The reference water levels are used on
inland waterway
A waterway is any navigable body of water. Broad distinctions are useful to avoid ambiguity, and disambiguation will be of varying importance depending on the nuance of the equivalent word in other ways. A first distinction is necessary bet ...
s to define a range of
water level
Water level, also known as gauge height or stage, is the elevation of the free surface of a sea, stream, lake or reservoir relative to a specified vertical datum.
Over long distances, neglecting external forcings (such as wind), water level ten ...
s allowing the full use of the waterway for
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...
. Ship passage can be limited by the water levels that are too low, when the
fairway might become too shallow for large ("target", "design") ships, or too high, when it might become impossible for the target ships to pass under the bridges. The goal of establishing the reference water levels is to balance the safety of navigation and economic value of the waterway
(for example, increase of the low level shortens the
navigation season but allows the use of larger ships). Reference levels are set up based on statistics obtained from the multi-decadal observations (typically 30 years).
Free-flowing rivers
Free-flowing rivers use the low navigable water level (also lowest navigable water level, LNWL) as a low reference water level. When the river is at or above LNWL, the ships of the target
draft
Draft, the draft, or draught may refer to:
Watercraft dimensions
* Draft (hull), the distance from waterline to keel of a vessel
* Draft (sail), degree of curvature in a sail
* Air draft, distance from waterline to the highest point on a v ...
can use its fairway. LNWL is usually defined as the water level that the river surface stays mostly above at the times when the river is navigable (e. g., during the ice-free season in Europe), so that the river level on average stays below the LNWL only for a fixed small portion of a year. For example, in Europe the limit of time below LNWL is 20 ice-free days per year (e. g., on
Rhine
The Rhine ( ) is one of the List of rivers of Europe, major rivers in Europe. The river begins in the Swiss canton of Graubünden in the southeastern Swiss Alps. It forms part of the Swiss-Liechtenstein border, then part of the Austria–Swit ...
) or, alternatively, corresponds to 5–6% of the ice-free period on European rivers (6% on
Danube
The Danube ( ; see also #Names and etymology, other names) is the List of rivers of Europe#Longest rivers, second-longest river in Europe, after the Volga in Russia. It flows through Central and Southeastern Europe, from the Black Forest sou ...
).
The highest navigable water level (HNWL) is defined as a water level that is exceeded for only a few days a year (usually 1%).
Regulated rivers, canals
The low reference level is usually not defined for
regulated rivers and
canal
Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...
s, as in these cases the depth of the navigation channel is guaranteed by design (sufficient margins are covering the variations of the flow).
The high reference water level (MHW) is sometimes set to accommodate the short-term variations (for example,
tidal effects, usually at the levels observed 1% of the time) and the effect of water level changes due to the operation of
locks and
weir
A weir or low-head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the water level. Weirs are also used to control the flow of water for outlets of l ...
s.
Equivalent water level
In Germany, an equivalent water level (, GlW) is the value of a
stream gauge that corresponds to the nominal
fairway depth
Fairway is a part of a water body (bay, harbor, river) containing the navigable channel (also known as a ''ship channel''), a route suitable for ships of the larger size (with draft closer to the draft limit).
Fairway depth, width, and heigh ...
(ideal minimum channel depth, TuGlW).
See also
*
Chart datum
*
Vertical datum
In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built stru ...
References
Sources
*
*
Nautical terminology
Navigation
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