2. Description

The rainwater garden is located on the grounds of Clandeboye Primary School in Bangor, Co Down,

within an existing green space interspaced with mature trees and close to a dense residential area

near the town centre. The sustainable urban drainage (SuDs) features were proposed by Northern

Ireland Water (NIW) to resolve two surface water management issues: an increase in storm water

volume and an existing culvert capacity issue resulting in out of sewer flooding in times of heavy

rainfall.

In parallel to the surface water issues in the area, NIW and its parent department, the Department

for Infrastructure (DfI) were keen to promote and demonstrate the effectiveness of SuDs within the

urban environment. Particularly innovative was the use of “soft SuDs” as a sustainable yet effective

tool to manage surface water and reduce both the risk of flooding and carbon footprint. NIW

successfully secured funding from the DfI Special Fund for SuDs demonstration projects.

The scheme’s location within the grounds of a local primary school added a significant challenge due

to concerns about the risk posed to children by the introduction of an open water feature, making

stakeholder buy in more difficult. Key to the acceptance of the project was the support by the School

Principal and Board of Governors who endorsed the project given its potential not only to help

The raingarden is fed from hardstanding areas around the school building, approx. 300m

2

of which is

directed into a storage tank with overflow pipework to a rainwater rill. The tank has a sluice gate

which when activates allowing 10l/s to flow into the rill for 20 minutes when the tank is at capacity.

The discharge is via a grassed feature mound which is shaped as a turtle with the water discharging

from the wooden carved turtles’ mouth.

The rill terminates at a circular stone sinkhole which conveys the flow underground via a pipe to

facilitate the access path to the school. The pipe terminates at pond A and water then overflows a

stone cascade wall to fill pond B. Pond A also receives the overland runoff from the existing school

hard surfaces playground area. The overland flow travels first through a bog garden which provides

pupils of Clandeboye Primary to add value to the scheme. The additional features included a spiral

mound with slide and landscape profiling and serve a dual purpose: to accommodate excavated

material from the swale and pond areas, thereby considerably reducing the project’s carbon

footprint, and to ensure no material required transportation off site.

4. How it works

The drainage strategy involves the interception of overland flows from hardstanding areas around

the school and attenuation of these flows before discharging into the Clandeboye Stream. The total

area of hardstanding is 2,702m

2

inflow to the ponds of 70l/s. The hardstanding area can be divided into three areas each of which

The geo-cellular tank was selected as source control for Area A runoff to retain the required volume

at the top of a natural slope. The tank is designed so that flows can be released via a valve

running down the slope. The design of the conveyance system provides amenity value to the school

children by a creating play feature where duck races can take place (Fig. 2).

The bog garden collects the runoff from Area B and allows flows to slowly percolate down the

natural slope into Pond A. This process filters the runoff removing sediment while also providing

12m

ensures that the maximum depth of water in the ponds does not exceed 450mm. The ponds are

planted with a mix of aquatic plants to form a boundary to the pond margins. The planting has been

selected to ensure that views to the ponds are not obstructed for safety reasons. A dipping platform

has also been provided to allow children safe access to the edge of the pond in supervised

conditions (Fig.1).

A swale was selected as the most suitable SuDs component for collecting the access road drainage as

it can follow the horizontal alignment of the road and collect flows along its length. The swale also

provides filtration of the runoff and a small level of infiltration as it conveys flows to the Clandeboye

Stream.

5. Specific project details

The raingarden project forms part of a wider drainage area plan (DAP) which involves the closure of

unsatisfactory combined sewerage overflows and aims to improve water quality of flows discharging

The ponds are the main focus of the scheme, providing most of the attenuation, the aesthetic focal

point and the primary educational space. The rill and associated underground storage tank provide a

safe and fun way for the children to interact with the water as it flows down to the ponds while the

swale provides a safe and easy method of drainage adjacent to the access road (Fig. 4).

6. Maintenance & operation

Overall ownership and maintenance of the feature falls to both the School and the Education

Authority. As part of a collaborative approach, it was agreed that NI Water will monitor and maintain

certain aspects of the feature over a four-year period following handover. Focus will be given to

ensuring vegetation survives the initial planting through to maturity phase, and that suitable design

features are in practice. This extended monitoring period was essential to ensure initial buy in,

reassure stakeholders of the commitment to make the project a success and guarantee short- to-

medium-term overall performance.

Additionally, continued visual inspection of the feature by all stakeholders will ensure maintenance

will be carried out as required, as opposed to reactionary maintenance typical of more conventional

solutions.

The effectiveness of the project in reducing the risk of flooding via overland flow in the downstream

cul-de-sac is currently being monitored by both NI Water and DfI Roads.

8. Benefits and achievements

The scheme successfully provides attenuation storage before discharging to Clandeboye stream via a

high-level overflow. The reduced run-off rate is evidenced in the observed resolution of the flooding

in the nearby cul-de-sac. Water quality is addressed at every point of entry into the attenuation

system. This includes vegetated slopes entering the swale, the gravel bog garden between the

playground and the ponds and gully traps on the pipelines discharging to the geo-cellular tank.

As well as the carbon reduction benefits normally associated with SuDs, the project also delivers a

number of benefits related to the additional carbon reduction strategies implemented to reduce the

its carbon footprint. These include removing surface water from the combined sewer network,

leading to energy savings on treatment, as well as reusing excavation material on site.

substantial lowering of the school’s accident rate, with play becoming more focused on the features

of the garden and away from the traditional games of “chasing” on hard playground surfaces, as well

a noticeable reduction in playground disagreements. Further work completed by voluntary groups

has included an outdoor classroom including a mud kitchen and sandpits which has complemented

the learning curriculum.

Clandeboye Primary actively encourage other schools within the area to tour the SuDS and

demonstrate the added value provided by the rain garden to the school curriculum. An official video

which celebrates the success of the project was also produced and can be found here.

9. Lessons learnt

Key challenges were predominantly linked to health and safety concerns. The Education Authority

raised concerns about the of risk to pupils attending the school and the potential of legal action

taken against the board should accidents occur. This opposition and challenge was welcomed by the

development team as a chance to influence at a strategic level within the Education Authority, with

the high level of scrutiny leading to the achievement of the best possible design. Due to the location

of the raingarden, community engagement meetings were held to discuss the project and planned

Key to the success of the project was the early inclusion, buy in and subsequent advocacy of the

school’s Board of Governors and in particular the headmistress, whose energy, vision and positive

approach to perceived obstacles secured the success of the project.

No interaction was required with the Local Authority, Ards and North Down Borough Council as the

scheme was constructed on grounds which belonged to the Education Authority. Interactions with

11. Project team