12 Road Drainage and the Water Environment 12.1 Scope of the Assessment 12.2 Statutory and Planning Context 12.3 Assessment Methodology 12.4 Baseline Conditions 12.5 Predicted Impacts 12.6 Mitigation 12.7 Residual Effects

12 Road Drainage and the Water Environment

12.0 Introduction

12.0.1 This chapter reports the findings of the assessment into the predicted impacts on the water environment associated with the Proposed Scheme corridor and the neighbouring area, principally resultant from the effects of road drainage.

12.0.2 In this context the water environment includes surface watercourses, water bodies, groundwaters and aquifers.

12.1 Scope of the Assessment

12.1.1 The purpose of the assessment has been to establish the nature and significance of the predicted impacts that occur as a result of the construction and operation phases of the Proposed Scheme. Such impacts relate to changes in flow characteristics, flooding regimes and the existing water quality.

12.1.2 The Stage 1 and 2 assessments identified a number of minor watercourses flowing north to south across the Proposed Scheme corridor, all of which form part of a local catchment drained by the River Pow. These surface waters form part of the wider Solway Firth greater catchment area located to the south of the study area. The Solway Firth is a SSSI and is designated under the Habitats Directive as an SPA and candidate SAC (refer to Chapter 7).

12.1.3 The Stage 1 and 2 assessments also identified a number of water bodies within 1 km of the Proposed Scheme corridor; the principal ones being at Kelhead Quarry and Kinmount House. It also established that the nearest designated floodplain and groundwater abstraction points were located 7 km to the east at Annan.

12.1.4 The assessments concluded that the ES should accordingly focus on the implications of the potential modification to the flow regimes and characteristics of the existing watercourses and the prospective impacts on water quality associated with potential sedimentation and contamination by pollutants (including accidental spillage) should the Proposed Scheme become operational. Sedimentation and contamination issues have been accordingly addressed in Chapter 15, Disruption Due to Construction. Potential consequent impacts on aquatic and marginal habitats and aquatic dependent species are addressed in Chapter 7, Ecology and Nature Conservation.

12.1.5 Whilst it was concluded there was no need to undertake a detailed flood risk assessment, consideration has been given to the effectiveness of the existing culverted watercourses under the A75 to determine if there is the potential for localised flooding upstream and by association if this risk would be transferred to the proposed culverts associated with the new alignment. Such a consideration extended to assessing the combined effect of having two proximal culverted sections along each affected watercourse; one serving the existing A75 and the other the Proposed Scheme.

12.1.6 In summary, the assessment has addressed the following topics identified in the DMRB:

  • effect of routine run-off on surface waters;
  • effect of routine run-off on groundwaters; and
  • pollution impacts from accidental spillages.

12.2 Statutory and Planning Context

12.2.1 The assessment has been informed by reference to the following directives and statutes.

  • EC Freshwater Fish Directive 75/659/EEC (1975).
  • EC Groundwater Directive 80/68/EEC (1980).
  • Water Resources (Scotland) Act (1991).
  • Water Drainage (Scotland) Act (1997).
  • Flood Prevention & Land Drainage (Scotland) Act (1997).

12.2.2 The Water Environment (Controlled Activities) (Scotland) Regulations (2005), or Controlled Activities Regulations (CAR), bring into effect the requirements of Section 20 of the Water Environment and Water Services Act (2003) insofar that, from the 1st April 2006, authorisation from SEPA will be required for the following activities:

  • discharges to all wetlands, surface waters and groundwater (replacing the Control of Pollution Act (CoPA) (1974));
  • disposal to land (replacing the Groundwater Regulations (1998));
  • abstractions from all wetlands, surface waters and groundwater;
  • impoundments (dams and weirs) of rivers, lochs, wetlands and transitional waters; and
  • engineering works in inland waters and wetlands.

12.3 Assessment Methodology

12.3.1 The assessment for the Proposed Scheme has been undertaken in accordance with the guidelines in Volume 11 Section 3, Part 10 of the DMRB (HA 216/06).


12.3.2 The following organisations have been consulted to determine existing data related to surface waters and groundwater within the study area:

  • Dumfries and Galloway Council; and
  • Scottish Environment Protection Agency (SEPA).

12.3.3 The corresponding responses are contained within Appendix B.

Effect of Routine Run-off on Surface Waters

12.3.4 The DMRB provides both a simple and detailed approach for assessing the impact of routine run-off on surface waters. The Proposed Scheme was initially subject to ‘simple assessment’ to determine if more a ‘detailed assessment’ would be required in light of potentially significant impacts. The assessment concluded that there would be a low risk of significant impact; thus a detailed assessment was accordingly not required.

12.3.5 The simple assessment method is based on Construction Industry Research and Information Association (CIRIA) Report 14227. The method assesses the dilution potential28 of the receiving waters and traffic flow29 as key parameters and is modified to allow for rivers having different River Quality Objectives.

Effect of Routine Run-off on Groundwater

12.3.6 The assessment is based on the consideration of the source/pathway/receptor protocol (a risk assessment approach to the evaluation of contaminated land30). The principles of the approach are that for there to be a risk of impact there must be:

  • a source, being a contaminant or a potential contaminant;
  • a receptor, being something or someone that may be ‘harmed’ by contaminants; these include ground and surface waters (grouped as Controlled Waters), ecological environment and most importantly human health; and
  • a pathway being a route taken by a source of contamination to cause harm to a receptor, where either:
    • the contaminant is causing significant harm to that receptor, or
    • there is a significant possibility of such harm being caused by that contaminant to that receptor.

12.3.7 Where the above approach is adopted for the water environment the resultant effect is that of pollution over contamination.

12.3.8 Where a source or pathway is identified, the assessment considers how the said source or pathway might be modified to reduce the risk/impact. In the case of the Proposed Scheme, the source has been accepted as being fixed; there being predicted levels of traffic and a consequent level of associated contaminants associated with the traffic. The assessment is conservative in that no allowance is made for the potential lowering of generated contaminants that are likely to be achieved as vehicle technology improves.

12.3.9 Where the source is identified as being discernibly harmful, modification or breakage of the pathway remains the basis for mitigation.

Pollution Impacts from Accidental Spillages

12.3.10 The assessment is based on a preliminary evaluation of the risk that there could be an accident involving the spillage of potentially polluting substances. This is followed by consideration of the risk that such an accident would result in the pollutant reaching the receptor.

12.3.11 Table 2.4 of Part 10 of Volume 11 of the DMRB defines three categories of pollution incident in which categories 1 and 2 are considered serious pollution incidents. The risk of a serious incident is expressed in terms of the annual probability of such an incident, which is in turn is used a basis for determining the acceptability of the risk or if mitigation is required to reduce the risk.

12.3.12 The norm adopted under the DMRB is a 1%-risk provided the identified receptor is not classed as being of exceptional sensitivity.

12.3.13 The method is based on an analysis of spillage incidents on motorways and trunk roads in England and Wales. It assumes that spillage accidents are distributed across the network in the same way as personal injury accidents involving HGVs are.

12.3.14 The DMRB provides a standard formula for assessing the annual probability of a spillage accident for each individual section of road:

PACC = RL x SS x (AADT x 365 x 10-9) x (%HGV ÷ 100)


PACC = the annual probability of an accidental spillage with the potential to cause a serious pollution incident.

RL = Road Length in kilometres.

SS = Spillage rates. Values are provided in Table D.1, Annex 1, Part 10 DMRB Vol. 11. 11.

AADT = Annual Average Daily Traffic (design year for new road).

%HGV = percentage of Heavy Goods Vehicles.

12.3.15 It further provides a formula for calculating the predicted annual probability of a serious pollution incident for each section of road:



PINC = the probability of a spillage incident with an associated risk of a serious pollution incident occurring

PPOL = the probability, given an accident, that a serious pollution event will result. The value is dependent on the sensitivity of the water course and how soon it can be reached by the emergency services. Values are provided in Table D.2, Annex 1 Part 10, Volume 11 of the DMRB.

12.3.16 The annual probabilities for each section of road draining into a watercourse are then added. If the risk does not exceed the acceptable risk, no further assessment is required under the simple assessment.

Significance of Effects

12.3.17 The significance of the effects arsing from the impacts identified by the three components of the assessment are determined by the magnitude and probability of the impacts and the importance of the affected receptor.

Importance of Identified Features/Attributes

12.3.18 Importance criteria adopted are detailed in Table 12.131.

Table 12.1 – Criteria for Evaluating Importance




Very High

Attribute with a high quality and rarity on a regional or national scale

Surface Water:

EC Designated Salmonid/Cyprinid fishery; RQO River Ecosystem Class RE1;
Site protected under EU; UK wildlife legislation (SAC, SPA, SSSI, Ramsar site)


Major aquifer providing regionally important resource or supporting site protected under wildlife legislation; SPZ1.


Attribute with a high quality and rarity on a local scale

Surface Water:

RQO River Ecosystem Class RE2;
Major Cyprinid Fishery;
Species protected under EU or UK wildlife legislation.


Major aquifer providing locally important resource or supporting river ecosystem SPZII.


Attribute with a medium quality and rarity on a local scale

Surface Water:

RQO River Ecosystem Class RE3/RE4;


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


Attribute with a low quality and rarity on a local scale

Surface Water:

RQO River Ecosystem Class RE5



Magnitude of Impact

12.3.19 The adopted magnitude criteria are detailed in Table 12.232. The assessment criteria range from major, moderate, minor and adverse to negligible and minor beneficial; there being no moderate and/or major benefits as assessed.

Table 12.2 – Magnitude of Impacts




Major Adverse

Results in loss of attribute and/or quality and integrity of attribute

Surface Water:

Potential high risk (Method A) and potential failure of Total Zinc and Dissolved Copper (Method B); calculated risk of pollution from an accidental spillage >2% annually (Method D); loss or extensive change to a designated fishery.


Loss of aquifer; potential high pollution risk score >250 (Method C); risk of pollution from accidental spillage >2% annually (Method D).

Moderate Adverse

Results in impact on integrity of attribute or loss of part of attribute

Surface Water: Potential high risk (A) and either potential failure of Total Zinc or Dissolved Copper (B); risk of pollution from accidental spillage >1% but <2% annually (D); partial loss of fishery.


Partial loss or change to an aquifer; medium pollution risk from run-off, score 150-250 (C); pollution risk from spillage >1% but <2% annually (D)

Minor Adverse

Results in some measurable change in attributes quality or vulnerability

Surface Water: Potential high risk (A) and no change in Total Zinc and Dissolved Copper (B); risk of pollution from spillage >0/5% and >1% annually (D)


Low risk of pollution from routine run-off (C), score <150; risk of pollution from spillage >0.5% and <1% (D)


Results in an impact on attribute but of insufficient magnitude to affect the use/integrity

Surface Water: Low risk or pollution from routine run-off (A) or spillage risk <0.5%.


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

Minor Beneficial

Results in some beneficial effect on attribute or a reduced risk of negative effect occurring

Surface Water: Low risk (A) and risk of pollution from spillage <0.5%.


No measurable impact and risk of pollution from spillage <0.5%


12.3.20 By combining the importance of the attribute and the magnitude of the predicted impact, the significance of the effects has been determined in accordance with the matrix shown in Table 12.333. The significance ratings can be adverse or beneficial. The same ratings have been used for effects prior to and with mitigation in place.

Table 12.3 – Significance of Potential Effects

Magnitude of potential impact

Importance of attribute

Very High





Very Large

Large/Very Large




Large/Very Large














12.4 Baseline Conditions

Catchment Area

12.4.1 The Proposed Scheme is located within a wider area that falls within the catchment of the River Annan, which itself discharges into the Solway Firth some 4 km south of the existing and proposed sections of the A75. The immediate study area for the Proposed Scheme is contained within the local catchment of the Pow River (Figure 11.1) A number of tributaries of Pow Water, including Hardgrove Burn, Stenries Burn and Glen Burn are located within the study area along with various drainage ditches.

12.4.2 The River Annan and Pow Water are both protected under EC Directive 78/659/EEC, and as such, are classified under the River Classification System.

Pow Water

12.4.3 Pow Water originates north of the existing A75 and to the west of Carrutherstown. It flows in a south-easterly direction discharging into the Solway Firth at Powfoot. The watercourse is culverted beneath the A75 at Whitecroft Gate; some 0.2 km east of Carrutherstown. In its lower sections, Pow Water is a classified salmonid watercourse with an important sea trout population. However, locally it was displaced and culverted as part of the original A75 and subsequently it has been extensively culverted in the fields to the north of the existing road. These works have reduced the attractiveness of the watercourse to wildlife; including fish (see Chapter 7).

12.4.4 Furthermore, there remains a potential unqualified risk, given the expanse of agricultural activity in the area, for diffuse pollutants from surrounding farmland to have impacted on the local water quality with consequent implications for Pow Water’s biodiversity.

Hardgrove Burn

12.4.5 Hardgrove Burn is a small tributary of Pow Water originating immediately north of the existing A75. It follows a north-south course close to the U81a. The watercourse is carried in a culvert beneath the A75 immediately to the east of the existing U81a past the point where it crosses the existing A75. No evidence of flooding upstream of the existing culvert has been observed or recorded throughout the planning and design phases of the Proposed Scheme.

Glen Burn

12.4.6 Also a tributary of Pow Water, the Glen Burn flows from farmland to the east of Nether Stenries, south to the A75, where it is culverted beneath the carriageway. No evidence of flooding upstream of the existing culvert has been observed or recorded throughout the planning and design phases of the Proposed Scheme. South of the A75, the burn flows south-westwards towards Upper Mains Farm. The confluence with Pow Water is some 3 km to the south of the A75. The north side of the burn within the study area is mostly fenced and has a mix of aquatic and bank side vegetation.

Stenries Burn

12.4.7 Stenries Burn is a minor tributary of Glen Burn. It runs alongside the existing access road from the farm to the existing A75. There is large seasonal wetland associated with the burn north of the existing A75, suggesting that the existing culvert beneath the trunk road is no longer capable of addressing changes in the upstream catchment/flow characteristics. South of the A75 the burn flows south towards Topmuir Farm. The watercourse throughout the study area is unfenced. Its banks show signs of poaching by livestock; a factor that is likely to contribute to sedimentation within the watercourse.

Water Quality

12.4.8 SEPA do not monitor water quality within the watercourses identified in the study area though investigations undertaken during route selection process did identify that one of the tributaries of the Pow Water was classified as B (Fair).

Existing Road Drainage

12.4.9 Other than for the short section of the existing A75 at the western end of the Proposed Scheme, surface water run-off from the existing carriageway flows across roadside verges and is collected via existing drainage ditches that run along each side of the trunk road; ultimately discharging to the local watercourses. There is neither provision for treatment, interception of runoff to mitigate the impacts of traffic related pollutants contained therein nor to address the potential for accidental spillage.


12.4.10 The sedimentary deposits that make up the underlying geology within the study area (see Chapter 13, Geology and Soils) form aquifers where flow is predominantly through fissures and discontinuities. They tend to be highly productive but are usually only of local importance; i.e. they are not extensive. There is also the potential for surficial shallow groundwaters to be associated with the peat deposits in the area.

12.4.11 The geotechnical desk study (Mouchel Parkman, 2004) indicated the presence of a number of issues and sinks to the north of the existing A75 beyond the western end of the route section. No such evidence was found within the Proposed Scheme corridor.

12.5 Predicted Impacts

12.5.1 The following components of the Proposed Scheme would have implications for water quality:

pollutant contamination associated with road related runoff and potential accidental spillage;

local modifications to channel alignments and profiles; and

temporary loss of vegetation cover leading to increased sedimentation during and immediately post construction.

Routine Run-off and Surface Waters

12.5.2 Calculation of the potential pollution impacts based on the simple assessment method provided in DMRB assumed the following. The assessment disregards the associated drainage mechanism that will be employed.

95%-ile river flow (Q95) – 0.15m3

River Ecosystem Class B (equivalent of RE2)

Road width – 13.2m

Road length – 2,800m

AADT two-way traffic flow – 15,380 vehicles/day (average value across the 3.6 km improvement section in 2010)

Run-off coefficient – 0.5

Rainfall depth – 12mm (based on Figure A1 in Annex 1, Part 10, Volume 11 DMRB)

Resultant road area – 36,960m2

Run-off volume from the road – 0.5 x (12 ÷ 1,000) x 36,960 = 221.76m3

River daily flow – 0.15 x 3600 x 24 = 12,960m3

Dilution rate – 12960 ÷ 221.76 = 58.44

12.5.3 Based on the diagram in Annex II, Part 10, Volume 11 DMRB a dilution rate of 58.44 for a Class B (RE2) watercourse with daily traffic flows of 15,380 vehicles would present a low risk to the local environment, with there being no requirement for further assessment.

12.5.4 Given the lack of any gauging data for any of the watercourses, no assessment of the impact of routine run-off to surface waters has been undertaken as part of this assessment.

Routine Run-off and Groundwater

12.5.5 The assessment has demonstrated that there is a pollutant derived source associated with the Proposed Scheme in the form of the surface water run-off. It has also demonstrated that there is a receptor in the form of a productive aquifer valued as being of local importance. The Proposed Scheme design includes balancing ponds, which serve to remove any potential pollutant pathways linkages. The balancing ponds will be sealed and made impermeable, with the outlets comprising simple interceptor points where the waters, once settled, would be further treated/ screened prior to discharge. However, the potential risk to a local aquifer thus would be low and the impact accordingly slight in accordance with Table C.3, Annex 1, Part 10, Volume 11 of the DMRB.

Pollution Impacts from Accidental Spillage

12.5.6 Two sections of road have been included in the calculation of the annual risk of an accidental spillage resulting in a serious pollution incident; namely the 3.6 km length of new trunk road and the 0.8 km length of the modified U81a. These comprise the limits of the Proposed Scheme.

12.5.7 The calculations for the annual probability of an accidental spillage with the potential to cause a serious pollution incident using the formula detailed in Paragraph 12.3.4 and the traffic data within Appendix C are indicated below (see footnote34.

PACC = 3.6 x 0.29 x (15,380 x 365 x 10-9) x 0.186 = 1.09 x 10-3 PACC = 0.8 x 0.93 x (129 x 365 x 10-9) x 0.046 = 1.61 x 10-6 12.5.8 The probability of a spillage accident with an associated risk of a serious pollution incident is calculated below.

PINC = 1.09 x 10-3 x 0.6 = 0.07%

PINC = 1.61 x 10-6 x 0.6 = 0.0001%

12.5.9 The sum of the annual probabilities has been calculated as being 0.07%; this representing the risk of a serious pollution incident occurring in any given year. This is based on a worst-case assumption that all the spillage will go to the same reach of the receiving watercourse.

12.5.10 The result is negligible in comparison to the acceptable threshold of 1% identified in the DMRB. The assessment concluded there would be no need for further work and that there would be no requirement for further mitigation.

12.6 Mitigation

12.6.1 Under Proposed Scheme proposals the existing A75 would be retained as a local road, whilst a new carriageway alignment is constructed to the south. This would result in an increase in total surface area and consequent increase in the volume of road derived surface-water run-off between Carrutherstown and Upper Mains. Transferring the existing trunk road traffic to a new section of road which will incorporate contemporary interception measures decreases the risk of establishing a pollutant pathway and a resultant impact on any of the local watercourses identified above.

12.6.2 These measures include filter strips in the roadside verges that carry the water to balancing ponds containing reed beds in the vicinity of the U81a and north of the trunk road near Upper Mains prior to discharge to local watercourses. Aside from the above measures there is no requirement for further mitigation as proved through the assessment above.

12.6.3 The contractor would be required to detail outfalls to ensure that appropriate angles of entry are achieved and baffles provided to dissipate the flow impacts on entry to the existing channels with the objective of avoiding undue erosion and scour at the outfalls.

12.6.4 Maintenance and management of the proposed drainage system and future maintenance works for the Proposed Scheme, which would involve working in proximity to watercourses, would be subject to SEPA’s Pollution Prevention Guidelines (PPGs).

12.7 Residual Effects

12.7.1 The assessment has demonstrated that there would be no significant residual effects in relation to road drainage and the water environment. It is noted that the transfer of vehicles onto a road with effective drainage provisions would constitute an improvement.