Open Channel Flow

common solution to flooding is

channelization

Irrigation or drainage ditches

commonly occupy formerly natural, straightened channels and excavated wider than original channel to eliminate overbank conditions

Hydraulic radius

cross sectional area/ wetted perimeter

modifications to reduce flooding include

decrease roughness (Manning's n) by removing obstructions or increase slope by straightening channel

Flow is steady if

depth (velocity) is constant over time

Flow is uniform if

depth is similar (velocity Δ) along a given reach

Flood reduction strategies

detention structures, channel modifications, or both

do-nothing and passive enhancemen

do not actively adjust the channel form

6 Channel management options

do-nothing, passive enhancement, threshold channel design, two-stage channel design, self-forming channel design, or natural channel design

In a Two-stage channel design, bank is

dry during low flows

stream power primary cause of

erosion

In Two-stage channel design sorting is

fine on benches & coarse particles in bed

Flow velocity (v) depends on

flow resistance

Laminar describes

fluid elements (layers) move downstream without mixing with other layers

turbulent describes

fluid elements move in irregular paths and become mixed

when Irrigation or drainage ditches are excavated wider than original channel to eliminate overbank conditions

fluvial processes usually deposit a bench to render narrower channel

Two-stage channel design improves

bank stability by support from bench at toe

Two-stage channel design improves ecology

because it is deeper during low flows with grasses and shade from benches

4 sources of flow resistance

(1) grain resistance, (2) form resistance, (3) channel resistance, (4) free surface resistance

Self-forming channel design creates

A sink for sediments that may result in nutrient or pollutant sequestration

Passive enhancement

Ceasing activities that are causing the problems and allow natural processes to recover the channel such as remove Arundo

Natural channel design

Construction of channel and floodplain to replicate the expected naturally-occurring conditions. raise channel bed or excavate floodplain to increase Channel floodplain connectivity. Possibly include meanders and pool-riffle sequences

Self-forming channel design

Establishes conditions for a channel-floodplain system to develop through time. Excavate a valley and allow vegetation and sediment to fill in.

Re > 2500

Flow is Turbulent

Re < 500

Flow is laminar

Re between 500 and 2500

Flow is transitional

Common open channel parameters

Hydraulic radius , Energy grade line

where lots of error can occur

Manning's n determination

Q = AU

Q is discharge (m3/s); A is crosssectional area (m2), & U is mean flow velocity (m/s)

discharge is measured by

Stream is divided into subsections. Velocity is measured at 0.2 and 0.8 of the depth in the middle of the subsection, and the average is used. For very shallow streams, one velocity measurement at 0.6 of the depth is used.

Threshold channel design

Trapezoidal design to accommodate a design discharge and ensure sediment is transported without erosion

Open Channel Flow is

a flow of water in channels that are directly open to the atomosphere

drawbacks of Two-stage channel design

increased width and initial earthwork that lead to potential problems with meandering

increasing slope by straightening channel

increases Velocity

submerged vegetation

is much less retardant to flows

Supercritical flow

is rapid

Subcritical flow

is tranquil

Darcy-Weisbach equation

more theoretically derived equation (based on physics)

Channel shape assumptions

rectangle or trapezoid can generate small errors 10%

Two-stage channel design

results in improved stability & function

channelization

results in whopper stream power and highly unstable channel

Energy grade line

slope of water surface

Differences in actively adjusting channel form

space required, how channel form is established, channel geometry parameters, measures to stabilize, & cost

Turbulent mixing

speeds up the channel-bed velocity & slows down the water-surface velocity•

Two-stage channel design reduces

stream power of high flows by spreading it over benches

Discharge (streamflow)

the flow rate of a stream

Reynolds number

used to distinguish laminar from turbulent flow

Manning's equation

v= k(R2/3S1/2)/n

flow rate of a stream

volume of water flowing past a cross section during a given time

Re = vRρ/μ

where v is flow velocity (m/s), R is the hydraulic radius (depth can be substituted) (m), ρis the density of water (1,000 kg/m3), and μ is the dynamic viscosity of water (0.00998 g/cm/s at 20°C)

v= k(R2/3S1/2)/n

where v is flow velocity, k is 1 in SI units and 1.49 in English units, R is the hydraulic radius (depth commonly is substituted), S is channel slope, and n is Manning's coefficient