Stormwater

Question 1

Design Objectives

Answer 1

• to regulate the storm surface run-off rate of flow and volume and to control groundwater levels and protect the quality of both

Question 2

Potential adverse effects of stormwater

Answer 2

• flood damage
• surface and channel erosion and sedimentation
• water pollution
• loss of bio-diversity
• damage to aquatic eco-systems

Question 3

Building Act Requirements

Answer 3

• safeguard people and property
• surface water from an event having a 2% probability shall not enter buildings
• drainage systems for the disposal of stormwater shall be constructed to convey surface to an appropriate outfall, avoid blockages, avoid damages

Question 4

Common rules relating to flood mitigation

Answer 4

• minimum 0.5m freeboard above the X year return period predicted flood level for all inhabitable floors
• defined secondary overland flowpaths
• pipe network must be designed for the Y year return period flood.

Question 5

Considerations

Answer 5

• future sea level rise
• climate change (higher intensity storm events rain)
• land settlement
• future development

Question 6

Acceptable Overland Flow Paths

Answer 6

• roads
• parks/reserves
• avoid private properties if possible (easily blocked)

Question 7

How to estimate flood flows

Answer 7

• Rational Method (Q = CIA commonly used)
• Auckland - Modified SCS method
• Hydrological and Hydraulic models

Question 8

Freeboard

Answer 8

provides contingency against:

• flood level estimation methods
• future climate change
• system failure (e.g. blockage)

Question 9

Why treat stormwater?

Answer 9

• protection of receiving environment
• public health
• recreational values (e.g. fisheries)
• aesthetics

Question 10

First Flush

Answer 10

• highest concentration in first third of rainfall event

- focus treatment on first flush contaminant removal

Question 11

Contaminant Management Approaches

Answer 11

• planning controls
• at-source treatment
• end of pipe treatment
• community education

Question 12

At-source treatment

Answer 12

treatment on site prior to discharge to network
Controls: swales, green roof
- minimise impervious area
- rain water tanks
- soakage

Question 13

End of Pipe

Answer 13

one device serves whole catchment

• wetlands, wet and dry ponds
• large proprietary device (e.g. sandfilter, downstream defender)

Question 14

Community Education

Answer 14

• changes behaviour
• promotes “ownership”
• can be critical to success

Question 15

Canterbury conditions

Answer 15

• soils can be very pervious
• good for discharge but not for treatment
• soakage devices can combine attenuation, treatment and discharge

Question 16

How to choose an approach

Answer 16

• site constraints
• financing
• consenting authority requirements
• ongoing responsibility and maintenance

Question 17

What information is needed?

Answer 17

• site infiltration characteristics
• source of contamination
• catchment areas impervious and pervious
• topographical survey
• existing services
• existing drainage/treatment

Question 18

General Rules of Erosion and Sediment Control

Answer 18

• 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 - sediment control devices