Design storms for urban drainage design' J. MARSALEK Hydraulics Division, National Water Research Institute, Burlington, Ont., Canada L7R 4A6 AND W. E. WATT Department of Civil Engineering, Queen's University, Kingston, Ont., Canada K7L 3N6 Received November 25, 1983 Revised manuscript accepted May 7, 1984 The design storm concept is well established in Canadian urban drainage practice, but appropriate use is hindered by an incomplete definition of design storms and their applications. To remedy this situation, it is recommended that design storms be described for various regions and a wide range of durations and return periods; these storms should be based on local Atmospheric Environment Service (AES) rainfall data, given for both the rational method and hydrograph model applications, and supplemented by specifications of the computational procedure and normal antecedent conditions. Such design storms would produce peak flows of approximately the same return period as that of the design flow. None of the existing design storms has all these features but an acceptable set of design storms could be developed using existing Canadian data. Key words: design storms, urban drainage, stormwater, hydrological design, precipitation, runoff computations. L'utilisation de pluies thkoriques dans la conception d'ouvrages reliCs i I'hydrologie urbaine est maintenant bien Ctablie au Canada. L'efficacitC de cette technique est toutefois limitCe parce qu'on a pas encore parfaitement dCfini ce que doit Ctre une pluie thkorique et i quelles fins elle doit 6tre utilisCe. Afin de remCdier i cette situation les auteurs suggerent d'employer des pluies thCoriques differentes pour les diverses rCgions du pays. DCveloppCes i partir de donnees locales de pricipitation recueillies par le Service Canadien d'Environnement AtmosphCrique, elle devront couvrir des dukes de precipitation trks variCes et satisfaire une grand nombre de pCriodes de retour en plus de prCdire un dCbit de pointe pour la mCme pCriode de retour que le dCbit de design. On devra suggCrer les mithodes de calcul appropriCes lesquelles devront pouvoir tenir compte des conditions existantes avant la precipitation. Aucune des pluies thtoriques actuellement utilisies ne rencontre tous ces objectifs mais il est probable que parmi un ensemble de pluies qui pourrait Ctre suggCrC il y en ait toujours une qui soit satisfaisante dans une rCgion donnCe et dans des conditions particulitres. Mots clPs: pluie thCorique, drainage urbain, ruissellement urbain, conception hydrologique, prkcipitation, Cvaluation du ruissellement. [Traduit par la revue] Can. J. Civ. Eng. 11, 574-584 (1984) Introduction The specification of a precipitation input for design purposes is well established as a criterion for the design of hydrotechnical structures. Early examples were the 'standard project storm' and the 'probable maximum precipitation' in the United States and, more recently, Hurrican Hazel and the Timmins rain (McMullen 1962) in Ontario. Often the precipitation input was linked implicitly to an antecedent soil moisture state and a computational technique for determining the design flood. In the area of urban storm drainage, a common example is the specification of a rainfall input of given return period and inlet time together with a computational technique, the rational method, and, implicitly, antecedent soil moisture conditions in the runoff coefficient. With the advent of stormwater modelling techniques, many urban design storms have 'This paper was presented at the 6th Hydrotechnical Conference, Ottawa, Ontario, June 1983. been proposed but none has received the acceptance of the early examples listed above. Therefore, an urban design storm, or more precisely an urban design storm event, can be defined as a some- what arbitrary specification of the temporal distribution of a precipitation input, which is used together with specified antecedent soil moisture conditions and a computational technique to determine design flows. Although the concept of an urban design storm is straightforward, a lack of accurate definition of design storms and uncertainty regarding areas of application have resulted in misuse. In this paper, attempts will be made to clarify the areas of application of design storms, to critically review design storms in common practice, and to recommend necessary features of urban design storms for Canada. Before doing this, however, consider whether the design storm concept should be employed at all. The justifications for using design storms are as follows. (i) Frequency analysis of actual flow records is not feasible in urban drainage projects because such records Can. J. Civ. Eng. Downloaded from www.nrcresearchpress.com by 216.208.156.69 on 06/05/13 For personal use only.