Kennington Reservoir Major Maintenance Project

Kennington Reservoir is a unique nature conservation and passive recreation area that is greatly valued by the community. 

In 2001 the City commissioned a specialist consultant to undertake regular monitoring and provide assessment reports on the dam wall. For some time now the reports have noted that upgrade works are required to ensure the dam wall is safe for the long term. 

The reports detail a number of issues with the dam and outline the works required to ensure the dam wall is safe.  The main issue is water seepage through the dam wall structure which over time could lead to a significant dam wall failure. 

The proposed works will be aimed at preserving the integrity of the reservoir and its ageing infrastructure to ensure it is safe for the community to use over the long term.  There are no concerns that the dam is currently unsafe, but it is clear that without intervention, the dam may become unsafe and unusable in the near future.

Based on recommendations from the inspection reports, the proposed rehabilitation project will include the following works:

• Replacement of the outlet pipe as it is at the end of its usable life
• Removal of vegetation from the upstream batter (wet side) as tree roots create seepage paths into the wall
• Reprofile and repair the shape of the upstream batter and line the face with a rock beaching to ensure no erosion
• Repair of the waterproof layer along the length of the wall to ensure no seepage through the wall
• Repairs to the damaged spillway and a short section of the downstream creek channel
• Repairs to the fishing platforms during the dam wall batter reprofiling

In December 2020, the City’s arborists and Engineering staff met with the design consultant to survey vegetation on the dam wall. Particular attention was paid to vegetation on the upstream batter, crest, and immediate downstream shoulder, as trees in those areas are considered to represent an enhanced risk to the dam wall.

The most appropriate design option to repair the dam was then collectively decided upon with consideration to the vegetation present on site. In this instance, a design solution was available which allowed retention of the trees of the downstream batter of the wall. In many other dam wall repair instances, that option is not available and the entire wall must be cleared of vegetation. Through this collaborative design approach, the City was able obtain a design solution that repaired the dam wall to an acceptable level of risk, whilst retaining much of the existing vegetation. The City sought opinion from other Victorian Dam Managers regarding its approach who confirmed we had struck the right balance.

A biodiversity assessment was also undertaken to assess the impact the works will have on the native vegetation. The report indicates there will be the equivalent of 0.7 hectares of vegetation requiring removal. This is made up of 101 varying sized remnant trees, 77 with a trunk diameter of 47cm or less and 24 with a trunk diameter greater than 47cm.

The trees require removal for the following reasons:

• Trees located on the upstream batter shoreline or upstream of the proposed cut-off trench, as they represent a piping risk
• Any dead or aging trees showing signs of decay on other areas of the dam wall, as they present a piping risk (rot and allow water to move through old tree root lines)
• Trees interfering with existing elements of the reservoir (e.g. causing damage to spillway, outlet, or limiting flows within the spillway channel)
• Trees in the way of the construction of new elements (new outlet, capping of old outlet, cut-off wall)

Removal of the required trees will include:

• Removal of tree stumps and tree roots to the extent of the tree drip line.
• Removal of tree roots 20 mm or larger in diameter outside of the tree drip line.
• Minimum depth of 1 m below the existing surface.

The City will engage vegetation removal contractors to remove the vegetation from site and any suitable vegetation will be reused on park areas or other City projects.

The City must follow the Department of Environment, Land, Water and Planning (DELWP) detailed pathway assessment for the removal of any native vegetation associated with this project. This process ensures retention of vegetation is avoided or minimised where possible, and where not possible, that the removals are offset.

Any offsets required for this project will be made using either the City’s internal offsets, or offsets purchased through a commercial provider. Both the design process, and any offset amount/location are assessed and approved by DELWP as part of the Planning Permit process.

A number of types of dam failure were extensively researched and modelled in 2018 in line with best practice for management of large dams to allow the City to manage the risks accordingly.

Wall fails during a major flood event (this is where there is a lot of inflow during a storm, which causes water to overtop the wall leading to erosion and a quick major failure of the wall) which would lead to:

• Up to 614 people exposed to flood water if occurs during the day
• Up to 244 people exposed to flood water if occurs during the night
• $26 million dollars in damage to residential, commercial and industrial buildings downstream
• 0.3% chance of a fatality

A normal day failure (called a sunny day failure) is when water finds its way through the wall along tree root lines, and over time gradually erodes out to create a larger hole eventually causing a major failure and leading to:

• Up to 56 people exposed to flood water if occurs during the day
• Up to 24 people exposed to flood water if occurs during the night
• $6.7 million dollars in damage to residential, commercial and industrial buildings downstream
• 0.01% chance of a fatality

The City engaged a specialist consultant to undertake a study into the aquatic biodiversity present within Kennington reservoir with the main fauna and flora species found to include:

• Fish – redfin, goldfish, tench, flat headed gudgeon, rainbow trout
• Turtles – long neck tortoises
• Crustacean – yabbie, shrimp
• Bivalve mollusc – mussels
• Beds of eel weed are present within the reservoir

The study suggested a minimum level that the water could be drawn down to during works, before any lasting impact would be felt by the biodiversity present. The proposed works have been designed in accordance with those recommendation.

The fishing platforms will be closed off and repaired during the works.  They will reopen for use once the works are complete.

Yes. You can continue to fish in areas that are not closed off to the public.

During the construction period there will be limited access for public use in the works area as follows:

• During the majority of the works, the walking path and access to the top of the wall area will be closed off. The path will be re-instated and re-opened once works are complete
• During the entire works period, the fishing platforms will be closed. The platforms will be repaired, reinstated and open for use once works are complete
• The pedestrian bridge over the spillway will be closed for approximately four to six weeks at times during the construction phase

A few other projects will be undertaken at the same time as the dam wall repairs, to make use of the already closed off area, and lowered water levels.

• Minor improvements to the fish habitat around the fishing platforms will be explored whilst the water levels are down. Improvements may include placement of dead trees in appropriate areas
• Installation of minor culverts under existing paths in crucial locations downstream of the dam wall

The Kennington Reservoir dam was constructed in 1861 when the government of the day

constructed two reservoirs along Grassy Flat Creek to provide a reliable water source for gold mining activities, market gardens and stock. However, the lower reservoir eventually failed and the site was later redeveloped into sports fields, now known as Strathdale Park.

In 1929 Kennington Reservoir was widened and repaired due to water seepage issues.   The reservoir has not had any major renewal works undertaken on it since that time. 

The reservoir was decommissioned in the late 1970’s and became the responsibility of the former Shire of Strathfieldsaye and then the City of Greater Bendigo following the 1994 Victorian Local Government Council amalgamations.

When full the reservoir holds 118 megalitres and covers approximately six hectares. Kennington Reservoir is fed by storm water runoff only and it is common for water levels to reduce during the dry summer months of the year.

The City uses water from the reservoir to irrigate recreation spaces downstream.

The City will re-commence works in April 2024 for an expected completion by March/April 2025.

The cost of the project is estimated at approximately $2 million.  Funding for the project will be considered as part of the City’s annual budget process.