The East Cape has a history of “hits” from decaying tropical cyclones causing widespread flooding and disruption. There are also other events that are more localised and result from a weather system dumping a large volume of water in a small area. A recent study showed that there is a trend of less of these events, but when they occur more rain falls. Most areas north of Ruatoria have a higher average rainfall than the south of the district and can cope with high rainfall events. The Waikura Valley for example can get 200mms in a 24 hour period without any adverse effects. The populated areas likely to be worst affected in a large event are the Poverty Bay Flats, Te Karaka and the area just north of Tolaga Bay. More intense localised events such as the 1985 Ngatapa flood and the 1977 Glenroy flood also caused substantial damage. Most of the Poverty Bay Flats, during inundation, is not subject to fast flowing water but huge areas are subject to ponding, which after the water recedes, will leave thick layers of silt damaging pasture and fences. In the city a number of riverbank properties are at risk. Extensive flooding depends upon the tides, potential overflow from the Waipaoa River, storm surges and the flood-peak times of the Taruheru and Waimata Rivers.

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 thatdesign 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 includea) 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.

This layer provides polygons of approximate flood areas derived from Sentinel-1 satellite radar imagery captured at various times on 14/02/2023 near the peak of the Cyclone Gabrielle event in the North Island of New Zealand. Areas of coverage for this layer include the Hauraki, Thames-Coromandel and Hastings Districts, and Napier City.

​(Note: More accurate and validated flood extent models for the Hawke’s Bay region can be obtained from Hawke’s Bay Regional Council - Flood Observation Extents.)

​This dataset was created by Toitū Te Whenua LINZ as part of its contribution to the emergency response, and is limited to areas that had a satellite pass over, and to the few areas of interest as we knew them at the time.

​**Accuracy** Users of this data should exercise caution . The accuracy of this data has not been validated. A subjective threshold was used to determine possible flood areas and eliminate false positives. Polygons in hilly areas or below a certain area size were excluded as likely false positives.

Please use this data as advisory only. The results have not been ground truthed and will contain errors and ommissions.

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 thatdesign 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 includea) 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.

This layer contains the DSM for LiDAR data in Gisborne and Hawke's Bay following Cyclone Gabrielle, captured from 26 February to 15 March 2023.

• The DEM is available as layer Gisborne and Hawke's Bay - Cyclone Gabrielle Post Flood LiDAR 1m DEM (2023)

• The Index Tiles are available as layer Gisborne and Hawke's Bay - Cyclone Gabrielle Post Flood LiDAR Index Tiles (2023)

LiDAR was captured for NIWA and the New Zealand Government by University of Canterbury and Christchurch Helicopters from 26 February to 15 March 2023. Coverage includes Aropaoanui River, Esk Valley, Heretaunga Plains (Ngaruroro and Tutaekuri Rivers), Mangahauini River (Tokomaru Bay), Tangoio (Te Ngarue Stream), Tukituki River, Uawa River (Tolaga Bay), Waipaoa River and Wairoa River. Data management and distribution is by Toitū Te Whenua Land Information New Zealand.

Data contacts: University of Canterbury - James Brassington and Justin Stout NIWA - Hamish Biggs, Emily Lane and Andrew Tait

Data comprises:

• DEM: tif or asc tiles in NZTM2000 projection, tiled into a 1:1,000 tile layout

• DSM: tif or asc tiles in NZTM2000 projection, tiled into a 1:1,000 tile layout

Vertical datum is NZVD2016.

Gisborne areas susceptible to coastal erosion, coastal flooding and/or tsunami (ASCE) layer as identified in a 2015 study commissioned by the Gisborne District Council.

The Gisborne District coastline extends some 500 km from Takararoa in the south to Omaruparoa in the North and comprises some 138 km of sandy and gravel beaches and 202 km of cliffed coastline. An Area Sensitive to Coastal Hazards (ASCH) line was developed by Gibb between 1994 and 1999 and shows areas that may be adversely affected by coastal erosion, coastal flooding and/or tsunami. The line is applied to areas where more comprehensive and locally specific Coastal Erosion Hazard Assessment have not been undertaken and indicates where prospective developers may be required to undertake further research on coastal hazards. New research is being undertaken on effects of tsunami and coastal flooding in the Gisborne District that will replace those components of the ASCH

Gisborne District Council areas susceptible to coastal erosion (ASCE) as identified by the Tonkin & Taylor study in 2015.

Gisborne District Council (GDC) is required to identify areas in the coastal environment that are potentially affected by coastal hazards, over at least 100 years, as required by the New Zealand Coastal Policy Statement (NZCPS) (DoC 2010). In 2014 NIWA developed a Coastal Calculator to derive coastal-storm inundation elevations and likelihoods as a result of combinations of elevated storm-tide, wave setup and wave runup, along the coastline of Gisborne District (Stephens et al. 2014). The GDC subsequently commissioned NIWA to map areas potentially affected by large and rare coastal storm inundation events at locations for which topographic data of sufficient resolution were available: Poverty Bay; Wainui; Tolaga Bay; Tokomaru Bay; Anaura Bay; Hicks Bay and Te Araroa.