How to define the location of a bank, or the width of a river (or stream)
Esplanade areas are frequently established along the banks of rivers and streams. In order to do so, and in order to establish if a river (or stream) has an average width of 3 metres or more, the position of the natural boundary along the bank must be determined.
The terms 'river' and 'bed' (of a river) are defined in s2 of the RMA along with how it is measured - at its annual fullest flow or level without topping its banks. This means that when measuring a 'river bed ' it will always be greater than a river's ordinary flow.
Subdivision of land with tidal boundaries
Generally, titles of older parcels of land with a tidal boundary (ie, the land is bounded by the sea or a tidal river or stream), will show the position of the tidal boundary as the mean high water mark (MHWM) as per s35 of the Crown Grants Act 1908. Notwithstanding the RMA, the boundary of the land being subdivided continues to be the MHWM and all of the land to the MHWM must be accounted for on the survey plan.
During subdivision when an esplanade reserve or esplanade strip is set aside from land with a tidal boundary, the boundary is mean high water springs (MHWS) as per s230 of the RMA. If the horizontal positions of MHWS and MHWM are not significantly different, the tidal boundary is labelled MHWM/MHWS.
In areas where the horizontal positions of MHWS and MHWM are significantly different, the land between the two positions is shown on a (redefinition) title survey plan as a separate parcel. Redefinition surveys precisely identify established land boundaries through an accurate survey. Under s237A of the RMA, this parcel, being land in the coastal marine area (ie, land below MHWS) is shown as vesting on deposit in the Crown, subject to any rule in a district plan or any resource consent which provides otherwise.
Methods of defining tidal boundaries
Tidal boundaries such as mean high water or MHWS may be determined by many different techniques. The approach taken will be influenced by the individual location and will depend on a number of factors including the type and value of the land, the hydraulic gradient and the required accuracy of the survey.
In low-value rural areas, physical evidence such as the position of a bank or cliff may be accepted without further investigation. In any case the physical evidence such as the toe of a bank, the edge of consolidated ground or the extent of a type of vegetation will be an important determining factor.
Where further confirmation is required, tidal transfer methods such as the Education Method or the Range-Ratio Method will add further confidence. These methods transfer tide levels from an established tide gauge at a standard port to the area of the survey using a temporary tide gauge and sea level observations. The LINZ website provides a discussion of methods for determining MHWS and the tidal level information to be used as part of MHWS determination for cadastral surveys. This includes the use of direct levelling by surveyors to establish the MHWS where it is in close proximity to a standard port tide gauge. This can only be used for a few kilometres either side of a standard port tide gauge, where the range of the tides is the same.
In any determination a combination of methods will provide the most reliable result.