The other main category is the very flat land common to alluvial plains. The absence of any appreciable gradient in this landscape means that rainfall will be very slow to drain away. A high water table is often associated with this type of land which restricts the soils capacity to absorb rainwater. The resultant ponding becomes a nuisance to property owners and road users. The stormwater system in these areas is required to collect the surface water and channel it to artificially sloped drainage networks to remove the excess. Pumps may be necessary to accelerate the natural flow. It is therefore important to consider the stormwater catchment as a whole and for the designer to understand the primary purpose of the drainage infrastructure. Stormwater management in the two areas can have opposing aims. The main objective on the flat alluvial plain is to reduce and remove the ponded water. In hilly areas a degree of ponding serves to detain the flows downhill and may be regarded as beneficial. Many urban areas consist of both types of topography. It has been estimated that in Blenheim around 60% of the urban stormwater catchment is on the very flat alluvial plain. Approximately 9% of the catchment is in the foothills of the Wither Hills. The remainder of the land is at an intermediate gradient, typically around 1:100. The shallow slope is created by the emergence Taylor River into the Wairau valley that formed an ancient alluvial fan. The technical detail of this topic is explored in greater depth in “Procedures for Reviewing the Blenheim Stormwater Capacity and Providing for New Areas (Draft)” (Williman). 4.1.2 Stormwater Conveyance Rain water that cannot penetrate the soil runs across the surface of the land and collects into a variety of channels. In urban areas the accumulated stormwater is conveyed through pipes, kerb channels on roadways and modified & naturally occurring water courses. The weather patterns in New Zealand are such that it is uneconomical to build specific infrastructure to accommodate flows from large storms that are infrequently experienced. The roadways therefore become useful alternative conveyance pathways. Many roads are specifically designed as secondary flow-paths for stormwater. On the flat plains the roads serve as detention ponds as there is often little gradient to permit a flow. Historically there has been little consistency on the size of storm that should be accommodated by the primary stormwater infrastructure and that which spills over into the secondary flow-path. Recommendations for the design of the piped water system have varied between 2 and 10 year Annual Return Interval(ARI) The final design is likely depend on a number of contributory factors - the permeability of the catchment area, the topography and the vulnerability/sensitivity of the surrounding properties. Industrial areas tend to have less permeable surfaces and business maybe severely interrupted by surface water flooding. In these instances the piped system maybe designed for a greater capacity and be able to cope with more frequent storm events. 4.1.3 Performance For the above reasons measurement of the performance of stormwater systems can be difficult. The base line set by the Building Act is the most definitive but strictly this still refers only to buildings constructed after 1991. The interpretation of ‘nuisance’ is reliant on case law. During storm events the Assets and Services department establish an Incident Management Centre to deal with flooding incidents and analysis of the customer complaints and issues received at the centre gives an indication of system performance. Table 5 is a statistical summary of the customer issues received during storm events since 2009. The data has been screened to remove rural issues outside of the stormwater system. Issues primarily relating to road access and traffic management have also been removed. Page 37