Beachmere Lake

Beachmere Lake is a constructed lake which plays an important role in stormwater and flood mitigation for the Beachmere community.  The lake also provides significant visual amenity for the surrounding residential properties and is connected to the ocean via a tidal exchange system, which assists in maintaining good water quality within an estuarine environment.

Council will be carrying out a number of improvements to Beachmere Lake to improve functionality.

Location

Kunde and Biggs Avenue, Beachmere 4510  View map

Google Map

Water circulation within Beachmere Lake.

Beachmere Lake water circulation

 

  1. Inlet valve

    Operation sensitive to lake water level.

  2. Outlet weir
    Critical for optimal operation of lake tidal exchange.

  3. Tidal flaps
    These flaps allow excess stormwater to drain into the swamp area but prevents flows from the swamp entering the lake system.

Inflow

The water intake from the sea is located along Kunde Street (indicated by the yellow line). The sea water only enters the lake through the pipe system when the tide level is 0.5m above mean sea level.

The combination of the size of the inlet pipes together with activation on the top part of the tide is designed to ensure the water entering the lake is sufficient to allow flushing, however is not so much that it causes the level of the lake to rise and impact on nearby homes.

Outflow

The outflow from the lake is located at the northern end of the lake and is controlled by a weir system. This system allows water to flow back to the sea (indicated by the purple line) for the period when the tide levels are below 0.5m.

The intake and out-take pipes operate continuously, responding to each high tide. Minor fluctuations in water level within the lake occur in response to tidal cycles (typically 100-150mm rise and fall per tide).

Stormwater overflow weir

Stormwater drains (for example curb side gutters) in the surrounding residential areas flow into the lake.

To cater for stormwater input into the lake during and after rainfall events, a stormwater overflow weir is provided at the southern end of the lake. If, due to rainfall events, lake levels exceed 0.8m, excess stormwater is discharged via this overflow. The overflow weir has been specifically designed to prevent high tide water entering the lake from the mangrove area south of Kunde Street. If water was to enter the lake from this location, impacts to homes may occur.

Beachmere Lake - Overflow weir

A water quality buoy is located within the lake. This buoy sends information on the quality of the water electronically to Council. In addition, an alarm system has been installed within the lake to alert Council to conditions that could potentially result in poor water quality.

Aerators (large bubblers) are also installed in the lake to assist with water circulation and the mixing of salt and fresh water. Typically, freshwater will sit above salt water in an estuarine environment. The aerators assist by introducing vertical mixing of the freshwater and saltwater, reducing the risk of the creation of poor water quality conditions.

In March 2019 the lake’s water quality was impacted causing fish in the lake to perish. An assessment of the lake’s circulation system was conducted, and water quality and algae tests occurred.

The analysis of these tests indicates that several factors contributed to the fish kill, including:

  • A significant rainfall event following a period of dry weather, resulting in the influx of nutrient-rich stormwater into the lake
  • Blockages and non-performance of the stormwater overflow system
  • Partial blockages within the tidal exchange system

Collectively, these factors resulted in the lake containing nutrient-rich stormwater for an extended period, leading to an algal bloom. The algal bloom affected dissolved oxygen concentrations in the lake, which contributed to the fish kill.

In response to this situation several actions have been put in place, including:

  • Inspection (via closed circuit camera) of all the pipework and circulation system components
  • Review of the functionality of the stormwater overflow weir
  • Review of the tidal exchange system and investigation into options to supplement the tidal exchange system

The 2019 fish kill has highlighted a shortcoming in the design of the lake’s tidal exchange system. While small fish, eggs and larvae can enter the lake, large fish cannot leave. The fish become trapped within the bound of the lake, with many species unable to complete their life cycle. Advice from specialists regarding options to manage the adult fish population within the lake is being sought. A key challenge with this is to provide an opportunity or mechanism for adult fish to leave the lake, while ensuring this does not adversely affect the tidal exchange system nor result in increased flooding risks for the surrounding low lying properties.

The current lake revetment wall is made of timber and is failing in various areas. An upgrade of this lake wall is planned for delivery in 2019/2020. Commencement of the project is on hold, awaiting the outcome of the CCTV inspection and other investigations in the lake. As appropriate, the scope of the wall replacement project may be extended to include necessary upgrades to the lake’s tidal exchange system.