Drainage FAQ — Forster

It depends on the job, from clearing one blocked stormwater pit to a full subsoil drainage system around a house. Relining a collapsed stormwater pipe is usually priced per metre plus a set-up charge that applies whatever the length, so a short section costs more per metre than a long one. Digging it up and replacing it is usually dearer again. Council requirements can add to the bill, such as a geotechnical report where an infiltrating raingarden is planned and high groundwater is possible, or an absorption system designed by a qualified civil engineer or surveyor on a block with no other outlet. Get two or three quotes, and ask for a drain camera inspection first.

Start with which water it is. Stormwater drainage carries rain off roofs, paving and the yard surface to the street system or a legal point of discharge. Subsoil (ag) drainage is slotted pipe in gravel and geotextile sock that catches groundwater before it reaches footings or gets under a slab. Ponding that lasts for hours usually means the yard falls towards the house or an old ag drain has silted up, and a wet yard often needs both looked at. Bureau of Meteorology averages for the Forster-Tuncurry Marine Rescue station show March as the wettest month, at 152.2 mm, and September the driest, at 65.8 mm, so a drain that copes in September can still struggle in March.

Only up to a limit. MidCoast Council's Site Stormwater Drainage Guidelines (February 2024) cap direct discharge to the kerb and gutter at 20 litres per second at any one discharge point in the 20% AEP storm, meaning a storm with a one-in-five chance of being reached or exceeded in any year. If a site discharges more than that, Council may require a connection to its underground pipe and pit system, or on-site detention can be provided or enlarged to bring the flow down to what is permitted. Ask whoever designs the system to check the flow from your roof and paving against that limit before work starts.

MidCoast Council calls that a low-level lot. For a single house that doesn't need on-site detention, its Site Stormwater Drainage Guidelines (February 2024) list steps in order, and Council must accept each is impractical before allowing the next. Step 1 is an existing Council drainage line inside the site, a charged line to the kerb, or an existing inter-allotment drain. A charged line needs more than 1.0 m of head from the roof gutter, or tank overflow, to the kerb invert, and no surface inlet pits can connect to it. Failing those, the next step is an absorption system designed by a qualified civil engineer or surveyor, with a Positive Covenant and Restriction on Use of Land registered.

Where the water sensitive design controls in Chapter 11 of the Great Lakes DCP apply, infiltrating raingardens are only allowed on sandy soil, with evidence of that soil required. They can't be used where groundwater is within 700 mm of the surface, and possible high groundwater needs a geotechnical report. The sides are lined to 600 mm deep with 0.75 mm HDPE or equivalent, and the base drains into the sandy subsoil. Along a boundary, or under 2 m from a building or driveway, the liner goes to 1 m. At 1 m or closer to a building, a geotechnical or structural engineer's certificate goes to the Certifying Authority. Check with MidCoast Council which DCP is in force.

It does if the trench goes through it. MidCoast Council's Acid Sulfate Soil Management Plan for Minor Works (October 2023) covers work that disturbs less than 10 tonnes of acid sulfate soil, including sewage management facilities and effluent land application areas. The dug-out soil has to be treated with agricultural lime within 48 hours, on a lime bed with non-acid-sulfate bunds. The rate is 87 kg per cubic metre for sandy soil (roughly 4.5 bags of 20 kg) or 240 kg per cubic metre (12 bags) for non-sandy soil. Hydrated or slaked lime can't be used without Council approval. Ask whether the lime and the 48-hour window are allowed for in the quote.