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WHITE PAPER

Capacities of Stacks and Horizontal Drains in Storm Drainage Systems

The sizing for storm drainage systems are dependent upon flow capacity equations used to calculate velocities and flow rates in pipe conduits. One of the variables in the equations is the coefficient of roughness. This paper explores how the roughness of different types of material will change the computational results that are dependent upon the roughness coefficient of the pipe.

The sizing for storm drainage systems has been committed to plumbing codes with mandatory sizing tables for engineering design. Although the plumbing codes allow for deviation based on recognized engineering practices and equivalency of effectiveness, the designs of storm drainage systems generally default to the plumbing codes. The sizing tables in the U.S. plumbing codes stem from either the ASA A40.8 National Plumbing Code from 1955 (ASME, 1955) or the computation from the National Bureau of Standards (NBS) Monograph 31 (Wyly, Eaton, 1961). It is probable that the National Plumbing Code was influenced by the research of Dawson and Kalinske, University of Iowa, published in Bulletin 10 that referenced the Manning equation (Dawson, Kalinske, 1937).

The original research of the NBS and the University of Iowa dated from the time when cast iron was the common pipe material for both the sanitary and storm drainage systems. Since then, plastics have emerged as a dominant material for plumbing systems. Therefore Section 1.0 begins with identifying the allowable types of material for storm drainage systems in three plumbing codes used in the US. This study considers how the varying roughness of the different types of material will change the computational results that are dependent upon the roughness coefficient of the pipe.

Section 2.0 examines the equations used to compute capacities for vertical and horizontal drains and assesses the outcomes. Better understanding of the equations will facilitate decision­making for sizing calculations. Three equations are introduced in this study, one for vertical drain capacity and two for horizontal drain capacity. The roughness coefficient for each equation is considered for two types of pipe material, PVC and cast iron, showing comparative results. Annular ratios for vertical stacks and flow depth for horizontal drains are also examined to consider the effect on pipe capacity.

Section 3.0 discusses the comparative differences in calculating flow rates when changing the roughness coefficient for two types of material. The results of this analysis and evaluation demonstrate the need to consider the roughness of pipe material when sizing storm drainage systems. 

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