Peak Flows and Stormwater Networks Design—Current and Future Management of Urban Surface Watersheds

António Freire Diogo,José Antunes Do Carmo

doi:10.3390/w11040759

Abstract

Stormwater urban drainage systems are typically designed in open channel flow. Pipe sewers must have enough capacity to transport maximum design flows for a given frequency of the project rainfall. The classic rational method or related procedures that are based on rational approaches are still currently used to a great extent, particularly for small urban drainage basins, and the pipes are frequently designed in uniform steady flow. Numerical integration of Saint-Venant equations for one-dimensional gradually varied unsteady flow allows the computation of waves’ progression along the pipes for given input surface hydrographs. This paper presents a comprehensive, systematic, simple, and original comparison between the peak flows that are achieved through simulation in unsteady flow using an implicit complete dynamic model, developed in the Laboratory of Hydraulics, Water Resources and Environment of Coimbra University, and those that are obtained with the classic rational method along urban drainage networks. Boundary conditions and some approximations typically considered in the methodologies are analyzed in detail. Classic rational approaches may underestimate the peak and design flows. Practical recommendations for the system design phase when rational approaches are used are also proposed. The need for indispensable requirements for suitable urbanization rules, intelligent management of surface runoff in urban basins, and control measures for the reduction of peak flows entering existing networks is confirmed and reinforced.

Highlights

Much research has been and is being currently conducted all over the world in the area of urban drainage systems
Conclusions response, a lower of concentration, and a higher intensity of precipitation must be considered, together with the use of the upper limit for the estimated value of C that is normally established in the Maximum design flows in stormwater networks that are designed in open channel flow using technical literature
The classic rational method for a given frequency of the project rainfall should be generally increased principles thepurpose, rational method arebound normally limited to small drainage basins

Summary

Introduction

Much research has been and is being currently conducted all over the world in the area of urban drainage systems. Issues, or measures related to the management or control of stormwater systems have been investigated, ranging from flood incidence analysis [2], or the evaluation of urban hydrologic changes [3], up to flood hazard studies in metro systems [4,5]. Areas of concern include: the reduction of peak flows entering the networks, for example by considering bioretention systems [6,7,8]; detention ponds associated to management strategies [9] and rainwater harvesting systems [10], with potential reuse of the stored rainwater; the creation of conditions at the level of the urban basins allowing controlled surface runoff, similar to those existing prior to urbanization [11,12]; the increase of permeable surfaces in the urban basins, such as through the development of green infrastructures and green roofs [13]; and the mitigation or possible treatment of some pollutants associated with the washing of streets and other impervious surfaces [14].

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Conclusion