Drainage Solutions FAQ — Central Coast

It depends on the job. A French drain along a fence line is priced mainly by the metre of trench. A pit-and-pipe stormwater system costs more and varies with length and depth, and with how far the water has to travel to an outlet. In the former Gosford area, Central Coast DCP 2022 (Chapter 3.1, Part C) doesn't normally require on-site detention for a single house, dual occupancy, alteration or second-storey addition. If the site's stormwater retention falls short of the DCP formula, an OSD system has to make up the shortfall, with a permissible site discharge of 8 L/s, and no more than 50% of rainwater tank volume can count towards it. That is the sort of detail that moves the price, so we walk the block, check the fall and the outlet, and price the job in front of us rather than one off a list.

Ag pipe (agricultural pipe) is the slotted or perforated plastic pipe that collects groundwater through its holes. A French drain is the whole system: a gravel-filled trench with ag pipe inside, wrapped in geotextile fabric to stop silt clogging it. People use the terms interchangeably, but a pipe laid in bare dirt is not a French drain. The same setup run around the footings to a gravity outlet or sump pump, together with better subfloor ventilation, is a common fix for damp and mould under a house when water sitting against the foundations is the cause. A contractor can check whether the damp is a drainage problem or something else, such as a plumbing leak.

It depends on the job and on which part of the Coast the property is in. Chapter 3.1 of Central Coast DCP 2022 is split into Part B for the northern area (the former Wyong LGA) and Part C for the southern area (the former Gosford LGA). Under Part C, no excavation is allowed within 2 metres of any drainage easement without approval from Council's Director City Services. Part C also says Council generally won't allow buildings or overhanging structures over a drainage easement or Council stormwater pipe, and piers along the easement or pipe must go down at least to the pipe's invert level. Central Coast Council can confirm what applies to your lot before any digging starts.

Central Coast Council's Woy Woy Floodplain Risk Management Study (DHI, final report 2022) gives part of the answer. It describes the Woy Woy sand flats as low-lying, with sandy soil that normally soaks water in well. But when the shallow groundwater table is high, rain doesn't soak in easily. The study says nuisance flooding in road reserves and private property generally sits for a couple of days, and in very wet periods, when the groundwater rises, it can stay for several weeks. A soakwell or absorption trench can struggle in exactly those spells. A contractor can also check for a low point, runoff from a neighbour's yard, or a blocked or collapsed pipe.

On site, a French drain along a fence line is often a 1 to 2 day job, and a full stormwater system with pits, charged lines and connections can take 3 to 5 days. For new development, the paperwork can take longer than the dig. Central Coast Council is also the water and sewer authority: a Section 305 application under the Water Management Act 2000 leads to a Section 306 requirements letter, which Council says typically takes 25 to 30 business days, then a Section 307 Certificate of Compliance. Council says that process is separate from, and additional to, the DA. Ask for a timeline with each quote.

In the former Gosford area, sometimes. Central Coast DCP 2022 (Chapter 3.1, Part C) sets a 4 metre setback for development beside a minor creek (catchment under 7.5 hectares) in existing subdivided areas where no easement exists over it. Where appropriate, the applicant may create and pipe an easement to remove that setback. The pipework must meet Council-approved design and construction specifications, and the works also need the consent of other relevant government authorities. For a major creek (catchment over 7.5 hectares) with no approved waterway design, the setback is at least 6 metres each side from the top of the natural bank. The former Wyong area falls under Part B instead.