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

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.

Integrated Hydrological-Hydraulic Model for Flood Simulation in Tropical Urban Catchment

“…The estimation of the rainfall-runoff model can be done using a modified rational method. The modified rational method is an advance from the rational method by generating the hydrographs that related to rainfall-runoff events in a particular area [44]. Globally, the modified rational method was applied to determine 1400 rainfall-runoff events at 80 watersheds in Texas by developed hydrographs [45].…”

Assessing Redundancy in Stormwater Structures Under Hydraulic Design

“…This indeed conveys a pivotal message that introducing extra capacity single-handedly without determining proper additional pipes positions in the system might even aggravate the hydraulic performance. From Figure 3a, it can be seen that further removal of pipes only up to the 10th loop in downstream could maintain the flood generation below 630 m 3 . This means that the 10th additional pipe in downstream can be also considered as a critical element since further pipe eliminations tremendously increased the surface flooded amounts (Figure 3a).…”

“…Though a comprehensive trade-off analysis is still required when it comes to the selection of optimal network configuration, Figure 4, which integrates construction costs and hydraulic response, is expected to assist engineers to make a more informed decision about the final layout/structure of the stormwater network. As shown in Figure 4a, having at least 10 loops in downstream up to 23 loops (networks 23 to 10) can provide the most efficient network configuration in terms of dispersing high runoff amounts (generating flooded volumes with minimum 618 m 3 to maximum 626 m 3 ) and total construction costs between €2,040,203 and €1,938,491 (shown in black box in Figure 4a). With respect to both flood behavior as well as total costs, these configurations can be determined as the most reliable solutions once one is willing to design a loop-branch-combined stormwater system to more desirably cope with functional failures.…”

“…Nevertheless, as aforementioned, the network number 26 with 26 additional alternative flow pathways, having a total cost equal to €2,070,702, might be even chosen as a good compromise candidate between costs and flood reliability compared with all other networks and regarding all scenarios thanks to more efficiently discharging the increased rainfall amounts out of the system. minimum 618 m 3 to maximum 626 m 3 ) and total construction costs between €2,040,203 and €1,938,491 (shown in black box in Figure 4a). With respect to both flood behavior as well as total costs, these configurations can be determined as the most reliable solutions once one is willing to design a loopbranch-combined stormwater system to more desirably cope with functional failures.…”

“…As far as hydraulic dimensioning is concerned, the storm sewer design of urban catchments is typically based on the steady-state methods, such as the classic Rational method due to its simplicity, quick implementation and few input parameters. However, these methods neglect complex flow conditions such as backwater effect and pressurized flow, and tend to imprecisely assess maximum design flows [3], especially when it comes to large networks as well as high intensity precipitation events. Furthermore, a looped structure cannot be assessed with such steady-state methods.…”