arch was probably cast in place and is one of the oldest structures of the Blenheim stormwater system. The fifth most common pipe material is a corrugated polyethylene drainage pipe recorded under the brand name Novaflo (NOF.) Used extensively in agricultural and rural drainage it has a number of advantages as it is light weight, easily handled and installed. It has reasonable strength and durability but is best suited to non-trafficked areas such as the road berm and private land. Sum of Length CC-RFPVC AC EW NOFOtherGrand Total <125mm 624 1,348 541 4711,266 310 4,558 150 - 180 8,549 6,5547,0702,280819 2,694 27,966 200 - 275 23,30623,1445,366 882 25 1,930 54,654 300 -350 20,7696,217 1,722 710 3,188 32,605 375 - 400 8,658 2,359 139 551 11,706 450-500mm 11,244 254 278 99 436 12,311 525-750mm 14,310 235 80 1,672 16,297 760-1050mm 5,643 20 327 5,990 >1050 2,908 104 3,012 Grand Total 96,01040,13115,1964,4422,10911,210 169,099 Figure 4 Stormwater Pipe Age and Material Pipe materials and standards of manufacture have evolved over the years. Modern materials are generally easier to handle, more robust and are constructed to a more consistent standard. However when a new material is introduced there is often a fall in the overall life expectancy of the pipeline as there is a lack of skill and experience in the installation limitations. There is a relatively large variety of pipe materials in the network. This is partly due to the history of different authorities that have administered stormwater across the region over the years. At various times the responsibility for stormwater infrastructure has been in the hands of drainage boards; town, county or borough councils and in some cases privately installed schemes have been adopted the Council. 4.1.1 Asset Capacity/Performance Perhaps the over-arching objective that has guided stormwater design was first defined nationally in the Building Act 1991 (superseded by the Building Act 2004.) It states that surface water from a storm with a 2% probability of occurring annually will not enter a building. Also surface water collected or concentrated by a building or site shall not cause nuisance from an event that has a 10% probability of occurring annually. The clauses regulate the building structure, floor levels and areas immediately surrounding the buildings. They also have clear implications for the stormwater infrastructure serving the building sites. The two must work together to achieve the target. The quantitative management of stormwater has two major components – detention/soakage and conveyance. In this respect a recent study of stormwater systems in Blenheim separated the two elements based on the topography of the area served. In hilly terrain the stormwater system needs to be designed to protect the lower lygpp from the run-off from the hillside. In these conditions rain water will naturally flow off the impermeable surfaces. It will accumulate and concentrate as it flows downhill and may become a threat to the properties downstream. In these circumstances the stormwater system must either retard the flow in order to limit the accumulation or intercept the flow and direct it to a suitable watercourse. Page 36