Road Drainage and the Water Environment

Introduction

Water quality and quantity is not only essential for flora and fauna that live within it and that are dependent on it but it is also a valuable resource for anthropogenic related activities such as fisheries, drinking, hydropower and recreation. Protection of the water environment, including surface water and groundwater is a high priority. In addition, watercourses and lochs form key features in the landscape (refer to Chapter 7: Landscape Effects) and support important habitats and species (refer to Chapter 6: Ecology and Nature Conservation).

The objective of this chapter is to evaluate the importance of the existing water features within the locality of the proposed scheme, identify potential impacts from the proposed scheme, assess the likely degree of impact and propose mitigation measures to reduce or control impacts or risks to the water environment. Chapter 6: Ecology and Nature Conservation covers information on species present in the study area, whilst this chapter focusses on the water environment itself.

Methodology and Criteria for Evaluation of Water Environment

The methodology used for the Road Drainage and the Water Environment assessment has been based on DMRB Volume 11, Section 3, Part 10, also referred to as HD 45/09 (Highways Agency, 2009).

The framework for assessing the importance of water environment features, the magnitude of impact and the significance of impact is detailed in Tables 8.1, 8.2 and 8.3. Professional judgement has also been used in the evaluation of features and the assessment of impacts.

The methodology for sizing the proposed new box culvert is given in Section 8.7.1.1. The method used for assessing potential impacts on water quality during operation is described in Section 8.7.1.3.

Table 8.1 Importance of water environment features (based on DMRB Volume 11, Section 3, Part 10 HD 45/09 Table A4.3 with fluvial geomorphological characteristics added

Very High - Attribute has a high quality and rarity on regional or national scale.

Surface Water:

Water Framework Directive (WFD) Status ‘High’.

Site designated under European or UK legislation (SAC, SPA, SSSI, Ramsar site, salmonid water).

Species protected by European legislation.

Water feature displaying no signs of previous modification and/or experiencing no morphological pressures at the current time, with natural fluvial and sediment transportation processes.

Groundwater:

Aquifer providing a regionally important resource or supporting site protected under European and UK habitat legislation.

Flood Risk:

Floodplain or defence protecting more than 100 residential properties from flooding.

High - Attribute has a high quality and rarity on local scale.

Surface Waters:

WFD Status ‘Good’.

Species protected under European or UK habitat legislation.

Water feature displaying no/very few signs of previous modification and/or experiencing no/very few morphological pressures at the current time, with natural fluvial and sediment transportation processes.

Groundwater:

Aquifer providing locally important resource or supporting river ecosystem.

Flood Risk:

Floodplain or defence protecting between 1 and 100 residential properties or industrial premises from flooding.

Medium - Attribute has a medium quality and rarity on local scale.

Surface Waters:

WFD Status ‘Moderate’.

Water feature displaying signs of previous modification and/or pressures, but is recovering towards a natural state. Mostly natural fluvial and sediment transportation processes occurring.

Groundwater:

Aquifer providing water for agricultural or industrial use with limited connection to surface water.

Flood Risk:

Floodplain or defence protecting 10 or fewer industrial properties from flooding. 

Low - Attribute has a low quality and rarity on local scale.

Surface Waters:

WFD Status ‘Poor’, ‘Bad’.

Artificial channels (such as drains) and previously natural but now highly modified channels with very limited signs of natural recovery or natural fluvial and sediment transport processes occurring.

Groundwater:

Unproductive strata.

Flood Risk:

Floodplain with limited constraints and a low probability of flooding of residential and industrial properties.

Table 8.2 Estimating magnitude of impact (based on DMRB Volume 11, Section 3, Part 10 HD 45/09 Table A4.4 with fluvial geomorphological impacts added)

Major Adverse - Results in loss of attribute and/or quality and integrity of attribute.

Surface Water:

Failure of both soluble and sediment-bound pollutants in Highways Agency Water Risk Assessment Tool (HAWRAT) and compliance failure with Environmental Quality Standard (EQS) values.

Calculated risk of spillage > 2% annually.

Loss or extensive change to a fishery.

Loss or extensive change to a designated nature conservation site.

Causes deterioration in the overall water body status or WFD quality elements and prevents the water body from achieving an overall status of ‘Good’. Failure of hydromorphological elements (morphology, quantity and dynamics of flow) as a result of the works. Loss or extensive damage to physical habitat due to extensive modification. Replacement of a large extent of the natural bed and/or banks with artificial material. Extensive change to channel planform.

Groundwater:

Loss of, or extensive change, to an aquifer.

Potential high risk of pollution to groundwater from routine runoff – risk score >250.

Calculated risk of pollution from spillages > 2% annually.

Loss of, or extensive change to, groundwater supported designated wetlands.

Flood Risk:

Increase in peak flood level (1% annual probability) > 100mm.

Moderate Adverse - Results in effect on integrity of attribute, or loss of part of attribute.

Surface Waters:

Failure of both soluble and sediment-bound pollutants in HAWRAT but compliance with EQS values.

Calculated risk of pollution from spillages > 1% annually and < 2% annually.

Partial loss in productivity of a fishery.

Prevents a water body from achieving an overall status of ‘Good’. Failure of one or more hydromorphological elements (morphology, quantity and dynamics of flow) as a result of the works. Partial loss or damage to habitat due to modifications. Replacement of the natural bed and/or banks with artificial material (total length is more than 3% of water body length).

Groundwater:

Partial loss or change to aquifer.

Potential medium risk of pollution to groundwater from routine runoff – risk score 150-250.

Calculated risk of pollution from spillages > 1% annually and < 2% annually.

Partial loss of integrity of groundwater supported designated wetlands.

Flood Risk:

Increase in peak flood level (1% annual probability) > 50mm.

Minor Adverse - Results in some measurable change in attribute quality or vulnerability.

Surface Waters:

Failure of either soluble or sediment-bound pollutants in HAWRAT. Calculated risk of pollution > 0.5% annually and < 1% annually.

Potential for failure in one of the hydromorphological elements (morphology, quantity and dynamics of flow) as a result of the works. Slight change/deviation from baseline conditions or partial loss or damage to habitat due to modifications.

Groundwater:

Potential low risk of pollution to groundwater from routine runoff – risk score < 150. Calculated risk of pollution from spillages > 0.5% annually and < 1% annually. Minor effects on groundwater supported wetlands.

Flood Risk:

Increase in peak flood level (1% annual probability) > 10mm. 

Negligible - Results in effect on attribute, but of insufficient magnitude to affect use or integrity.

Surface Waters:

No risk identified by HAWRAT (passes both soluble and sediment-bound pollutants).

Risk of pollution from spillages < 0.5%.

No direct engineering impact but potential indirect impact due to proximity of the watercourse to the proposed route options, such as pollution by sediment release or reduction in riparian corridor.

Groundwater:

No measurable impact upon aquifer and risk of pollution from spillages < 0.5%.

Flood Risk:

Negligible change in peak flood level (1 % annual probability) <+/- 10mm.

Minor Beneficial - Results in some beneficial effect on attribute or a reduced risk of negative effect occurring.

Surface Waters:

HAWRAT assessment of either soluble or sediment-bound pollutants becomes Pass from the existing site where the baseline was a Fail condition.

Calculated reduction in existing spillage risk by 50% or more (when existing spillage risk is < 1% annually).

Potential for improvements in one of the hydromorphological elements (morphology, quantity and dynamics of flow) as a result of the works. Slight change/deviation from baseline conditions or partial improvement or gain in riparian or in-channel habitat. Note: beneficial impacts will only arise on impacted/modified/artificial water features. The greatest improvement will occur on water features that have a uniform morphology, acting as a transfer (larger watercourses) or sink (minor watercourses with limited flow and overgrown vegetation) of sediment and no signs of active fluvial processes.

Groundwater:

Calculated reduction in existing spillage risk by 50% or more to an aquifer (when existing spillage risk < 1% annually).

Flood Risk:

Reduction in peak flood level (1% annual probability) > 10mm.

Moderate Beneficial - Results in moderate improvement of attribute quality.

Surface Waters:

HAWRAT assessment of both soluble and sediment-bound pollutants becomes Pass from an existing site where the baseline was a Fail condition.

Calculated reduction in existing spillage by 50% or more (when existing spillage risk is > 1% annually).

Provides improvements in the water body that could lead to it achieving an overall status of ‘Good’. Improvement in one or more hydromorphological elements (morphology, quantity and dynamics of flow) as a result of the works. Partial creation of both in-channel and riparian habitat. Removal of an existing superfluous structure or artificial channel bed/bank.

Groundwater:

Calculated reduction in existing spillage risk by 50% or more (when existing spillage risk is > 1% annually).

Flood Risk:

Reduction in peak flood level (1% annual probability) > 50mm.

Major Beneficial - Results in major improvement of attribute quality.

Surface Waters:

Removal of existing polluting discharge, or removing the likelihood of polluting discharges occurring to watercourse.

The water body would improve in status from the current overall water body status and the improvements could lead to achieving ‘Good Status’. Extensive creation of both in-channel and riparian habitat, vastly improving the water body from baseline conditions. Removal of an existing superfluous structure or artificial channel bed/bank.

Groundwater:

Removal of existing polluting discharge to an aquifer or removing the likelihood of polluting discharges occurring.

Recharge of an aquifer.

Flood Risk:

Reduction in peak flood level (1% annual probability) > 100mm.

Table 8.3 Estimating the magnitude of impact in relation to the importance of the attribute

Importance of Attribute	Negligible	Minor	Moderate	Major
Very High	Neutral	Moderate/Large	Large/Very Large	Very Large
High	Neutral	Slight/Moderate	Moderate/Large	Large/Very Large
Medium	Neutral	Slight	Moderate	Large
Low	Neutral	Neutral	Slight	Slight/ Moderate

Policy and Regulatory Framework

This Road Drainage and the Water Environment assessment has been undertaken in accordance with DMRB Volume 11, Section 3, Part 10 (HD 45/09). It has also taken into consideration the legislation, guidance and policies listed below.

Design Manual for Roads and Bridges, Volume 11, Section 3, Part 10, HD 45/09.

EU Directive 2000/60/EC: Water Framework Directive (WFD).

Water Environment and Water Services (Scotland) Act 2003 (WEWS Act).

Flood Risk Management (Scotland) Act 2009.

Scottish Planning Policy, Scottish Government, 2014. pp. 57-60 (includes a section on Managing Flood Risk and Drainage).

SEPA Policy No. 19: Groundwater Protection Policy for Scotland V3, November 2009.

SEPA Policy No. 22: Flood Risk Assessment Strategy.

SEPA: The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended): A Practical Guide. Version 7.3 June 2016.

SEPA: The River Basin Management Plan for the Scotland River Basin District 2015-2027 (2015).

The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended).

Study Area

The study area included all surface waters within the proposed scheme footprint as well as beyond, where connectivity existed. Groundwater bodies were similarly identified within the proposed scheme area. Surface and groundwater protected areas were also identified within the proposed scheme area. Activities licensed under the Controlled Activities Regulations were also identified within 1.5 km of the site. Refer to Figure B2 in Appendix B.

Character of the Existing Baseline

Surface Water Features

The Allt Lagain Bhain is a watercourse within the River Ness catchment (Figure 8.1 and Figure 8.2). It rises in hills to the north of the A887 and flows in a general southwards direction under the A887 to join the River Moriston (Figure 8.3). It is approximately 3 km in length. Also refer to Section 6.5.1.2 and Figure 6.7 in Chapter 6: Ecology and Nature Conservation. The River Moriston flows in a north-easterly direction from the Cluanie Dam to the west to the shore of Loch Ness in the east.

The River Moriston and the Allt Lagain Bhain downstream of the A887 is designated as a SAC, the qualifying interests of which are:

• freshwater pearl mussel; and
• Atlantic salmon.

More detailed information on the Special Area of Conservation (SAC) is described in Chapter 6: Ecology and Nature Conservation, Section 6.5.1.

SEPA hold no data on the Allt Lagain Bhain, but have identified the River Moriston as a heavily modified water body (HMWB). This is because of flow regulation pressures resulting from Dundreggan Dam (located approximately 5 km downstream from the proposed scheme and part of the Garry-Moriston Hydroelectric Power Scheme) and morphological alterations due to forestry. In 2014, SEPA classified the reach of the river from Dundreggan Dam to Bun Loyne as having an overall condition of Good.

There is also a small un-named watercourse, less than 1 m wide, culverted under the A887 trunk road to the west of the bridge (see Figure 6.9 in Chapter 6). This small watercourse has been evaluated overall as being of Medium importance but it is important to note that it flows into the SAC designated River Moriston.

The Allt Lagain Bhain has habitat suitable for salmon and trout spawning upstream and downstream of the bridge (refer to Chapter 6: Ecology and Nature Conservation). The surface water resource upstream of the road bridge is, therefore, considered overall to be of High importance. Owing to the SAC designation (see Chapter 6) downstream of the road bridge, the Allt Lagain Bhain at this location is considered overall to be of Very High importance.

The River Moriston is considered to be of Very High value due to its designation as an SAC for Atlantic salmon and freshwater pearl mussels.

Land use in the immediate vicinity of the A887 Allt Lagain Bhain Bridge is dominated by native broadleaved woodland. The wider catchment of Allt Lagain Bhain includes rough pasture and coniferous forest. Some of the terrain is rocky particularly in the upland areas. It is considered that land use in the Allt Lagain Bhain catchment is unlikely to significantly adversely affect water quality in Allt Lagain Bhain. There have been recent works to upgrade the power lines nearby. The works could have posed a risk of adverse impact on the watercourse; this is no longer likely since the main construction works have largely been completed although further works to remove old power lines are likely to be required. There is forestry within the catchment of the River Moriston which could pose a risk of some limited adverse impacts on the water body. However, there are no forestry pressures recorded on the River Moriston (SEPA Water Environment Hub).

Groundwater

The proposed scheme is within the North Highland Groundwater as identified on the SEPA Water Environment Hub (SEPA, 2016). This groundwater body was classified by SEPA in 2014 as having an overall status of Good. It was classified as Good for water flows and levels and Good for water quality. It is forecast by SEPA to have an overall status of Good in 2021, 2027 and beyond. It is also designated as a Drinking Water Protection Area (DWPA).

British Geological Survey (BGS) mapping (2013) shows the bedrock geology in the area as Tarvie Psammite Formation – Psammite. Psammite normally refers to metamorphosed sandstones and BGS refers to this particular Psammite as “well bedded, flaggy, fawn to brown and white quartzose psammite, where predominant pelites (metamorphosed mudstones) pass into predominantly psammitic lithologies”. The superficial geology (deposits overlying the bedrock) at the location comprises Glaciofluvial Ice Contact Deposits, Devensian – gravel, sand and silt. Nearer the river, there are also areas of Alluvium – sand, gravel and boulders. If groundwater is present in the alluvium, it is likely to be in hydraulic continuity with the River Moriston.

The nearest wells marked on 1:25,000 Ordnance Survey (OS) mapping are located 0.8 km to the north-east of the proposed scheme location and are associated with the settlement of Dundreggan. SEPA CAR authorisations (see Section 8.5.6) for the area do not include these wells.

The groundwater has been evaluated as being of High importance as it forms part of a DWPA and is likely, at least to some degree, to be in hydraulic continuity with the Allt Lagain Bhain and the River Moriston.

Hydrology and Flood Risk

Much of the area on the south side of the A887 is floodplain associated with the River Moriston and is shown as an ‘area at risk of flooding from rivers’ on the SEPA Floodmap (SEPA, 2016). The land beside the burn upstream of the A887 road bridge is not shown to be at risk of flooding. The land slopes steeply up from the burn on both sides and during extreme storm events, high flows in the burn are likely to be contained within these slopes until and if the road level is reached.

The flow in the river is artificially regulated by the Dundreggan Dam 5 km downstream. There are other hydropower structures upstream, i.e. Cluanie Dam, controlling flow of the River Cluanie and River Moriston; Loch Loyne Dam controls flow on a tributary, the River Loyne.

Allt Lagain Bhain is approximately 3 km in length with its source on the western slopes of Meall na Doire, a hill rising to 380 m north of the A887. A tributary of similar size, the An Leth-allt, joins Allt Lagain Bhain approximately 1 km upstream from the A887 bridge. A small un-named watercourse flows into Allt Lagain Bhain 0.5 km upstream of the road bridge. The catchment comprises open moorland, rough pasture, coniferous forest and, near the lower reaches, native broadleaved woodland. Allt Lagain Bhain flows within a steep-sided valley just upstream of the road bridge.

SEPA have assessed the Allt Lagain Bhain as having a catchment of less than 3 km² and therefore it has not been incorporated into the SEPA Flood Map (SEPA Flood Map website, 2016). The area is relatively undeveloped and SEPA hold no records of historical flooding in the vicinity although this does not necessarily mean there have been no flooding problems in the area.

The area at risk of flooding from Allt Lagain Bain is limited in extent by the steep-sided valley and no properties are located in very close proximity. Consequently, it has been evaluated as being of Low importance. The floodplain of the River Moriston has been evaluated as being of High importance as there are properties within Glen Moriston in the general vicinity of the proposed scheme and the floodplain. Downstream of Allt Lagain Bhain Bridge and upstream of Dundreggan Dam, these properties are located on land slightly elevated above the floodplain (approximately 0.5 km to 5 km downstream of the proposed scheme).

Fluvial Geomorphology

Allt Lagain Bhain is a typical highland watercourse with a small catchment and relatively high energy, having cut a steep V-sided dell before flowing into the River Moriston. The source of the watercourse is on moderately high hills up to approximately 380 m AOD located north of the proposed scheme with its elevation being close to 115 m AOD at the A887 road bridge. It flows through a mixture of heath, moorland and commercial forestry plantations. The bed of the burn comprises angular pebbles with some loose rock. It is likely that during spate conditions, considerable material is brought downstream from the upper parts of the catchment as much of the watercourse will act as a sediment transfer zone. The channel has been modified at the location of the trunk road bridge. As would be expected, the longitudinal gradient noticeably slackens as it approaches the floodplain of the River Moriston. Strictly in terms of fluvial geomorphology, Allt Lagain Bhain is considered to be of High importance as the watercourse has a largely natural planform and bed and has typical characteristics of a highland watercourse with only very localised modifications.

The River Moriston is classed by SEPA as a heavily modified water body (HMWB) for water storage for hydro power generation. Strictly in terms of fluvial geomorphology, it has been assessed as of Medium importance.

The small un-named watercourse that flows into the Allt Lagain Bhain is considered to be of Low importance in terms of fluvial geomorphology.

Water Quality

The SEPA Water Environment Hub indicates that the River Moriston achieved high status for water quality in 2014 indicating that it meets water quality requirements under the Water Framework Directive. Although Allt Lagain Bhain is not classified by SEPA (i.e. it is not routinely monitored for water quality), during the site visit on 21^st January 2016, the water was clear, indicating a low suspended solid loading. There had been snow on preceding days and snow was still lying on the ground when the site was visited. Flow was relatively low for winter and similar to flows observed during summer. Some microbial growth was noted on a previous survey but it is likely that this was of natural origin rather than ‘sewage fungus’.

Existing CAR Authorisations and Private Water Supplies in the Vicinity

Information on existing CAR Licences and Registrations in the vicinity of the proposed scheme was obtained from SEPA in April 2015. There are 17 existing CAR Licences / Registrations for locations within 1.5 km of the proposed scheme as shown in Table 8.4 and Figure B2 in Appendix B. None of these authorisations are for drinking water abstraction and none of them are from groundwater; they are all surface water abstractions. It should be noted that abstractions do not require a Licence or Registration if they are for less than 10 m³ of water per day.

Table 8.4 Existing CAR Licences and Registrations within 1.5 km of Proposed Scheme

Ref. on Figure B2 in Appendix B	Licence / Registration Number	NGR	Activity
1	CAR/L/1003069	NH 30088 12684	Fish farm freshwater tank or hatchery
2	CAR/L/1024870	NH 29975 12579	Abstraction fish production
3	CAR/R/1007738	NH 32100 14350	Sheep dip onto land
4	CAR/S/1017394	NH 32110 14320 and NH 32100 14350	Sheep dip onto land
5	CAR/R/1089500	NH 30477 13923	Bridging culvert
6	CAR/R/1089499	NH 30450 13998	Bridging culvert
7	CAR/R/1058025	NH 30840 13142	Sewage (private) primary
8	CAR/R/1081083	NH 30857 13104	Sewage (private) primary
9	CAR/R/1040120	NH 30550 12990	Sewage (private) primary
10	CAR/L/1112111	NH 32361 13401	Abstraction return
11	CAR/R/1097846	NH 30350 12530	Sewage private secondary
12	CAR/R/1099138	NH 30290 12480	Other effluent
13	CAR/R/1076976	NH 30140 12430	Sewage (private) primary
14	CAR/R/1073232	NH 30010 12770	Sewage (private) primary
15	CAR/R/1016121	NH 29842 12789	Sewage (private) primary
16	CAR/R/1037463	NH 29806 12776	Sewage (private) primary
17	CAR/R/1038440	NH 29910 12790	Sewage (private) primary

There are a number of registered sewage discharges within 1.5 km, however, none of these are to the Allt Lagain Bhain catchment. The nearest of the CAR Licence / Registration in Table 8.4 is approximately 400 m to the south of the proposed scheme (CAR/R/1058025) and it is for a private sewage discharge. The fish farm located approximately 1.3 km south west of the proposed scheme has a CAR Licence for an abstraction and for the fish farm / hatchery. In addition to the above CAR authorisations, there are three known private water abstractions within 1.5 km of the proposed scheme (information received from Highland Council Environmental Health department). These are as follows (the letter references refer to Figure B2 in Appendix B);

• A: 3 Torgoyle Crescent, (ID: 30004) Domestic, NH 30569 12975, water supply type: hill loch.
• B: Inchmore Distillery, (ID: 30010) Domestic, NH 30171 12450, water supply type: stream.
• C: Allt Ruadh, (ID: 30017) Domestic, NH 32136 14354, water supply type: spring.

A summary of key water features and their importance is given in Table 8.5.

Table 8.5 Water features and importance

Feature	Aspect	Importance
Allt Lagain Bhain upstream of A887 bridge	Water quality	High
Allt Lagain Bhain downstream of A887 bridge	Water quality	Very High
River Moriston	Water quality	Very High
Small un-named burn	Water quality	Medium
Allt Lagain Bhain	Fluvial geomorphology	High
Small un-named burn	Fluvial geomorphology	Low
River Moriston	Fluvial geomorphology	Medium
Allt Lagain Bhain flood area	Hydrology and flood risk	Low
River Moriston floodplain	Hydrology and flood risk	High
Groundwater	Water quality	High
Groundwater abstraction	Water quality and quantity	High
Surface water abstractions	Water quality and quantity	High

Potential Impacts and Mitigation during Construction

Potential Impacts during Construction

Hydrology and Flood Risk

The works will involve removal of vegetation within the working area and disturbance of soil. There will be some unavoidable compaction of the soil as a result of tracking of site vehicles and plant. As a result, surface runoff from the working area is likely to be increased. However, the working area will be relatively small in relation to the catchment of Allt Lagain Bhain and a very small fraction of the River Moriston catchment. Runoff control measures are proposed in Section 8.6.2.

Fluvial Geomorphology

During the construction phase, the in-channel works and provision of a temporary bridge have the potential to adversely affect fluvial geomorphology as a result of the following:

• Alterations to channel morphology, flow patterns and sediment dynamics during the construction of the new bridge, demolition of old bridges and associated channel modifications (such as channel realignments).
• Sediment release during in-channel works, site clearance operations and earthworks in the vicinity of water features. This could result in reduced morphological diversity due to smothering of channel bed by sediment, an increase in turbidity and loss of active features such as gravel deposits.
• Reduced bank stability during the demolition of the old bridges and construction of the new bridge, which will require vegetation clearance on the banks of the watercourses. This could result in increased bank erosion and associated sediment release.
• Disturbance of existing channel bed forms and morphological features as a result of in-channel working.
• Temporary removal of riparian habitat and floodplain connectivity due to construction activities and access.
• Disturbance of banks leading to increased scour and deposition.
• Disturbance of the channel bed resulting in an unnatural alteration to the longitudinal gradient.
• Risk of creating a knick point which could migrate upstream and cause further bank and bed instability if culvert not designed correctly.
• Changes to the flow regime as a result of diversion of flows / over-pumping during the in channel works.
• Risk of scour where the channel is diverted or over-pumping is undertaken.

The main risk during construction is likely to be scour and erosion and associated deposition of sediments downstream. This also poses a risk to water quality as discussed in the following section.

Water Quality

During construction there will be a risk of pollution from the following:

• Removal of vegetation, excavation and disturbance of soil, in-channel working and watercourse bank works would pose a risk of mobilisation of soil particles during rainfall events. Such sediment, particularly if composed of fine material could interfere with the gills of fish and affect other aquatic organisms. Such material would also be deposited either downstream in Allt Lagain Bhain or in the River Moriston. Deposition of sediments on the beds of watercourses can be detrimental to spawning grounds of fish, particularly salmonids with a risk of smothering redds. There is also a risk that such sediment could flow overland or via the construction site drainage system into Allt Lagain Bhain, the nearby un-named watercourse or potentially into the River Moriston.
• Spillage or leakage of oil, fuel or chemicals. There is a risk of leakage of oil or fuel such as hydraulic oil from excavators, fuel from refuelling and leakage from static plant such as generators and pumps. Chemicals used on site are likely to include shutter release oils which are used to facilitate removal of wooden shuttering from concrete cast on site. Waterproofing chemicals will also be used.
• Spillage of uncured concrete, cement or mortar. Where concrete is mixed and poured on site, there would be a risk of spillage. There would also be a risk of spilling dry cement and additives.
• The physical disturbance of the bed and banks of Allt Lagain Bhain could have detrimental effects on the structure of the burn channel and lead to mobilisation of sediment leading to increased suspended solid concentrations downstream.

The private water supplies from surface water (labelled A and B on Figure B2 in Appendix B) and the abstraction licensed under CAR (labelled 2 on Figure B2) will not be affected as they are well upstream of the working area.

Groundwater

During construction, activities could pose a risk of pollution to groundwater. For example, spillage or leakage of oil, fuel and chemicals on permeable ground could pass though soil and into groundwater.

There are no known drinking water abstractions within or immediately adjacent to the working area. The nearest known private water supply abstraction from groundwater (Private Water Supply C on Figure B2 in Appendix B) is located approximately 1.1 km from the working area and upslope from the A887. Consequently, no impact is predicted on drinking water.

Fish

The works have the potential for temporary impacts on Atlantic salmon and other species of fish during construction. There is likely to be some restriction to fish passage during the in-channel works but this will be temporary in nature as the in-channel works will be restricted to outside the salmon spawning season (15^th October to 31^st May is the period considered to be the spawning season). Mitigation measures will be put in place as described in Section 6.7.3.3 and 6.7.3.5 of Chapter 6 (Ecology and Nature Conservation) and 8.6.2.

Proposed Mitigation during Construction

CAR authorisation will be required for works within Allt Lagain Bhain. SEPA has advised that an engineering simple licence will be required for two activities under the CAR legislation. The two activities are:

• bridge construction; and
• watercourse realignment.

Hydrology and Flood Risk

The contractor will be required to implement measures to control runoff from the site working area. Measures will include Sustainable Drainage Systems (SUDS) to attenuate runoff flows to greenfield rates and these will also be designed to provide pollution control as described in the Water Quality section. Where practicable, a buffer strip will be retained alongside the banks of Allt Lagain Bhain, the un-named burn and along the banks of the River Moriston.

The temporary bridge will be designed to accommodate flood flows. The works will take account of flows within Allt Lagain Bhain and if flood flows occur work will be temporarily postponed within the area influenced by such flows. Demolition of the bridges will not be undertaken during flood flows.

Fluvial Geomorphology

The contractor will be required to implement the following mitigation measures:

• Reduce the risk of scour by providing temporary scour protection or controlling flows to avoid bank and bed erosion.
• Where flows are diverted or over pumped, reduce the risk of scour through scour protection or controlling flows to avoid bed and bank erosion.
• Avoid unnecessary disturbance of the bed and banks.
• Retain as much of the bank side vegetation as practicable to reduce risk of scour and help to reduce ingress of suspended sediment from site runoff.
• Realignment of Allt Lagain Bhain: The creation of a new channel for Allt Lagain Bhain will be carried out in the dry with the flow diverted or continuing to run in the existing channel but separated from the new channel works. The works will follow advice given in SEPA Engineering in the Water Environment Good Practice Guide: Temporary Construction Methods (2009). Once the new channel has been formed, the flow should be allowed to gradually enter the new channel to avoid scour and prevent mobilisation of large amounts of sediment. If practicable and the programme allows, vegetation should be allowed to colonise the banks of the new channel to help stabilise the bare earth. If this is not practicable, consideration should be given to stabilisation of the banks with biodegradable materials such as coir. The bed of the new channel should be formed using bed material from the abandoned section of the existing channel, if practicable. The contractor will be required to carry out this work in a sensitive manner to reduce the impacts on aquatic organisms such as macro-invertebrates (i.e. not to disturb any more of the existing bed than is necessary to carry out this activity). The transfer of bed material will be undertaken outside the salmonid fish spawning season (15^th October to 31^st May is the period considered to be the salmonid spawning season).
• It is recommended that a review of the contractor’s method statements is undertaken by those with appropriate water environment experience so that all appropriate mitigation is included.
• In addition, the design team will engage a fluvial geomorphologist to assist in the detailed design of the culvert and channel realignment.

Water Quality

The contractor will be required to implement the following mitigation measures during the construction period:

• Compliance with the conditions of the SEPA CAR licence.
• Compliance with relevant SEPA Pollution Prevention Guidelines (PPGs), in particular:

• PPG 4: Treatment and disposal of sewage where no foul sewer exists.
• PPG 5: Works and maintenance in or near water.
• PPG 6: Working at construction and demolition sites.
• PPG 8: Safe storage and disposal of used oils.
• As far as it is applicable to the site: PPG 22: Incident response – dealing with spills.
• Vehicle washing must be carried out in strict accordance with PPG 13: Vehicle washing and cleaning.

• Compliance with advice in SEPA Engineering in the Water Environment Good Practice Guide: Temporary Construction Methods (2009).
• Incorporation of relevant good practice measures included in CIRIA guidance documents CIRIA C648 (2006a) and C649 (2006b).
• The contractor will be required to prepare a method statement for prior approval by SEPA as to how they intend to carry out the works including specific aspects of the work including installation of the temporary bridge, demolition of the existing bridges, construction of the box culvert, realignment of the channel and scour protection measures.
• Restrict vegetation removal and excavation / disturbance of soil to the minimum required to carry out the work. Avoid unnecessary tracking of vehicles and plant. Where feasible keep a buffer of vegetation alongside the banks of the watercourse. Vegetation must be disposed of safely and legally in compliance with waste management regulations. It must not be disposed of in the channel.
• Refueling on site is to be undertaken on an impermeable surface within an impermeably-bunded secure area. Where there is a risk of dripping oil or fuel (e.g. at refueling site, under generators and other static plant or equipment), drip trays must be placed to catch any drips. The drip trays must be appropriately maintained to avoid overflow and any contaminated water disposed of off-site at an appropriately licensed disposal facility. The drip trays must have a capacity of 110% of the fuel tank supplying the static plant or equipment.
• All site vehicles and plant must be appropriately maintained in accordance with best practice.
• Vehicles and plant must be kept out of the watercourse unless there is no practicable alternative. Disturbance of the bed and banks of Allt Lagain Bhain must be kept to the minimum practicable for the works to be undertaken.
• The concrete works must be carried out in the dry to avoid contact of uncured concrete and unset waterproofing materials with the burn water.
• Works within the watercourse must be undertaken outside the salmonid fish spawning season. In-stream works are to be undertaken from 1^st June to 14^th October to avoid the fish spawning season (15^th October to 31^st May). The overall construction works are anticipated to take approximately nine months. It will be critical to programme the works carefully to allow for the in-channel works outside the salmonid spawning season. Fish are to be removed according to good practice methods prior to dewatering of the channel (refer to Chapter 6: Ecology and Nature Conservation).
• Measures are to be put in place to reduce the risk of suspended solids entering Allt Lagain Bhain, the small un-named watercourse and the River Moriston. Such measures will include installation of silt fencing along the margins of the Allt Lagain Bhain and the small un-named watercourse above the top of the channel and on the site boundary between the site and the River Moriston.
•  Appropriate measures must be employed to control overland flow and site drainage. These measures must be able to accommodate heavy rainfall events as well as preventing the discharge of high suspended solids concentrations into Allt Lagain Bhain, the small un-named watercourse or the River Moriston. In order to achieve these requirements, the contractor will be required to incorporate SUDS into the site drainage to settle out suspended solids to avoid unacceptably high concentrations entering Allt Lagain Bhain, the small un-named watercourse or the River Moriston. It will be a requirement that such SUDS be designed and installed based on CIRIA guidance C753: The SuDS Manual (CIRIA, 2015).
• During construction of the new A887 box culvert, the contractor will be required to implement measures to avoid pollution to Allt Lagain Bhain from debris, uncured concrete, cement, mortar, additives, oil, fuel, chemicals and suspended solids.
• The type of protection measures would be dependent on the method of demolition. If scaffolding is used at any stage, double sheeting and debris nets or an equivalent system would be required.
• During installation of the temporary bridge and in-channel working, the work must be carried out as far as practicable using methods to avoid damage to the banks of the watercourse.
• Appropriate control monitoring points shall be identified to provide data on the characteristics of surface water within the wider catchment.
• Baseline water quality shall also be assessed for the Allt Lagain Bhain, the unnamed watercourse and the River Moriston (near the confluence with Allt Lagain Bhain). Parameters for analysis will include suspended solids, dissolved oxygen, temperature, dissolved and total metals, pH, ammonia, conductivity, turbidity and total petroleum hydrocarbons.
• Water quality will be monitored upstream and downstream of the works through daily visual inspections to observe and record whether any oil, construction debris or increased turbidity is present. The visual inspections will be assisted with the use of a portable meter to measure turbidity, pH and conductivity. Temperature and dissolved oxygen will also be measured. In the event of observation of oil, construction debris or noticeably increased levels of turbidity downstream compared with upstream, SEPA must be informed within 24 hours of the incident. Suitable actions must be taken immediately to determine the cause of the problem and to rectify the situation.
• During the full construction period, chemical water quality monitoring will be undertaken on Allt Lagain Bhain, the River Moriston and the small un-named watercourse on a fortnightly basis or as otherwise agreed with SEPA. Samples will be sent to a United Kingdom Accreditation Service (UKAS) laboratory. Parameters are to include suspended solids, dissolved oxygen, temperature, dissolved and total metals, pH, ammonia, conductivity, turbidity and total petroleum hydrocarbons. Results will be compared against those from the baseline assessment and from appropriate control monitoring points. The detection limits must be sufficiently low to determine whether compliance with Environmental Quality Standards (EQSs) is being achieved. If any samples show that an EQS for a parameter has been exceeded, SEPA must be informed. Suitable actions must be taken to investigate and rectify the situation if it is due to the proposed scheme construction works.
• Biological (macro-invertebrates) water quality monitoring is to be undertaken prior to the works commencing, on at least two occasions (preferably between April and September inclusive) and following construction in order to determine the long-term pollution status of the site prior to, during and post construction. Samples are to be taken upstream of the works and downstream on each occasion in order to allow comparison.

Groundwater

A number of the mitigation measures described above for protection of water quality will also reduce the risks of pollution to groundwater. For example, compliance with relevant SEPA PPGs will also help to protect groundwater. The controls relating to refueling and provision of drip trays under static plant with potential to leak oil will also assist in protecting groundwater. In addition, the appropriate maintenance of site vehicles and plant will be important in reducing leakage of oil and fuel which otherwise could pose a risk to groundwater.

Fish

As described in Chapter 6 (Ecology and Nature Conservation) the following mitigation measures will be put in place during construction to protect Atlantic salmon and other fish species:

• No in-channel works during the fish spawning season (15^th October to 31^st May).
• Electro-fishing to be carried out safely to remove fish prior to channel realignment and in channel works.

Potential Impacts and Mitigation during Operation

Potential Impacts during Operation

The proposed scheme predominantly involves the provision of a new submerged invert box culvert and widening of the existing single track road to standard single carriageway width. The overall length of the proposed widened road is approximately 160 m which will increase the impermeable area from approximately 800 m² (average width 5 m) to approximately 1120 m² (average width 7 m). Consequently, the runoff volume during a storm event will increase as a result of the proposed scheme and measures will need to be included in the drainage design to accommodate the resulting additional flow (see Section 8.7.1.1 and 8.7.2.1 below).

Hydrology and Flood Risk

Sizing of the box culvert has been determined using methodology based on IH124 (Flood Estimation for Small Catchments, Institute of Hydrology, 1994) (see Appendix F). This is considered suitable for small catchments (Technical Flood Risk Guidance for Small Catchments, SEPA, 2015). As described in the mitigation section the submerged invert box culvert will be designed to convey a 1 in 200 year event (plus 20% for climate change) and an additional 600 mm freeboard. As a result, the proposed scheme should not increase flood risk upstream. The new box culvert may allow increased flows to be conveyed downstream to the River Moriston but the contribution from the flow in the small Allt Lagain Bhain watercourse to the overall flow in the River Moriston based on catchment sizes would be minimal. Correspondence with SEPA in 2014 confirmed that they had no concerns regarding the design of the bridge in terms of flood risk.

In addition, flows along the River Moriston are partially controlled by hydroelectricity power generation schemes, as described in Section 8.5.3.

Fluvial Geomorphology

The provision of a new submerged invert box culvert, associated scour protection and channel realignment has the potential to affect the fluvial geomorphology of the watercourse in a number of ways. They include the following two beneficial impacts and single adverse impact:

• Increase in channel cross-sectional area at the crossing, which previously constrained flow. This is a permanent beneficial impact, which will encourage natural bed to form by allowing natural geomorphological fluvial and sediment transport processes to occur.
• Potential for improvement in channel planform from realignment provided appropriate geomorphological input is used in designing the new channel.
• Potential for initial increased scour with associated increased deposition downstream but eventually reaching equilibrium provided new bridge and channel realignment are designed to consider geomorphological processes.

The potentially adverse impact will be controlled by the mitigation measures proposed below in Section 8.7.2.2.

Water Quality

Road runoff tends to contain a number of pollutants including sediments, hydrocarbons (from oil and fuel) and metals such as iron, copper and zinc from wear of car parts. Particulates can include carbon, rubber, plastics, grit, silt, rust and metal filings. In winter, especially in Scotland, road runoff often contains de-icing salt. Impurities in rock salt can include metals such as cadmium. Road salt may enhance release of toxic metals from sediments.

The DMRB (Highways Agency, 2009) Method A (Assessment of Pollution Impacts from Routine Runoff to Surface Waters) has been used to assess potential impacts from routine runoff on water quality. Method A entails the use of a Microsoft Excel based tool to assess water quality of watercourses receiving road drainage. This Highways Agency Water Risk Assessment Tool (HAWRAT) has been used to estimate the potential effects of road drainage discharge on Allt Lagain Bhain (see Appendix E). The assessment uses a tiered approach. The first tier estimates the concentration of key pollutants (see below) in highway runoff. These concentrations are then compared with Runoff Specific Thresholds (RSTs) for soluble pollutants and Threshold Effects Levels (TELs) for sediment-bound pollutants to assess whether there could be an impact, not taking account of dilution from the receiving watercourse. If the predicted concentrations do not exceed the threshold values, there is no need for further assessment using HAWRAT. However, if the RSTs or TELs are exceeded for any parameter, the second tier of the assessment needs to be carried out.

HAWRAT provides an estimate of water quality and is not designed to accurately predict concentrations of pollutants. Consequently, sensitivity testing of the results was carried out whereby key input parameters, particularly the 95-percentile flow (flow exceeded for 95% of the time) were changed to see the effect on HAWRAT results. Where results are close to failing thresholds, sensitivity testing proves useful in assessing whether failures could occur if key input parameters have been over or under estimated. The HAWRAT tool has been developed using data from roads where traffic flows were greater than 10,000 AADT. The A887 has an AADT of less than 1000 but for the purposes of the HAWRAT assessment has been placed in the band 10,000 to 50,000 AADT (i.e. the lowest category available). As a result the tool is likely to overestimate pollutant concentrations in this case.

For each key soluble pollutant, two RSTs are used, namely RST 24 hour and RST 6 hour. These are used to protect aquatic organisms from short-term exposure to soluble pollutants. The RST 24 hour is designed to protect aquatic organisms against worst case conditions whereas RST 6 hour is designed to protect against more typical exposure conditions. Copper and zinc are used as indicative pollutants as they are commonly present at detectable concentrations in highway runoff and they are known to have toxic effects on aquatic organisms such as fish and macro-invertebrates above certain concentrations.

Tier 2 calculates predicted concentrations of key pollutants following dilution within the receiving watercourse. If RSTs or TELs are still exceeded, the third tier of HAWRAT is undertaken to assess the level of mitigation required.

The key pollutants assessed in HAWRAT are as follows:

• dissolved copper;
• dissolved zinc;
• sediment-bound copper;
• sediment-bound zinc;
• sediment-bound cadmium;
• sediment-bound total polycyclic aromatic hydrocarbons (PAH);
• sediment-bound pyrene;
• sediment-bound fluoranthene;
• sediment-bound anthracene; and
• sediment-bound phenanthrene.

PAHs are hydrocarbons generally derived from oils and fuels present in highway runoff. The latter four pollutants in the above list are specific types of PAH.

In Scotland, mitigation measures in the form of SUDS are normally a legal requirement under CAR for new road schemes; even if Tier 1 predicted no failure, SUDS would need to be incorporated into the drainage design.

The results of the Tier 1 HAWRAT assessment (Refer to Appendix E) indicate that concentrations of key pollutants in the highway runoff would be likely to exceed RSTs for soluble pollutants (dissolved copper and dissolved zinc).

Consequently, the assessment was extended to Tier 2 to assess whether dilution in the watercourse would be sufficient to reduce key pollutant concentrations to below the RSTs. The results of the Tier 2 assessment predicted that the dilution in Allt Lagain Bhain at low flow (95-percentile flow) would be sufficient to dilute soluble copper and zinc concentrations to below the relevant RSTs. Sensitivity testing was undertaken entailing reducing the permeable area draining to the outfall to zero and reducing the input parameter for 95-percentile flow considerably to below 0.001m³/s from the estimate of 0.009m³/s based on Institute of Hydrology 1992 guidance (IH108: Low flow Estimation in the UK). With these input values reduced to this extent, predicted failures of the zinc RSTs can begin to occur.

Sediment-bound pollutant concentrations are predicted to be below the TELs.

The presence of the River Moriston SAC downstream was highlighted in the HAWRAT assessment. The HAWRAT calculations suggest that following mixing and dilution within the Allt Lagain Bhain, soluble pollutants would be within the limits required. However, sensitivity testing suggests that on rare occasions, concentrations of soluble zinc could be close to exceedance within Allt Lagain Bhain downstream of the discharge point. There is also a low risk that sediment-bound pollutants could be slightly elevated. These results should be treated with caution as they probably over-estimate the pollution risk because the calculations assume that traffic flows are considerably higher than the actual flows.

Although the HAWRAT assessment does not in itself indicate the need for pollution control for routine runoff, SUDS will be required under the CAR legislation. The proposed SUDS will also help to protect the water quality of the River Moriston SAC. Sediment removal will be a key aim of the SUDS in order to protect the gravel bed of Allt Lagain Bhain and the bed of the River Moriston.

The proposed road widening extends beyond the small un-named watercourse. If discharges are proposed to this watercourse, there would be a risk of pollution and this would be addressed by the provision of SUDS as outlined in the Mitigation section.

Spillage risk has been assessed using the Method D given in DMRB Volume 11 Section 3 Part 10 and included as part of the HAWRAT assessment tool. The results of the spillage risk assessment indicate that there is a very low risk, i.e. an annual probability of less likelihood than 0.00001 (1 in 100,000 years) of a serious spillage resulting in a serious pollution event. This is largely because of the low traffic flows with less than 1000 Annual Average Daily Traffic (AADT) flow (Transport Scotland, 2016) along the A887 at the location and the lack of junctions within 100 m of the proposed scheme. Even when a sensitivity test is undertaken where the input risk factor is increased, the spillage risk is still very low.

The private water supplies sourced from surface water (labelled A and B on Figure B2 in Appendix B) and the abstraction licensed under CAR (labelled 2 on Figure B2) will not be affected as they are upstream of the proposed scheme.

Groundwater

The trunk road has a low traffic flow as described in Section 8.7.1.3 above and the pollution risk to surface water is assessed as being low. The risk to groundwater from routine runoff is also considered to be low. As discussed, spillage risk is very low and this applies to groundwater as well as surface water. The proposed scheme will not increase the risk of pollution to groundwater compared with the existing situation.

The nearest known groundwater abstraction for drinking water is approximately 1.1 km from the proposed scheme and therefore will not be affected by the proposed scheme.

Fish

Following completion of the bridge, fish will continue to be able to move upstream and downstream of the bridge. However, the bridge will be wider than the existing and a wider invert without any mitigation would result in a lower depth of flow. This is unlikely to be an issue in winter during relatively elevated flows but during summer months when flows are likely to be lower, the reduced depth of flow could pose a barrier to migratory fish movement upstream and downstream. The bridge design will incorporate a low flow channel to ensure migratory fish passage is not affected.

Proposed Mitigation during Operation

Proposed mitigation measures during the operational phase include:

Hydrology and Flood Risk

The following measures will be included in the design:

• Provision of SUDS based on guidance given in CIRIA C753: The SuDS Manual (CIRIA, 2015) to provide suitable attenuation of flows prior to discharge to Allt Lagain Bhain.
• The box structure will be designed to convey flows from a 1 in 200 year flood event plus 20% to allow for climate change. In addition, a freeboard of 600 mm has been included to allow for large debris such as branches to flow through the bridge during spate or flood flows.

Fluvial Geomorphology

The following mitigation measures are proposed to reduce the impact on fluvial geomorphology during the operational phase.

• Appropriate geomorphological input will be used to design a new channel realignment under the new bridge. Realignment should be limited as far as practicable. This design will consider the natural geomorphological processes in this location and limit the risk of undermining of the structure and downstream environmental impacts. The realignment design may incorporate a low flow channel which will reduce the risk of siltation.
• Appropriate geomorphological input will be used to design the box culvert so that it is at least as wide as the natural watercourse width in this location and that a suitable gradient underneath the road is adopted.
• The box culvert will have a submerged invert to encourage the deposition of sediment and formation of a more natural bed under the road. The box culvert and scour protection will be designed with the aim of avoiding a step in bed level developing at the downstream end of the box culvert or scour protection.
• Scour protection will be required to protect the structure of the box culvert, particularly the side walls. The scour protection will be designed with the aim of reducing the risk of scour immediately upstream and downstream of the hard protection. This will include tying in the upstream and downstream ends of the bank protection into the natural bank as far as practicable.
• Consideration should be given to the use of green-bank protection immediately upstream and downstream of the hard engineered scour protection in order to merge the engineered bank into the natural bank.
• Follow best practice identified in the following:

1. SEPA’s Position Statement to support the implementation of the Water Environment (Controlled Activities) (Scotland) Regulations 2005: WAT-PS-06-02: Culverting of Watercourses (SEPA, 2006);
2. SEPA’s Engineering in the Water Environment Good Practice Guide: Bank Protection Rivers and Lochs (WAT-SG-23) (SEPA, 2008a);

• SEPA’s Engineering in the Water Environment Good Practice Guide: Intakes and Outfalls (WAT-SG-28) (SEPA, 2008b); and

1. SEPA’s Position Statement to support the implementation of the Water Environment (Controlled Activities) (Scotland) Regulations 2011: WAT-PS-07-02: Bank Protection (SEPA, 2012).
2. SEPA’s Engineering in the Water Environment Good Practice Guide: River Crossings (WAT-SG-25) (SEPA, 2010a).
3. SEPA’s Engineering in the Water Environment Good Practice Guide: Sediment Management (WAT-SG-26) (SEPA, 2010b).

• SEPA’s Engineering in the Water Environment Good Practice Guide: Temporary Construction Methods (WAT-SG-29) (SEPA, 2009)

• It is recommended that a review of the contractor’s method input statements is undertaken by those with appropriate water environment expertise so that all appropriate mitigation is included.

Water Quality

Based on the importance of the downstream aquatic habitat (River Moriston SAC), facilities to promote the settlement of suspended solids must be provided prior to discharge to Allt Lagain Bhain. In any case, SUDS are required for new and widened roads in Scotland under CAR. The following mitigation measures are proposed:

• Two levels of SUDS (as far as practicable, in line with the SUDS Manual, CIRIA, 2015) to promote settlement of suspended solids and removal of a proportion of dissolved metals within the road drainage system prior to discharge to Allt Lagain Bhain.
• Provision of SUDS prior to any discharge to the small un-named watercourse.

The risk of a serious spillage causing a serious pollution event is less than 0.00001 probability of occurrence in any one year. Consequently, there is no requirement for major spillage containment. In view of the sensitivity of the water environment in the vicinity, it is proposed that the SUDS include simple provision for containment of small spillages such as check dams (low dams which slow the flow of drainage water). They should be incorporated within a linear SUDS such as a vegetated ditch or swale. If ground conditions allow, the base of the SUDS should be lined with an impermeable liner for a few metres upstream of the check dam.

Groundwater

The mitigation measures to protect water quality will also assist in protecting groundwater. In terms of spillage control, the proposed lining of linear SUDS for a few metres upstream of check dams will reduce the risk to groundwater in the unlikely event of a major spillage resulting from a vehicular collision.

Fish

As described in the Ecology and Nature Conservation chapter (Section 6.7.3.5) the bed of the channel will not have a hydraulic drop either upstream of downstream of the new bridge and no obstacle to fish migration will be created. A low flow channel will be provided along the invert of the new bridge to facilitate fish passage upstream and downstream during low flows.

Summary and Residual Impacts during Construction and Operation

Residual impacts on the water environment are shown in Table 8.6 (construction period) and Table 8.7 (operational period). Impacts are adverse unless stated otherwise. Table 8.6 indicates that some construction activities following mitigation could still result in effects of Moderate significance (i.e. a significant effect in the context of the EIA Regulations), largely because of the Very High importance of Allt Lagain Bhain downstream of the bridge and the River Moriston. These impacts, however, are likely to be temporary, localised and short-term in nature. The potential for medium-term effects relate to the mobilisation of sediments and subsequent deposition of such sediments downstream and this risk would be controlled by the proposed mitigation measures which incorporate current best practice. It is also anticipated that in the long-term, erosion and deposition would revert back to the equilibrium similar to that which currently exists. Therefore, the long-term impact is not anticipated to be significant. It is considered that with the implementation of the proposed mitigation measures, the proposed scheme will comply with relevant legislation and policy in relation to the water environment.

Table 8.7 indicates that, with the implementation of the proposed mitigation measures, there should be no significant effects from the proposed scheme during the operational phase.

Table 8.6 Residual impacts during construction period

Potential Impact	Feature	Aspect	Import-ance	Mitigation	Magnitude	Significance
Decrease in water quality from site runoff	Allt Lagain Bhain upstream of A887 bridge	Water quality	High	Silt fences; SUDS.	Minor, short-term	Slight
Decrease in water quality from site runoff	Allt Lagain Bhain downstream of A887 bridge	Water quality	Very High	Silt fences; SUDS.	Minor, short-term	Moderate
Decrease in water quality from site runoff	River Moriston	Water quality	Very High	Silt fences; SUDS.	Negligible, short-term	Neutral
Decrease in water quality from site runoff	Small un-named burn	Water quality	Medium	Silt fence; SUDS.	Minor, short-term	Slight
Restriction of flows	Allt Lagain Bhain	Fluvial geomorphology	High	Avoid over-restriction of the channel.	Minor, short-term	Slight
Disturbance of banks	Allt Lagain Bhain	Fluvial geomorphology	High	As far as practicable, retain existing bank vegetation along Allt Lagain Bhain.	Minor, temporary (vegetation should re-establish following works)	Slight
Increased scour and subsequent deposition in burn	Allt Lagain Bhain	Fluvial geomorphology	High	As far as practicable, retain existing bank vegetation along Allt Lagain Bhain. Care during diversion works and any over pumping. Temporary scour protection.	Minor, short-term with potential for medium-term	Slight
Increased deposition resulting from scour of Allt Lagain Bhain banks and bed and transport of sediments downstream to river	River Moriston	Fluvial geomorphology	Medium	As far as practicable, retain existing bank vegetation along Allt Lagain Bhain. Care during diversion works and any over pumping. Temporary scour protection.	Minor, short-term with potential for medium-term	Slight
Increased surface water runoff from working area leading to elevated watercourse flows	Allt Lagain Bhain flood area	Hydrology and Flood Risk	Low	SUDS	Negligible, short-term	Neutral
Increased surface water runoff from working area leading to elevated watercourse flows	River Moriston floodplain	Hydrology and Flood Risk	High	SUDS	Negligible, short-term	Neutral

Table 8.7 Residual impacts during operational period

Potential Impact	Feature	Aspect	Import-ance	Mitigation	Magni-tude	Signifi-cance
Decrease in water quality from site runoff	Allt Lagain Bhain upstream of A887 bridge	Water Quality	High	Two levels of SUDS.	Negligible, long-term	Neutral
Decrease in water quality from site runoff	Allt Lagain Bhain downstream of A887 bridge	Water Quality	Very High	Two levels of SUDS.	Negligible, long-term	Neutral
Decrease in water quality from site runoff	River Moriston	Water Quality	Very High	Two levels of SUDS.	Negligible, long-term	Neutral
Decrease in water quality from site runoff	Small un-named burn	Water Quality	Medium	Two levels of SUDS.	Negligible, long-term	Neutral
Spillage from vehicular collision resulting in pollution to burn	Allt Lagain Bhain upstream of A887 bridge	Water Quality	High	SUDS with simple spillage control.	Negligible, long-term	Neutral
Spillage from vehicular collision resulting in pollution to burn	Allt Lagain Bhain downstream of A887 bridge	Water Quality	Very High	SUDS with simple spillage control.	Negligible, long-term	Neutral
Changes in flood risk	Allt Lagain Bhain	Hydrology and Flood Risk	Low	New crossing designed to accommodate flood flows.	Negligible, long-term	Neutral
Changes in flood risk	River Moriston	Hydrology and Flood Risk	High	N/A	Negligible, long-term	Neutral
Changes in cross-sectional area at new crossing structure, risk of knick point	Allt Lagain Bhain	Fluvial Geomorphology	High	Use of geomorphologist to assist in detailed design so that planform and culvert are designed using best practice.	Negligible beneficial, long-term	Neutral
Changes in scour and deposition	Allt Lagain Bhain	Fluvial Geomorphology	High	Following realignment, the burn is likely to adjust through natural processes.	Negligible, long-term	Neutral
Change in planform of channel	Allt Lagain Bhain	Fluvial Geomorphology	High	The realignment to a naturalised planform should provide a benefit.	Negligible beneficial, long-term	Neutral
Pollution risk	Groundwater	Water quality	High	The pollution control measures to protect surface water will also assist in reducing the risk to groundwater.	No change	Neutral