Stormwater Conference 2024

T. Strang, Wellington Water Ltd


For manholes to be functional yet cost-effective means they need to be large enough but not over-sized. The ‘design art’ presented here starts with the basics of structural integrity, referring to agreed national approaches then taking these forward to develop updated formulas that allow accurate spreadsheet-based sizing of manholes.

A critical design criterion is the minimum radius of manhole benching – which varies over our 45 national design codes from a maximum of 3.0 x internal pipe diameter in Auckland to an "anything goes if you can build it” approach. Adopting a ‘high’ value for the minimum radius will reduce losses but increases construction costs markedly. For this paper, expected hydraulic losses are analysed based on research by Shukry (1950), Ackers (1959) and Frost (2006). Losses are compared to current NZ and international approaches, allowing a recommendation for a cost-effective balance between performance and cost.

National Standards may be on-hold for now, but the designs must go on and more than ever it is critical that infrastructure is affordable yet fit-for-purpose. A robust manhole design methodology is something we all need to have in our toolboxes, an easy first-step for alignment on future joint standards and more of an ‘art’ than might be first appreciated.