River waves · building, running, surfing

Glossary: technical terms around the river wave

Hydraulic engineering, water law and the surf scene each have their own language. This glossary explains the terms that appear in our articles, in expert reports and in submissions to the authorities – briefly, without formulas, with a link to the article that explains it in more detail. Sorted alphabetically by the English term, with the German term in brackets.

A–F

Common use (Gemeingebrauch)

The right to use a body of water without a special permit – for bathing, swimming, travelling in small boats without a motor. In Baden-Württemberg it is governed by § 20 of the state Water Act. Surfing on a natural wave falls under it; erecting an installation in the river does not, for that a water-law approval is needed. → Ecology and permits

Discharge (Abfluss)

The volume of water flowing through a river cross-section per second, measured in cubic metres per second (m³/s). In a natural river the discharge varies daily and is the most important control variable of any river wave: too little water gives no wave, too much an unsurfable or dangerous one. Key figures are MQ (mean discharge), MNQ (mean low-water discharge) and MHQ (mean flood discharge). → Hydraulics

Duty of care (Verkehrssicherungspflicht)

The duty of whoever operates an installation or creates a source of danger to take the necessary and reasonable precautions against harm. For a river wave: run-out zone, rules, briefing, supervision, shutdown during floods. → Operators and operation

EU Water Framework Directive (Wasserrahmenrichtlinie, WRRL)

The EU directive of 2000 that requires a "good status" for all bodies of water and prohibits deterioration. Every intervention in a river, including a wave, is measured against it; the assessment is made per water body according to biology, chemistry and river structure. → Ecology and permits

Feasibility study (Machbarkeitsstudie)

The investigation of whether and how a wave can work at a site: discharge and flow duration curve, hydraulics, floods, ecology, permit prospects, costs. It stands at the beginning of every project and is usually prepared by a hydraulic engineering firm or a university. → Feasibility study

Fish pass (Fischaufstieg)

A structure that allows fish to overcome a weir: a staircase of pools, a near-natural bypass channel or a rough ramp. Also called a fish ladder or fishway. → Ecology and permits

Fish passage (Durchgängigkeit)

The possibility for fish and other aquatic organisms to pass a structure in the river upstream and downstream. The EU Water Framework Directive requires it; a fish pass, a rough bed ramp or a lowerable ramp provides it. In Nuremberg the wave was therefore built with a fish pass. → Ecology and permits

Flow duration curve (Dauerlinie)

A curve showing on how many days a year a certain discharge is reached or exceeded. From it one can read on how many days a wave with a certain water requirement would run – the surfable-days calculation. → Feasibility study

Froude number (Froude-Zahl)

The figure that decides whether water flows "subcritically" (calm, deep) or "supercritically" (fast, shallow). It relates the flow velocity to the propagation speed of a surface wave, which depends on the water depth. Below 1 the flow is subcritical, above 1 it is supercritical; a surfable wave needs supercritical flow before the jump, as a rule of thumb about Fr 2.3 and more. Named after the English engineer William Froude. → Hydraulics

G–R

Gauge (Pegel)

A measuring station at which the water level and discharge of a river are recorded continuously. For the Rems there are gauges in Schorndorf and Neustadt; their long-term series are the basis of every surfable-days calculation. → The Winterbach site

General order (Allgemeinverfügung)

An administrative act addressed to a definable group of people, for example everyone who surfs at a particular spot. The City of Munich has regulated surfing at the Eisbach in this way since 2010 – with surfing hours, a leash rule and a minimum age, but without an operator. → Case study Eisbach

Headwater (Oberwasser)

The water upstream of a weir or ramp – impounded, slow, higher than the tailwater. The difference in height between headwater and tailwater (the head) drives the wave. → Hydraulics

Hydraulic jump (Wechselsprung, hydraulischer Sprung)

The abrupt transition from supercritical to subcritical flow: fast, shallow water meets slow, deep water and throws up a standing wall of water. Every standing wave is a hydraulic jump (in German "Wechselsprung" or "hydraulischer Sprung"). Its shape depends on the Froude number before the jump. → Hydraulics

Leash

The cord between the surfboard and the surfer. Standard in the sea, controversial at river waves because it can catch on rocks or structural parts; after the fatal accident at the Eisbach in 2025, Munich requires a leash that releases under load, Pforzheim bans it altogether. → Operators and operation

Lifeguard (Rettungsschwimmer)

A person with a lifesaving qualification (from the DLRG or Wasserwacht, usually silver) who safeguards surfing operations. The clubs in Pforzheim, Augsburg and Nuremberg staff every session with trained lifeguards. → Operators and operation

Lowerable ramp (Absenkbare Rampe)

A wave ramp that can be moved hydraulically completely out of the water or down onto the river bed. During a flood the discharge cross-section stays clear, and fish can pass; in Pforzheim and Nuremberg the ability to lower the ramp was a condition of the permit. → Wave types and technology

Operating body (Träger)

The organisation that is legally responsible for a wave: contractual partner of the authority, holder of the permit, bearer of the duty of care. At the German river waves a non-profit club, in Salzburg the city. → Operators and operation

Polder

In hydraulic engineering, a diked area that can be flooded deliberately to hold back water during floods. The initiative used to call the concrete blocks (stepping stones) in front of the Winterbach structure the "polder"; as that is misleading, this site variant is now called the "stepping stones" variant. → The Winterbach site

Rapid Surfing

Surfing on standing waves as a competitive sport, with its own rules, German Championships since 2019 and a European Championship since 2026. Not Olympic – ocean surfing is the Olympic sport. → Rapid surfing

River bed (Sohle)

The bottom of the river channel. Its shape downstream of a weir helps decide whether a wave stands; in Thun one of the two waves forms at a drop in the river bed. A bed sill (Sohlschwelle) is a fixed step in the river bed. → Wave types and technology

Roller (Walze)

A reverse current in which water at the surface runs back upstream and forms a rotating body of water. Small rollers are part of every breaking wave; large rollers downstream of weirs hold people and objects fast and are the classic danger at weirs. A good surf wave has a roller that carries instead of holding on. → Hydraulics

Run-out zone (Auslaufzone)

The area downstream of the wave into which surfers are washed after a fall. It must be free of obstacles such as boulders, piers and shallows and have a safe exit – one of the central safety requirements for a built wave. → Operators and operation

S–Z

Slot

A booked time window for surfing, usually 60 to 90 minutes for eight to twelve people, supervised by a wave warden and lifeguards. Slots are the organising principle of all newer river waves. → Operators and operation

Standing wave (Stehende Welle)

A wave that stays in one place while the water flows through it – in contrast to the travelling ocean wave. Physically a hydraulic jump. Also called a river wave, stationary wave or, in the scene, simply "wave". → Hydraulics

Subcritical flow (Strömender Abfluss)

Flow state with a Froude number below 1: calm, deep, like the Rems on most days. No wave forms from subcritical water; it first has to be brought to supercritical flow by impoundment, constriction or a ramp. → Hydraulics

Supercritical flow (Schießender Abfluss)

Flow state with a Froude number above 1: the water is fast and shallow, disturbances cannot propagate upstream. It forms at the foot of a weir or on a ramp and is the prerequisite for a hydraulic jump. Opposite: subcritical flow. → Hydraulics

Surfable days (Surftage)

The number of days a year on which a wave runs at a sufficient but not too high discharge. It follows from the flow duration curve and the operating range of the installation; Bern named a target of 100 days for its project. → Feasibility study

Tailwater (Unterwasser)

The water downstream of a weir or ramp. Its depth determines where the hydraulic jump stands and what the wave looks like: tailwater that is too deep "drowns" the wave, tailwater that is too shallow turns it into a roller. → Hydraulics

Water-law permit (Wasserrechtliche Erlaubnis)

The official approval for the construction and operation of an installation in a body of water under the Federal Water Act and the state Water Act. Converting a weir or installing a ramp is in addition a watercourse modification that needs a planning approval procedure (Planfeststellung) or a plan permit (Plangenehmigung) from the water authority. → Ecology and permits

Wave warden (Wellenwart)

The person in charge of a slot, called "Wavemaster" in Hanover and "Wellenwärter" in Augsburg: controls the ramp, gives briefings, enforces the rules and decides when surfing is off. A volunteer, usually with lifeguard training. → Operators and operation

Weir (Wehr)

A structure across the river that impounds the water – for hydropower, for regulation or historically for mills. Fixed weirs have an unchangeable crest, movable weirs have flaps or gates. Strictly speaking, the "Remswehr" in Winterbach is not a weir but the outlet structure of the Schorndorf/Winterbach flood retention basin: at normal water levels the Rems flows straight through; only during floods do gates close the opening and impound the valley. It is the starting point of the Remswelle. → The Winterbach site

Frequently asked questions

What is the difference between a river wave, a standing wave and a stationary wave?

None – the terms mean the same thing: a wave that stands in one place in flowing water. "Rapid Surfing" is the name of the sport practised on it.

What does Froude number 2.3 mean?

That the water before the jump flows 2.3 times as fast as a surface wave could propagate at this water depth. From about this value, experience shows, a strongly breaking, surfable wave forms; well above it the roller becomes too strong.

Why is the roller below a weir dangerous?

Because the water flowing back at the surface keeps pulling a body back towards the weir. Built surf waves are shaped so that the roller carries and the current behind it leads into a clear run-out zone.

Sources: Ven Te Chow, Open-Channel Hydraulics, 1959 (classification of the hydraulic jump) · University of Innsbruck, Unit of Hydraulic Engineering, Newsroom 01.04.2022 · Wassergesetz für Baden-Württemberg (Baden-Württemberg Water Act), § 20 (common use) · Directive 2000/60/EC (Water Framework Directive) · Landeshauptstadt München, Allgemeinverfügung Surfen Eisbach (general order on surfing at the Eisbach) of 08.05.2026 · Nürnberger Dauerwelle e.V., Leinewelle e.V., blackforestwave e.V., Surffreunde Augsburg e.V., operating rules and FAQ (see Operators and operation) · Wasserverband Rems, "Der Verband" · City of Bern, Projekt Flusswelle (surfable-days target; see Feasibility study)

Last updated: 15 October 2026 · Initiative Remswelle