What is the difference between a standing wave and a traveling wave?
Traveling waves can travel over great distances (think ocean waves), however, the water itself shows little horizontal movement; it is the energy of the wave that is being transmitted, not the water. In a stationary wave system, (think river waves) the water is moving predominately in a horizontal fashion with significant velocity at a shallow water depth.
These two different types of waves can be generated in wave pools worldwide. We wanted to run this piece because there is a lot of confusion when talking about traveling wave pools and stationary (rapid, standing, etc. wave pools). So much so that the water fun godfather himself Tom Lochtefeld reached out to us to share a few notes detailing the distinctions between each. Read on!
MOVING or TRAVELING WAVES
A moving or traveling wave occurs on the free surface of water originating from an initial disturbance combined with gravitational restoring forces. For example, the ripples that occur when a small rock is dropped into still water, or the waves from a wind event seen breaking on a beach. Surface gravity waves can travel over great distances, however, the water itself shows little horizontal movement; it is the energy of the wave that is being transmitted, not the water. Instead, the water particles move in circular orbits, with the near-surface size of the orbit equal to the wave height. As a wave passes, water moves forwards and up over the wave crests, then down and backwards into the troughs, so there is little horizontal movement. As the wave progresses up a reef incline, the orbital water motion transforms from deep-water circles into shallow-water elongated ovals, which (assuming proper bathymetry) will over-top and break in a spilling or plunging fashion. In a surf pool, a mechanical device creates the initial disturbance that transforms into a gravity wave that radiates from this impulse up and inclined reef towards the beach and ideally breaks at a predominately oblique angle with suitable characteristics for surfing. This concept of orbital gravity wave motion that reacts to changes in reef water depth and ultimately breaks at an oblique angle progressively towards the beach is the key differentiator between stationary and gravity waves.
STATIONARY, RAPID OR DEEPWATER-STANDING WAVES
In a stationary wave system, the water is moving predominately in a horizontal fashion with significant velocity at a shallow water depth. This condition is technically classified as a supercritical stream flow. There is no orbital movement of water particles that translate into creating the stationary wave that is surfed, in fact, radiant gravity water waves are oftentimes swept downstream, e.g., if you throw a rock into a stationary wave’s water flow the ripples cannot propagate back upstream since the flow velocity exceeds the radiating wave velocity. From a hydrodynamic perspective, there are two main sub-classes of stationary wave systems. The first generates a high velocity water flow that sheets upwardly over an inclined surface with the rider ‘surfing’ over this inclined sheet-flow, e.g., the ‘FlowRider’. Alternatively, there are stationary wave systems that start with a supercritical shallow water stream flow that enters deeper water to form a hydraulic jump and form a standing wave, e.g., a river wave or artificially a ‘Unit’ wave pool. This wave form occurs when the depth of water flow changes from supercritical to a subcritical state. A rider surfs on the inclined supercritical side of the water flow that forms the hydraulic jump. This process is manifest in natural river style waves, e.g., the Zambezi. Notwithstanding, under both classes of stationary wave systems the water surfed is predominately moving horizontally at a supercritical velocity, which is the key point of distinction from the gravity wave phenomenon found in orbital moving / traveling wave system.
Tom Lochtefeld is the OG wave pool geek and a core surfer. For the past 30 plus years, he’s found ways to have fun and surf outside of the ocean. Beyond the creation of systems like SurfLoch, he co-founded Raging Waters waterpark and invented the FlowRider. Through the early nineties, he built WaveHouse which was the first place surfers could gather in a dedicated space to ride a human-made wave – a ‘sheet’ wave in reality.
As Surf Park Management CEO Skip Taylor says, “Tom really deserves to be recognised for all his work in this field as the Godfather of modern surf parks. Tom’s extensive background in the surf park business and making wave pools of all types shows he’s super passionate about it.“