11. Natural Hazards Volume One stopbanks, river diversions, detention dams, stopbank erosion protection measures, river channel clearing, channel excavation channel training, flow control gates and other flood mitigation measures. [R, D] Policy 11.1.5 – Enable the maintenance of existing Marlborough District Council administered flood defences and other Council initiated flood mitigation works. Given the population and community infrastructure that relies on the protection provided by existing Council administered flood defences, it is important that flood defences be maintained to the standards specified in Policy 11.1.4. This policy signals that the maintenance of the flood defences and other flood mitigation works will be enabled. Maintenance could involve works in the river bed or floodway, or on the landward side of flood banks. Regional and district rules will therefore both be required to implement the policy. These rules will assist to provide for the social and economic wellbeing and safety of the communities protected by the Council administered flood defences. The designation of Council administered floodways also enables any maintenance works that would otherwise be covered by Section 9(3) of the RMA to occur (see Policy 11.2.1). [R] Policy 11.1.6 – Recognise and provide for gravel extraction as a means of mitigating the adverse effects of gravel deposition in river beds. Gravel that naturally accumulates in river beds can act to impede flood flows and encourage bank erosion. There is a history of strategic removal of accumulated gravel in Marlborough’s rivers (especially the Wairau River) to maintain the floodway capacities specified in the standards of Policy 11.1.4 and reduce the potential for bank undercutting and erosion. This extracted gravel also provides a significant resource that is used in road construction and maintenance and the construction industry. Provided the adverse environmental effects of gravel extraction are avoided, remedied or sufficiently mitigated, the removal of gravel from the river bed in these circumstances has positive outcomes. The rate and permanence of gravel accumulation is variable and depends on the occurrence of floods and other characteristics specific to each river. A single large flood can significantly change the amount of gravel available and the location of extraction. Regular riverbed monitoring and observations are undertaken to determine the acceptability of gravel extraction at any location and any extraction operation will need to be limited in duration so that its impact can be monitored. [R] Policy 11.1.7 – Mitigate the adverse effects of gravel extraction on ecological and recreational values, water clarity and bank stability by: (a) avoiding, where practicable, extraction from the wet bed of any river; (b) placing limits on: (i) the timing of operations (especially to avoid bird nesting); (ii) the method of extraction; (iii) the location of the extraction and access to the location; (iv) the amount of gravel that can be extracted; and (v) the length of time over which the extraction can occur. The main adverse effects caused by the extraction of gravel from river beds are disturbance of bird nesting, impacts on fish habitat, disturbance of recreational activity, sedimentation causing reduced clarity of water, river bank erosion and its potential impact on existing structures located in the riverbed. These effects can be minimised by controlling the method, location, timing, amount and duration of the gravel extraction operation as set out in (a) and (b). 11 – 4