Waitarakao Washdyke lagoon and catchment - once a thriving network of streams, swamps, and wetlands - has been highly modified over the years. The way in which water moves through the catchment has therefore changed and we now need to review how it will work best in the future.
Farmland nearest the coast (east of Seadown Road) is supported by a network of drains referred to as the Seadown Drainage Rating District. These drains all eventually feed into the northern end of Waitarakao Washdyke Lagoon and during periods of low or ‘normal’ rainfall this drainage scheme provides the only regular input of freshwater that flows directly into the lagoon.
The Seadown Drain is the largest of this network of drains, and flows immediately on the landward side of the coastal stopbank. This drain supports farming to the north of the lagoon and is part of the mātaitai. Its fate is tied to that of the coastal stopbank; when the stopbank is shifted in response to coastal erosion the Seadown Drain must also shift in location. Changes to the drainage scheme will change the whole lagoon’s water flow (hydrology).
Flows from streams
The freshwater stream flow input to Waitarakao Lagoon is via Washdyke Creek, which crosses beneath State Highway One. Washdyke Creek is the downstream reach of a number of tributary catchments including Papaka, Rosewill and Oakwood streams.
These stream tributaries extend as much as 10–12 km west and northwest of the lagoon. The upper parts of the streams are rolling downs or low hill country that is farmed with isolated residential development. The streams exit the rolling hill/gully type parts of their catchments on the Washdyke Flat (traversed by Washdyke Flat and Cartwrights Roads) up-plain of the industrial area. This flat land is riddled with old watercourses and depressions, which will carry flood flows during heavy rainfall events.
Washdyke Creek (during 'normal' conditions) does not flow directly into Waitarakao Lagoon but is picked up by a ring drain circling the outside of the lagoon and taken directly to the piped outlet point to sea at the south end of the lagoon. When flows in the creek are slightly elevated after rainfall, the creek overtops the ring drain and discharges straight to the lagoon.
Stormwater run-off following rainfall will discharge from the residential, commercial, and industrial development land into Waitarakao Lagoon (in some cases via Washdyke Creek) via a combination of piped or open stormwater drains. In major rainfall events, stormwater flooding can impact lower parts of the developed catchment.
Seawater influences the catchment in multiple ways. High tides push up through the piped lagoon outlet and ring drain and lift the lagoon level (at low tide the lagoon can mostly empty out in a reverse of the same process). In moderate coastal storm/high swell events the gravel barrier between the lagoon and sea can be overtopped and waves will push seawater into the lagoon filling it up. And lastly, in larger/extreme coastal events, seawater can breach or overtop the stopbank to the north (as well as overtopping the gravel barrier) causing more widespread flooding of rural and/or industrial land.
In any situation where there is an excess of water due to seawater overtopping, heavy or prolonged rainfall (or river and stream flooding) the lagoon will rise. When the lagoon is at high levels, stormwater discharges will back up in the industrial area and the Seadown Drain system will cease flowing to the lagoon and floods. This situation can be alleviated by either a natural or mechanical breach of the gravel barrier to lower the lagoon level (but only if sea conditions allow this).
The catchment currently contains just a single outlet for discharging water to sea; a pipe at the south end of the lagoon. This means all water in the catchment must pass through the lagoon area in its journey to the ocean – including industrial stormwater and farm drainage. The pipe’s location has impacts on the reef environment, and its height above sea level controls the lagoon level and enables the Seadown Drain scheme to function.
Sea level rise and coastal erosion threaten the outlet’s ability to function as it does currently. Changes to the outlet will have other significant effects on wider catchment hydrology.