Infrastructure Strategy will feed into streams that pass through the existing urban area and to some extent are 1.2 Stormwater quality part of the urban drainage infrastructure. Development in the headwaters of these Rainfall landing on urban areas will mobilise a wide range of contaminants — oils and catchments requires careful design to ensure there are no adverse effects greases from roads and parking areas, litter and urban waste, tiny but significant downstream. amounts of metals from roofs and spouting, and an assortment of chemicals and This became very apparent with the development of a large retail park and proposed organic matter that wash off hard, impervious surfaces. If the stormwater cannot soak residential development in an area that drains to Murphy’s Creek. The downstream into the ground within a property, it is piped and channelled to the nearest waterway. residents were very concerned over the increased input of stormwater and the This could lead to significant deterioration in the quality of the natural waterway. potential for both flooding and deterioration of water quality. An independent arbitrator Stormwater from industrial areas can be a particular hazard, but accidental and worked with the Council and the residents to resolve these concerns using a structured sometimes deliberate spillages from residential areas can also cause acute pollution decision making process. Over the course of 18 months a mutually acceptable solution problems. was reached. Stormwater and wastewater pipes are often laid underground in close proximity to The solution included many features that may be expected to be included in future each other. The potential for both systems to leak, and for stormwater to become stormwater designs. Land within the new development has been assigned for contaminated with sewage, is high. This is particularly the case following an stormwater detention. This will detain part of the run-off and help reduce the peak flow. earthquake, as both systems can be damaged by ground movement. Regular Dished, grass swales will provide a flow path for stormwater whilst at the same time monitoring of stormwater outfalls provides useful information on the integrity of both increasing infiltration into groundwater. This natural process will provide primary systems treatment by removing some of the suspended solids and other contaminants from the In 2017 a government supported project was initiated by the Council’s Environmental run-off. A regulated portion of the flow will be directed into the existing Murphy’s Creek Science and Monitoring Department to monitor the water quality of the Taylor River as and the remainder directed to the Taylor River through a new stormwater pipe. it passes through Blenheim. The Assets & Services Department opted to undertake a Climate change complementary sampling programme of the stormwater outfalls to the river. The outcome of the project will help to guide future decisions on pipe rehabilitation and Strategies are being developed to manage the effects of more intense storms. Building stormwater treatment. bigger infrastructure and ensuring renewed pipes are sized to meet the new standards will be important. Advanced mathematical models are used to project future Many of the existing stormwater outfalls are managed under a range of different stormwater flow patterns and ensure infrastructure is designed with sufficient resource consents. The Blenheim Stormwater Strategy includes a proposal to contingency to accommodate the current uncertainties. A good understanding of consolidate the legacy of consents into a new consent structure that meets both the historic rainfall patterns and the powerful hydraulic models allow different combinations current and foreseeable standards and reflects the land use plans for the town. of factors to be simulated. Non-infrastructure alternatives can be modelled and the engineering solutions adjusted. The most recent sub-divisions on the periphery of Blenheim have included especially designed swales (shallow dished drainage channels), porous water courses and, in The use of overland flow paths along roads can avoid the expense of installing one instance, a wetland to provide treatment of urban run-off through filtration and additional stormwater infrastructure, but roads and adjacent properties need to be entrapment of particulate matter. The cost of installation resides with the developer as designed appropriately to provide this function. Stormwater detention areas can make a condition of Resource Consent, however maintenance, renewal and monitoring will an important contribution to stormwater management by removing the peak flows and become an on-going operational cost to the Council and included in future budgets. allowing stormwater to drain away once the peak flood waters have receded. Maximising the potential for soakage into the ground is useful, as are roof water There are numerous patented treatment systems that can be installed on the collection tanks. The code of practice for subdivision could be amended to encourage reticulation system to intercept contaminants in the urban stormwater. Due to their greater stormwater management for new developments. compact nature they can be an effective treatment for particulate matter to retrofit to existing outfalls. 2018-2028 Long Term Plan Page 202