Marlborough District Council Roading Assets - Activity Management Plan 2015 - 2018 SECTION 1 EXECUTIVE SUMMARY 1.6.3 Drainage Facilities 1.6.3.1 Physical Description Drainage Assets The roading network administered by the Marlborough District Council (Council) has 6043 culverts, 306.6km of kerb or kerb and channel, 25km of lined channel, and 1,2367km of unlined channel. Other drainage facilities consist of run-off control bunds, flumes, soak pits, sumps and piped mains. Many of the urban systems are piped to discharge in the Council stormwater mains while in rural areas stormwater discharge is through soakage pits and roadside swales. The following tables are a breakdown of drainage facilities: Description Number Total Length (m) Culverts 6043 64,942 Soak pits 38 - Sumps (rural) 75 - Kerb / kerb and channel 2595 306,630 Stormwater channel, lined 270 25,158 Stormwater channel, unlined 1938 1,236,812 Table 6.3.1a: Drainage Facilities by Type Culvert size range Asset length (m) % of total length <300mm dia 3157 4.9% 300mm – 440mm 35302 54.4% 450mm – 570mm 8658 13.3% 600mm 850mm– 7441 11.5% >900mm 10348 15.9% Table 6.3.1b: Culvert Lengths Extracted from RAMM Database Sept 2014 Environment Rainfall varies across the region with an average in excess of 2500mm per annum in the north of the region (Rai / Sounds) and less than 700mm per annum in the south of the region (Seddon / Ward). In the high rainfall areas and in those with steep bush-clad topography the function of the drainage systems is normally adequate due to the self-cleaning nature of this environment. Where the topogapys flatter or has man-made forest the drainage systems are prone to blockage by sedimentation or by slash from de-forestation. The management of these high risk drainage facilities is difficult during periods of heavy rain, due mainly to the inability to get access to the source of the problem while muddy water is overtopping a structure. The problem is specific and of high risk in those areas where Council face floodwater related litigation from owners below the road. The likely impact of global warming on the region will be moderate with the weather being warmer and wetter. Over the next 70 to 100 years the temperatures could be up to 2.5ºC warmer. Most of the region could be up to 10% wetter whilst, because of prevailing northwest weather patterns, the eastern coastal areas could be up to 10% dryer. There is not expected to be any significant effects experienced within the next 10 years. 1.6.3.2 Age and Current Condition 1.6.3.2.1 Surfaced Water Channels (Kerb and Channel) A comprehensive RAMM condition rating survey of Council footpaths and Kerb and Channel was undertaken in 2010/2011. This survey was carried out only in the urban areas of Blenheim and Picton and represents 263km out a total RAMM length of 306km, which is 87% of the known surface water channels that are recorded in RAMM. 30 September 2014 Page 58 of Section