• drainage basin: basic unit for collection and distribution of water and sediment (pp. 135-137; 147-153)

• stream flow is ultimate end product of runoff generation

• agenda:

  • discuss generation of runoff

  • examine stream channel geometry

  • explore the mechanics of flow

• class discussion: pp. 137-140

• channel width: w

• channel depth: d

• cross sectional area: a

• wetted perimeter: P_w

• hydraulic radius: r = a/P_w

• 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

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

flow conditions: laminar or turbulent flow

       

Reynolds Number provides an approximate measure of flow conditions

Re<500 = laminar flow         Re>2000 = turbulent flow

Re = v x r x r/m

v = average flow velocity

r = hydraulic radius

r = density of water

m = viscosity

b. friction with bed and banks

• increased roughness causes increased resistance

• roughness elements include measures of:

  • grain size

  • microtopography (e.g. ripples, bars)

  • gross channel shape

3. Manning equation

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

F = v / sqrt(g x d)

v = average flow velocity

g = acceleration due to gravity

d = average depth

F<1 subcritical or tranquil flow (deep, slow flow)

F=1 critical flow

F>1 supercritical or rapid flow (shallow, fast flow)

hydraulic drops and hydraulic jumps

• Importance

  • flow velocity and depth intimately related

  • obstructions result in non-uniform and unsteady flow

  • flow/depth conditions affect turbulence

  • flow/depth conditions affect aquatic habitat

• Stream flow ultimate end product of runoff generation

• Flow characteristics affected by channel geometry

• Velocity = balance between driving forces (gravity, gradient) and resisting forces (viscosity, friction)

• Flow velocity and depth are inversely related

• Velocity varies in 4 dimensions:

  • with distance from bed

  • with distance from banks

  • downstream

  • over time

• Flow conditions (velocity, turbulence) ultimately impact amount & type of work river does