• stream flow is ultimate end product of runoff generation

• variations in flow affect amount & type of geomorphic work a river can do

• stream channel geometry

• mechanics of flow

1. Channel width: w

2. Channel depth: d

3. Cross sectional area: a

4. Wetted perimeter: P_w

5. Hydraulic radius: r = a/P_w

6. Channel gradient (slope): s

Q = w x d x v = a x v

1. Driving forces

a. gravity: 32 ft/sec/sec or 9.8 m/sec/sec

b. channel gradient

2. Resisting forces

a. viscosity: resistance of a fluid to a change in shape

1) molecular viscosity: resistance due to friction between individual water molecules as they collide and slide past one another

• affected by temperature and suspended sediment

• laminar flow

2) eddy viscosity: resistance due to friction along eddy lines

• turbulent flow

• turbulence: frequency and magnitude of changes in water velocity as water is interchanged in eddies

3) Reynolds Number

• provides an approximate measure of flow conditions

  Re<500 = laminar flow

  Re>2000 = turbulent flow

• Re = v x r x r/m

  where: v = average flow velocity

  r = hydraulic radius

  r = density of water

  m = viscosity

b. friction with bed & banks

• increased roughness causes increased resistance

• roughness elements includes measures of grain size, microtopography & gross channel shape

3. Manning equation

where v = average flow velocity

n = Manning roughness coefficient

r = hydraulic radius

s = channel slope

restricted to uniform flow: constant depth and velocity along some length of channel of constant cross-section and slope

1. F = v / sqrt(g x d)

where: v = average flow velocity

g = acceleration due to gravity

d = average depth

2. Interpretation

F<1 subcritical or tranquil flow

deep & slow flow

F=1 critical flow

F>1 supercritical or rapid flow

shallow and fast flow

3. Importance

• affects velocity and depth (non-uniform and unsteady flow)

• affects turbulence

• affects aquatic habitat

1. Flow velocity varies in 4 dimensions

• with distance from bed

• with distance from banks

• downstream

• over time

2. Channel geometry affects turbulence & thus velocity

3. Hydraulic jumps & drops affect velocity & depth

4. All of these ultimately impact amount & type of work a river does