Glossary¶
Hydraulic invariance — The principle whereby the peak flow discharged from an area must not increase as a result of its transformation.
Hydrological invariance — Extension of the principle to the total volume, not just to the peak flow.
Runoff coefficient (φ) — Fraction of the rainfall that becomes surface runoff. It is about 0.9–1.0 for impervious surfaces, 0.2–0.3 for pervious surfaces.
Weighted runoff coefficient (φpond) — Mean of the φ values of the individual surfaces, weighted by area.
Weighted area (Spond) — Equivalent impervious area, equal to Stot × φpond.
Rainfall depth-duration-frequency (DDF) curve — Relationship between rainfall depth, duration and return period, in the form h = a·tn.
Return period (T) — Mean interval between two events exceeding a given threshold. An event with T = 50 years has a 1/50 probability of occurring in any year.
GEV — Generalised extreme value distribution, used to derive the curve parameters as a function of the return period.
Hyetograph — Time distribution of rainfall intensity during the design event.
Position coefficient (r) — In the Chicago hyetograph, the relative position of the peak within the duration.
Rainfall losses — Subtraction of losses (infiltration, interception, storage) from the gross rainfall, to obtain the net rainfall.
Curve number (CN) — Parameter of the SCS method that summarises permeability, land use and antecedent conditions.
AMC — Antecedent moisture condition of the soil: I dry, II average, III wet.
Time of concentration (tc) — Time taken by water falling at the hydraulically most remote point to reach the outlet section.
Hydrograph — Time distribution of the flow at the outlet section.
Nash model — Representation of the catchment as a cascade of n linear reservoirs with storage constant K.
Attenuation — Effect of a storage facility that temporarily accumulates water and reduces its outflow peak.
Discharge limit (Qu,lim) — Maximum flow allowed into the receiving water body, imposed by the regulation or by the operating authority.
Head (H) — Water depth above the discharge device, which determines its flow.
Specific storage volume (w) — Storable volume per unit area, expressed in m³/ha, distinguished between pre- and post-development.
Hydraulic criticality zone — Classification of the territory (A, B, C) which in Lombardia determines the minimum volume and the unit discharge limit.
Development class — Size-based classification of the development, which determines the calculation method proposed by the regulation.
Jurisdiction profile — In HID, the set of rules, constraints and parameters of a country or a region.
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