northampton north west relief road

70021598-001 JUNE 2020 INTERNAL

Northamptonshire County Council

NORTHAMPTON NORTH WEST

RELIEF ROAD

Drainage Network Water Quality Assessment


NORTHAMPTON NORTH WEST RELIEF ROAD


TYPE OF DOCUMENT (VERSION) INTERNAL

PROJECT NO. 70021598

OUR REF. NO. 70021598-001

DATE: JUNE 2020


NORTHAMPTON NORTH WEST RELIEF ROAD


WSP

The Mailbox Level 2

100 Wharfside Street

Birmingham

B1 1RT

Phone: 0121 352 4700

WSP.com

NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council

QUALITY CONTROL

Issue/revision First issue Revision 1 Revision 2 Revision 3

Remarks

Date June 2020

Prepared by F Marlow

Signature

Checked by J Goodwin

Signature

Authorised by J Goodwin

Signature

Project number 70021598

Report number V1.0


CONTENTS

1. INTRODUCTION 1

1.1. PURPOSE OF THE ASSESSMENT 1

1.2. STUDY AREA 1

1.3. METHODOLOGY 2

2. PROPOSED DRAINAGE STRATEGY 4

2.2. PROPOSED WATER TREATMENT 5

3. IMPACT OF ROUTINE RUNOFF ON SURFACE WATER BODIES 7

3.1. METHODOLOGY AND DATA 7

3.2. RESULTS 8

CUMULATIVE ASSESSMENT 9

IMPACT OF INCREASED SPILLAGE RISK ON SURFACE WATER BODIES.

10

3.3. METHDOLOGY AND DATA 10

3.4. RESULTS 10

4. SUMMARY 11

5. REFERENCES 12

TABLES

Table 2-1 – Description of highway drainage catchments for the Proposed Scheme and their

outfall locations. 4

Table 3-1 – HEWRAT assessment results for the Proposed Scheme. 8

Table 3-2 – HEWRAT cumulative assessment results for the Proposed Scheme. 9

WSP NORTHAMPTON NORTH WEST RELIEF ROAD JUNE 2020 Project No.: 70021598 | Our Ref No.: 70021598-001 Northamptonshire County Council

Table 3-3 – Spillage risk assessment results for the Proposed Scheme. 10

FIGURES

Figure 1-1 - Overview of the Site. 2

Figure 2-1 - Overview of the highway drainage elements of the Proposed Scheme. 6

APPENDICES

IMPACT OF ROUTINE RUNOFF ASSESSMENT DATA

RISK OF SPILLAGE ASSESSMENT DATA


GLOSSARY AND ABBREVIATIONS

Terminology Abbreviation Description

Annual Average Daily Traffic Flow

AADT Total volume of two-way vehicle traffic of a highway or road for a year divided by 365 days passing a specific point in a 24-hour period.

Baseline The existing conditions which form the basis or start point of the environmental assessment.

Catchment The area contributing flow to a point on a drainage system.

The Construction Industry Research and Information Association

CIRIA The construction industry research and information association.

Design Manual for Roads and Bridges

DMRB A series of 15 volumes that provide standards, advice notes and other documents relating to the design, assessment and operation of trunk roads, including motorways in the UK.

Environmental Quality Standards

EQS Maximum permissible annual average concentrations of potentially hazardous chemicals, as defined under the WFD.

Environmental Statement Addendum

ESA The Environmental Statement for the Environment Impact Assessment process was written in June 2019. An addendum has been submitted in June 2020 to incorporate further changes to the Proposed Scheme and the changes to the DMRB released in August 2019.

Heavy Goods Vehicles

HGVs Large vehicles with a weight over 3.5 tonnes.

Highways England Water Risk Assessment Tool

HEWRAT A Microsoft Excel application designed to assess the acute and chronic risks of pollution related to the intermittent nature of road runoff and associated with soluble and sediment bound pollutants.

Mitigation Term used to indicate avoidance, remediation or alleviation of adverse impacts.

North West Relief Road

NWRR Northampton North West Relief Road if the subject of this study. Scheme

Proposed Scheme The scheme design for the Northampton North West Relief Road used as the basis for environmental assessment and reporting.

Residual impacts Residual impact means the environmental impact after the provision of mitigation measures, if any.

Sustainable Drainage System

SUDs A sustainable drainage system (SUDs) is designed to reduce the potential impact of new and existing developments with respect to surface water drainage discharges

WSP NORTHAMPTON NORTH WEST RELIEF ROAD JUNE 2020 Project No.: 70021598 | Our Ref No.: 70021598-001 Northamptonshire County Council

Water Body A body of surface water, or a body of groundwater. The WFD defines discrete surface water bodies, but not necessarily a whole river, while groundwater bodies should be distinct.

Water Framework Directive

WFD Wide-ranging European environmental legislation (2000/60/EC). Addresses inland surface waters, estuarine and coastal waters and groundwater. The fundamental objective of the WFD is to maintain "high status" of waters where it exists, preventing any deterioration in the existing status of waters and achieving at least "good status" in relation to all waters by 2027.

Water quality The chemical and biological status of various parameters within the water column and their interactions, for example dissolved oxygen, indicator metals such as dissolved copper, or suspended solids (the movement of which is determined by hydrological process and forms geomorphological landforms).


EXECUTIVE SUMMARY

The Drainage Network Water Quality Assessment informs Chapter 15 Road Drainage and the Water

Environment of the Environmental Statement. The assessment has been conducted in accordance

with the Design Manual for Roads and Bridges (DMRB) LA 113; local planning policy; as well as

consultation with the Environment Agency.

The assessment outlines the potential impacts associated with the Proposed Scheme on the

chemical quality of the receiving surface water features and assesses the impact of the proposed

mitigation measures within the surface water management strategy. The assessment uses the

Highways England Water Risk Assessment Tool (HEWRAT) to assess routine runoff and the

method set out in Appendix D of DMRB LA 113 to assess pollution impacts from accidental spillage.

The results of the HEWRAT assessment indicate that the Proposed Scheme passes the

assessment for acute impacts of soluble pollutants and chronic impacts of suspended sediments

with the proposed mitigation measures in place. The results of the assessment of long term pollution

impacts to the receiving water environment indicates that the annual average pollutant

concentrations for zinc and copper are below the Environmental Quality Standard (EQS) thresholds

as set out under the Water Framework Directive (WFD) and as summarised within the DMRB.

The method in Appendix D of DMRB LA 113 assesses the pollution impacts from accidental spillage

on the receiving watercourse. The DMRB recommends that an annual probability of a serious

pollution incident occurring of less than 1% would be acceptable. The Proposed Scheme passes

Method D without taking the proposed mitigation measures into account. The annual probability of a

serious pollution incident occurring is further reduced with the mitigation measures included in the

design.

The results of both the Method A and Method D assessments demonstrate that there will not be a

significant impact on the receiving surface water features as a result of the Proposed Scheme.

Impacts of routine runoff on groundwater bodies (method in Appendix C of DMRB LA 113) is scoped

out of this assessment. The proposed drainage basins are lined to minimise the risk of infiltration

and the superficial geology at the location of the receiving land drains has low permeability and

therefore impacts to groundwater through infiltration are not expected.

NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council Page 1 of 12

1. INTRODUCTION

1.1. PURPOSE OF THE ASSESSMENT

1.1.1. The Drainage Network Water Quality Assessment informs Chapter 15 Road Drainage and the Water

Environment of the Environmental Statement Addendum (ESA) and supports the planning application

for the Northampton North West Relief Road (Herein referred to as the ‘Proposed Scheme’).

1.1.2. The assessment has been conducted in accordance with the Design Manual for Roads and Bridges

(DMRB) Volume 11, Part 10 (LA 113); local planning policy; as well as consultation with the

Environment Agency (EA).

1.1.3. The Proposed Scheme will increase the impermeable road surface area and alter the current traffic

flow regime through the creation of a new road section consisting of a single two-lane carriageway

and three new roundabouts. These changes have the potential to impact the volume and quality of

surface water runoff to the receiving water features. The purpose of the assessment is to assess the

potential impacts associated with the Proposed Scheme on the chemical quality of the receiving water

features and to assess the impact of the proposed mitigation measures within the surface water

management strategy.

1.1.4. The assessment focusses on the potential risks associated with the operational phase of the Proposed

Scheme and does not consider potential risks during the construction phase. The potential impacts to

the chemical quality of water features during the construction phase are assessed within the Road

Drainage and the Water Environment Chapter of the ESA.

1.2. STUDY AREA

1.2.1. The Proposed Scheme is located to the north of the town of Northampton, an overview of the

Proposed Scheme is provided in Figure 1-1. The Proposed Scheme consists of a single two-way

carriageway design.

1.2.2. The Main Carriageway of the Proposed Scheme runs along a north-south alignment to the west of the

River Nene. To the north, it will connect to Sandy Lane and the A5199 Welford Road, via a new

roundabout. To the south, the Proposed Scheme crosses Brampton Brook and the railway line before

connecting into a new roundabout, east of Grange Farm, which will provide access to the proposed

Dallington Grange residential development. This element of the Proposed Scheme measures

approximately 1570m.

1.2.3. From the new roundabout to the north a second element of the Proposed Scheme, hereafter referred

to as the Causeway, will cross the River Nene parallel to the existing A5119 crossing, and connect to

the A5119 and Brampton Lane via a second roundabout. The existing A5119 crossing will remain but

will be downgraded to a pedestrian and cycle access route only, therefore the Causeway will become

the A5119. This element of the Proposed Scheme measures approximately 320m.

WSP JUNE 2020 Project No.: 70021598 | Our Ref No.: 70021598-001 Page 2 of 12 Northamptonshire County Council

Figure 1-1 - Overview of the Site

1.2.4. The proposed scheme forms part of a wider strategy to complete the ring road around the northern

sector of Northampton that will ultimately connect the A4500 Weedon Road and M1 Motorway

Junction 16 to the West, with the A34 Kettering Road and Kettering to the north.

1.2.5. The Proposed Scheme will unlock further development of 6,600 houses and over 3,000 jobs in

Northampton and relieve pressure that already exists on the local highway network in northern

Northampton and nearby villages that suffer high volumes of traffic and associated disturbance.

1.3. METHODOLOGY

1.3.1. The assessment of impacts to water quality is defined by the methodology set out in the DMRB.

1.3.2. The Highways England Water Risk Assessment Tool (HEWRAT) has been used to determine the

effect of routine runoff on receiving watercourses. This is the simple assessment method provided in

the DMRB which considers several factors including impermeable road area, and low flows and

dimensions of the receiving watercourse. The HEWRAT has been used to assess all new outfalls as

part of the Proposed Scheme. The HEWRAT assesses the likely quality of scheme-generated surface

water runoff against the Environmental Quality Standards (EQS) given in the Water Framework

Directive. The results of this assessment are summarised in Section 3.

1.3.3. The risk of a potentially polluting spillage event is calculated using equations and factors provided in

Appendix D of DMRB LA 113. This method calculates the probability of a spillage event with an


associated risk of a serious pollution incident occurring. This firstly calculates the probability of a

spillage occurring with the potential to pollute, and secondly calculates the probability of the pollutant

reaching and impacting a receiving water body.

1.3.4. An assessment of potential risks to groundwater resources as set out in in Appendix C of DMRB LA

113 has not been completed in this assessment because the proposed basins will be lined therefore

infiltration to ground is not expected. In addition, the geology of the area comprises Alluvium and

Whitby Mudstone Formation which is expected to substantially limit potential for infiltration. Therefore,

an assessment of potential risks to groundwater associated with discharge to the land drain which will

receive highway runoff (before discharging to the River Nene) has not been undertaken.


2. PROPOSED DRAINAGE STRATEGY

2.1.1. Infiltration of runoff to ground is not considered viable therefore the drainage strategy proposes to

drain highway runoff to the River Nene. Attenuation features have been designed to accommodate

the 1 in 200yr flood event with 65% climate change allowance with additional freeboard. This is in

accordance with the guidance in the DMRB CG501i and has been agreed with the Lead Local Flood

Authority (LLFA). All attenuation features will discharge at a restricted rate of 5 l/s (each). This rate

has been agreed with the LLFA.

2.1.2. There are four new outfalls and three new attenuation basins proposed and detailed in Table 2-1. The

location of these elements is shown in Figure 2-1.

2.1.3. The Proposed Scheme does not connect to any existing drainage systems and therefore these are

not scoped into this assessment.

Table 2-1 – Description of highway drainage catchments for the Proposed Scheme and their

outfall locations

Outfall Number Highway Catchment Receiving Watercourse Treatment Features

1 Brampton Lane roundabout (Catchment 1a)

The Causeway (Catchment 1b)

River Nene Catchment 1a discharges directly to the Nene via a new surface water drainage ditch.

Catchment 1b will be attenuated in Basin 1 prior to discharge to the same new surface water drainage ditch.

2 Sandy Lane roundabout and Main Carriageway

River Nene, via an existing land drain

Filter drains and Basin 2

3 Main Carriageway River Nene, via an existing land drain

Filter drains and Basin 3

4 Main Carriageway River Nene Filter drains.

Attenuation will be provided in oversized pipes. Discharge to the Nene will be via a SuDS swale feature also serving the Dallington Grange development. Potential opportunity to combine attenuation with the attenuation basin serving the new Dallington Grange development.


2.2. PROPOSED WATER TREATMENT

2.2.1. Catchments 1b, 2 and 3 will receive treatment through the use of the attenuation basins. Catchment

4 will receive treatment through the use of a swale. These features will allow for settlement and

entrapment of suspended solids. Filter drains will be implemented along straight sections of the Main

Carriageway and therefore provide treatment of highway runoff for catchments 3 and 4, as well as

some parts of catchment 2 (assumed 50% of the catchment for the purposes of the HEWRAT

assessment). Catchment 1a and 1b will be drained by gullies.

2.2.2. Discharge from catchment 4 will be coordinated with the drainage system serving the Dallington

Grange development. Currently it is proposed that a swale between the borrow pits and the railway

line will receive highway runoff from catchment 4 and divert it towards the River Nene. There may also

be opportunity to discharge runoff from catchment 4 to attenuation basins serving the Dallington

Grange development. The construction of these features and outfall 4 will be coordinated so that

temporary outfalls are not required.


Figure 2-1 - Overview of the highway drainage elements of the Proposed Scheme

.


3. IMPACT OF ROUTINE RUNOFF ON SURFACE WATER BODIES

3.1. METHODOLOGY AND DATA

3.1.1. This assessment reviews the effect of pollution from routine runoff on the River Nene. Consideration

has not been given to specific risks to the short section of existing land drain that will receive runoff

from basins 2 and 3 as this is a drainage feature with ephemeral flow and low ecological value.

3.1.2. This assessment uses the Highways England Water Risk Assessment Tool (HEWRAT) which is

described in more detail in Appendix A of DMRB LA 113. This inputs to this tool are:

▪ Details about the receiving watercourse:

• Annual Q95 river flow

• Base Flow Index (BFI)

• River width

• Bed width

• Manning’s n

• Channel side slope

• Channel bed slope

• Presence of downstream structures or sensitive areas

• Ambient background copper concentration

• Water hardness

▪ Details about the proposed scheme / current arrangement:

• Annual Average Daily Traffic flow (AADT)

• Impermeable area

• Permeable area

• Existing mitigation

• Proposed mitigation

3.1.3. Annual Q95 flow is derived from HR Wallingford’s LowFlows2ii software. The software uses a

catchment boundary to determine the low flow data. Catchment boundaries have mostly been derived

from the FEH Web Serviceiii catchment boundaries, with some adjustments based on available LiDAR

dataiv. The FEH Web Service also provides a value for BFI for each downloaded catchment.

3.1.4. Ambient background copper concentration and water hardness is set to the default value for the

HEWRAT assessment for the River Nene as water quality data has not been provided or obtained.

3.1.5. Channel width has been informed by the topographic data used in the hydraulic model for the River

Nene.

3.1.6. AADT was extracted from the traffic model for both the baseline arrangement and the Proposed

Scheme. This data was based on the design year traffic flows expected in 2036.

3.1.7. Impermeable and permeable area, as well as mitigation measures, were taken from the Proposed

Scheme drawings and drainage strategy.

3.1.8. Water treatment elements described in Section 2.2 are included as existing mitigation in this

assessment as they are embedded in the design. Each of these features have treatment efficiencies


as stated in Table 8.6.4N3 of DMRB CG501 which have been applied in the HEWRAT. For any

treatment features located downstream of another, a 50% reduction in efficiency has been applied as

per guidance in CIRIA C609v.

3.1.9. All input parameters used in this assessment are presented in Appendix A.

3.1.10. Each of the outfalls in Table 2-1 have been assessed quantitatively using HEWRAT. In addition, a

cumulative assessment has been carried out at the point where discharge from Outfall 4 joins the

River Nene, to identify the cumulative impacts to water quality from the entirety of the Proposed

Scheme on the River Nene.

3.2. RESULTS

3.2.1. The HEWRAT has two stages of assessment: Tier 1 and Tier 2. Tier 1 is a high level assessment

based on river width. If the Tier 1 assessment fails, then Tier 2 is carried out using more channel

dimensions. The assessment is undertaken first without and then with proposed mitigation to assess

the reduction in impact to water quality from copper, zinc and suspended solids. Step 3 of the

assessment is where the mitigation has been included.

3.2.2. The results for each tier indicate whether the test has exceeded the acceptable limits for copper and

zinc and indicate how much sedimentation is required for the test to pass on suspended solids. For

the Proposed Scheme, the Tier 1 assessment was passed and therefore the Tier 2 assessment was

not carried out. Table 3-1 summarises the results for each individual outfall assessed. The full results

suite is presented in Appendix A.

Table 3-1 – HEWRAT assessment results for the Proposed Scheme

Outfall Step Soluble Pollutants Sediment EQS Assessment

Acute impact assessment of copper

Acute impact assessment of zinc

Chronic impact assessment of sediment

Annual average concentration of copper (μg/l) due to road runoff

Annual average concentration of zinc (μg/l) due to road runoff

1 Step 2 Pass Pass Pass 0.00 μg/l - Pass

0.01 μg/l - Pass

Step 3 Pass Pass Pass 0.00 μg/l - Pass

0.01 μg/l - Pass


0.04 μg/l - Pass


0.03 μg/l - Pass


0.01 μg/l - Pass


0.01 μg/l - Pass


Outfall Step Soluble Pollutants Sediment EQS Assessment




Annual average concentration of copper (μg/l) due to road runoff

Annual average concentration of zinc (μg/l) due to road runoff


0.00 μg/l - Pass


0.00 μg/l - Pass

CUMULATIVE ASSESSMENT

3.2.3. In accordance with DMRB methodology, the HEWRAT assessment should include a cumulative

assessment that considers other outfalls located within 100m and 1km of each that drain to the same

watercourse catchment. With relevance to the Proposed Scheme this applies to outfalls 1, 2, 3 and 4

for the cumulative assessment of outfalls within 1km, and outfalls 2 and 3 for the cumulative

assessment of outfalls within 100m. The results of the cumulative assessment are shown in Table 3-

2 below.

3.2.4. To determine the cumulative effects, the total area of all catchments draining into the River Nene via

each outfall was calculated. The percentage removal of copper and zinc and the percentage

settlement of sediment was determined by weighting the proposed mitigation with the catchment

areas.

Table 3-2 – HEWRAT cumulative assessment results for the Proposed Scheme

Outfalls Step Soluble Pollutants Sediment EQS Assessment




Acute Impact assessment of copper

Acute Impact assessment of zinc

2 & 3 Step 2 Pass Pass Pass 0.02 μg/l - Pass

0.05 μg/l - Pass


0.04 μg/l - Pass

1, 2, 3 & 4

Step 2 Pass Pass n/a 0.02 μg/l - Pass

0.06 μg/l - Pass

Step 3 Pass Pass n/a 0.02 μg/l - Pass

0.04 μg/l - Pass

3.2.5. The assessment indicates that the Proposed Scheme passes all aspects of the assessment of

potential cumulative effects.


IMPACT OF INCREASED SPILLAGE RISK ON SURFACE WATER

BODIES.

3.3. METHDOLOGY AND DATA

3.3.1. This assessment reviews the probability of a spillage event occurring and the potential for this event

to pollute receiving water bodies, in this case the River Nene. This is a calculation using a formula

provided in Appendix D of DMRB LA 113. The inputs to this calculation are:

▪ Road grade (motorway, urban trunk road, rural trunk road),

▪ Road type (roundabout, slip road, side road, crossroads, road with no junction),

▪ Road length,

▪ Road location (rural or urban),

▪ AADT for each road type, and

▪ Percentage of heavy goods vehicles (%HGV) use for each road type.

3.3.2. AADT and %HGV data was extracted from the traffic model for both the baseline arrangement and

the Proposed Scheme. This data was based on the design year traffic flows expected in 2036. AADT

for roundabouts was taken as the highest AADT of the arms of the roundabout.

3.3.3. Road grade, type and length was derived from maps and the designations are shown in Appendix B.

3.3.4. Water treatment elements described in Section 2.2 are included as existing mitigation in this

assessment as they are embedded in the design. Each of these features have a spillage risk reducing

factor as stated in Table 8.6.4N3 of CG501 (DMRB) which have been applied in this assessment.

3.3.5. The calculation was carried out for each proposed outfall. All input parameters are presented in

Appendix B.

3.4. RESULTS

3.4.1. A summary of the results of the spillage risk assessment for the Proposed Scheme are presented in

Table 3-3. The Proposed Scheme passes the spillage risk assessment as all outfalls have a

probability of a spillage event occurring of less than 0.5%. Therefore, no additional mitigation is

required in order to minimise spillage risk to an acceptable level. However, mitigation, such as manual

penstocks, is welcomed to reduce the risk of spillage further.

Table 3-3 – Spillage risk assessment results for the Proposed Scheme

Outfall Embedded mitigation Spillage Risk

1a None 0.008%

1b Attenuation basin 0.003%

2 Filter drains, attenuation basin 0.006%

3 Filter drains, attenuation basin 0.000%

4 Filter drains, swale 0.001%


4. SUMMARY

4.1.1. The introduction of a new highway can impact water quality of nearby water bodies which receive

highway drainage. This assessment determines if the proposed drainage system of the Proposed

Scheme is likely to cause a significant adverse effect to receiving waterbodies. This assessment has

considered impact to surface water quality from changes to routine runoff and from changes to the

risk of a spillage event with potential to pollute waterbodies.

4.1.2. The assessment of routine runoff was conducted using HEWRAT. This assessment determined that

the proposed system is acceptable and does not significantly adversely affect the River Nene.

4.1.3. The assessment of spillage risk was conducted using the calculations provided in Appendix D of

DMRB LA 113 and covered the entire length of the Proposed Scheme and adjoining junctions. This

assessment determined that the proposed system is acceptable for the River Nene.

4.1.4. The proposed mitigation measures that will be included in the design have been taken into

consideration within the assessments. As a result, the proposed mitigation measures are deemed to

be appropriate.

4.1.5. The HEWRAT and spillage risk assessments will also support the standalone WFD assessment and

the Chapter 15 Road Drainage and the Water Environment of the Environmental Statement for the

Proposed Scheme.

4.1.6. The assessment of impacts of routine runoff on groundwater was not carried out as the Proposed

Scheme includes lined basins, preventing infiltration, and the geology of the area is considered to

have a permeability insufficient to pose risk to groundwater.


5. REFERENCES

i Design Manual for Roads and Bridges, Volume 4, Section2, Part 3 (CG 501) ii Wallingford HydroSolutions 2010 iii UK Centre for Ecology & Hydrology 2020, fehweb.ceh.ac.uk iv Defra Survey Data Download 2020, environment.data.gov.uk/DefraDataDownload v Wilson, S., Bray R., Cooper, P., (2004) Sustainable Drainage Systems, Hydraulic, structural and water quality advice CIRIA C609

https://environment.data.gov.uk/DefraDataDownload/?Mode=survey

Internal

IMPACT OF ROUTINE RUNOFF

ASSESSMENT DATA

Internal


Table A-1 – Single outfall HEWRAT input data and assessment for routine runoff

Outfall reference Outfall 1 Outfall 2 Outfall 3 Outfall 4

Section of road Causeway Main Carriageway Main Carriageway Main Carriageway

Easting 473414 473477 473477 473851

Northing 265231 264416 264416 263656

Receiving watercourse

River Nene River Nene River Nene River Nene

AADT DS banding >10,000 and <50,000

>10,000 and <50,000

>10,000 and <50,000

>10,000 and <50,000

Climate region Warm / Dry Warm / Dry Warm / Dry Warm / Dry

Rainfall site Huntingdon Huntingdon Huntingdon Huntingdon

Base flow index 0.53 0.53 0.53 0.53

Hardness (mg CaCO3/l)

Worst case (hard) Worst case (hard) Worst case (hard) Worst case (hard)

Q95 flow (m3/s) 0.14 0.14 0.14 0.14

Impermeable areas drained to the outfall (ha)

0.8027 2.4889 0.7068 0.2686

Permeable area drained to the outfall (ha)

0 0 0 0

River width (m) 14 12 12 12

Downstream structure within 100m of outfall?

No No No No

Discharge in or within 1km upstream of a designated site

No No No No

Proposed Mitigation

Treatment for solubles (%) Copper

50

Treatment for solubles (%) Zinc

22.5 45 70

Outfall reference Outfall 1 Outfall 2 Outfall 3 Outfall 4

Restricted discharge rate (l/s)

5 5 5 5

Settlement of Sediments (%)

19 40 85 100

Tier 1 Results

Step 2

Acute impacts - Copper

Pass Pass Pass Pass

Acute Impacts - Zinc

Pass Pass Pass Pass

Chronic Impact - Sediments

Pass Pass Pass Pass

Annual average concentration - Copper

0 μg/l 0.01 μg/l 0 μg/l 0 μg/l


0.01 μg/l 0.04 μg/l 0.01 μg/l 0 μg/l

Step 3

Acute impacts - Copper

Pass Pass Pass Pass

Acute Impacts - Zinc

Pass Pass Pass Pass

Chronic Impact - Sediments

Pass Pass Pass Pass


0 μg/l 0.01 μg/l 0 μg/l 0 μg/l


0.01 μg/l 0.03 μg/l 0.01 μg/l 0 μg/l


Table A-2 – Cumulative HEWRAT input data and assessment for routine runoff

Outfall reference Outfall 2 & 3 Outfall 1, 2, 3 & 4

Section of road Main Carriageway Causeway and Main Carriageway

Easting 473477 473851

Northing 264416 263656

Receiving watercourse River Nene River Nene

AADT DS banding >10,000 and <50,000 >10,000 and <50,000

Climate region Warm / Dry Warm / Dry

Rainfall site Huntingdon Huntingdon

Base flow index 0.53 0.53

Hardness (mg CaCO3/l) Worst case (hard) Worst case (hard)

Q95 flow (m3/s) 0.14 0.14

Impermeable areas drained to the outfall (ha)

3.1957 4.2670

Permeable area drained to the outfall (ha)

0 0

River width (m) 12m 12m

Downstream structure within 100m of outfall?

No No

Discharge in or within 1km upstream of a designated site

No No

Proposed Mitigation

Treatment for solubles (%) Copper 3

Treatment for solubles (%) Zinc 27 25

Restricted discharge rate (l/s) 10 20

Settlement of Sediments (%) 50 47

Tier 1 results

Step 2

Acute impacts - Copper Pass Pass

Outfall reference Outfall 2 & 3 Outfall 1, 2, 3 & 4

Acute Impacts - Zinc Pass Pass

Chronic Impact - Sediments Pass n/a


0.02 μg/l 0.02 μg/l


0.05 μg/l 0.06 μg/l

Step 3

Acute impacts - Copper Pass Pass

Acute Impacts - Zinc Pass Pass

Chronic Impact - Sediments Pass n/a


0.02 μg/l 0.02 μg/l


0.04 μg/l 0.04 μg/l

Internal

RISK OF SPILLAGE ASSESSMENT

DATA

Internal

Figure B-1 - Spillage Assessment Road Types


Table B-1 – Spillage risk assessment for the Proposed Scheme

Road label Catchment Receiving watercourse / drainage system

Length of road draining to outfall

Road Type Junction type Response time

Traffic flow (AADT 2-way)

% HGV Serious spillage factor

Risk of accidental spillage

Probability factor

Risk of pollution incident

1.1 1a River Nene 0.123

Urban trunk road

Roundabout <20 minutes 32677 1.1 5.35 0.009%

0.45 0.004%


Urban trunk road


0.45 0.002%


Urban trunk road


0.45 0.001%

Catchment 1a Total Spillage Risk 0.008%

Spillage reduction factor 0

Catchment 1a Total Residual Spillage Risk 0.008%

1.4 1b River Nene 0.023

Urban trunk road


0.45 0.001%


Urban trunk road


0.45 0.002%


Urban trunk road

No Junction <20 minutes 32677 1.1 0.31 0.000%

0.45 0.000%


Urban trunk road


0.45 0.003%

Catchment 1b Total Spillage Risk 0.005%

Spillage reduction factor 0.6

Catchment 1b Total Residual Spillage Risk 0.003%

2.1 2 River Nene 0.197

Urban trunk road


0.45 0.006%


Urban trunk road


0.45 0.000%


Urban trunk road


0.45 0.003%


Urban trunk road


0.45 0.000%


Urban trunk road

Roundabout <20 minutes 8093 3 5.35 0.005%

0.45 0.002%

Road label Catchment Receiving watercourse / drainage system

Length of road draining to outfall

Road Type Junction type Response time

Traffic flow (AADT 2-way)

% HGV Serious spillage factor

Risk of accidental spillage

Probability factor

Risk of pollution incident


Urban trunk road


0.45 0.001%


Urban trunk road


0.45 0.000%


Urban trunk road

No Junction <20 minutes 8093 3 0.31

0.000% 0.45 0.000%

Catchment 2 Total Spillage Risk 0.013%


Catchment 2 Total Residual Spillage Risk 0.006%


Urban trunk road


0.45 0.001%




Urban trunk road


0.45 0.000%


Urban trunk road


0.45 0.003%

Catchment 4 Total Spillage Risk 0.003%



The Mailbox Level 2

100 Wharfside Street

Birmingham

B1 1RT

wsp.com

northampton north west relief road

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