Hydraulic Calculation
Properly draining rainwater is no longer just a technical necessity, but a true design challenge, accentuated by climate change and the intensification of extreme weather events.
Today, stormwater management is an integral part of the design of public works—such as roads, ports, and airports—as well as private contexts, from parks to residential and commercial centers.
Based on the installation location, the reference technical standard EN 1433 defines which channels to use according to their intended use and expected loads (from pedestrian traffic to airport vehicles). However, understanding which section to choose and how to manage water discharge within the drainage network requires specific expertise and advanced calculation tools.
The Technical Approach: How to Calculate Hydraulic Flow Rate
The hydraulic flow rate (Q), expressed in liters per second (l/s), represents the quantity of water flowing through the channel. This flow rate varies spatially as a function of flow inputs (rainfall) and discharges. Inputs are generally distributed along the length of the collector based on the contributing catchment area, local rainfall, and the absorption characteristics of the drainage surfaces, while discharge sections, where outflow is assumed to be free, can be introduced at one or more points along the collector.
Stormwater collection channels are sized and verified by solving the equations that express the conservation of mass, energy, and momentum, under the assumption of steady flow and incompressible fluid, which take the following form:
where s (m) is the longitudinal coordinate running along the individual channel section in the direction of the flow itself and q (l/s/m) is the distributed flow rate assumed to flow within it.
A fundamental parameter in the calculation is the material roughness: the greater the internal friction of the channel, the greater the energy loss of the fluid. This is why the choice of construction material directly affects the efficiency of the system.
Greenpipe Software for Hydraulic Sizing
To provide a practical, precise, and scientifically validated tool, Greenpipe has developed—in collaboration with the University of Padua—a calculation model for hydraulic flow rate and drainage network sizing.
Based on the data provided by the client—size of the drainage area to be treated, length of the channel section, type of surface, precipitation (the result of a previous hydrological study), slopes, and any upstream inflows—our team is able to calculate the maximum filling level of the channel as a percentage of the section height, as well as the flow rate trend, current velocity, and bed shear stress “τ”.
All results are presented in the form of graphs for clear and immediate interpretation.
For the same “study,” several valid design solutions can coexist: from smaller section channels with multiple distributed discharges to larger channels with a final discharge.
The choice will depend on both technical considerations related to water velocity within the channel (which must remain within certain limits) and bottom shear stress (which measures the self-cleaning capacity of the channel), as well as economic considerations and specific characteristics of each project.
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