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Pegasus Roadside Swale Information Sheet

Updated: Mar 28, 2022

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A swale is a grassed channel that runs along the edges of most roads at Pegasus. Swales collect storm runoff, remove sediment and discharge water to ground. They provide treatment of stormwater and protect groundwater.

How does a swale operate?

  • Stormwater from roads enters the swale through openings in the kerb. It collects in the channel and soaks into the ground beneath.

  • A buried gravel soakage trench runs beneath each swale. During smaller storms, stormwater can be expected to pond within the swales. After the storm ends, water will stay in the swale for up to 48 hours.

  • In larger storm events, stormwater will overflow from the swales and enter the soakage trench through the grate. An ‘Enviropod’ traps any sediment or litter washed through the grate. The stormwater then passes into the buried gravel soakage trench.

  • In floods greater than 10-year intensity, water will flow along roads and designed secondary flow paths to the lake or ECMA.

What maintenance is required?


WDC owns the swale in front of your property and will periodically remove any sediment and debris that accumulates. WDC will also regularly inspect the swales and soakage trenches beneath. Please report any swales that do not drain within 48 hours to the Waimakariri District Council (03) 311 8900.


To keep the swales functioning correctly, property owners should do the following:

  • Mowing – Grass should be kept short enough to reduce the fire risk but long enough to provide a healthy grass cover. Any cuttings should be removed from the swale, while bare patches of swale should be reported to the council for remedial works. If you do not mow your swale, WDC may have it mowed. However it will not be as frequently or to the same standard as what you would expect for lawn areas on your own property. It is recommended that you mow the swale in front of your own property when you mow your own property’s grassed areas.

  • Watering – To keep vegetation cover healthy, the swale may be watered in dry periods.

How do I install a driveway?

Driveways can be installed across swales. Before beginning any work on your driveway, you must contact WDC who can supply a standard driveway detail.

Pegasus Swale Engineering Diagrams

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