Week 8: Design - Treatment Systems Flashcards
What considerations need to be made in terms of site suitability of infiltration (soakage basins)?
- Medium sized catchments, generally multiple properties. Placed at lower end of catchment.
- Manage peak flow attenuation
- Suitable to eliminate risk to cold-water fisheries
- Pretreatment to avoid failure
- Away from public pathways
- Out of overland flow paths
- > 300 mm above highest seasonal GW table
- Special considerations for slopes > 1:4
- Suitable soils for drainage
- Overflow connection to public stormwater system
What are some performance design considerations to be made for bioretention basins?
- Uniform flow over basin bottom (no slope)
- Careful plant selection (moisture, wetting frequency, filtration, evapotranspiration, maintenance, lifespan)
- Inlet design to minimise litter and debris
What are some safety design considerations to be made for bioretention basins?
- Avoid tripping hazards (e.g., provide bridges)
- Subsidence
- Avoid planting impinging on access routes
- Maintenance
What are some methods to test infiltration rates and soil?
- Geological logging (test pits/boreholes)
- Monitoring nearby infiltration systems
- Test devices
- Double ring infiltrometer, Guelph permeameter, Inverse auger hole
Outline the steps for sizing a bioretention basin (CCC).
- First flush volume (Vff)
- First flush basin water surface area (Aff)
- Storm average runoff flow rate (Qavg)
- Basin floor infiltration flow rate (Qif)
- Underdrain flow rate (Qud)
- Storm total volume (Vs)
- Basin live storage volume (V_LS)
- Basin total surface area(A_B)
- Number of soakage chambers
(In terms of size) rain gardens are suitable for:
- Small street scale catchments
- Larger subdivision catchments
- Maximum device area of 1200 m^2 is recommended for even flow distribution, even infiltration and maintenance practicality
What are the steps to sizing a bioretention basin?
Auckland City Council
- Determine minimum area requirements for infiltration, ponding and media based on catchment size.
- Size d of various different sections for detention requirements. Can also increase L or W.
- Size L and W of storage/pond to achieve desired retention requirements if current are not satisfactory.
- Check required retention volume can be infiltrated/evapotranspired in required time.
What is the general applicability of rain gardens?
Hint: “CCC recommend rain gardens in…”
- CCC recommend rain gardens in existing site redevelopments or where there are space constraints
- Prefer rain gardens over proprietary devices due to higher amenity, improved filtration, and provision of flow attenuation at similar costs.
- For greenfield developments prefer wetlands and basins due to higher amenity and lower cost
- Good option for street renewals, can utilise parking spaces, be used for road narrowing, flow can drop into existing stormwater pipes.
What is the specific applicability of rain gardens
- Base at least 300 mm > highest GW table
- Transition layer > GW level for submerged design
- Typically GW should be at least 800 mm below surface
What are some further design considerations to take for a rain garden (other than sizing).
Hint: “Inlets, dual inlet/outlet,…”
- Inlets
- Dual inlet/outlet
- Sediment forebay
- Edge support
- Overall plan and underdrains
- Underdrain standpipe (and entry, with tree, outlets)
- Scruffy dome overflow
- Layer composition
Swales are suitable for:
- Small to medium catchments
- > 1 m above seasonal groundwater
- Slope < 8%
- Reasonable strength and drainage to withstand wetting/drying cycles
- > 1 m from property boundaries
- Contaminated land requires liners
What are the steps to sizing a swale?
- Determine design flow rates using Rational method.
- Initial dimension of swale at WQF.
- Determine cross sectional flow area and hydraulic radius.
- Determine effective length. Calculate flow velocity, flow. Check HRT.
- If HRT or depth is not acceptable, adjust.
- Design for 10% AEP.
- Give overall dimensions.
Swales with steep slopes require:
Check dams
Swales with shallow slopes require:
Underdrains
