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Services & Drainage

Inadequate Drainage Design

Inadequate drainage design is a systemic defect — where the stormwater system was designed or constructed with insufficient capacity for the contributing catchment area and local rainfall intensity. Unlike blocked drainage (which can be rectified by cleaning), design deficiency requires hydraulic re-assessment and physical upgrade of the infrastructure. In Class 2 buildings, drainage design deficiencies are often not apparent until a design storm event — typically the 1 in 100-year storm — reveals that the system cannot cope, causing flooding that damages the building and its occupants.

Common Signs

  • Persistent flooding at car park entries and basement levels during storm events
  • Surface ponding across the site in areas that should drain freely
  • Overflowing pits during events that appear to be of moderate intensity
  • Balcony and podium areas flooding before the drainage outlets are engaged
  • Surcharging at the connection to the council stormwater system
  • Water entry to the building during any significant rain event
  • Drainage infrastructure that is visibly undersized relative to the catchment area

Common Causes

  • Drainage system designed to an inadequate storm return period (e.g. 1 in 5 year rather than 1 in 100 year)
  • Hydraulic calculation errors in original design
  • Increased impervious area from subsequent development not reflected in drain sizing
  • Incorrect rainfall intensity data used in the hydraulic design
  • Inadequate gutter and outlet sizes for the roof catchment area
  • Incorrect pit spacing relative to inlet capacity
  • Failure of the hydraulic consultant to account for building configuration in the catchment area

Risk of Neglect

  • Basement flooding causing significant damage to property, vehicles and building services
  • Insurance claims and coverage disputes for repeated flood events
  • Building Commission and council intervention for systemic drainage failure
  • Strata liability for damage to lot owner property from flooding
  • Ongoing remediation cost escalation if flooding recurs without hydraulic upgrade
  • Safety risk to occupants from rapidly rising water in basement areas
  • Potential requirement to obtain development consent for drainage upgrades

Inspection Requirements

Download Inspection Checklist
  • Engage a hydraulic engineer to conduct a full hydraulic assessment of the stormwater system.
  • Collect CCTV inspection data of all existing stormwater infrastructure.
  • Measure all pipe diameters, pit sizes, falls and inlet grate open areas.
  • Confirm the contributing catchment area to all drain elements.
  • Run hydraulic modelling using local design rainfall intensity data from the Bureau of Meteorology.
  • Compare the modelled capacity against the design storm requirement under AS/NZS 3500.3.
  • Identify all elements of the system that are deficient in capacity.
  • Confirm the capacity of the downstream council stormwater system at the connection point.
Start Inspection Checklist

Create a project inspection record, add defect locations, upload photos, and save notes for scope writing.

Typical Repair Methodology

  1. 01.Engage a hydraulic engineer to prepare a drainage upgrade design based on the hydraulic assessment findings.
  2. 02.Submit a development application where council consent is required for infrastructure upgrades.
  3. 03.Upsize deficient stormwater pipes in accordance with the hydraulic engineer design.
  4. 04.Install additional pits and grate inlets to increase surface inlet capacity.
  5. 05.Install overflow grates or scuppers at all locations where surcharging may cause internal flooding.
  6. 06.Regrade surfaces to improve falls to drainage inlets where inadequate grading is a contributing factor.
  7. 07.Install pumped drainage systems in basement areas where gravity drainage to the council system is not achievable.
  8. 08.Confirm the completed system performance through hydraulic testing and CCTV inspection.
  9. 09.Record all upgrade works with as-built drawings, hydraulic engineer certification and photographs.

Related Repair Systems

  • Hydraulic drainage design and assessment services
  • Stormwater pipe upsizing programmes
  • Stormwater pit and inlet upgrade systems
  • Basement sump and pump systems
  • Surface drainage and grading improvement systems
Open Repair Systems →