Flood Hazard Areas

Flood hazard assessments have been carried out for several areas in the District. These include the Poverty Bay Flats, Gisborne urban area, and the Mangatuna/ Wharekaka Area for the Hikuwai/Uawa River. The flood hazard varies across liable areas. Generally towards the edge of the flooded area depths are shallow and floodwaters move at slow speeds. Therefore the degree of hazard is low. However floodwaters are generally deep and flow swiftly in the vicinity of the main river channel and other major flood flow paths. These areas generally have a high degree of flood hazard with silt and debris deposition. The process of assessing flood hazard, firstly involves a study into flood behaviour. This involves estimating discharge for the various sized floods and the determination of water levels, velocities and depth of flooding. Then secondly a `design flood standard' is selected. The determination of that `design flood standard' balances the social, economic and ecological considerations against the consequences of flooding. If the standard is too low development will be inundated relatively frequently with greater damage. If the standard is too high land will incur unwarranted controls. The selection of the design flood standard depends on flood behaviour, landuse and consequences of larger floods. The level of protection offered by flood mitigation works may be different from the design flood standard adopted for land use planning. That level is dictated by economics of the situation or physical limitations of the site. It is prudent to assume that floods may occur greater than the ability of protection works to contain them. The design flood standard is intended to reduce the impacts of such floods, by avoiding or limiting development which would be affected. Flood Overlay categories include a) Flood Hazard Overlay 1 (River and Floodway): These are the main routes for floodwaters. They include all watercourses and adjacent berms liable to regular flooding. Floodwaters could be deep and fast flowing. These are areas unsuitable for regular human occupation. Floodway areas are areas which even if only partially blocked would cause a significant redistribution of flood flows. Care needs be taken not to alter the level of the land in a way which could divert floodwaters and cause adverse effects. Activities which could trap sediment in a flood and build up the river berms should also be avoided. b) Flood Hazard Overlay 2A (Moderate/High Hazard Areas): Similar to Flood Hazard Overlay 2 except that: i. ii. The flood hazard varies between “moderate” and “high”; and Flood warning systems and evacuation plans provide some measure of protection to residents Within this overlay some areas are unsuitable for permanent habitation, while others may be suitable subject to the practicality of evacuation routes and the potential numbers to be evacuated. c) Flood Hazard Overlay 2 (High Hazard Areas): Flooding in high hazard areas is associated with flow over stopbanks and roads and deep overland flow confined to narrow valleys. Floodwaters could cause structural damage to buildings and in extreme cases light framed houses could be swept away. Heavy silt deposition can occur. These areas are generally unsuitable for permanent habitation. Care needs be taken not to alter the level of the land in a way which could divert floodwaters and cause adverse effects. Activities which could trap sediment in a flood and build up the river berms should be avoided. d) Flood Hazard Overlay 3 (Flood Ponding Areas): This contains low-lying areas or basins subject to occasional but relatively deep flooding. Generally floodwaters would be slow moving or stationary. For Poverty Bay these areas have been flooded in 1985 and/or 1988. Ponding areas store floodwaters during major rainfall events. Infilling of these areas may divert and raise the level of floodwaters elsewhere. e) Flood Hazard Overlay 4 (Areas Liable to Flooding): contains areas on floodplains that have previously been flooded. For Poverty Bay that is flooding from the 1985 and/or 1988 floods. For the Mangatuna/ Wharekaka area it is flooding from the 1988 flood. For the Waimata Taruheru and Turanganui Rivers and the Waikanae Creek it is flooding from the 1977 and/or 1985 flood. f) Flood Hazard Overlay 5 (Flood Fringe Areas): contains areas that have not previously flooded but are expected to be flooded under design flood standard conditions. Generally water would be shallow and slow moving. These areas are generally suitable for permanent habitation as flooding should not cause structural damage. However floor levels need to be high enough for inhabitants to remain safely in houses until effective evacuation can take place. Care needs be taken not to alter the level of the land in a way which could divert floodwaters and cause adverse effects. g) Flood Hazard Overlay 6 (Old River Loops): These areas are old river loops that can be flooded to depths exceeding 1m. They are not generally suitable for residential occupation because the depth of water could cause difficulties in evacuation. Care needs be taken not to alter the level of the land in a way which could divert floodwaters and cause adverse effects. h) Flood Hazard Overlay 7 (Urban Stormwater Flood Hazard Area): These areas are affected by flooding from local streams and drains in design flood conditions. The stormwater reticulation system within the Gisborne urban area is presently undergoing an upgrading programme and the extent of this area may be able to be reduced when this programme is complete. However, work on this has only just begun and therefore the 1977 and 1985 floodspread maps are to be used until then as the basis of this overlay area. i) Flood Hazard Overlay 8 (Urban Ponding Areas): Urban ponding areas store floodwaters during major rainfall events. Infilling of these areas would put extra stress on urban reticulation systems or require expensive upgrading of such systems. j) Flood Hazard Overlay 9 (Urban Floodways): These are main routes for floodwaters. They include all rivers, streams and watercourses and adjacent berms liable to flooding. Floodwaters could be deep and fast flowing. Floodway areas are areas which even if partially blocked would cause a significant redistribution of flood flows. Care needs to be taken not to cause adverse effects by diverting or impeding floodwaters.