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A river to examine
Appropriate equipment to traverse a riparian environment
Understanding States of Flow
Discussing examples and real world application of Flow States
In this lesson, Trevor will be diving deep into the flow states of water in the river. Understanding flow state is a critical building block to understanding how rivers actually work and how to accurately identify river features. This lesson builds on some of the fundamental principles we laid down in our beginner hydrology lessons and expands upon them by identifying the 3 states of flow which are:
Super Critical Flow
Sub-Critical Flow
Critical Flow
Before we are able to dive into what each flow state is we need to lay the ground work about the Froude Number or Fr. This number is a value that represents a ratio of gravitational force to inertial force. and it is best described using the following formula:
Fr = V/√gD
V = Velocity
g = Gravity
D = Depth of the cross section of flow
This equation sets the basis for how we define flow states in a channel as follows:
When Fr > 1 the flow is in a super critical state
When Fr = 1 the flow is in a critical state
When Fr < 1 the flow is in a sub-critical state
Super critical flow is most easily defined by high velocity water. Another element of super critical flow is that any disturbance or obstacle in the flow has little to no effect on the water level upstream. The high velocity of the super critical flow will also create tell-tale signs like what boaters call tongue lines or pour overs. One of the best examples and most common places we see super critical flow is at the top of low head dams spilling over. Due to the relatively high velocity of super critical flow, it is easy for people standing in super critical flow to be knocked over and swim. Therefore, if you are crossing a stream or find yourself standing in the river being able to recognize super critical flows can help prevent you from getting swept downstream by fast moving water.
On the opposite end of the spectrum of flow we have sub-critical flow. This flow is most commonly characterized by tranquil, low velocity water. In river environments though we often see this water appearing as highly chaotic due to often multiple intersecting currents, irregular obstacles, and random features on the bottom of the river. Another defining feature of sub-critical flow is that any obstacle or disturbance to the flow will cause a rise in water level to propagate upstream. So, if there is an area of sub critical flow backed up by a downstream boulder it will cause the water to rise significantly upstream of the obstacle. The most common place you would see this is a pour over drop backed up by a rock. This can force more water towards the drop and make it seem much worse than if the rock was not present.
At the intersection of sub and super critical flows you will find a zone of critical flow. This area is typically short in distance and although the ratio of gravitational to inertial force is equal all of that pressure is often violently dispersed into turbulent whitewater. These zones of critical flow are where all the action happens in rivers and are also called hydraulic jumps. In our lesson on hydraulic jumps, we will discuss the characteristics that make up a hydraulic jump and how they work. For now, though understanding where these zones are will help form the basis for understanding where a jump is likely to occur.
The equation above is pretty academic and far too impractical to solve for in the field so boaters are forced to use some general rules to help understand what they are seeing in the water. While all water in a river is turbulent water looking for common characteristics of flow will help you identify where to go in the river.
Super Critical Flow: Look for the fastest moving current in a specific area. Also look for smooth water moving rapidly or submerged obstacles causing water to fall quickly over a distance. Pour overs, tongue lines, and swift water moving downstream are all tell-tale signs of super critical flow. Any water moving at a high velocity downstream relative to other water around it is in a Super Critical state.
Sub-Critical Flow: Glassy flat water, eddies, water moving upstream, or rolling tumbling water that seems to lack clear direction in flow are all telltale signs of sub-critical flow. Look for water that is moving relatively slowly compared to other water. You can also see subcritical flow when an obstacle is pushing a large volume of slower moving water back upstream especially after a drop in elevation.
Critical Flow: Look for large features including waves, holes, and curling whitewater. Critical flow zones will generally be characterized by the largest outstanding features in a particular flow area. Looking for any submerged obstacles with super critical flow coming over the top of them then suddenly falling into slower water. Being able to identify critical flow will help you locate and analyze hydraulic jumps to determine the dangers associated with their navigation.
When navigating rivers people often use the phrase “Shoot the V” what they are really saying is aim for the strongest area of super critical flow and charge down it. Properly identifying and aiming for super critical flow can help you gain momentum by putting you into faster moving water. That fast-moving water can give you the edge you need to punch large features (zones of critical flow) by giving you extra purchase and increasing your relative velocity. This is especially useful in big water rivers or anywhere that there is a large hydraulic feature that you need to push hard through to successfully complete the rapid. For more about this please review the momentum video from the Beginner Rafting Series.
On the flip side of this “catching an eddy” requires you to accurately identify and paddle into a zone of Sub-Critical flow. alternatively, you may want to surf your raft so you will have to locate a zone of sub-critical flow leading into a zone of critical flow. By catching the sub-critical flow, sometimes called slack water, you can slow down your momentum and avoid hitting obstacles too fast. This is especially useful in lower volume creeks or rapids filled with obstacles. For more on this you can see our eddy catching video which discusses properly catching eddies.
We want to say a huge thanks n this video to Dr. Rollin Hotchkiss from BYU who has been doing some fantastic work on identifying lowhead dams throughout the country. if you would like to help support his work you can download the free app and report lowhead dams to the database.
