Estuary Report Cards
Estuary report cards provide a hydrodynamic classification for individual New Zealand estuaries and key statistics that describe physical and hydrodynamic characteristics for the each estuary and its catchment.
The cards show a list of key statistics, a photograph of an example of the estuary type and schematics of the estuary type that show key morphometry and hydrodynamic properties of the estuary.
In the side view the relative levels of tidal versus river forcing, scale according to the size of the large blue and green arrows respectively. The presence of wind-wave resuspension, wind-driven circulation and ocean swell resuspension are indicated by the thin arrows. The top view shows the overall shape of the estuary and intertidal areas (stippled). The elevation and plan views are not to scale and the schematics represent average conditions for the estuaries.
Source of data
The estuary classification is a hydrodynamic category as determined by the New Zealand Estuarine Environment Classification.
The statistics for each estuary are a selection of variables extracted from the Estuarine Environment Classification database. The database is populated with physical variables descriptive of climate, oceanic water mass, basin morphometry, oceanic forcing (tides), river forcing (freshwater inflow), characteristics of the contributing catchment (including topography, geology and land cover), and factors that vary within an estuary including depth, tidal currents, and wind waves. The database has over 50,000 cells relating to 440 estuaries. Data mining and computation using numerical and analytical models and GIS was used to derive the database. Variables were derived from various sources including NIWA’s DEM (30 m cell size) of New Zealand, the 1:50,000 Digital Topographic Database, New Zealand Land Resource Inventory (NZLRI), the New Zealand EEZ Tidal Model, digital files of the RNZN hydrographic charts, and various publications and reports.
Defining estuaries
The classification uses a broad definition for estuaries to include the many different types of coastal water bodies that need to be managed. It follows Day’s (1981) variation of Pritchard’s (1967) definition and defines an estuary as: “A partially enclosed coastal body of water that is either permanently or periodically open to the sea in which the aquatic ecosystem is affected by the physical and chemical characteristics of both runoff from the land and inflow from the sea”. This definition includes estuary types and coastal water bodies described in other New Zealand classifications (e.g., Healy and Kirk 1982, Hume and Herdendorf 1986, 1993) as drowned river valleys, lagoons, coastal lakes, fjords, and river mouths. It includes features variously named on the NZMS 1:50,000 topographic maps as estuary, creek, firth, inlet, gulf, cove, river, bay, lagoon, harbour, stream, fjord, sound, port, arm, small craft retreat, haven, and basin.
We identified basins at least 0.5 km long working at a mapping scale of 1:50,000. The shoreline is taken from the LINZ 1:50000 database. This is approximately high water line (and maybe nearer spring tide level than neap tide level, but it depends on the state of the tide when the aerial photos were taken, and also on what was taken to be the shoreline e.g., water level, wet line or edge of vegetation). The seaward boundary or mouth of an estuary is the location where the water body meets the ocean. This was drawn at an inlet constriction, or where the shoreline diverges up or down coast. The seaward boundary was easily defined in most situations. Seaward boundaries were difficult to define for estuaries with funnel-shaped mouths. In these few cases the boundary was drawn to follow the general trend of the hard shore coast either side of the mouth. The up-stream limit of the estuary was easy to define in most cases as the location where the coastlines met on 1:50,000 topographic maps. However, in river dominated elongate estuaries we defined the upstream boundary as the upstream limit of salinity intrusion under average tidal and river flow conditions at HW. Where there was no salinity data, we made an expert decision based on anecdotal knowledge, the location of freshwater intakes and geomorphic information such as the location of river bars or where tributaries enter the system.
The computation and mapping procedure involved identifying/defining the estuaries on the NZMS 1:50,000 maps, defining low tide and high tide areas at 1:50,000 scale, and extracting the areas as GIS shape files.
Definition of statistics
Estuary name - Estuaries are named in the EEC database using names from the NZMS 1:50,000 topographic maps. Where the estuary is not named on the maps, the estuary was named after the major river input(s).
Spring tide range (m) – The tidal range for a mean spring tide (metres).
Spring tide prism (m3) - The tidal prism (P) is the volume of water entering an estuary on the flood or incoming tide. The tidal prism was calculated from the product of spring tidal range (Ts) and the area of the estuary at mid tide (Am) as:
P = Ts x Am
using predictions from NIWA’s New Zealand EEZ tidal model (Walters et al. 2001). The model was used to extract tide range (M2 and S2 tidal constituents) information at the mid point of the seaward extent of each estuary or at the closest model node to the midpoint.
The area of the estuary at mid tide (Am) was calculated as:
Am = (Ahw + Alw)/2
where the high tide (ht) and low tide (lt) areas were sourced from the 1:50,000 topographic electronic maps.
Width of mouth (m) - The seaward boundary or mouth of an estuary is the location where the water body meets the ocean. This was drawn at an inlet constriction, or where the shoreline diverges up or down coast. The seaward boundary was easily defined in most situations. Seaward boundaries were difficult to define for estuaries with funnel-shaped mouths. In these few cases the boundary was drawn to follow the general trend of the hard shore coast either side of the mouth.
Catchment area (km2) – The area of the land catchment that drains into the estuary as defined on a 50m DEM.
Shoreline length (m) - The length of the shoreline of the estuary, excluding the distance across the mouth of the estuary, but may include the length across the upper boundary cut-off (e.g., the line across the upper limit of river mouth estuaries). The upper limit of the estuary is the limit of estuarine intrusion.
Intertidal area (% of HW area) – The area of estuary exposed at spring low tide and calculated from the GIS shape files derived from the 1:50,000 LINZ DTDB.
Mean depth (m) - The total estuary volume at spring high water divided by the estuary area at high water spring tide.
Estuary area at high tide (m2) – The total water area at spring high tide derived from the coastline of the 1:50,000 LINZ DTDB. Estuary basins were identified as being at least 0.5 km long working at a mapping scale of 1:50,000. The high water line (MHWS) defines the shoreline of the estuary. The accuracy of this number depends how well LINZ defined the high water line from the aerial photographs.
Volume at spring tide (m3) - The volumes for 169 estuaries have been computed from the RNZN chart bathymetry and GIS TINs. For the remainder of the estuaries and where there was no bathymetry data the volume was estimated as the sum of the volume at low water and the spring tidal prism.
Thalweg length (m) - The length of the thalweg (main channel) of the longest branch of the river from the mouth to the upper limit of the estuary.
Mean annual river discharge (cumecs) - Mean annual river flow into the estuary. An annual runoff surface (mm/km2/yr) has been estimated for the whole of New Zealand at a spatial resolution of 1 km2 using a water-balance model, based on rainfall and potential evapotranspiration. We used the GIS to sum this surface within the catchment of each estuary to estimate annual runoff because most estuaries lacked flow-measuring stations on their inflowing streams and rivers