QLD – Revisions to The Manual For Assessing Consequence Categories and Hydraulic Performance of Structures

Author: Michael Marley.

The Department of Environment and Heritage Protection has recently completed a review of the Manual for Assessing Hazard Categories and Hydraulic Performance of Dams (Manual).

Until 2000, all dams in Queensland were regulated under the Water Resources Act 1989. Dams above a particular threshold of risk were required to have a Waterworks Licence with associated conditions regarding design, construction, inspections and maintenance.  The Water Act 2000 excluded dams containing “hazardous waste” which were instead to be regulated under the Environmental Protection Act.  The resultant regulatory framework required all dams on mines to have a hazard category assessment that considered the consequences of dam spill and dam wall failure on humans, the environment, stock and infrastructure. Dams assessed as significant or high hazard category were specifically regulated under the Environmental Authority and required design, construction supervision and annual inspection by a Registered Professional Engineer, Queensland (RPEQ) with competence in dam engineering.

Initially the effectiveness of this framework was limited because it was not specified who was to undertake the hazard category assessment.  However for the past several year’s conditions in Environmental Authorities have required certified assessment by a registered engineer with competence in dam engineering, and this has improved the capture and management of dams where the consequences of failure would be significant.

Recent modifications to the earlier versions of the Manual include:

• Replacement of the requirement to assess “hazard categories” by the requirement to assess “consequence categories”;
• Replacement of “Dams” with “Structures” and the related inclusion of levees as regulated structures;
• Specific reference to regulated structures to contain flowback wastes resulting from hydraulic fracturing activities;
• Exemptions from the requirements of the Manual if the structure is to contain waste for no longer than 24 months (and meet certain other conditions with respect to volumes stored);
• Exemption from the requirements of the Manual in the case of structures to contain wastewater from hydraulic fracturing activities if they meet the above requirements AND the lining is certified by an RPEQ as being able to contain the wetting front;
• Alternatively (to the RPEQ providing site specific certification that the structure is able to contain the wetting front) an RPEQ may certify a “pro forma” design that will contain the wetting front.
• Assessment and documentation of the Consequence Category by a “suitably qualified and experienced person” for the following failure event scenarios;
  • Failure to Contain – Seepage (new structures only)
  • Failure to Contain — Overtopping (new and pre-approved structures)
  • Dam Break (new and pre-approved structures)
  • Limiting specific contaminant trigger levels for consequences related to higher risk dams associated with the “failure to contain – seepage” scenario to pH and electrical conductivity.
  • Introduction of a new definition of “Extreme Storm Storage” (ESS)
  • For each of the failure scenarios, assessment of the potential for :
    • Failure of a structure placing lives at risk due to dwellings or workplaces being in the failure impact zone;
    • Downstream consequences, including but not limited to failure of other structures that may be affected by flooding;
    • The consequences of such cascade failure for other structures;
    • The impact of both on-site and off-site environmental values;
    • Long term potential adverse effects due to release of contaminants to groundwater systems and soil profiles;
    • Potential consequential effects on surface water systems; and
    • Storage releases that may chemically interfere with waters used as sources of drinking water.
    • Despite any exemptions otherwise applicable, a failure impact assessment in accordance with the requirements of the Water Supply (safety and Reliability) Act 2008 if the structure is a dam more than 10 metres high and meets certain other criteria in terms of storage capacity and maximum surface area.

The revised consequence category assessment (dam break scenario) provides that a high consequence category results if “people are routinely present in the failure path and if present, loss of life to greater than 10 people is expected”.  Included as a footnote is a particularly concerning caveat that “ people routinely be present in the failure path” should refer to people other than site personnel engaged by the resource operation and located on the tenements and tenure associated with the resource operation.  These personnel are considered to be adequately protected under the Coal Mining Safety and Health Act 1999 and Regulations 2001.

Attempts by some concerned RPEQs to have this caveat deleted, based on experience that dam failure is presently not well addressed under internal mine safety procedures, were unsuccessful.

It is important to note that the Manual requires certification by an RPEQ of the following reports relating to Regulated Structures:

• Consequence Assessment Reports
• Design and Design Plans
• As Constructed Drawings
• Construction Report
• Annual Inspection Report.

In addition to these certification requirements, as the design and construction of Regulated structures constitute separate “engineering services” as defined in the Professional Engineers Act 2002 , Queensland (PE Act), both design and construction are required under this Act to be undertaken or directly supervised by RPEQs who are qualified and experienced in the relevant class of work.  It is not therefore sufficient simply for the end result to be certified by an RPEQ as required by the Manual.  The requirement for engineering services to be undertaken or directly supervised by RPEQs under the PE Act is extended by the requirement of the Manual to include RPEQ certification of the final result.

This article does represent the views of any organisation.  It has been written from the perspective of professional engineers’ experience and observations of change in regulatory context and expectations of significance to professional dam engineers.