River wave formation at an abrupt drop

Most great manufactured river waves occur where fast and smoothly moving water meets an abrupt drop. The science of river wave formation at an abrupt drop has been researched since at least the 1950s. This article explains how river waves form at abrupt drops and key considerations for designing these waves. You can see information on flat bottom wave formation from a previous article. All information is under a Creative Commons Attribution-ShareAlike 4.0 International License to encourage the creation of more good waves and can be used for no cost.

Examples of manufactured static waves at abrupt drops

These are four manufactured static waves that form where fast and smoothly moving water meets an abrupt drop in the river bed.

Scout wave, USA (2023) by REP and Salida

Left photo from Paddling Life. Right photo from Endless waves.

Annecy Wave, France (2021) by Hydrostadium and Surfeurs d’eau Douce

Left photo from Annecy Wave. Right photo from Surfeurs d’eau Douce.

Donsurf, Russia (2020) by Donsurf

Both photos from Donsurf.

Flosslaende, Germany (1972) by Munich surfers.

Left photo from Buster Surfboards. Right photo from Stephan Rumpf via Suedduetsche.

How waves at abrupt drops form

The images below are excerpts from research articles that detail the stream flows lines of waves at abrupt drops.

The above image is an excerpt from Effect of adjustable flaps on river surf waves at abrupt drops by Helge Fuchs (2017). Arrows added to show flow directions. Top image is from the side and bottom image is from above. The images are aligned. These images show a kicker at the edge of the drop. The kicker can help make the wave taller but waves still form without a kicker.

Excerpt from N. Kawagoshi & W. H. Hager (1990) Wave type flow at abrupt drops, Journal of Hydraulic Research, 28:2, 235-252.

Key design components

Waves at abrupt drops have the following key design components.

  1. Smooth walled and bottom channel
    1. Allows the water to accelerate with minimal turbulence.
  2. Ramp
    1. Accelerates the water.
    1. See Ramp Angles for Waves by Surf Anywhere for more information.
  3. Horizontal transition at the end of the slope
    1. Redirects the water to horizontal and allows the formation of a wave rather than a recirculating hydraulic.
    1. Transition may be flat or have a kicker at the end. A kicker will make the wave steeper.
  4. Channel walls
    1. Prevents tailwater from entering the wave from the side and collapsing the wave.
    1. Walls allow the wave to be much taller before collapsing. Once the channel wall is below the surface of the tailwater, the wave quality and maximum wave height start decreasing.
    1. See Effect of Channel Walls on Waves by Surf Anywhere for more information.
  5. Vertical drop at the end of the transition.
    1. Provides depth under the wave for an underwater eddy that helps make the wave larger.
  6. Hard surface under the wave for scour protection
    1. Stops the circulating water under the wave from digging into the bottom.
  7. Tailwater control
    1. A structure downstream that keeps the tailwater high enough to make a wave.
    1. The tailwater control can also prevent the river flow from lowering the river bed and lowering the tailwater elevation.
    2. The structure can be simple like a layer riprap in the river bed with no concrete or complex like an adjustable structure.

Images of abrupt drop waves with key components numbered.

2. Ramp
3. Flat transition
4. Channel walls prevent tailwater entry into channel
5. Abrupt drop in river bottom

Left photo from Endless Waves. Right photo from Surfeurs d’eau Douce.

Left photo from Donsurf. Right photo from Buster Surfboards.

How to prevent a retentive hydraulic

On an abrupt drop wave, a recirculating hydraulic will form when the tailwater elevation is too high above the horizontal transition. As the tailwater elevation increases, the pressure from the tailwater increases until causes the wave to collapse. See Tailwater Effect on Waves by Surf Anywhere.

 Image from video by Practical Engineering.

The images below show the change in wave shape as tailwater rises. The only change is tailwater elevation increasing. The tailwater from the video does not raise high enough to create a recirulating hydraulic.

Images from video by Dominik Puckert.

The images below show tailwater elevation increase causing a recirculating hyrdaulic.

Images from Surf Anywhere.

To ensure good waves across a range of tailwater elevations and prevent the formation of a recirculating hydraulic, the depth of tailwater over the horizontal transition must be controlled. See Easy Tailwater Management with Multi Adjustable Kicker System by Surf Anywhere. There are a few basic ways to manage tailwater elevation and all these methods impact each other.

  1. Control the tailwater elevation. Some controls include:
    • Wide river downstream with hard underwater structure spanning the river
      • Hard structure sets the tailwater level.Wide river allows increased flows to spread out and minimize tailwater elevation rise.
    • Adjustable structure downstream
      • Adjustable structure can be raised when flows are low to keep tailwater higher.
      • Adjustable structure can be lowered when flows are high to keep tailwater lower.
  2. Control the horizontal transition elevation.
    • Image below shows waves forming at very different tailwater elevations by changing transition elevation.

Images from Surf Anywhere.

  1. Raise channel walls above tailwater elevation.
    • Prevents tailwater from flooding the wave and collapsing the wave. The image below shows how removing a channel wall allows tailwater to enter from the side which collapses the wave.

Images from Surf Anywhere.

  1. Increase the headwater elevation.
    • Higher headwater elevation provides more velocity to the wave which counters the tailwater pressure and prevents the wave collapse.

Simple control of horizontal transition

All manufactured river waves manage tailwater. The simplest waves have limited tailwater changes and just build a static structure at the height for the tailwater. The most complex waves raise and lower the entire wave ramp to meet the tailwater elevation. See Types and Costs of Adjustable Waves by Surf Anywhere.

Below is a simple and relatively low cost method to control the elevation of the horizontal transition and prevent retentive hydraulic formation over a wide range of tailwaters. This design could be relatively easily retrofitted to existing waves. The key design components are the adjustable kickers in purple. The adjutable headwater gate is not necessary.

Low tailwater and fixed horizonal transition forms wave

Medium tailwater and first adjustable kicker forms wave

High tailwater and second adjustable kicker forms wave

The designs above show an adjustable headwater gate. Keys points about the adjustable crest gate:

  1. Adjustable headwater gate is used to increase the headwater elevation and maintain the headwater to tailwater drop.
  2. Having a gate allows much of the structure to be lowered and the drop minimized during high flow events.
  3. The headwater gate is not always necessary depending on the range of flows and headwater and tailwater elevation changes.

These concepts of river wave formation are proven with decades of successful use and allow the creation of simple and complex river waves.