Quantifying the impact of high density deployments of rain gardens on the drainage network
Authors: Cotterill Sarah 
Published: February 2023

&lt;p&gt;Raingardens are a nature based sustainable drainage system (SuDS) that seek to reduce the flow into the drainage network and reduce the likelihood of the combined sewer network becoming overloaded. SuDS are often considered more adaptable than &amp;#8216;harder&amp;#8217; engineered systems (Woods-Ballard et al., 2007) and offer multiple benefits beyond diverting rainwater including a habitat for urban pollinators, horticultural opportunities and increased well-being. However, like many SuDS, they are often designed using propriety software to specify installation requirements, and are subsequently implemented with little or no monitoring to evaluate their performance (Melville-Shreeve et al., 2018). Monitoring could be used to demonstrate whether an installed raingarden is under- or over-performing.&lt;/p&gt;
&lt;p&gt;There are some, but limited, examples of good monitoring of sustainable drainage systems (Cotterill and Bracken, 2020; Ellis et al., 2006). As such, a series of laboratory and field-based experiments have been set up to quantify the impact of raingardens on the drainage network; to optimise their design and operation; and to enhance the opportunities for education and engagement activities within communities (e.g. lesson plans for schools, citizen science projects etc).&lt;/p&gt;
&lt;p&gt;Preliminary studies in a laboratory scale planter showed that peak flow was reduced by 10.9 &amp;#8211; 34.4% in the raingarden, during saturated conditions, when the planter was subjected to rainfall intensities of 3.75 mm/h, 4.5 mm/h and 5.25 mm/h. These intensities were chosen to represent yearly storm events that would be expected in Ireland (i.e. less than a 1 year return period).&lt;/p&gt;
&lt;p&gt;In the field, one hundred raingardens have been installed in north Dublin as part of Bi Urban&amp;#8217;s Rain Garden Project, funded by the Bonneville Environmental Foundation, Dublin City Council and the Local Authority Waters Programme (LAWPRO), with support from Wavin Ireland (Bi Urban, n.d.). Five of these have been selected for monitoring; featuring tipping bucket raingauges, turbine flow meters, weather stations and soil moisture probes. The volumetric flow rate in and out of the raingardens will be monitored for a period of 12 months alongside contextual information relating to temperature, humidity, rainfall, windspeed and soil moisture. This will help to build an evidence base to demonstrate the impact these assets can have on the drainage network, including both the total volume attenuated and the rate of peak flow reduction, as well as how this performance may be affected by antecedent conditions.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References: &lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;Bi Urban (n.d.) NatureX Rain Garden Project. Available online at: https://www.biurban.ie/rain-garden-project [ accessed 29/12/22]&lt;/p&gt;
&lt;p&gt;Cotterill, S., and Bracken, L. J. (2020) &#039;Assessing the effectiveness of Sustainable Drainage Systems (SuDS) : interventions, impacts and challenges.&#039;, Water., 12 (11). p. 3160.&lt;/p&gt;
&lt;p&gt;Ellis, C., Cripps, R., Russ, M., and Broom, S. (2016) Transforming water management in Llanelli, UK. Proceedings of the Institution of Civil Engineers - Civil Engineering.169 (1): 25-33&lt;/p&gt;
&lt;p&gt;Melville-Shreeve, P., Cotterill, S., Grant, L., Arahuetes, A., Stovin, V., Farmani, R. and Butler, D. (2018), State of SuDS delivery in the United Kingdom. Water and Environment Journal, 32: 9-16.&lt;/p&gt;
&lt;p&gt;Woods-Ballard et al., (2007) The SuDS Manual. CIRIA C697. London.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;