Bridge Manual Waterway Design: An Update

Bridge and major culvert (>3.4m² waterway area) design in New Zealand has long used the NZ Transport Agency’s Bridge Manual, currently in its 3rd edition. Within it, Section 2.3 deals with waterway design which in turn heavily references the Austroads Waterway Design Guide – A Guide to the Hydraulic Design of Bridges, Culverts and Floodways for understanding hydraulic conditions, and Bridge Scour (Melville & Coleman, 2000) for the assessment of scour and design of countermeasures.

The development of design guidance for minor culverts and stormwater systems in the Transport Agency’s Highway Structures Design Guide revealed updated design practices that were also relevant to major culverts and bridges. Furthermore, the Waterway Design Guide has now been withdrawn and replaced by the Austroads Guide to Road Design Part 5B: Drainage – Open Channels, Culverts and Floodways and more recently, the Austroads Guide to Bridge Technology Part 8: Hydraulic Design of Waterway Structures. In response to this Beca, for the Transport Agency, have reviewed Section 2.3 of the Bridge Manual and many revisions are proposed that will affect the way that bridges and major culverts are designed for waterway actions in the future.

The purpose of this paper therefore is to report on the outcomes of the review including the issues identified and the updates proposed. The updates proposed will:

i. Include references to recent Austroads documents and fish passage and debris assessment guides

ii. Emphasise the influence of design constraints derived from environmental and river management practices

iii. Recognise the relevant local/regional council’s role in setting levels of service, determining what is an acceptable effect and determining hydrological and hydraulic methods (including modelling)

iv. Expand the hydrological clauses for the use of HIRDS rainfall data, the use of maximum probable development conditions and clarify runoff coefficient/time of concentration references

v. Clarify climate change requirements, including sea level rise

vi. Include further performance requirements for major culverts

vii. Recognise the importance and influence of downstream boundary conditions on the performance of a design, and

viii. Include guidance for scour assessments where the bed materials are cohesive.

2019 Stormwater Conference

During the review, several residual issues were identified that the Transport Agency is taking under consideration, including:

i. The need for a stand-alone waterways design guide rather than cross-referencing multiple guidelines and documents

ii. Removing the Serviceability Limit State (SLS 1) requirements pertaining to rip rap design

iii. Confirming the Transport Agency’s expectation of scour protection under Ultimate Limit State (ULS) conditions

iv. Interpretation of the 2018 Ministry for the Environment climate change guidance for use in design

v. Inclusion of guidance for bridges subjected to coastal and marine conditions

vi. Inclusion of requirements relating to waterway diversions, and

vii. Preparing a technical specification for rock rip rap including materials testing and standard size/mass grading envelopes.