About Lesson
Some common terms describing flow in open channels
- Steady flow:
- Refers to the condition in which flow characteristics at any point don’t changes with time. i.e. dv/dt = 0, dy/dt = 0.
Here, V denotes the velocity of flow
y denotes the depth
t denotes the time
- Unsteady flow:
- When the flow characteristics change with time at a channel cross-section, the flow is unsteady.
- Uniform Flow:
- When the channel flow is steady and the mean velocity is the same at each succeeding cross-section, the flow is uniform.
- Non-uniform flow:
- When the mean velocity changes from cross-section to cross-section, the flow is non-uniform.
- Wetted perimeter:
- It is the sum of length of that part of the channel sides and bottom which are in contact with water,
i.e P = b + c+ c = b+ 2c
- Cross-sectional Area (a):
- It refers to the area of the wetted section of the channel.
- Hydraulic radius:
- It is the ratio between the cross-sectional area of the stream and its wetted perimeter.
i.e. R = a/p = a/ (b+2c)
- Velocity of flow in channel is directly proportional to the square root of the hydraulic radius. e. V α
- Hydraulic Slope (S):
- It is the ratio of its vertical drop (h) for a length of (l) of the channel. i.e. s = h/l
- The velocity of the flow in channel varies directly with the square root of the hydraulic slope . i.e. . V α
- Free Board:
- It is the vertical distance between the highest water level anticipated in the design and the top of the retaining banks.
- It is done to prevent overtopping of structures because of wave action or damage due to trampling or other unforeseen condition.
Note:
Generally,
When Soil is stiff clay = ½ : 1
When soil is stiff sand = 2:1
- Angle of Repose:
- If a mass of the loose dry soil is allowed to fall freely to the ground, the soil heap formed by it will attain a conical shape.
- The maximum angle of the slopping sides of the cone measured from the horizontal is called angle of repose.
Note: Channel side slope < angle of repose ; channel side will be stable.