Traditional methods proved to be either inappropriate, or too expensive, for monitoring of the small scale rain garden planter project at Hebden Bridge Town Hall Courtyard (see case study).
So, Geoff Sweaney of Wetland Engineering devised a bespoke system for us, and the first results are very encouraging!
The inlet and outlet are observed by a timelapse camera, whose footage Geoff has painstakingly interpreted into a video and graph.
The video footage can be viewed below, or on our YouTube channel at https://www.youtube.com/watch?v=SD_Toqq55nU. The speeded up timelapse video shows the inlet filling quickly and high (to the black line) – if you then quickly flick to watching the outlet (it’s speeded up), you can see a slower and lower peak.
Interpretation of results
We can see that the orange line, representing the outlet from the planter, shows a much longer, shallower curve.
Results from this rainfall event show a delay in the peak flow of about 4 minutes, and reduction in the level of the peak of 57%.
The area underneath the blue graph runs 8:10 to 9:28 (1hr 18) and orange 8:24 to 10:21 (1hr 57) i.e. there is a delay to the end of the flow period of 53 minutes, and increase in overall outfall time of 39 minutes, or 50%.
In summary, the outlet has a lower peak, and more gradual release of water, than the inlet – textbook slowing the flow!
The planters make a small contribution on their own, but these results have significant implications if lots of people get involved in retrofitting SuDS interventions throughout the Calder Valley.