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Break point

Point at which an intersecting line crossing the main reach defines a closure of the uphill basin.


Critical inclination

Free flow surface height with an assigned capacity Q for which the specific energy E is in respect to the bed is minimal.


Critical rain height

The rain height obtained from the rain model for a duration equal to tc (time of concentration).


Effective rainfall

The rain water quantity that remains available at the surface after the real evapotranspiration loses.
The effective rainfall PE are given by the deduction between rainfall and real evapotranspiration ETR.


Free flow

The hydro-current that flows in natural streams or artificial canals.


Free rise

The height from riverbed to surface that the current can reach without overflow.


High flood hydrogram

Graphic representation of the progression of flow in a given break point, related to given recurrence interval.


Hydraulic radius

The ratio between the area of liquid A and the wetted surface perimeter.


Hypsographic curve

The representation on the Cartesian axis of the medium heights hi (the Y axis) of a basin, relative to two contiguous elevation levels, and the partial axis Ai (the X axis), delimited by two  elevation levels that above to hi.



The infiltration I represents the water quantity that penetrates the soil surface feeding the underground water flow. The infiltrated water quantity reaching the water table surface is called effective infiltration IE.


Outflow Coefficient

The ratio for a given break point closure between the value d, outflow and the rainfall's quantity.


Permanent motion state

The gradually varied permanent flow is the one encountered in a free current with gradual variations of the cross section. Naturally, capacity remains constant while speed and cross section vary along the axis of flow.


Potential and total evapotranspiration

The evapotranspiration and the water loss that take place through the physical process of evaporation and the biological one, of plants  (perspiration). The water quantity consumed by atmospherically evaporation (potential evaporation)and plants' activity (perspiration) is indicated to be potential evapotranspiration (ETP). When the water loss through  evapotranspiration is inferior to RFU (available hydro reserve) we're dealing with real evapotranspiration ETR.


Rain model

For a given recurrence interval, this represents by the relation between height of rain  h and time t. (duration)


Recurrence interval

The period within which intensity I is on average, exceeded only once –  I being the maximum capacity
The Recurrence interval’s values for some hydraulic structures are shown in the table below:




Recurrence interval [years]

Bridges and river dykes


Stream dykes






Urban sewer


Tubular footbridge and small river footbridge


Underground street passage


Gutter and main street groove



Riverbed at high inclination.

Riverbed where uniform current is  fast.


Riverbed at low inclination.

Riverbed where is verified that the uniform current is slow.



Streaming, R, is the portion of PE that feeds the surface efflux QS and is conveyed into the surface flow network.


Time of Concentration

Longest time of travel for a drop of water to reach the watershed outlet


Uniform motion

A flow can move in uniform motion state within a break point closure if the inclination of the riverbed is equal to that of the water surface.


Wetted surface

The cross section of the reach's surfaces along which the liquid flows.