7 Corridor Evaluation
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Preliminary Options Report Scheme Assessment Report 1 - Constraints Report 7 Corridor Evaluation 7.1 Road Design Categories 7.1.1 Road Design Standard The Design Manual for Roads and Bridges (DMRB) Volume 6; Section 1; TD 9/93 Highway Link Design states that All-Purpose dual carriageways shall be designed to permit light vehicles to maintain the design speed. Subject to traffic conditions, light vehicles can overtake slower moving vehicles throughout. Five categories of dual carriageway are defined, three of which refer to 2 lane all purpose dual (D2AP) carriageway: Category 5 – This is the lowest category of dual carriageway, which normally represents an alternative to single carriageways. The vertical alignment should follow the topography closely with the horizontal alignment phased to match. All junctions should be at-grade, with roundabouts at the more heavily trafficked intersections, although where economically / environmentally feasible, grade-separated solutions should be provided. Category 6 – In this category, gaps in the central reserve for turning traffic are not permissible, and major / minor junctions shall not be used. Minor roads shall be stopped up, provided with left in / left out connections, or grade-separated without connection. Major intersection types, which may include roundabouts, will be determined by site conditions, traffic demand, and economic / environmental effect. However, the high costs of delays caused by roundabouts will normally result in more economic grade-separated solutions at the higher end of the range. The combined vertical / horizontal alignments should follow the topography as much as possible, without purposely achieving a “motorway” type flowing alignment. Category 7a – This is the highest category of Dual 2-lane All-Purpose road, where all intersections, both major and minor, shall be grade-separated. A smooth flowing alignment is required for sustaining high speeds. Although the specific category to be considered as part of this study was not specified in the brief, Roads Service has indicated a preference for a high standard dual carriageway. 7.2 Review of Preliminary Corridor Options Given the scale of the project, a large number of potential corridors (links) were identified and are described in Chapter 3. To condense these down to a single preferred corridor, evaluation of the preliminary corridor (link) options has involved a two stage approach. 796036/0000/R/006 Version 3 198 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report Step 1 comprised exclusion of options where major and/or cumulative significant constraints precluded them from further consideration. The remaining corridors that were brought forward to the next step are displayed in Figure 7.1 to 7.3 in this report and Drawings 796036-0800-D-00310 to 796036-0800-D-00329 in Volume 2. Step 2 involved a detailed evaluation whereby all planning and engineering design criteria were assessed and reviewed against those corridor options that emerged from Step 1. From that detailed evaluation, a preferred corridor emerged. This corridor is displayed on Figure 7.4 to 7.6 in this report and on Drawings 796036- 0800-D-00330 to 796036-0800-D-00339 in Volume 2. 7.3 Step 1 Evaluation of All Corridors The Step 1 evaluation involved the evaluation of all the corridor links against major constraints, or cumulative significant constraints such as: • International and national nature Unacceptable environmental impact. conservation designations. • Requirements for extensive Unacceptable social impact and land cost property demolition. • Areas of very high / steep ground. Unfeasible alignment and earthwork. • Areas of extensive peat. Very expensive to engineer and unacceptable ecological impact. • Areas of nationally designated Unacceptable environmental impact. landscapes. • Major impact upon river flood Unacceptable flooding impact. plain. • Major impact on sites with cultural Unacceptable environmental impact. heritage value. Details of these constraints are discussed in Chapters 2, 3, 4 and 5. The options that were consequently excluded during this step and the basis for their exclusion are scheduled below. 796036/0000/R/006 Version 3 199 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report 7.3.1 Section 1 New Buildings to South of Strabane Six Corridors were rejected in this section the major constraints were identified as follows: Node – Node Corridor Link Location Constraint(s) 102-103 West of existing A5 at New Bready archaeological Area (see Buildings to Drumenny via 5.6.2) Bready Foyle floodplain and SAC/ASSI (see 5.6.4) 103-121 West of existing A5 at Foyle floodplain and SAC/ASSI Ballydonaghy to Greenbrae and McKean’s Moss ASSI (see 5.6.4) Peat bog (see 4.3.9) 115-120-116 East of existing A5 at Milltown High ground, high relief (see Burndennet to Ballee via 4.3.9) Artigarvan Proximity to Sperrins AONB (see 5.6.5) 115-109-121 East of existing A5 at Milltown Severance between Burndennet to Greenbrae via Ballymagorry/Artigarvan (see Ballymagorry 5.6.8) Impact on Foyle SAC/ASSI (see 5.6.4) 116-119 Liskinbwee to east of Sion Mills High ground, high relief (see (Carrigullin) 4.3.10) Proximity to Sperrins AONB (see 5.6.5) 121-104-117-111-112 Existing A5 between Greenbrae Impact on floodplains, Mourne and Sion Mills via Strabane River SAC/ASSI (see 4.3.10& 5.6.4) Significant property loss in Strabane and potential impacts on large number of residents (air/noise/severance) (see 5.6.1/2/6) Table 7.3.1-1: Corridor Links Eliminated in Section 1 Drawing numbers 796036-0800-D-00320 to 00322 show the Section 1 corridor links that were evaluated at the corridor selection workshop. 796036/0000/R/006 Version 3 200 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report 796036/0000/R/006 Version 3 201 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report 7.3.2 Section 2 - South of Strabane to South of Omagh 5 links were rejected from this section: Node – Node Corridor Link Location Constraint(s) 112-217 Existing A5 from Sion Mills to Adverse impact on towns and north of Ardstraw Heritage, especially Sion Mills and Victoria Bridge (see 5.6.2) 213-214 Douglas Bridge to east of Encroaches into Sperrins AONB Mountjoy (see 5.6.5) 207-214 Mountjoy to node 214 214-216 East of Mountjoy to east of Proximity to Sperrins AONB Omagh (see 5.6.5) 208-211 Existing A5, from Fairy Water Significant property loss in bridge to south of Omagh Omagh and potential impacts on large number of residents (air/noise/severance) (see 5.6.1/2/6) Table 7.3.2-1: Corridor Links Eliminated in Section 2 After the elimination of node to node link 213-214 a further corridor link was developed following the nodes 218-213-208. Geographically, heading south from Douglas Bridge, this link runs east of the Strule River before connecting into the existing A5 corridor at Newtownstewart. Drawing numbers 796036-0800-D-00323 to 00325 show the Section 2 corridor links that were evaluated at the corridor selection workshop. 796036/0000/R/006 Version 3 202 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report 796036/0000/R/006 Version 3 203 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report 7.3.3 Section 3 - South of Omagh to Aughnacloy Of the initial 22 corridor link options in section 3, 18 links were removed from further consideration; although a further 4 potential hybrid corridors were identified for inclusion. (Note some node links in the following table cover more than one corridor) Node – Node Corridor Link Location Constraint(s) 319 Tattykeeran Deroran Bog SAC / ASSI (see 5.6.4) 211-313 Omagh Significant property loss in Omagh and potential impacts on large number of residents (air / noise / severance) (see 5.6.1/2/6) 321-324 Martray House Martray House and Lough (see 5.6.2) Slievemore Ridge – excessive earthworks (see 4.3.15) 306-308 West of Aughnacloy - Ravellea The Thistle, Ravellea (see 5.6.2) Substantial low-land raised peat bog (see 4.3.16) River Blackwater floodplain (see 4.4.8) 216-313 Ballynamullan, Golan, Substantial low-land raised peat Donaghanie bog (see 4.3.14) 317-318 210-301 Seskinore, Tullyvally, Tattyreagh Substantial low-land raised peat 210-325 Glebe, Mullaghmore bog (see 4.3.14) Table 7.3.3-1: Corridor Links Eliminated in Section 3 Following the elimination of these nodes an additional link and a further three potential hybrid corridors were identified for inclusion, development and discussion at the workshop. They are as follows: S3H10: Nodes 203 – 301 – 325 – 315 – 316 – 306 – 307 – 308 An additional link was identified linking nodes 301 - 325 – 315. Geographically it extends from Seskinore eastwards to Letfern and on to Curr on the existing A5. S3H11: Nodes 210 – 312 – 313 – 314 – 315 – 316 – 306 – 307 – 308 S3H12: Nodes 210 – 312 – 318 – 320 – 321 – 322 – 316 – 306 – 307 – 308 These two corridors are hybrid forms of the original corridors S3B01 and S3H06 respectively, which was removed because it went to the west of Aughnacloy (nodes 306-308 - see above). This corridor instead extends to the east of Aughnacloy passing through nodes 306 – 307 – 308. S3H13: Nodes 210 – 312 – 318 – 320 – 321 – 323 – 307 – 308 796036/0000/R/006 Version 3 204 ©Mouchel 2008 Preliminary Options Report Scheme Assessment Report 1 - Constraints Report This corridor is a hybrid form of the original corridor S3H06, which was removed because it went to the west of Aughnacloy (nodes 306-308 - see above). This corridor instead extends to the east of Aughnacloy passing through nodes 306 – 307 – 308, having passed just west of Ballyreagh and east of Martray Lough. Drawing numbers 796036-0800-D-00326 to 00329 show the Section 3 corridor links that were evaluated at the corridor selection workshop.