Stormwater Drainage

Stormwater drainage collects rainwater from roofs and paved areas and carries it to the public system or to on-site treatment and storage: surface pits and grates, channel drains, underground pipes, subsoil drains, on-site detention tanks that hold water and release it slowly, retention and harvesting tanks, gross pollutant traps and treatment devices, bioretention basins and swales, and the overland flow paths that carry water safely when pipes are full. Roof gutters and downpipes have their own page.

Stormwater is designed for a storm of a given size. When pipes and pits are full, water must flow safely across the surface along overland flow paths, away from buildings. Fences, walls, landscaping and new structures that block those paths send floodwater into buildings, basements and car parks.

What shortens life:

• Sediment and litter from car parks, construction and landscaping.
• Tree roots entering joints.
• Traffic over pits and grates not rated for it.
• Lack of maintenance of detention and treatment devices.

The pipes usually outlast the building; the maintainable parts (grates, treatment media, detention screens and orifices) set the real maintenance cycle.

Blockage

Leaves, litter, sediment and debris block grates, pits and pipes; water then ponds on pavements, enters buildings and basements, and overflows into neighbouring land.

Root intrusion and pipe damage

Roots enter pipe joints and grow into masses that block flow and crack pipes; ground movement displaces joints; pipes crushed by traffic or construction collapse.

On-site detention failure

Detention tanks release water through small orifices that block with debris; tanks fill with sediment and lose storage; access covers are buried or sealed. Failed detention increases flooding downstream and on the site itself, and in many places the owner has a legal obligation to maintain it.

Treatment device failure

Gross pollutant traps and filters fill with litter and sediment and then bypass, discharging pollutants; clogged media stops working.

Blocked overland flow paths

Fences, retaining walls, landscaping, sheds and raised paving built across overland flow paths force floodwater into buildings and basements during large storms.

Subsoil drain failure

Silted or root-blocked subsoil drains let groundwater build up behind retaining walls, under slabs and around footings, causing damp, heave and wall failure.

Illegal connections

Stormwater connected to the sewer overloads it in storms; sewer connected to stormwater pollutes waterways. Both are breaches.

Capacity shortfall

Systems sized for older rainfall data or smaller paved areas are overwhelmed when sites are extended, paved over, or when design rainfall intensities are revised upward.

On site

• Ponding on paved areas and around pits after rain; silt and debris marks.
• Grates broken, missing or sitting proud; pits full of sediment.
• Water entering basements, car parks or ground floors in storms.
• Fences, walls and landscaping across low points.
• Detention tank access covers missing, buried or sealed.
• Treatment devices overflowing or full.

In the records

• Stormwater drawings and design.
• On-site detention design and maintenance obligations.
• Cleaning and maintenance records.
• Flood history.
• CCTV surveys.

• Erosion, scour and sediment fans at outlets and on slopes below the site.
• Water discharging onto neighbouring land or footpaths from the site.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A civil or hydraulic engineer, with a drainage contractor for cleaning and CCTV.

• Drawings and obligations review: stormwater design, detention and treatment requirements, and any covenants or council conditions.
• Pit and pipe inspection: lift grates, measure sediment, CCTV pipes.
• Detention tank inspection: access, orifice plates, screens, sediment, storage volume.
• Treatment device inspection: litter and sediment levels, media condition.
• Overland flow path survey: obstructions and levels against design.
• Subsoil drain flushing and inspection.
• Connection check for illegal cross-connections.

The report should state system condition, flood risk, compliance with detention and treatment obligations, and a maintenance plan.

• Capacity check of the system against current design rainfall and the paved area now draining to it, especially after extensions.
• Storm observation: where practicable, observe the site in heavy rain to see where water actually goes.

• Cleaning programme: scheduled cleaning of grates, pits, pipes, detention tanks and treatment devices, with extra visits after storms and leaf fall.
• Pipe repairs: root cutting as an interim, then relining or excavation and replacement of damaged pipes.
• Grate replacement with load-rated grates suited to the traffic, and heel-safe and bicycle-safe patterns in pedestrian areas.
• Detention restoration: clear or replace orifice plates and screens, remove sediment, and reinstate accessible covers so maintenance can be done.
• Overland flow path restoration: remove, lower or open fences, walls and landscaping that block flow, and regrade where needed so water passes the building safely.
• Subsoil drain renewal behind retaining walls and under pavements.
• Capacity upgrades: larger pipes and pits, additional inlets, or added detention where flooding recurs.
• Treatment device servicing and media replacement.
• Correct illegal connections between stormwater and sewer.

Cleaning and inspection recur, often annually and after major storms, and are best carried as recurring maintenance. Pipe repairs are priced by length; relining by length and diameter; grates and pits per item. Detention restoration is a lump sum, often modest unless access must be rebuilt. Overland flow path works range from minor fence changes to regrading. Capacity upgrades are lump-sum civil projects. Treatment media is replaced on a cycle.

Cleaning is repairs and maintenance; detention and treatment compliance is code compliance; flood protection works are risk-driven; replacement is lifecycle.

• Australia. AS/NZS 3500.3 covers stormwater drainage; Australian Rainfall and Runoff sets design rainfall and flood guidance; councils impose on-site detention and water quality conditions, often backed by covenants on title requiring owners to maintain them.
• United Kingdom. Approved Document H covers rainwater drainage; the CIRIA SuDS Manual sets sustainable drainage practice; planning conditions and, in Wales, statutory approval regimes require SuDS and their maintenance.
• United States. Local stormwater codes and permits under the Clean Water Act set on-site management; maintenance agreements are often recorded against property; plumbing codes cover building storm drainage.

In strata and condominium schemes in all three countries, site stormwater, detention and treatment systems are common property, and maintenance obligations recorded on title bind the owners corporation; failure to maintain can lead to council orders and liability for flooding of neighbours.

• Stormwater drawings and design.
• On-site detention and treatment obligations, covenants and maintenance plans.
• Cleaning and maintenance records.
• CCTV surveys.
• Flood history.
• Council correspondence on stormwater.

• Records of flooding events and insurance claims.
• Council orders or notices about stormwater.
• Records of site extensions or paving since the stormwater design.