Proposal for Northern Extension of Northolt Tunnel sift report

`Code 

1 - 

Accepted

Uncontrolled when printed 

INTERNAL INFORMATION 

Northern Extension of Northolt Tunnel SIFT Report  

 

 

Northern Extension of Northolt Tunnel SIFT Report  

C222-ATK-DS-REP-020-000034  

Revision – P06 

 

                         

                                            Page 47                                                

 

                                                                                       Uncontrolled when printed 

 

INTERNAL INFORMATION 

 

Location: 

Colne Valley 

OPTIONS CONSIDERED: 

Option A  

The Proposed  Scheme as submitted in the hybrid Bill 

(Northolt Tunnel to West Ruislip/ Viaduct across the Colne 

Valley/ Chiltern tunnel east of M25 between jnc 16 and 

17) 

Option B  

Above ground section between Northolt and Chiltern tunnels 

placed underground in bored tunnels with provision for 

connection spurs to Heathrow 

Option C 

As per option B but with no passive provision for a connection 

to Heathrow

 

OPTION DESCRIPTION 

 

 

 

Key Sustainability 

Issue 

Topic 

STAGE: 

Constructio

n or 

Operation 

EDA 

Considered 

(incl. Topic 

and Ref no.) 

Comment 

QUALITATIVE IMPACT DESCRIPTION 

and/or QUANTITIVE ASSESSMENT 

RATI

NG

 

QUALITATIVE IMPACT DESCRIPTION 

and/or QUANTITIVE ASSESSMENT 

RATI

NG

 

QUALITATIVE IMPACT DESCRIPTION 

and/or QUANTITIVE ASSESSMENT 

RATI

NG

 

century garden at Denham Grove (De Vere 

Hotel) (moderate adverse effect); 

•  Buried archaeological remains of potential 

prehistoric date and of low value off 

Tilehouse Lane (moderate adverse effect); 

and 

•  Buried remains of a World War II searchlight 

battery at Corner Hall (moderate adverse 

effect) 

Op 

Operational impacts have been identified for Savay 

Farm (an asset grouping of high value) and a 

scheduled monument located south of Savay Farm 

as a consequence of a change in setting related to 

operational noise. This will constitute a medium 

adverse impact resulting in a major adverse effect. 

o 

There are no impacts associated with the operation of 

Option B.  

 

+ 

As Option B.

 

+ 

1.1.3 

 

Biodiversity 

Const 

EC1, 

EC2,EC4, 

EC5, EC7, 

The viaduct passes through the Mid Colne Valley Site 

of Special Scientific Interest (SSSI) resulting in the 

loss of ancient woodland and wetland, which are 

designating features of the site; effects that are 

assessed as significant at the national level. 

 

The combined effect of woodland, wetland and 

grassland loss and disturbance of the 

breeding bird assemblage will result in a permanent 

adverse effect on the integrity of the Mid Colne 

Valley Site of Metropolitan Importance that will be 

significant at the county/metropolitan level. 

 

Land required for construction within the Tilehouse 

Gravel Pits Biological Notification Site (BNS) will 

result in disturbance to birds having a permanent 

effect on the integrity of the site that is significant at 

the county/metropolitan level. 

1.1.4 

 

About 0.9ha (19%) of the River Colne east of 

Denham BNS is within land required for construction 

of the Proposed Scheme where National Grid 

overhead power lines will be realigned. This will 

o 

The tunnel section passes under the majority of the 

features listed in Option A (except at the vent shaft 

locations, see below) and has no direct impact on them. 

Depending on tunnel depth and construction, there are 

potential risks to groundwater sensitive features, such as 

the River Colne and the lakes themselves.  

 

The proposed vent shafts located at c29+400 and c26+400 

are not located within any designated site. The 

construction of the vent shafts may lead to the loss of 

broadleaved plantation woodland, hedgerows and arable. 

 

Overall the construction impact of this option is 

significantly reduced compared to Option A.

  

 

+++  As Option B.

 

+++ 

---

`Code 

1 - 

Accepted

Uncontrolled when printed 

INTERNAL INFORMATION 

Northern Extension of Northolt Tunnel SIFT Report  

 

 

Northern Extension of Northolt Tunnel SIFT Report  

C222-ATK-DS-REP-020-000034  

Revision – P06 

 

                         

                                            Page 48                                                

 

                                                                                       Uncontrolled when printed 

 

INTERNAL INFORMATION 

 

Location: 

Colne Valley 

OPTIONS CONSIDERED: 

Option A  

The Proposed  Scheme as submitted in the hybrid Bill 

(Northolt Tunnel to West Ruislip/ Viaduct across the Colne 

Valley/ Chiltern tunnel east of M25 between jnc 16 and 

17) 

Option B  

Above ground section between Northolt and Chiltern tunnels 

placed underground in bored tunnels with provision for 

connection spurs to Heathrow 

Option C 

As per option B but with no passive provision for a connection 

to Heathrow

 

OPTION DESCRIPTION 

 

 

 

Key Sustainability 

Issue 

Topic 

STAGE: 

Constructio

n or 

Operation 

EDA 

Considered 

(incl. Topic 

and Ref no.) 

Comment 

QUALITATIVE IMPACT DESCRIPTION 

and/or QUANTITIVE ASSESSMENT 

RATI

NG

 

QUALITATIVE IMPACT DESCRIPTION 

and/or QUANTITIVE ASSESSMENT 

RATI

NG

 

QUALITATIVE IMPACT DESCRIPTION 

and/or QUANTITIVE ASSESSMENT 

RATI

NG

 

result in a permanent adverse effect on the integrity 

of site that is significant at the county/metropolitan 

level. 

 

Denham Country Park LNR will also be affected 

during construction. Approximately 10ha (52%) of 

this site is within land required for construction of 

the Proposed Scheme where National Grid overhead 

power lines will be realigned. This extent of habitat 

loss is a high proportion of the LNR and it will result 

in a permanent adverse effect on site integrity that is 

significant at the district/borough level. 

 

In the Fray’s Valley Local Nature Reserve, 

approximately 4.2ha (6% of the LNR) of woodland 

and grassland will be removed where National Grid 

power lines will be realigned. The extent of habitat 

loss is relatively small in relation to the site’s size, 

however, as part of the precautionary assessment, it 

is assumed that the woodland and grassland lost is a 

habitat of principal importance. Vegetation 

clearance could therefore result in an adverse effect 

on site integrity that is significant at the 

district/borough level. 

1.1.5 

 

Dew’s Dell Site of Biological Importance lies partly 

within land required for the construction of the 

Proposed Scheme. Construction will result in the loss 

of approximately 1ha (11%) of the site, which will 

result in a permanent adverse effect on the integrity 

of site that is significant at the district/borough level. 

 

The Scheme will disturb two locations where 

coralroot (Nationally Scare) has been recorded; 

effects that are assessed as significant at the county/ 

metropolitan level. 

  

The Scheme will disturb two locations used by 

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