Preferred Options Report Scheme Assessment Report 2 2 Existing Conditions 2.1 Sources of Information 2.1.1 Drainage and Hydrology The information used to compile this section and the associated drawings referenced within Volume 2 was sourced from the following: • Consultations with key statutory bodies including Rivers Agency, Roads Service Maintenance Section Offices, Northern Ireland Water (NI Water), Loughs Agency and Londonderry Port and Harbour Commissionaires. • Field work assessments of watercourses, floodplains and drainage catchments. • Collation of historical flooding data. • Fluvial design flow estimations based on Flood Estimation Handbook (FEH) Software. • Existing Flood Risk Assessments and consultations with Rivers Agency Consultants. • Preliminary hydraulic models (currently being refined). Potential flood extent adjacent to river corridors is further delineated using information provided by the Mouchel Geotechnical Team. Alluvium drift geology mapping indicates the possible extent of floodplains arising from long-term historical flooding. It is noted that this data does not represent flooding associated with hydraulic restrictions such as culverts, bridges and retaining walls. The NI Water infrastructure discussed in paragraphs 2.2.3, 2.3.3 and 2.4.3 includes drainage outfalls at numerous locations along the length of the Preferred Corridor. Discharge to watercourses from existing NI Water infrastructure is not considered further in the current section. The hydrology and drainage engineering review assumes that NI Water infrastructure will not be utilised/incorporated into the proposed drainage solutions for the A5 WTC. In considering the existing drainage and hydrological conditions within the Preferred Corridor, desk based assessments have been supplemented with site visits and watercourse inspections. The desk based assessment utilised the information sources outlined in the previous section, with specific inputs/methodologies outlined below. 2.1.1.1 Primary Rivers Agency Data There are a number of discrete Units within Rivers Agency, each providing specific information that has been utilised in the current study. 796036/0000/R/011 Version 2 12 ©Mouchel 2009 Preferred Options Report Scheme Assessment Report 2 Rivers Agency GIS Unit have provided: • River Centre Line Data • Designated Watercourses • Areas of Historical Flooding • Rivers Agency Flood Defences • Location of Rivers Agency HEC-RAS Models • Rivers Agency Lidar Coverage Data • NI Strategic Flood Maps In addition, the Planning Advisory Unit have provided assessment guidelines and existing HEC-RAS models for the following watercourses: • Ballygawley Water • Burn Dennet • River Derg • Fairywater • River Finn • River Mourne • Quiggery Water • River Strule Rivers Agency information relating to historical flooding has been reviewed and it is noted that the data set is incomplete, in particular between Newtownstewart and Strabane. The information has been digitised by Rivers Agency from aerial photographs depicting actual flood event, it is noted that there is considerable uncertainty in the return periods associated with individual flood events. The Rivers Agency strategic flood maps were also made available to the public in November 2008. These maps have been consulted as part of this assessment and depict the extent of the predicted 1 in 100 year and the 1 in 100 year plus climate floodplains. 796036/0000/R/011 Version 2 13 ©Mouchel 2009 Preferred Options Report Scheme Assessment Report 2 Historical flood data for Londonderry, Strabane, Omagh and surrounding areas was also gathered from newspaper archives and microfiche held in Londonderry, Strabane and Omagh public libraries. Internet research was also undertaken. 2.1.1.2 Flood Risk Assessment & Previous Flooding Incidences Previous studies that have been reviewed include the River Finn Flood Study (N14/N13 Junction) - Mott MacDonald (Report and ISIS Model), Harcourt Development (Strabane) flood risk assessment report, available through the DoE Planning Service and Newtownstewart by-pass flood risk assessment report. Consultations have been held with Roads Service Maintenance Section Engineers to collate data relating to the existing A5 and flooding. Reports relating to other roads projects previously undertaken within the study area have also been provided by Roads Service. These include Omagh Throughpass and Strabane Bypass Phase 3 (which has been subsumed into this scheme). 2.1.1.3 Tidal Data The northern extents of the A5 WTC study area include the River Foyle north of Strabane which flows into Lough Foyle. Lough Foyle is tidal and exerts a tidal influence on the River Foyle as far inland as the Rivers Mourne and Finn. Consequently, consideration of the tidal influence on these rivers adjacent to Strabane has been undertaken. Londonderry Port and Harbour Commissionaires were consulted in relation to tidal records for Lough Foyle. The Admiralty tidal statistics for Derry Port and Lisahally Port were reviewed, together with a port location plan (refer to Figure 2-1 below). River Foyle (Lisahally) Standard Harmonic Port Londonderry Secondary Harmonic Port Figure 2-1: Derry Port Location Plan 796036/0000/R/011 Version 2 14 ©Mouchel 2009 Preferred Options Report Scheme Assessment Report 2 To facilitate an understanding of the tidal effects on the Foyle River, tidal gauging was undertaken at 2 locations along the Foyle system between 28/08/08 and 05/09/08. The purpose of these gauges, in conjunction with the permanent tidal gauge at Derry (sited over a spring tide) was to gain an appreciation of the deformity of the tide as it progressed up the fluvial system. 2.1.1.4 Site/Watercourse Inspections The watercourses along the route of the existing A5 were identified, marked in GIS and mapped for site survey. Rivers Agency data was cross referenced with the FEH database and ordnance survey mapping. An additional 594 watercourse surveys have also been undertaken within the Preferred Corridor of the A5 WTC with information pertaining to watercourse shape, size and gradient collated. Typical site survey information is presented in Figure 2-2. Figure 2-2: Sample from Site Survey Information Database 2.1.2 Existing Drainage The existing A5 highway drainage will continue to be a maintenance responsibility for Roads Service regardless of the alignment of the new road. Through discussions with Roads Service Maintenance Section Offices and Rivers Agency, it is known that sections of the existing A5 have been prone to historical flooding. These areas are described in the relevant section paragraphs below. 2.1.3 Hydrology & Flooding Overview Preliminary hydrological reviews have ascertained the principal rivers and associated drainage basins within the A5 WTC study area. The main drainage basins are 796036/0000/R/011 Version 2 15 ©Mouchel 2009 Preferred Options Report Scheme Assessment Report 2 shown on drawing 796036-0500-D-00038. The principal watercourses are outlined in drawings 796036-0500-D 00011, 00014, 00017, 00020, 00023, 00026, 00029, 00032, 00035 in Volume 2. The watercourses to the north of Ballygawley feed into the main Foyle Basin which comprises the Strule River, Mourne River and River Foyle. The River Blackwater catchment to the south does not form part of the Foyle basin. 2.1.4 Structures Inspection Methodology Data on the structures on the existing A5 was gathered by desk study and on site inspections. This process comprised: • A study of available client data, including inspection and maintenance records from the previous 5 years, the original construction records and “as built” drawings. It should be noted that all of this data was not available for every existing structure. • A “walk through” survey which identified all the known bridges and located all the retaining walls and minor structures (less than 1.8m span) on the route. • Detailed condition inspections of the major structures, using the HA BE/11 forms as well as the new draft HA “Performance Measurement of Highway Structures” (PMHS) system of inspection. It should be noted that the inspections carried out were intended to produce condition survey results to inform decisions made regarding use of the existing structures on potential on-line sections of the proposed A5 scheme. Therefore, an inspection methodology was developed to ensure that significant defects were reported and recorded in such a way that issues which needed particular consideration during the design phase were readily apparent. Consequently, the results do not necessarily align with the Roads Service condition survey reporting system as described in Roads Service Bridge Management System (RSBMS), which is aimed at informing a long term maintenance regime. Due to the need to assess the costs associated with the ‘Do Nothing’ option as well as the fact that all or some of the A5 may have been widened on line as part of the A5 WTC proposals, it was considered essential to identify the condition of the existing pavements along the route. The maintenance costs associated with the option of retaining the existing A5 as an A class road (i.e. the Do Nothing option) will be incorporated into Stage 3 Traffic & Economics work. It is also the case that the existing A5 structures will continue to be a maintenance responsibility for Roads Service regardless of the alignment of the new road 2.1.5 Pavements Due to the need to assess the costs associated with the ‘Do Nothing’ option as well as the fact that all or some of the A5 may have been widened on line as part of the 796036/0000/R/011 Version 2 16 ©Mouchel 2009 Preferred Options Report Scheme Assessment Report 2 A5 WTC proposals, it was considered essential to identify the condition of the existing pavements along the route. The maintenance costs associated with the option of retaining the existing A5 as an A class road (i.e. the Do Nothing option) will be incorporated into Stage 3 Traffic & Economics work. It is also the case that the existing A5 carriageway will continue to be a maintenance responsibility for Roads Service regardless of the alignment of the new road. This summary deals with the assessment of the condition of the existing pavement.