Civil Engineering Stormwater Drainage

Supplementing the INTERNAL stormwater design covered by our Hydraulic Services team (including the downpipe network for gutters, rainheads, sumps, emergency overflows, planter box drainage, balcony outlets and terrace drains), erbas™ can also provide EXTERNAL stormwater drainage design to address local Council Development Application (DA) requirements including on-site detention (OSD) storage, rainwater storage, stormwater quality improvement devices (SQIDs), Water Sensitive Urban Design (WSUD) measures, subsoil & pump systems, erosion & sediment control plans, and a design certificate provided by a Chartered Professional Engineer (CPEng) listed on the National Engineering Register (NER) of Engineers Australia.

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)