Swale Doing Its Job

It’s been very wet recently so the ground is saturated and further rain isn’t soaking in. These are exactly the conditions under which the swale is designed to temporarily store rainwater before letting it gradually drain away. (The rainwater collected from the roof of the house and from the paving around it is directed to the end of the swale next to the house.)

Normally any rain percolates into the subsoil before getting to the end of the swale but not today: there’s standing water all the way to the end.

Swale holding rainwater

Since much of the ground is clay the assumption was that the swale would primarily provide a means to transport water to the field drainage ditch, holding it back temporarily with an orifice plate on the outfall. In reality much of the water percolates through the bottom of the swale and this is the first time I’ve seen anything discharging from the outfall.

Small trickle of water from outfall from swale

Week 77, Day 5

Week 77, Day 5:

  • More work on the swale for the rainwater drainage, sorting out the western (outlet) end
    • The structural engineer who designed the swale wasn’t able to predict to what extent it would act as a soak-away hence they needed to specify an outlet
    • In theory an outlet is required because a swale is primarily a means to transport water, not to disperse it – the idea is that the grass growing in the swale will slow the flow of water but it will still travel the 50m to the far end and will then need somewhere to go
    • A 60mm orifice plate (not yet installed) will restrict the flow from the outlet, so in the event of a huge storm the water will back-up and then disperse gradually
    • There’s a flap valve on the far end of the pipe to prevent water flowing back into the swale in the event of the ditch getting blocked
Week 77, Day 5

Week 77, Day 5

Dam wall at western end of swale; orifice plate not yet installed on outlet

Swale outlet to field drainage ditch, with GRC head wall and flap valve