Flooding and overland stormwater flow impact assessments | Brisbane City Council

Flooding and overland stormwater flow impact assessments

The level of detail required to describe proposed works may vary depending on the level of development approval sought. A Registered Professional Engineer QLD (RPEQ) should scope the requirements to ensure an appropriate assessment of the flooding and overland stormwater flow impacts and proposed works is provided for that particular development.

The following technical guides can assist applicants and consultants in the preparation of these documents.

Flood Study - minor hydraulic assessment

Typically, minor hydraulic assessment is required for applications within Brisbane City Council's mapped overland flow paths where the primary concern is related to flood immunity, for catchments <30ha or where average flow depths are shallow (typically <500mm). Minor alterations to the floodplain are permissible under a minor assessment at the discretion of Council.

Flood Study - major hydraulic assessment

Typically, major hydraulic assessment is required for larger catchments and creeks, although this assessment may also apply to applications in overland flow paths where flood impacts and flood immunity are being assessed in regard to:

  • the effect of filling or buildings on flooding and overland flow
  • effects of structures such as bridges, culverts on hydraulics
  • major flow piped solutions (e.g. +20 year average recurrence interval (ARI) pipelines)
  • any development within a waterway corridor
  • where retention or detention basins and new open channels are proposed.

The following tables outline the general requirements for hydraulic reporting and assessment.

Table 1 - Flood Study content

All Flood Studies must address the detailed assessment requirements contained within the Flood planning scheme policy, but should also include and address the following information.

  1. The Flood Study which must be signed, dated and referenced by a Registered Professional Engineer QLD (RPEQ) who has overseen the modelling for the design of development.
  2. Existing site survey plan from a a registered surveyor to scale with cadastral boundaries, ground levels, existing structures, trees, fences, kerb and road levels, pipe invert levels and pit surface levels shown.
  3. Plan and sections of the development proposal clearly showing habitable, non-habitable levels, proposed fill levels and extent and relevant ground levels.
  4. Catchment plan to scale with hydrological calculations.
  5. Plan to scale, showing the location of all hydraulic model cross sections.
  6. Drawing showing the cross sections of any filling or excavation.
  7. Report describing modelling methodology (including justification of assumptions used and any limitations) and results:
Minor assessment Major assessment
  • tables showing differences between existing and proposed scenarios for water level, velocity and flow discharge
  • cross sections with flood level and energy grade line
  • plan of ground levels of any fill or excavation
  • flow calculations.
  • tables showing differences between existing and proposed scenarios for water level, velocity and flow discharge
  • cross sections with existing/design levels, design flood level and energy grade line
  • plan of ground levels for any fill or excavation
  • for 2D hydraulic models provide Digital Elevation Model (DEM) with blockages shown, roughness map, water depths with velocity vectors and hydrographs at hydraulic controls
  • HGL analysis of any proposed pipeline and other hydrological calculations and/or modelling results.

Table 2 - standards for Flood Studies

Survey data
  • where any hydraulic control is present that may influence flood levels at the site, surveyed levels of that structure will be required
  • survey is to extend beyond the most downstream boundary of the site to ensure the boundary condition chosen does not influence results through the site
  • Council Aerial Laser Survey (ALS) maybe used for 2D modelling, or for 1D modelling where access onto adjacent lots is restricted.
Hydraulic models
  • where a Major Assessment and flow is difficult to describe by 1D principles, a 2D hydraulic model must be used
  • downstream boundary condition is to extend significantly beyond the site boundary to ensure it does not influence flood levels through the site
  • cross section spacing should be chosen to accurately denote floodplain obstructions and other hydraulic controls.
Assessment of impacts
  • impact assessment should generally be based on the existing and proposed roughness values to allow interpretation of changes to flood depth/velocity
  • 50 year ARI flood is design standard for overland flow paths
  • 100 year ARI design standard for creeks, river and storm tide.
Flood immunity
  • refer to Flood overlay code and the Coastal hazards overall code for flood immunity requirements
  • manning roughness is to be 0.10 to set flood immunity requirements for overland flow paths
  • manning roughness is to be 0.15 to set flood immunity requirements for creeks.
Piping overland flows Generally piping of all overland flows as a solution is impractical and not permitted, as:
  • provision of an overland flow path to account for larger flows or blockages and the piped network extending downstream to a suitable outlet will be required
  • design will need to conduct a HGL analysis based on downstream 50 or 100 year ARI flows (e.g. top of kerb HGL assumption at the next downstream pit)
  • surcharging of flows back into an overland flow path is not permissible as it concentrates flow and may result in sedimentation problems in the pipe
  • capturing surface flows is unlikely without a detention basin as blockages will always occur.
Easements
Stormwater detention
  • underground tanks are only permissible for Material Change of Use (MCU) applications where the tank is not within Council ownership
  • no underground tanks to be proposed for residential Reconfiguring of a lot (RoL) applications, or in any existing/proposed Council road
  • rainwater tanks on residential freehold lots are not permissible for stormwater detention, except for management of frequent flows < 1 year ARI
  • sizing of detention systems is to be via hydrological Models (using Rational Method is not permissible, as the critical duration storm is often much greater than the calculated time of concentration).
Building over or near stormwater
  • where a structure overhangs a flowpath, an undercroft (clearance) is required between the estimated flood level and the underside of any structure to provide for maintenance and ensure sufficient clearance for debris and any slung services
  • minimum undercroft vary dependent upon the flood hazard. Specific details are identified in the Flood overlay code
  • in the case of houses, undercroft should ensure that the flooded areas under the house cannot be considered habitable or non-habitable areas (e.g. restrict clearance to < 2.1 m).

Flood risk assessment

In some cases, when determined by assessment against either the Flood overlay code or the Coastal hazard overlay code, a flood risk assessment may be required. Information to be contained within a flood risk assessment may be found in the Flood planning scheme policy and the Coastal management planning scheme policy.

04 July 2018