River regime

River regime can describe one of two characteristics of a reach of an alluvial river:

The latter is described by the fact that the discharge through a river of an approximate rectangular cross-section must, through conservation of mass, equal

Q = \bar{u} b h

where Q is the volumetric discharge, \bar{u} is the mean flow velocity, b is the channel width (breadth) and h is the channel depth.

Because of this relationship, as discharge increases, depth, width, and/or mean velocity must increase as well.

Empirically-derived relationships between depth, slope, and velocity are:[1]

b \propto Q^{0.5}
h \propto Q^{0.4}
u \propto Q^{0.1}

Q refers to a "dominant discharge" or "channel-forming discharge", which is typically the 1–2 year flood, though there is a large amount of scatter around this mean. This is the event that causes significant erosion and deposition and determines the channel morphology.

The variability in discharge over the course of a year is commonly represented by a hydrograph with mean monthly discharge variations plotted over the annual time scale. When interpreting such records of discharge, it is important to factor in the time scale over which the average monthly values were calculated. It is particularly difficult to establish a typical annual river regime for rivers with high interannual variability in monthly discharge and/or significant changes in the catchment's characteristics (e.g. tectonic influences or the introduction of water management practices).

Classification

There are three basic types of regimes (Pardé, 1955):

Simple regimes

Simple regimes can be nival, pluvial or glacial, depending on the origin of the water.

Glacial regime
Glacial regime of the Kander (Switzerland)

The glacial regime is characterised by:

It is found at high altitudes, above 2,500 metres (8,200 ft). Example: Rhône at Brigue.

Nival

The nival regime is similar to the glacial, but attenuated and the maximum takes place earlier, in June. It can be mountain or plain nival. The characteristics of the plain nival (example: Simme at Oberwi) are:

Pluvial

The pluvial regime is characterized by:

It is typical of rivers at low to moderate altitude (500 to 1,000 metres or 1,600 to 3,300 feet). Example: Seine.

Tropical pluvial

The tropical pluvial regime is characterized by:

Mixed régimes

Nivo-glacial
Nivo-pluvial

Example: Issole

Pluvio-nival

Example: Mississippi.

Complex regimes

The complex regime is characteristic of large rivers, the flow of which is diverly influenced by numerous tributaries from different altitudes, climates etc. The influences diminish extreme discharges and increase the regularity of the mean monthly discharge from upstream to downstream.

References

  1. 1 2 Luna B. Leopold, M. Gordon Wolman, John P. Miller. (1995). Fluvial processes in geomorphology. New York: Dover Publications. ISBN 0-486-68588-8.
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