How exactly is a water level

Flow

The Flow or also "Water flow"(Symbol: Q) is the amount of water that flows through a certain cross-section, e.g. a canal or a river bed, per unit of time. The flow is specified in m³ / s or (e.g. in the case of pipelines) in liters per second Its value ("today is 4.5 cubic") can be used to assess the paddling ability of a river.

In contrast to the runoff (see below), the flow is independent of the allocation to a catchment area. It is calculated from the cross-section of the river or stream bed (which must of course be known) at different water levels and the flow velocity at the respective water level. This speed is measured with a kind of propeller that is lowered into the water from above. At levels of this kind, a line is stretched across the river between two steps on the right and left bank, far above head height, on which the propeller hangs ("cable crane system"). With a winch system it can be cranked over the water or back to the bank, and at the same time it is possible to lower it into different water depths (down to the bottom). The measuring stick for the corresponding level is painted on one of the steps, and up on the slope there is the little station house with the measuring transducer.

Another possibility is the measurement at a weir crest, whereby the overflow height serves as a measure of the amount of water overflowing. Such a weir separates the Schmalen Luzin from the Carwitzer See in the Feldberger Seenlandschaft (Mecklenburg-Western Pomerania) in Carwitz. Weirs measure very precisely with simple means, but are at risk from paddlers who do not want to carry around, but simply pull their boat over the steel edge of the weir crown. However, this edge was measured with millimeter precision during construction. If you drag festival boats over them, the steel plate will sink over time and the series of measurements is over!

The flow rate is not to be confused with the Drain or "Discharge height"(Symbol: R), which Wikipedia puts in a pot with the flow. It describes the amount of water that flows over a certain area (the" catchment area ") per unit of time. The runoff includes the surface water as well as the seepage into the groundwater But not evaporation. Metaphorically speaking, the runoff is the amount of water that flows or seeps from the field into the river, while the flow means the amount of water that has arrived in the river. The runoff is given in liters per square meter (1 l / m² = 1 mm water height = 10,000 l / ha = 1000 m³ / km²), or rather, estimate or calculate it as precisely as possible.The two quantities are closely related (the flow is a subset of the runoff), but because If it does not rain evenly in the catchment area and the soil retains different amounts of water, the runoff says nothing about the paddling of a body of water.

level

With level (actually "water level") one means the height of the water level of a body of water. Water levels are given in centimeters or meters. Their indication ("Neuendorf gauge is 32 cm today") is also an indication of the paddling ability of a body of water. In addition, the current depth of the fairway is determined with the aid of the water level or a decision is made via a high water alarm.

Attentive water sports enthusiasts often notice an upright bar level, an inclined level leaning against the bank wall or the graduation on a bridge pillar. Traditional level slats are made of cast or wrought iron and, in order to be recognized from the ship, are in black and yellow or black and white in color and 2 or 5 cm graduation. There are also other measuring methods such as floats, compressed air probes, ultrasound, etc .; Wikipedia explains some of them under Water Level Measurement Technology. Some levels indicate their value on a large digital display.

Each level relates to a zero point that is specially defined for each level. In order to obtain only positive values ​​for the water level, this "level zero point" (PNP) or "level zero" is usually measured so that it is below the lowest possible low water level ("positive level"). When the water level rises, the number increases here and falls when the water level falls.

These zero points were of great importance until the 1950s, as their height was carefully determined and could therefore be used as a reference point for further land surveys. They were chosen so "that the level can maintain the altitude given to it as long as possible for all times" [1]. (Only when the entire ground subsided in mining areas, as in the Ruhr and Zwickauer Mulde, or when the erosion deepened the river bed faster than expected, as in the Rhine, was it decided to change a single level zero.)

Levels are permanently installed and are serviced every spring. So you could actually establish a relationship between water level and flow for a location. But rivers (even canalized) change their depth over the decades. Each river does this in its own way: if it carries little sediment and if many meander loops have been pierced during straightening, the river slowly flushes its bed deeper over the years, as happens with the Elbe and Rhine. Other rivers, such as the Oder, carry a lot of sediments with them, so that the river bed rises slowly over time. Still other, natural rivers reshape their beds with each flood, wash away here and deposit there again, like the Tagliamento. If the river bed has been removed, level values ​​below zero can occur at low tide, so that level information with a minus in front of it comes about ("minus level"). With them it should be remembered that the numerical value increases when the water level falls. Conversely, the number becomes smaller when there is flood.

In other places again, a strong flood can push a sand or gravel bank in front of the gauge so that it lies dry at normal or low water levels and then always shows the same value (namely the height of the dry river bed) and only works during floods, when the place is flooded again. (At Tagliamento, for example, you can see both). For this reason, level measurements usually do not (or no longer) indicate the actual depth of the water at the point.

So wrote pleasure paddlers in the folding boat forum on July 29th. 2019 on the Saale: "Incidentally, the reference values ​​for the hall level have apparently changed. At the moment, Rudolstadt displays 44 cm water level and 4.29 cubic meters per second flow rate. The river there used to have 8-10 cubic flow rate at 44 cm water level So sediments washed up by floods, which changed the display. I am writing this because 44 cm in Rudolstadt is stated in earlier postings and in the wiki as a well paddled level. This is apparently no longer the case.
For this reason, among other things, I always orient myself, if possible, on the flow rate and not on the level measured with the measuring stick. In contrast to the water level on the permanently installed measuring rod, the measured flow rate always remains objectively exact, regardless of whether the river removes material at the point of the river bed or washes up (which often happens at level measuring points). "

Guideline values ​​for some rivers can be found in the DKV guides, mostly in the form of gauge heights, sometimes in the form of flow values ​​("can only be driven when water is released from the dam xy, required 8-12 m³ / s") or experience ("if that is below the The flat section of the bridge is mobile, the water flow is sufficient for the entire route "). The level service of the Canoe Association of North Rhine-Westphalia worked like this in the 1980s. "The authorities publish the daily water levels of all levels in Germany in the yearbooks. If the minimum water level is known for a certain body of water, a mathematical comparison is used to determine which water level is then at a nearby call level. We call such a reference level Thing, because we have just put it in relation. The whole thing is a bit more complicated to determine how it sounds here, but it works anyway. So that no one from Castrop-Rauxel drives to Prüm and only finds a damp forest path, we have to go to the Calculations built in a "fear surcharge" of 5 cm. Of course, everything only works if there is a corresponding general weather situation, a thunderstorm cannot be used in this way. Below the minimum level given in our table, the streams are still navigable on some days, since everything is just a comparison calculation is. " (REUTER 1982) Today, the water level is indicated on the water level page of the association linked above with a colored marking (red-green), which indicates the navigability at this level.

In the case of tidal rivers, levels refer to a mean water level and fluctuate between positive and negative figures.

At this point the abbreviations of various water levels:

  • NNW = Lowest low water (lowest water level ever measured; is usually named with a date)
  • NW = Low water (lowest measured water level in a number of years)
  • MNW = Mean low water (average low water level, measured over a number of years)
  • MW = Mean water level (average water level measured over a number of years)
  • MHW = Mean flood (average flood level, measured over a number of years)
  • HW = Flood (highest measured water level in a number of years)
  • HHW = Highest flood (highest water level ever measured; is usually named with a date)

Equivalent water level

The skippers know how to deal with the change in the reference level values:

"The Rhine gauges are such a thing in themselves. At one point or another they actually indicated the existing water depth. In 1848, the Binger gauge was set at the same level as the Binger Loch level at that time. For example, the Bingen gauge showed 1.50 m , there were also 1.50 m fairway. In the meantime, however, the fairway has been deepened in various places, so that the original foundation is no longer correct. Today the levels are compared with the 'equivalent water level'. This level W. together with experience and measurement then gives the skippers as the unloading depth [2] to: Mainz level or Binger Loch + 80 cm for the Middle Rhine area. "(GARSKI 1962 [3])

Equivalent water levels (GlW, with "L" in the middle) were introduced on the Rhine in 1908 and are defined as "water levels that occur when discharges are determined to be equivalent along a stretch of river. They represent a low water level that has been around twenty on the Rhine over many years ice-free days in the year the respective reference levels are not reached. " (Wikipedia) The equivalent water level is therefore a reference value for assessing the depth of the fairway on the basis of selected levels that are important for shipping, the "reference level". Because of the different shape of the river bed, a GlW is only valid for a certain section of the river. The value of the equivalent water level for a section of the Rhine also guarantees a certain minimum fairway depth, which assumes a fixed value of between 1.90 m and 2.50 m for the respective section of the Rhine. The captain can quickly calculate the actual fairway depth based on the GlW value valid for the respective section of the route and the current water level: In Düsseldorf, for example, the GlW is 0.97 m, which guarantees a fairway depth of 2.50 m for the Rhine near Düsseldorf. If the level drops to 0.77 m, the guaranteed fairway depth is still 2.50 m - (0.97 m - 0.77 m) = 2.30 m.

The natural changes in the river bed of the Rhine force the Central Commission for Navigation on the Rhine (CCNR) to redefine the equivalent water levels in the individual river sections every ten years. The additional 80 cm for the Binger Hole mentioned by GARSKI in 1962 should therefore have long since been different.

The Elbe and Weser also have equivalent water levels, as do the Oder. Wikipedia has its own article on this.

experience

Specialists can calculate the flow velocity of a migratory river if they know the gradient, the flow, the flow cross-section and the roughness of the river bed and download a suitable calculation program. (After a tip from Spartans in the folding boat forum on May 27, 2009) If you want, you can use the formulas for flow meters and measuring weirs, for example. B. read here.

To answer the question "Can I still paddle on river X at this value?" The indication of the flow rate alone is useless without considering the character of the river and referring to it. This includes B. how fast the level of the respective river or stream falls or rises. "Flow rates give a clue if you have an idea of ​​the river's character, such as the width and shape of the river bed, gradient, flow speed, etc. [...] This is the same for water levels in cm, because these do not mean that the river is everywhere is the same depth or flows at the same speed. No level indication says anything about how diverse the river is. Only in contrast to these, the flow rate in itself represents an indication of the amount of water. "If [...] there are no clues, how the level information is to be interpreted, [...] I look at the flow rates and try to draw conclusions based on the character of the river. This approach has often been of practical use to me when no documented experience was available. I am always amazed that this option is not used if there are no other indications [...]. Personally, I use the flow rates to decide whether or not to go for rivers that I have never paddled before or that I have no documentation about experiences with gauges. Either I have already seen the river or I am trying to find out as much as possible about it (shape, size and structure of the river bed, gradient, etc.) [...]
However, if you neither have experience in estimating flow rates nor know the character of the river, the specification in cubic meters per second is not really of much use. So it is worth paying attention to and remembering the cm³ / s when driving on rivers. With increasing experience, you get a certain idea of ​​how many cubes could result in which conditions in rivers with a defined (!) Character. That as a constructive suggestion.
How the river will then be exactly at which point ... the adventure of exploring it yourself is not taken for anyone. And also the ability to cope with the respective conditions. "(Quotation for pleasure paddlers in the folding boat forum of 9.9. 2015)

"The flow rate gives a more objective point of reference. The level in cm is set arbitrarily and in itself has absolutely no meaningfulness. It always requires a reference point, i.e. empirical values ​​as a comparison. With some experience and knowledge of the body of water (gradient, width and character of the river bed ) the flow rate can give an idea of ​​how much water is in the stream even without empirical values ​​as a reference point. " (Quote for pleasure paddlers in the folding boat forum on July 4th, 2017)

Long-term paddlers of a river can use the information on the flow to estimate whether it is worth starting and provide information accordingly. If you do not know the nature of the river in question, the question can neither be definitively answered with flow nor with level height, the remaining uncertainty only disappears when you see the river. Quote from a paddling geo-ecologist: "The best is still the Stone Age method: take a string, wrap it around a stone and throw it in. When you pull it out, you can see whether you can still paddle or not." :)

Literature and Links

  • On this page you will find numerous long-term series of flow measurements from rivers from all parts of the world! For North Asia there is a finer breakdown at arctic.net, for Africa directly at grdc.sr.unh.edu, North American series can be found here or there.

swell

  1. ↑ Guidelines for the construction of hydrological stations - water level stations. Akademie-Verlag Berlin (East) 1953, p. 11.
  2. ↑ "Unloading depth" means the draft of a ship corresponding to a certain loading condition. The unloading depth affects the density of shipping traffic on the Rhine and Danube: if the level is low, the ships can load comparatively few goods in order to still be able to use the fairway. However, since the goods still have to be transported, more ships are used - the traffic is stronger when the water level is low.
  3. ↑ Ekkart Garski: The water levels from the Rhine - Mainz 160, fallen 3. In: "Kanu-Sport" 22/1962, p. 454 f.
  • Busch, Karl-Franz (Ed.): BI-Taschenlexikon Wasser. Bibliographisches Institut Leipzig 1985, p. 10, p. 86 and p. 225 f.
  • Lexicon technology and exact natural sciences. Fischer Taschenbuch Verlag Frankfurt am Main 1972, ISBN 3-411-01155-6, Volume 1, p. 31
  • Meisinger, Richard Merian: What does the level say? "Kanu-Sport" 6/1936, p. 76 (to the different level zero points and the interpretation of the numerical values)
  • Meyers Konservationslexikon, Verlag des Bibliographisches Institut, Leipzig and Vienna, 4th edition 1885-1892, Volume 16, p. 435, can be read online here (To use the level zero for measurements)
  • Neef, Ernst (Ed.): The face of the earth. Brockhaus reference work Physical Geography. F. A. Brockhaus Verlag Leipzig 1978, p. 371
  • Reuter, Adolf: Level and what's behind it. "Kanu-Sport" 3/1982, p. 58 f. (To measure the flow and how the water level service from the canoe association NRW works)
  • Guidelines for the establishment of hydrological stations - water level stations (= publications of the Meteorological and Hydrological Service of the German Democratic Republic No. 13). Akademie-Verlag Berlin (East) 1953 (to the different types of levels and measurement methods)