Why not a working depth?

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

Opinion

Eastland Port says some ships cannot be fully loaded because of extreme low tides due to high barometric pressure.

Which indicates there is not sufficient water between the keel and the harbour floor.

We should be raising eyebrows because it suggests that Eastland Port’s expensive dredging programme is not effective at maintaining a working depth in the port.

We are told recent low tides at Gisborne have been 20 to 30 centimetres lower than usual.

I refer to Land Information New Zealand: “Barometric pressure: Tide predictions are computed for a standard barometric pressure of 1013 hectopascals (hPa) or millibars. A difference from the average of 1 hPa can cause a difference in height of 1 centimetre. Changes in sea level due to barometric pressure alone seldom exceed 30 centimetres (about one foot in the old imperial) but, as such circumstances are usually associated with adverse weather conditions, the actual change in sea level is often much greater.” (Such as with storm surge.)

Each year the height of high and low tides rises and falls according to the well-known lunar cycle — 13 orbits of the Moon which produce two sets of spring and neap tides each month, and the twice-daily sets of tides.

The most extreme tides are when the Moon is at its closest to Earth (perigee); when Earth is closest to the Sun) perihelion; and when Sun and Moon are lined up facing Earth. As most people will have heard, we have just had a so-called Super Moon — a full Moon at its closest to Earth, and an eye-catching yellowish presence as it rises over the horizon.

These astronomical and tidal events happen each and every year, and so does high barometric pressure over summer — it is not unusual. We have experienced highs of 1030mb twice in the past month.

If Eastland Port cannot maintain a working depth, it does not bode well for the future as ships get larger and sit deeper in the water.

Roger Handford

(PS: Regarding the town/harbour bridge clip-on proposal — has anyone planned what to do when tree/flood debris needs to be cleared?)