Do you live in Auckland? Do you have a question or problem you want solved? Phoebe Falconer answers your queries every Thursday.
Email "Ask Phoebe" at newsdesk@nzherald.co.nz.
KEY POINTS:
I'm wondering about the "water catchment areas" along Esmonde Rd near the motorway intersection, and another two areas just north of this interchange. A lot of work has gone into constructing them, surrounding them with new plants, and now that they're finished they all look very nice and tidy. But what exactly are they, and what is their purpose?
Marilyn Squire, Takapuna.
They are stormwater ponds. They act as retention ponds, to delay the time taken for stormwater to run off roads into local streams. This reduces the effect of flooding during heavy rain.
The ponds also help treat runoff by acting as sedimentation pits. If there are road works in the area, as there have been around Esmonde Rd, there can be a substantial amount of earth in runoff. This, if left untreated, would contaminate rivers and streams, and eventually the harbour. It is also a safety issue if it gets on to local roads. The stormwater ponds allow suspended solids like soil to settle out, and the clean water is discharged.
The smaller ponds to the north of the intersection are treatment trenches, which work the same way but on a smaller scale.
The plants are there for a purpose as well. The roots help trap nutrients and sediment, and of course the plants enhance the view.
Auckland's largest sedimentation tank is at Grafton Gully.
It was built to deal with the runoff generated during the building of the motorway lanes that run through the bottom of the valley. Now that construction there has finished, the tank has been hooked up to the council's stormwater system.
Every time there is heavy rain the southbound left lane of Great South Rd, just past the rail overbridge in Penrose, is completely under water. This causes havoc at evening peak hour and has been for years. Can something please be done?
Garry Shearer, Auckland.
Stormwater can really cause problems, can't it? This area of Penrose relies on ground soakage to get rid of stormwater from the roads. The water goes into catchpits and is piped away to soak holes, where it drains through basalt rock into the underground water table.
If the rain is heavy and prolonged, the soak holes get overwhelmed and can't cope. The result is road flooding, which over time will drain away.
The council is looking at better ways of improving stormwater management in the area.
