiting factor influencing native crayfish distribution. At present the habitat suitable for crayfish at Costessey is extremely limited. The creation of chalk stream riffles will create one of the major habitat types preferred by crayfish. Submerged tree roots, which occur on the top section of the LHB, form important ‘ woody debris’ habitat. The creation of the ‘Faggot Berm’ on the LHB will increase the available habitat for crayfish, especially juveniles.

To minimise impacts of the works on Crayfish it is proposed that a hand search / netting survey is carried out immediately prior to the start of the works in August-October. This will have to be conducted by a licensed surveyor under Section 5 of the Wildlife &Countryside Act (1981) . Coincidentally this is the peak time of activity for crayfish, and hence the best time to ascertain their presence or absence. Should any crayfish be caught they will be re-located upstream away from the works. Upstream of the site is faster flowing gravel bed habitat below the Costessey Mill. If large numbers of crayfish are found by hand searching, traps may have to be deployed as a more efficient method of capture and re-location. Survey and trapping methodology from the recent publication ‘Guidance on Works Affecting White-clawed Crayfish’ (EN 2000) will be followed. As water voles are present on site it is recommended that ‘Vole-friendly’ traps are used. These are not traps but artificial refuges, comprising clusters of tubes.

The following ‘best practice’ will be adopted during the works to protect white-clawed crayfish.

• Disturbance to river banks kept to a minimum.
• Works on the channel and banks will be done in short sections.
• Works are to be conducted where current crayfish habitat is unsuitable
• Any crayfish accidentally removed during works to be re-located upstream to areas of equal habitat quality.
• Larger flints to be placed on riffles to act as refuges from high flows.

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6.2. Potential Impacts on BAP Species

1) Water Vole & Otter

The Project commissioned Norfolk Wildlife Trust (NWT) to conduct a vole and otter survey in December 2003.

The summary of the survey results, assessment and recommendations is below. A full copy is provided in Appendix 4.

Survey results

Water vole

A water vole colony occupies parts of both banks within the section identified for in-channel and bank modifications. Burrows and latrines, which indicate usage by water voles as core areas for breeding, were located.

Otter

No active otter holts or resting sites were located during the survey
There is a potential holt/resting site along the right-hand bank and areas along and adjacent to the left-hand bank could also support holts/resting sites.

Impact Assessment & Recommendations

The primary point of conflict relates to potential damage to, or obstruction of, water vole burrows which are protected under the Wildlife & Countryside Act
It is recommended that potential impacts on water voles be minimised by adopting three approaches:

Adherence to recommended limits to timing of works.
Implementation of operational best practice procedures for contractors throughout the works

Implementation of specific mitigation measures in relation to riffle installation and bank re-profiling where there is a potential impact on water vole burrows

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Water vole

Impact Mitigation Recommendation 1
Removal and damage to vegetation along the river banks during riffle installation works and any bank re-profiling should be minimised in order to ensure that water vole food sources and concealment is maintained into the winter period.

Impact Mitigation Recommendation 2
The riffle installations and any associated bank re-profiling should be completed no later than the end of November in order to avoid the important water vole over-wintering phase (December-February)

Impact Mitigation Recommendation 3
During the riffle installation process, implement an operational 3m exclusion zone for machinery, in parallel with the right-hand bank.
Mark the boundary of the exclusion zone clearly using coloured tape.
In addition to the implementation of buffer zones along the bank to prevent accidental damage or obstruction to burrows during riffle installation and bank re-profiling, general operational best practice procedural guidelines should also be adopted.

Impact Mitigation Recommendation 4
During any materials reclamation operations along the right-hand bank, implement an operational 3m exclusion zone for machinery, in parallel with the river bank.
Mark the boundary of the exclusion zone clearly using coloured tape.

Impact Mitigation Recommendation 5
Ensure that the potential impact on water vole burrows resulting from an increase in channel water levels following riffle installation is minimised.
Water levels should rise gradually, over a minimum period of 4-5 days
In relation to Riffles 1 & 3, the impact on water vole burrows of bank re-profiling requires mitigation

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There are two options:

Undertake bank re-profiling to create two-stage channel and implement measures to mitigate impact on water vole burrows that will be damaged or destroyed.

Drop two-stage channel approach to compensation of in-channel capacity lost.
The compensation for loss of in-channel capacity foregone can be achieved by widening and deepening two existing ditches along the right-hand bank, upstream and downstream of Riffle 3 respectively (where water vole burrows are not present).
This option also has the advantage that no mitigating measures are required

Preferred option is 2.

Drop proposal to construct two-stage channel along right-hand bank at Riffles 1 & 3 as this will remove water vole habitat and incur mitigating measures

Create compensatory valley-floor flood capacity by widening and deepening two existing ditches along the right-hand bank, upstream and downstream of Riffle 3 respectively (where water vole burrows are not present)
In relation to Riffle 2, bank re-profiling does not impact on water vole burrows so mitigation is not required
Impact Mitigation Recommendation 6
Drop proposal to widen the ditch mouth along the left-hand bank immediately upstream of Riffle 2.

Preferred option

Opt instead to reconnect and open out to the main river channel, two ditches along the right-hand bank located upstream and downstream of Riffle 3

Otter

The proposed works do not impact upon any otter holt or resting sites as none are known to be in current use. The timing of the works does not have any implications in terms of otters
A licence issued by DEFRA is not required provided there are no active holt or resting sites in current use prior to commencement of the works

Impact Mitigation Recommendation 7
Re-check the large bramble patch along the right-hand bank, which constitutes a potential otter holt/resting site, for signs of current usage one month prior to the commencement of works

Impact Mitigation Recommendation 8
During the installation of Riffle 2, implement an operational 10m exclusion zone for machinery around the large bramble patch along the right-hand bank, which constitutes a potential otter holt/resting site
Mark the boundary of the exclusion zone clearly using coloured tape

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Important Note:

NWT’s preferred option at riffles 1 and 3 is to drop the creation of a two-stage channel, preferring the creation of more flood plain storage.

However concerns from the Environment Agency over reductions in channel capacity, requires the creation of 2-stage channels as a prerequisite condition to be incorporated into the design prior to submitting application for Land Drainage Consent.

This has been incorporated into the Norfolk Wildlife Trust report which recommended the following mitigation measures at Riffles 1&3. This proposal is fully supported by the project.

Impacts in relation to bank re-profiling in association with riffle installation.
Re-profiling is proposed along the right-hand bank in order to create a two-stage channel. The aim is that a two-stage channel will provide volume compensation for loss of channel capacity due to riffle installation.

Direct damage to or destruction of burrows and disturbance to voles within them will ensue within Riffle sections 1 and 3 where water vole burrows are present along the right-hand bank (three and two burrows respectively).

In relation to Riffles 1 & 3, the impact on water vole burrows of bank re-profiling requires mitigation.
No water vole burrows were located along the right-hand bank corresponding to Riffle 2, so no impacts are envisaged in relation to the construction of a two-stage channel

In relation to Riffle 2, bank re-profiling does not impact on water vole burrows so mitigation is not required
Options for mitigation of impacts at Riffle 1 & 3
Once colonised by marginal aquatic vegetation, berms (shelves) created by the construction of a two-stage channel potentially yield benefits for water voles by providing additional habitat. However, the right-hand bank corresponding to Riffles 1 and 3 is a steep earth bank providing opportunities for burrowing – this steep bank profile would be lost and water vole breeding/burrow habitat removed from the river bank.

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There are two options:

Undertake bank re-profiling to create two-stage channel and implement measures to mitigate impact on water vole burrows that will be damaged or destroyed.

Drop two-stage channel approach to compensation of in-channel capacity lost.
Create compensatory river valley floor flood capacity by expanding the width and depth of two existing ditches along the right-hand bank adjacent to Riffle 3.

Option 1 –
Implement measures to mitigate the impact of bank re-profiling at Riffles 1 & 3

Displacement & exclusion technique
This involves the complete removal of bank side vegetation and emergent marginal vegetation growing in the river channel. Removal of vegetation renders the habitat unsuitable, encouraging water voles to vacate burrows, dispersing to surrounding locations.

The method requires the availability of suitable adjacent habitat for displaced water voles to move into

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Timing

Late August/September is an optimal period for undertaking the displacement technique and the technique will also still be feasible during early October.

The technique should not be attempted during the period November-February

Displacement & exclusion technique procedure

Stage 1. The locations of burrows should be marked with flagged canes before work begins.
Remove all current signs (droppings, latrines, etc.) of water vole activity.

Stage 1. Must be undertaken by a competent surveyor/ecologist.

Stage 2. Remove all vegetation within the entire displacement zone encompassing the length of bank supporting the burrows plus 10m in all directions both linearly and laterally away from the river bank.
Emergent marginal aquatic vegetation, within the channel, should also be removed to water level if present
All contractors used in the displacement procedure to be fully briefed.
A hand-held brushcutter should be used taking care not to damage banks or damage/block water vole burrows.

Stage 3. Rake the vegetation and remove off-site.
Rake to achieve bare earth or as near as possible. This is important as water voles may remain if vegetation is left in situ, as this material can still provide food and concealment from predators.
Stages 2 & 3 to be supervised by a competent surveyor/ecologist.

Stage 4. Leave displacement zone undisturbed to allow any water voles present to vacate.

Stage 5. After four days, erect water vole-proof fencing around displacement zone to form an exclosure (which prevents any water voles subsequently trapped from returning to their burrows once they have been released outside the exclosure).

Stage 5. To be supervised by a competent surveyor/ecologist.

Stage 6. Re-survey the exclusion zone for fresh signs of water vole activity four days after the vegetation has been completely removed.
Set baited live-capture traps in suitable locations (adjacent to newly located water vole field-signs or where signs were located prior to strimming, with a minimum of one trap per 5m along bank) within the exclosure, to capture any animals remaining. Traps to be checked at least three times per day.

Stage 6. Must be undertaken by a competent surveyor/ecologist.
Any animals displaced by the works will be able to locate suitable adjacent habitat along the left-hand bank. During late August/September the water vole population will be both at its highest level and most mobile, with juveniles dispersing from the colony.

Trapping & translocation procedure

If fresh signs of water vole activity are located after four days, this indicates that the burrows may not have been completely vacated and this will be confirmed should any water voles subsequently be trapped.
However, it is expected that the displacement technique procedure should displace any water voles present and that trapping will be negative (see Bennett et al. 2001).
Traps to be set for a minimum of five days and trapping to continue for a period of five consecutive days when no water voles are captured before clearance given to commence bank re-profiling works.
Exclosure to remain in situ (to prevent water voles returning), until works commence.
Any water voles trapped inside the exclosure to be translocated to a nearby site (that has been identified as suitable prior to commencement of the works).
This procedure must be undertaken by a competent surveyor/ecologist.
The success of translocating water voles is as yet unproven (Bennett et al. 2001)

In keeping with the sustainability ethos of the project. The recommendation of NWT to create more compensatory flood storage / habitat has been retained in the design with the creation of the Off-river refuges. These features will remain, as well as the 2-stage channels. The refuges are to be profiled to a ‘vole friendly’ design. (See Technical specification 4.0). There are no mitigation measures required for this option.

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6.3 - General Impacts

Timing of Works

The project works are to be conducted in the period August/September/October. This takes into account more generalised considerations on the effects on bird breeding, and salmonid and coarse fish spawning seasons. The timing also links in with mitigation measures proposed for white-clawed crayfish and water voles. This time period is also optimal due to the fact that the banks will be at their driest and plant machinery will not get stuck and cause damage to grassland habitat.

Plant Access

Plant will arrive from the farm track leading from Costessey Mill. Specific considerations to SAC features are mentioned earlier in this report. Before work commences all personnel involved with the project will be fully briefed and maps will be provided showing access routes that will also be marked to ensure no unnecessary damage occurs on site . Machinery will track along the river bank away from marshy areas and features such as vole burrows. Recent works that REEF project managed on the Stiffkey adopted this best practice.

There maybe additional temporary impacts from tracking machinery to and from work areas. As the RHB marshes do not support grassland of high conservation value, this is unlikely to be a significant concern. There may be also be a further temporary impact arising from the disposal of slubbings from the off-river refuges. This material will be spread within reach of the excavator. The marsh is semi-improved, and does not support species rich grassland. Spreading of slubbings is an established practice that has occurred for many cycles of both Environment Agency and Internal Drainage Board maintenance cycles.

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6.4 – Impact Conclusions

Analysis of available ecological and fisheries data suggest that the stretch of river that forms the scheme to be in poor ecological condition. Data from the River Rehabilitation Feasibility Study conducted by the Environment Agency in 1998 suggests the river fishery is in a state of decline. Survey work forming Phase 2 of the Feasibility study identified poor habitat quality as a principle cause of this decline.

The long term impacts of the scheme will be positive for ecological and fisheries considerations. The impacts on protected species are also concluded to be positive in the long term.

There may be temporary negative impacts on populations of water voles in areas where contractors will be working. The project fully supports the recommendations of Norfolk Wildlife Trust to mitigate any such impacts.

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NB:The above information is available in Acrobat PDF form:
6. Environment Impacts.pdf [212k]