On-Site Detention (OSD)

Many local Council authorities do not have available funding or space to provide “off-site” retarding basins to manage the effects of increasing development and flows in their limited pipe networks, so they often impose a requirement on developers to temporarily store water “on-site” and restrict the rate of outflow from a property with a small diameter outlet. Several different calculation methods for detention design are adopted as acceptable by various Councils.
erbas™ is able to provide design advice to the project team on OSD components, such as:
- Site Storage Requirement (SSR), or the volume contained in above ground basins or below ground tanks
- Permissible Site Discharge (PSD), or the limited rate of flow that can leave a site after a site is developed
- High Early Discharge (HED), a method to reduce volume by quickly achieving a significant flow
- Freeboard, a vertical allowance to protect internal floors from water intrusion
- Provision for emergency overflow, when an outlet is blocked or an extreme storm event occurs


erbas™ is able to provide design advice to the project team on rainwater tank components, such as:
- Charged or pressurised drainage (permitted by some Councils), where gutters are higher than tanks
- First Flush diverters, used to prevent the initial layer of dirt washed from a roof from entering a tank
- Storage volume based on BASIX or water balance calculation (using location, usage & roof area)
- Pump systems to provide adequate pressure at a hose tap or fixture

Stormwater Quality Improvement Devices (SQIDs)

erbas™ is able to provide design advice to the project team on treatment measures, such as:
- Pit insert mesh baskets to collect litter and gross pollutants
- Hydrodynamic settling tanks to capture suspended solids and fine particles conveyed in the water
- Oil separation units to capture grease or hydrocarbon pollutants
- Cartridge filtration units to remove excessive nutrients such as nitrogen and phosphorous

Water Sensitive Urban Design (WSUD)

erbas™ is able to provide advice to the project team on “environment-friendly” measures, such as:
- Vegetated or landscaped swale depressions or channels to promote natural soil infiltration
- Raingardens or bioretention storage basins with selected media layers to improve subsurface flows
- Permeable or porous paving (where underlying soil is free-draining) to reduce surface runoff
- Reduction of demand from public supplies, using rainwater storage or water-efficient fittings and fixtures

External Drainage in Conjunction with Civil Engineering Structures

erbas™ is able to provide design advice to the project team on common drainage measures, such as:
- Subsoil drainage for retaining walls and basements to relieve underground water pressure
- Provision of pump systems when gravity stormwater disposal is not practical
- Recommendation for easement routes to drain low-lying properties
- Absorption trench design (where soil infiltration characteristics are suitable)
- Overland flow assessment within a development site
- Hydraulic grade line analysis, accounting for energy losses through pipes and pits to predict surcharge
- Coordination of site drainage with other engineering disciplines (flood modelling consultants, geotechnical engineers, structural engineers, civil road pavement and bulk earthworks engineers)