By Paul Imm
Due to factors like climate change and intensified urban development, insurance claims in Canada from water damage exceed claims from fire, theft, and other natural disasters combined. This is compounded by the fact that the average cost of each home insurance claim is rising significantly because more homeowners are furnishing their basements as a living space.
Because of this, the Insurance Bureau of Canada recognized the need to better understand their exposure from municipal-side factors and they were motivated to develop the Municipal Risk Assessment Tool (MRAT). This pilot project assesses the impact of severe weather events on urban drainage systems and identifies where potential municipal infrastructure vulnerabilities may exist.
One aspect of sewer and culvert design which may come under scrutiny is how the Manning formula is used to determine a pipe size that is sufficient to carry a design peak flow. More specifically, design engineers need to use an appropriate flow resistance coefficient, or Manning’s n roughness coefficient. Many jurisdictions across Canada seem to agree that a Manning’s n Value of 0.012 or 0.013 is appropriate for all smooth wall pipe. This includes concrete, polyvinyl chloride (PVC) and high-density polyethylene (HDPE) pipe with a smooth interior surface.
Surprisingly, a small number of Canadian municipalities are willing to ignore the best practices adopted by many of these jurisdictions and still allow the use of a Manning’s n Value as low as 0.009 for plastic pipe.
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Design engineers must keep in mind that the Manning formula, developed in 1890, is an empirical relationship with a roughness coefficient n Value that actually varies with pipe size, slope of the pipe, and flow depth. Engineers must also recognize that the Manning’s n Value of 0.009 is derived from laboratory studies that examined small pipe sizes, or is based on clean water flowing through short sections of new pipe, not in-service sewers.
Similar laboratory studies with concrete pipe have also resulted in Manning’s n Values of 0.009. However, the concrete pipe industry has traditionally promoted the use of a more conservative n Value to account for the differences between laboratory testing and actual installed conditions.
Engineers familiar with pipe design know that flow resistance in a pipe is significantly affected by more factors than just the pipe material. Based on lab experiments, field studies, and hydraulic theory, it is evident that all hydraulically smooth wall pipes should be designed with the same Manning’s n Value, regardless of pipe material.
For a simplified and conservative method of estimating pipe capacities and flow velocities in actual working conditions, a constant Manning’s n Value of 0.013 for all smooth wall pipe materials will account for the head losses at manholes, pipe joints and fittings, and alignment changes. Sediment deposits and slime buildup inside an in-service pipe also result in the roughness coefficient being essentially the same for all pipe materials.
Any jurisdiction that still allows the use of an unrealistically low Manning’s n roughness coefficient for a sewer or culvert design may be unintentionally putting its own infrastructure and residents at risk of future sewer backups and flooded basements.
Paul Imm, P.Eng., is with the Canadian Concrete Pipe & Precast Association. This article appears in ES&E’s February 2016 issue.
