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