The reference water levels are used on

inland waterway A body of water, such as a river, canal or lake, is navigable if it is deep, wide and calm enough for a water vessel (e.g. boats) to pass safely. Such a navigable water is called a ''waterway'', and is preferably with few obstructions against dir ...

s to define a range of water levels allowing the full use of the waterway for navigation. Ship passage can be limited by the water levels that are too low, when the

fairway Fairway may refer to: * Fairway (golf), part of a golf course *Fairway (navigation), a part of a water body with navigable channel *Fairway (horse), a Thoroughbred racehorse *Fairway, Gauteng, South Africa *Fairway, Kansas, United States *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 In the water transport, the navigation season (or shipping season) is a period of the year during which a waterway is open for navigation. The opening and closing dates of the season are determined annually by the waterway administration to accommod ...

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 ves ...

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 ; french: Rhin ; nl, Rijn ; wa, Rén ; li, Rien; rm, label=Sursilvan, Rein, rm, label=Sutsilvan and Surmiran, Ragn, rm, label=Rumantsch Grischun, Vallader and Puter, Rain; it, Reno ; gsw, Rhi(n), including in Alsatian dialect, Al ...

) or, alternatively, corresponds to 5-6% of the ice-free period on European rivers (6% on

Danube The Danube ( ; ) is a river that was once a long-standing frontier of the Roman Empire and today connects 10 European countries, running through their territories or being a border. Originating in Germany, the Danube flows southeast for , ...

). 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 fl ...

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 Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

effects, usually at the levels observed 1% of the time) and the effect of water level changes due to the operation of

locks Lock(s) may refer to: Common meanings * Lock and key, a mechanical device used to secure items of importance * Lock (water navigation), a device for boats to transit between different levels of water, as in a canal Arts and entertainment * ''Lo ...

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 river level. Weirs are also used to control the flow of water for outlets of l ...

Equivalent water level

In Germany, an equivalent water level (german: Gleichwertiger Wasserstand, GlW) is the value of a

stream gauge A stream gauge, streamgage or stream gauging station is a location used by hydrologists or environmental scientists to monitor and test terrestrial bodies of water. Hydrometric measurements of water level surface elevation (" stage") and/or v ...

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 height The ...

(ideal minimum channel depth, TuGlW).

References

Sources

* * Nautical terminology {{water-transport-stub