Stormwater

Question 1

Design Objectives - Stormwater

Answer 1

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.

Question 2

Adverse Effects of Stormwater design on the Environment

Answer 2

• Flood damage
• Surface and channel erosion and sedimentation
• Water pollution
• Loss of bio-diversity
• Damage to aquatic ecosystems

Question 3

Minimum Levels of Service - Building Act

Answer 3

• 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

Question 4

Minimum Levels of Service - Considerations

Answer 4

• Future sea level rise
• Climate change (higher intensity storm events)
• Land settlement
• Future development

Question 5

Consent to Discharge

Answer 5

• 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

Question 6

Design Outcome - Detail

Answer 6

Acceptable overland flowpaths:

• Roads
• Parks/Reserves
• Avoid private properties if possible (as easily blocked)

Question 7

Common Methods to Estimate Flood Flows

Answer 7

• Rational Method: Q=CIA
• Auckland: Modified SCS method
• Hydrological and hydraulic models (MIKE Flood, HECRAS, INFOWORKS)

Question 8

Freeboard

Answer 8

Provides contingency against:

• Flood level estimation methods
• Future climate change
• System failure (blockage)

Question 9

Rational Method

Answer 9

Q = CIA

• Q = runoff in L/s
• C = runoff coefficient
• I = average rainfall intensity (mm/hr)
• A = area of catchment (ha)

Question 10

Why treat Stormwater

Answer 10

• Receiving environment protection
• Public health
• Recreational values (fisheries)
• Aesthetics

Question 11

First Flush

Answer 11

• Highest concentration in first third of rainfall event

- Focus treatment on first flush contaminant removal

Question 12

Contaminant Management Approaches

Answer 12

• Planning Controls
• At-source treatment
• End of pipe treatment
• Community educaiton

Question 13

At-Source Treatment

Answer 13

Treatment on-site prior to discharge to network.

• LID devices: Rain gardens, green roofs, biofiltration trenches
• Proprietary device sized for site (sandfilter/catchpit filter)

Question 14

At-Source Controls - Hydrological

Answer 14

• Minimise impervious area
• Rain water tanks
• Soakage

Question 15

End of Pipe

Answer 15

One device serves whole catchment:

• Wetlands, wet and dry ponds
• Large proprietary device (sandfilter, downstream defender)

Question 16

Community Education

Answer 16

• Changes behaviour
• Promotes ownership
• Can be critical to success

Question 17

Canterbury Conditions

Answer 17

• Soils can be very pervious
• Good for discharge but not for treatment
• Soakage devices can combine attenuation, treatment and discharge

Question 18

How to choose approach

Answer 18

• Site constraints
• Financing
• Consenting Authority requirements
• Ongoing responsibility and maintenance

Question 19

Stormwater System Deisgn

Answer 19

• Site infiltration characteristics
• Sources of contamination
• Catchment areas (impervious and pervious)
• Topographical survey
• Existing services
• Existing drainage/treatment

Question 20

General Rules for Erosion and Sediment Control

Answer 20

• Minimise disturbed area
• Divert clean flows around the site
• Provide a buffer to watercourses
• Restore disturbance asap/cover temporarily
• Minimise erosion (e.g. use of contours drains on slopes)
• Prevent sediment exiting site using sediment control devices

Question 21

Design Project - Site Characterisation

Answer 21

• Location of natural waterways, springs, bores, wells or wetlands
• Location of existing drainage and overland flow pathways
• Identify high value features
• Current and future flood risk
• Catchment impervious areas
• Landuse change
• Relevant catchment boundaries
• Soil characteristics