Spillway upgrades are most often undertaken following dam safety risk assessments, and may comprise enlargement of the spillway capacity and/or rising of the dam itself. Changes to the methods used for extreme flood estimation, and the assessment of risks to both the downstream population and the dam owner, are driving spillway upgrades around the world, particularly in Australia.

In Australia, there has been an increase in the remodelling of existing dams, as opposed to new dam construction. This is primarily due to revisions of the Australian National Committee on Large Dams (ANCOLD) Guidelines relating to dam safety and spillway capacity, as well as the need to augment stored volumes in order to meet increasing population demands.

Spillway works, whether new or remodelling/augmentation, are the primary safety feature of the dam. The ability to pass the design flood is paramount to the safety of the asset, to meeting consumer demand, and to the safety of the downstream population who would be impacted by a dam failure. The hydraulic adequacy and life of the spillway structure must be assured to achieve the safety, risk and consequence levels of the controlling statutory authorities. The safe operation of spillways and constant monitoring of conditions is an ongoing feature of the owner’s activities.

The following examples demonstrate the diversity of dam projects, from large dams in remote areas to smaller storages nearer to population centres where spillway and dam upgrades have been required. These examples show how the level of risk arising from the works was managed.

Darwin River Dam Raising

The Darwin River Dam, located in Australia’s Northern Territory, is the principal source of water supply for the City of Darwin and the surrounding areas.

The dam is 27 m high with a crest length of 560 m and upstream and downstream slopes of 2.5 H to 1 V and 1.9 H to 1 V respectively. The spillway is located on the left abutment of the dam.

The spillway currently comprises: a 260 m wide channel with a concrete sill control section; a 100 m long approach channel; and an unlined discharge channel terminating in a silt trap/stilling basin.

To meet the growing water demands of the region’s expanding population, SMEC was engaged to explore options to increase Power and Water Corporation’s (PWC) total water supply capacity. Dam upgrade works were undertaken in order to raise the full supply level of the dam.

Project scope included: the construction of a new ogee spillway structure on top of the existing crest structure; an apron slab at the downstream toe of the ogee; abutment slabs on the left and right sides of the crest structure and a sub-surface drainage system.

Project design requirements included: ensuring the overflow section and dissipaters were designed to maximise the outflow and minimise erosion; providing anchors and dowels to hold down the spillway overflow section; designing drainage provisions to minimise the uplift pressure; identifying the effects of flood events on the Darwin River floodplain; and completing a geological investigation and survey to determine the areas of weathered rock that required treatment in order to reduce erosion under spillway flow.

St Georges Dam Upgrade

SMEC was engaged to carry out a risk assessment of St Georges Dam in Central Victoria, Australia.

The embankment dam is 16 m high and 78 m long, and consists of earthfill with a puddle clay core and an upstream face of rock beaching and a grass covered downstream face. Originally, the dam had two spillways with near vertical walls. The approach and discharge channels are both unlined.

The dam safety assessment indicated that: the embankment stability did not meet steady seepage, rapid drawdown and seismic stability criteria; there is a risk of failure of the embankment by internal erosion and piping due to the absence of filters in the dam; and the two spillways were estimated to have a combined capacity to pass a flood with an annual exceedance probability of only (AEP) 1 in 350, a significant short fall of the required 1 in 100,000 AEP.

The risk assessment resulted in the following scope of works: construction of a bund across the existing secondary spillway; lowering the dam height; retrofitting the dam with a two stage filter; construction of a control structure along the dam crest to act as the new secondary spillway; placing rockfill on the downstream slope to facilitate flow over the embankment; and decommissioning the outlet system.

These works reduced both the likelihood and the consequences of failure of the dam.

Blowering Dam Safety Upgrade

Blowering Dam, located in New South Wales, Australia, is a 112 m high earth and rockfill irrigation and hydropower dam with a capacity of 1,628 GL. An 80 MW capacity hydroelectric power station is located at the toe of the dam.

As the existing storage does not have probable maximum flood (PMF) capacity, State Water NSW adopted a two-stage approach to upgrade the storage in order to meet flood capacity requirements.

SMEC was engaged by State Water to undertake the detailed design and documentation of the Stage 1 upgrade works: immediate reduction of risk to the downstream communities to an acceptable level. The Stage 1 works entailed increasing the discharge capacity of the existing spillway by raising the embankment and spillway walls.

SMEC’s detailed analysis identified the need for significant modification of the spillway walls, including the removal of the clay core adjacent to the embankment. With the core removed, the embankment would be at a greater risk of overtopping and breaching than currently exists, and precautions were put in place to manage this risk.

The works required excavation of the embankment at the spillway wall to a depth of 10 m, incurring considerable risk of breach if a flood occurred. This was managed by timing the works to coincide with low reservoir levels, as well as the construction of temporary cofferdams around the works. The timing proved effective with the spillway flowing just two weeks after completion of the works.

Kangaroo Creek Dam

SMEC undertook a safety review of the Kangaroo Creek Dam, a 63 m high concrete face rockfill dam located in South Australia. South Australian Water Corporation (SA Water), the owner of the dam, is undertaking significant upgrade works to ensure its compliance with updated guidelines set by the Australian National Committee on Large Dams. To meet these new guidelines, the capacity of the spillway needs to be increased to pass the Probable Maximum Flood (PMF), the design flood requirement.

SA Water had previously implemented modifications to limit flows up to the 1 in 200 AEP event in the River Torrens to around 210 m3/sec, and increase the spillway capacity to 1850 m3/sec. The works also resulted in the storage capacity of the reservoir being reduced from 24,400 ML to 18,700 ML by lowering the Full Supply Level (FSL).

SMEC identified a range of options, with the preferred approach being to widen the existing spillway using an ogee shaped crest in conjunction with raising the crest level of the dam.

As the review progressed, potential cost effective alternative options were identified and considered in detail, namely: adoption of a Piano Key (PK) shaped weir crest in lieu of the existing ogee crest; and the use of Hydroplus Fusegates.

The review process resulted in the development of three concept designs for increasing the spillway capacity of the dam. This comprised the preferred arrangement arising from the earlier safety review, as well as the two alternative arrangements listed above.
A number of additional dam safety requirements and specific design constraints were identified in addition to the need to increase the flood capacity (generally associated with resistance to strong seismic loading). SA Water requires these to be addressed as part of the Upgrade Works in order to reduce the residual risks to satisfy ANCOLD tolerable societal risk criteria.

Remedial measures to address the additional identified dam safety requirements and design constraints were developed to a concept design level, and incorporated with the three concept designs that were developed for upgrading the spillway capacity of the dam. Accordingly, the three options developed are considered principally to be dam safety upgrade options, but are distinguished by their arrangement for the flood capacity upgrade. These options were developed so as to achieve a consistent standard of protection to the dam.

Report by Bob Evans, Senior Engineer-Hydraulic Structures, and Dr Gamini Adikari, Chief Technical Principal-Dam Safety, SMEC-Australia & New Zealand Division.

SMEC provides comprehensive design and construction solutions for all major dam types and has extensive experience in the fields of dam safety, rehabilitation, analysis of existing structures and remedial works design. The company’s multidisciplinary expertise encompasses all aspects of dam investigation, design, construction, project management, construction supervision and safety management. SMEC has global experience in the complex and critical issues related to individual dam locations and environments, and provides conceptual development, detailed design and construction supervision to deliver successful project outcomes.