FAQ

The rock bund should remain under the water even when the lake is at a low water level. However, the low water level does vary and it is possible that in some circumstances (especially in the instances of prolonged hot summers without rain) the rock bund may be somewhat above the low water level. Lowering the height of the bund could impact its effectiveness in providing separation and fostering water flow through the lake. To address this, the bund will be located behind a group of trees, and this, together with planting of additional fringing vegetation will restrict visibility and access. If the rock bund is consistently exposed due to low water levels, the design of the rock bund can be modified to balance issues of visual impact, lake separation and access. There is also the option to install a short conservation-style fence to further restrict access if this becomes an issue in the future.

The below map shows the four catchment areas that drain into the lake, taking in a total area of over 70 hectares. The proposed design includes stormwater pre-treatment at each of the four catchment inlet points through a wetland system, which includes sedimentation basins and vegetated filtration wetlands.

There will be an opportunity for the local residents and the wider community to participate in community planting and weeding days that the City will hold as part of the Neil McDougall Park annual natural areas maintenance works. This will be advertised on the City's website, and we'll also get in touch with everyone who's participated in this project to let them know.

Although it would not be possible to completely eliminate the mosquito population from Neil McDougall Lake, the proposed works should help to control the population through the following:

• Increased water circulation will help prevent the formation of stagnant pockets of water, which are ideal for mosquito breeding
• Improved water quality with less organic matter and nutrient run off will hamper mosquito development
• Regular maintenance of all structures to help minimise water stagnation and mosquito breeding
• Regular maintenance of vegetation to prevent any build up of dead, floating vegetation or overgrowth
• Development and implementation of a mosquito monitoring program for Neil McDougall Lake

Check out the 'Phosphorous Awareness Project' by SERCUL (South East Regional Centre for Urban Landcare), where you'll find lots of information about the impact of too many nutrients in the Swan and Canning River systems and wetlands, and what you can do to reduce those levels.

Over the summer period there is usually a recurrence of a dense Lemna (duckweed) mat that covers the lake, as in common in wetlands and lakes across Perth.

Over the last few years, a toxic cyanobacteria (blue-green algae) bloom has followed the removal of Lemna, due to the higher water temperature and increased availability of nutrients, which is often further exacerbated by the nutrient-enriched stormwater runoff entering the lake. Improving the water quality will make the lake look better, reduce public health risks and provide a long-term, sustainable solution for the reduction of the nutrient load in the lake. This in turn will aid the management of duckweed and blue-green algae blooms.

Duckweed mats are common in still water bodies and do not pose a risk to public health. They indicate high nutrient loads in the water.

Blue-green algae blooms sometimes produce potent cyanotoxins that can pose serious health risks to people and pets. During toxic algal blooms the City warns the public not to enter the lake due to health risk through placement of appropriate signage and articles in the local newspaper.

Improving the water quality in Neil McDougall Lake involves a range of water sensitive urban design principles and includes several treatment components designed to address both the continual inputs of polluted stormwater and poor lake circulation. All treatment components are located fully within the park and are designed to largely operate with minimum maintenance requirements.

The treatment components include the following:

1. Construction of sedimentation zones and vegetated filtration zones at all stormwater inlets to treat ‘first flush’ and low flow stormwater as far as practicable before discharge to the lake.
2. Construction of a treatment wetland adjacent to the lake to treat lake water via a pumped recirculation system, as well as stormwater from the catchment.
3. Installation of aeration pumps at two locations to improve lake circulation and oxygenation within the lake.
4. Planting fringing lake vegetation to prevent the direct entry of pollutants (e.g. bird droppings carried in surface water run-off).
5. Interception and treatment of groundwater via planting of deep rooted plant species.

Please see the Draft Planting Species List for full details of the species the City will be using. Local provenance seed and vegetative material from the City’s natural reserves will be used whenever possible to generate the required plant stock. Swan Coastal Plain provenance seed and vegetative material (i.e. from the Swan Coastal Plain, but from outside the local City of South Perth area) will be used only if local provenance material is not available.

Four planting mixes have been specifically tailored for their designated position and function (i.e. nutrient uptake, enhancement of biodiversity, amenity) and their proximity to the water.

Planting Mix 1: Fringing Vegetation

This species mix is designated for the areas surrounding the Neil McDougall Lake and contains a variety of species that will increase nutrient uptake both in and outside of the lake, enhance habitat for wildlife and increase amenity. The design recognises the importance of maintaining the views to the lake and this will be addressed in careful arrangement of taller species during implementation. A number of sites surrounding the lake have been chosen to retain vistas with only very low sedges and herbaceous plants to be planted in front. These areas are clearly outlined in the concept plan.

Planting Mix 2:Vegetated Filtration

This mix is focused on species that are able to grow under permanent inundation, have a high nutrient uptake (predominantly nitrogen and phosphorous) and provide habitat for aquatic fauna and birds.

Planting Mix 3:Surface Flow (SF) and Sub Surface Flow (SSF) Wetlands

This mix will be used in areas that will support a greater variety of sedges and rushes to assist with a greater array of pollutant removal, nutrient uptake, as well as suiting variable hydrological conditions (i.e. inundation and drying).

Planting Mix 4:Landscape Integration

This mix contains a number of flowering species typical of damp land and transitional Banksia woodland habitats of Bassendean dunes. This selection of species is low height to maintain good views to and across wetlands, to the walking pathways and the lake beyond.

The City held a drop in information and feedback session on Thursday 7 March from 4-6pm. Thank you to all those who came along to discuss the preliminary designs.

If you have further questions about the preliminary design, please contact the City on 9474 0777 or at enquiries@southperth.wa.gov.au. 

The feedback is being analysed and a report detailing the outcomes of the engagement is being prepared. The City is also currently undertaking environmental studies at the lake to determine the presence of acid sulphate soils and to investigate site hydrology (groundwater levels). 

The outcomes of these studies together with the feedback received from the community on the preliminary design will be considered and the design will be revised accordingly.

The Neil McDougall Lake preliminary design proposes the removal of several mature trees as part of the project works, primarily introduced willows, and a small number of flooded gums. The removal of these trees will allows for the creation of stormwater treatment and lake water treatment wetlands, and are shown on the preliminary design. The details are as follows:

The installation of one low flow pump and two circulation pumps in the deep water lake zone are proposed. The low flow pump will pump lake water to the constructed/main wetland to improve the water quality of the lake.

Floating circulation pumps will aerate water of the lake to ensure adequate circulation coverage. These will be anchored to the lake bed with flexible pipework connections for power, and will be scheduled to operate during the day so that the potential noise does not disturb local residents in the evenings.

The use of solar powered pumps has been investigated but these are not suitable for this project.

Rock rip-rap is when rocks are placed in a protective layer to stabilise embankment slopes and protect shorelines from erosion. The rock rip-rap helps the lake treatment systems retain their shape during ongoing operations and sets a defined perimeter around the sedimentation basin for dredging purposes.

A rock bund is a kind of wall or divider used to separate one area from another. In this case, a rock bund is proposed at the southern end of the lake to create a defined anti-clockwise directional water movement for improved circulation of the lower water level flows. The rock bund will be constructed from similar rock to the rock rip-rap and would be installed by laying rocks below the water level providing flow separation. The rock bund will completely submerged.

The construction phase is proposed for early 2020 and it is anticipated that all planting and works will be completed by mid-2020.

The City’s natural areas maintenance team will carry out the maintenance of the vegetation, and all equipment such as pumps and aerators will be maintained by qualified contractors. Annual maintenance will coincide with low water levels to make access to the equipment more efficient.