EESC323 - Rivers Final Exam

Mannings n

Indicates the effects of flow resistance by:

- bed roughness

- Presence of vegetation

- amount of sediment etc

The larger the number the higher the roughness, increases with turbulence

Hydraulic Radius (sheer stress)

R = A/2y + w

du Boys (Sheer stress)

sheer stress at the bed of the river, approximated using du Boys formula

T = p.g.r.s

Mannings n Equation

Q = (A x R^2/3 X S^1/2) / n

or

n = (R^2/3 x S^1/2) / v

Supercritical and subcritical conditions

Defined by Froudes number:

Subcritical <1

Supercritical >1

Transitional = 1

Reynolds number

Allows you to compare the characteristics of bed form type and flow through determining a dimensionless critical velocity gradient

Re = depth x velocity/ kinematic viscosity

Character of Laminar flow

Smooth and orderly

Re <500

Shape = parabolic

Character of Turbulent flow

Random and chaotic

shape = logarithmic

Re > 750

Density p (rho) is

Mass per unit volume

At a stream gauge identify what is continuously being measured and how it is converted into discharge?

measures water level and periodically velocity is measured to known water levels and a rating curve between stage and discharge is developed.

As a channel narrows with distance downstream, would a constant discharge exhibit:

steady, non-uniform flow.

why?

Steady Flow: This means that the flow parameters (such as velocity, discharge, etc.) do not change with time at any given location along the channel. Since the discharge is constant, the flow is steady.

Non-Uniform Flow: This refers to the variation of flow parameters (such as velocity and depth) along the length of the channel. As the channel narrows, the cross-sectional area decreases, which results in an increase in flow velocity to maintain the constant discharge (according to the continuity equation Q=A⋅V This variation in velocity with respect to the position along the channel makes the flow non-uniform.

A Froude Number is a dimensionless number that describes

The flow velocity and the velocity of a gravity wave

Why does water in a sloping channel not continue to accelerate downstream?

Roughness in the form of distortion resistance AND Boundary roughness from vegetation and bedforms.

Solid objects can withstand a certain degree of stress without deforming. What happens when water is subjected to even slight stress and is water a true Newtonian fluid?

It will flow as water, and is a true Newtonian fluid.

What happens to velocity as width decreases and slope stays constant?

Velocity increases

Continuity Equation:

According to the continuity equation Q=A⋅V

Q is the discharge (constant in this case), A is the cross-sectional area, and V is the velocity. When the width (and hence the cross-sectional area) decreases, the velocity must increase to keep the discharge constant.

Manning's n represents?

Flow resistance caused by bed roughness, the presence and flexibility of vegetation, the effects of bends, the amount of sediment or debris carried by the flow.

Shear stress could be BEST defined as

Tangential force per unit area.

Dissolved Load

Carried in solution, consists of dissolved ions and minerals

Suspended load

fine particles suspended in the water column, visible as muddy or cloudy

Bedload

courser particles moving along the stream bed by rolling, sliding or hopping

Stream power

Omega = pgQs

p = density of water

g = gravity = 9.81 (m/s^2)

Q = discharge (m^3/s)

s = slope

Components of geomorphic work

Erosion: ability of the stream to detach and remove material from the bed banks, higher stream power = increased ability to erode

Transport movement of sediment downstream. Stream power influences the size of particles able to be transported

Deposition: settling of sediment when the streams energy decreases. Lower stream power can lead to sediment deposition that forms features such as point bars.

Factors influencing stream power

Discharge (Snow melt), slope (steeper) and channel morphology (narrowing = increase velocity)

Geomorphic implications

channel incision: downcutting and deepening

lateral erosion: erode banks - lead to meandering

formation of landforms: terraces, floodplains, alluvial fans and deltas.