KEEPING OUR RAIN GARDENS SAFE – AUCKLAND TRANSPORT’S SAFETY BULLETIN

A. Sharma, Senior Environmental Specialist, Auckland Transport

ABSTRACT

Auckland is experiencing an exponential growth and across the region, development is occurring at a rapid pace. As Auckland grows so does the demand for maximized intensification of land space for housing and other ancillary infrastructure (such as roads) to support the wellbeing and connectivity of the population. This leaves limited space to provide for better designed and integrated stormwater treatment solutions in accordance with the operational Auckland Unitary Plan (AUP(OP)) and Auckland Council’s (AC) GD01(2017) guideline document for water quality and hydrology mitigation especially for the road corridor. One example of this type of device is a raingarden. Raingardens in the road corridor are intended to provide road runoff treatment and retention (i.e., volume loss). However, due to space constraints there has been an unprecedented implementation of small and deep raingardens along the road corridor for detention in new greenfield developments that are to be vested in Auckland Transport (AT).

Safe transport is vital to our city’s success and is a primary objective in street design.  Vision Zero of AT acknowledges human error but doesn’t accept that death and serious injury as an acceptable outcome. Poorly designed raingardens can also pose a safety hazard for pedestrians and cyclist. This usually results from conflict between the desire to maximize the lot yield of the development and still meet consent objectives. Insufficient education on design guides and lack of communication with AT on the design of the device prior to engineering approval and vesting stages, also contributes.

Recent safety related incidents in medium density housing developments have shown that raingardens can pose a safety hazard when they include sudden or steep drop-offs to the raingarden bed particularly when installed adjacent to kerb/footpaths/cycleways. This hazard can be exacerbated when insufficient space is allowed, resulting in the depth being increased above the GD01 recommended minimum to ensure the required storage volume is captured. This leads to sudden vertical or steep drop offs from the berm/footpath/cycleway and creates a fall hazard. The same hazard is also present when raingardens are located adjacent to parking areas as there is the risk to people exiting vehicles. Raingardens can pose a safety hazard when the drop-off from adjacent berm/footpath exceeds 150mm.

In response to the serious injuries associated with poorly design raingardens AT has developed a safety bulletin which specifies design requirements intended to ensure road user safety. Design requirements seek to minimize the fall hazard and apply to both new and existing devices. It has been produced to supplement the various AT and AC design codes, safety requirements (Council Code of Practice) and guidelines, in accordance with the H&S at Work Act 2015 and regulations 2016.

This Safety Bulletin is therefore aimed at guiding the industry on safe design and installation of raingardens for all users. This approach applies to raingarden works covered by:

i) the Land Development Engineering Plan Approval (EPA) process;

ii) any AT approved works programme/project involving upgrading and redevelopment of road infrastructure works;

iii) third party developed roads and other infrastructure to be vested to AT;

iv) retrofit alteration of raingardens that are considered high priority due to risk to road users;

v) any other AC or AT approval process which involves raingardens.

Hazard mitigation requirements for raingardens can differ depending on whether the raingarden is proposed in a greenfield or brownfield development. In greenfield developments there are generally no existing space constraints (other than a desire to view the road as lost land which should be minimised) and hence optimal design measures can be implemented. In brownfield developments where road boundaries are set there can be existing constraints which can mean designs must instead include mitigation measures.

Alongside some of the proposed mitigation methods in the safety bulletin and mentioned below, AT’s Bioretention Design provides specific advice for raingardens to be located within the road corridor and vested in AT. Auckland Transport does not support the use of raingardens for detention only (i.e., where there is no statutory requirement for treatment and retention is unable to be achieved due to ground conditions) as these have been shown to be costly to maintain and ineffective. Therefore, other alternative devices should be employed as a part of the treatment train approach to provide a similar function as roadside raingardens for detention purposes.

Where the height difference from the raingarden bed to surrounding berm/path exceeds 150mm, raingardens should be designed with 1:3 (vertical: horizontal) battered side slopes from the top edge of the raingarden down to the raingarden bed. This slope is generally considered pedestrian safe in both national and international standards. The side slopes must be formed using a suitably stable material to prevent slumping/settlement over time, which will increase the vertical drop. Mulch cannot be used to form the side slopes as this quickly settles and slumps.

The raingarden media generally settles over a 6 – 12-month period as water flows through the raingarden. To mitigate settlement (and a hazardous vertical drop) raingarden media should be installed in layers no greater than 300mm and be thoroughly watered prior to the installation of the next layer.

The internal width of the roadside rain garden should be no less than 2m (excluding battered sides) to ensure that adequate planting successfully establishes and matures. Planting provides a contaminant treatment function as well as visual safety barrier to pedestrians and road users. Planting can reduce the hazard if someone were to fall into the raingarden. However, this is not considered sufficient mitigation as plants can take years to properly establish and are not necessarily permanent solutions as plants may dieoff due to droughts or improper installation, leaving the raingarden without mitigation.

A buffer strip a minimum of 500mm on the footpath side and on the roadside of the raingarden should be provided. The roadside buffer will help mitigate the hazard posed to people exiting vehicles next to raingardens. The buffers should be grassed to provide a visual cue that clearly indicates it is not meant for pedestrians/cyclists. Buffer strips should also be considered where a short upstand along the edge of the raingarden is required or has been installed.

Where width constraints already exist within the berm (brownfield developments) and a buffer of minimum 500mm between the rain garden and carriageway edge is unable to be formed, parking restrictions adjacent to the rain garden should be provided to prevent passengers alighting from parked vehicles directly into the rain garden.

Many raingardens have already been constructed throughout the Auckland region which may pose a risk to the public and could need retrofitting to mitigate safety hazards. Various factors will influence the choice of retrofit option and each raingarden will likely need to be considered on a case-by-case basis.

A further description of each option is provided below:

Option 1

Option 1 involves using available space around the raingarden to construct 1:3 battered side slopes from the existing media bed outward. Changing the dimensions (i.e., narrowing and lengthening) of the raingarden can be considered to maintain the effective infiltration area and ponding volume.

Option 2

Option 2 involves raising the media bed level of the raingarden to reduce the height of the drop-off and constructing 1:3 battered slopes up to the edge of the raingarden if required.  Raising the raingarden bed allows the lost surface area of the infiltration to be minimized, compared to just battering down to the existing level, thereby maintaining a similar treatment efficiency.

Option 3

The treatment function of some raingardens may be considered critical, and reduction or removal of their treatment capacities may be considered unacceptable. A physical barrier, installed to height of at least 500mm above the kerb line is considered as a last resort option. Physical barriers are particularly useful where hazardous raingardens are located adjacent to areas pedestrians use to cross roads such as raised pedestrian crossings.  However, these should be reviewed on a case-by-case basis. Careful consideration is required when proposing this option for raingardens adjacent to cycleways as they could introduce a new hazard.

Option 4

In some instances, the above options are not feasible or desirable, meaning that the only viable option is for the raingarden to be disestablished. The raingarden can either be removed entirely or can be filled with media up to the footpath level, and the kerb cutout filled in, and either replanted as a standard street garden, or grassed to match the surrounding berm environment.

In conclusion, it is essential that the decision to construct raingardens within the road corridor is made based on a comprehensive options assessment for stormwater treatment devices. This assessment is required to follow evaluation principles and desired outcomes in accordance with AT and AC design guides, Council’s Code of Practice and AUP(OP). If raingardens are deemed to be appropriate via a suitable options assessment, designers should focus on the design for and relationship between the raingarden and the adjacent road space addressing a range of safety, structural, maintenance and operational aspects. To support the designers, AT has developed a raingarden design safety bulletin that identifies acceptable safety measures, particularly treatment for the edges of raingardens in context to all road within the AT System and is available to be shared.