Stormwater Flashcards
Design Objectives - Stormwater
Is to regulate the storm surface run-off rate of flow and volume (quantity) and control groundwater levels and to protect the quality of both; to the extent that agreed levels of service are maintained and any adverse effects on the environment are no more than minor.
Adverse Effects of Stormwater design on the Environment
- Flood damage
- Surface and channel erosion and sedimentation
- Water pollution
- Loss of bio-diversity
- Damage to aquatic ecosystems
Minimum Levels of Service - Building Act
- Safeguard people and property
- Surface water resulting from an event having a 2% probability shall not enter buildings
- Drainage systems for the disposal of stromwater shall be constructed to convey surface to an appropriate outfall, avoid blockages and avoid damage
Minimum Levels of Service - Considerations
- Future sea level rise
- Climate change (higher intensity storm events)
- Land settlement
- Future development
Consent to Discharge
- New stormwater systems require approval form the District Council and consent from Regional Council (Environment Canterbury)
- Discharge consent may be needed for both construction and operation
- Consent conditions vary between Regional Councils
- Management Plans often are required to demonstrate compliance
Design Outcome - Detail
Acceptable overland flowpaths:
- Roads
- Parks/Reserves
- Avoid private properties if possible (as easily blocked)
Common Methods to Estimate Flood Flows
- Rational Method: Q=CIA
- Auckland: Modified SCS method
- Hydrological and hydraulic models (MIKE Flood, HECRAS, INFOWORKS)
Freeboard
Provides contingency against:
- Flood level estimation methods
- Future climate change
- System failure (blockage)
Rational Method
Q = CIA
- Q = runoff in L/s
- C = runoff coefficient
- I = average rainfall intensity (mm/hr)
- A = area of catchment (ha)
Why treat Stormwater
- Receiving environment protection
- Public health
- Recreational values (fisheries)
- Aesthetics
First Flush
- Highest concentration in first third of rainfall event
- Focus treatment on first flush contaminant removal
Contaminant Management Approaches
- Planning Controls
- At-source treatment
- End of pipe treatment
- Community educaiton
At-Source Treatment
Treatment on-site prior to discharge to network.
- LID devices: Rain gardens, green roofs, biofiltration trenches
- Proprietary device sized for site (sandfilter/catchpit filter)
At-Source Controls - Hydrological
- Minimise impervious area
- Rain water tanks
- Soakage
End of Pipe
One device serves whole catchment:
- Wetlands, wet and dry ponds
- Large proprietary device (sandfilter, downstream defender)
