Download Slide 1

Part 2
Some Basic
Aspects of
CHANNEL
HYDRAULICS
• The volume of water that passes by any given
point along a watercourse is called “Q”, for
quantity of flow. It is generally expressed in
units of cubic feet per second (cfs) or cubic
meters per second (m3/s).
MANNING’S EQUATION for
Open Channel Flow (1889)
• Where:
•
Q = Flow Rate, (ft3/s)
•
v = Velocity, (ft/s)
•
A = Flow Area, (ft2)
•
n = Manning’s Roughness Coefficient
•
R = Hydraulic Radius, (ft)
•
S = Channel Slope, (ft/ft)
Hydraulic Depth
and Radius
• In terms of frictional
head losses, the
perimeter is important.
Hydraulic radius, Rh, is
defined as the area of
the flow section divided
by the wetted perimeter,
Pw, which is shown on
the figure at left and is
written as: Rh = A/Pw
Manning’s n for natural channels
• For main channels with clean, straight,
full stage, no rifts or deep pools navg =
.030
• For mountain streams with channel bed
of gravels, cobbles, and few boulders
navg = .040
• For flood plains with scattered brush,
heavy weeds, navg = .050
• For excavated earthen channel, clean
and recently completed navg = .018
• Trapezoidal channels are commonly excavated for
flood control because they have predictable
characteristics
• Over time, these man-made channels can aggrade and
fill with sediment, diminishing their design capacity
• Flow data is measured at discrete points along a
watercourse, known as gaging stations. Velocity
data is usually measured during high flows on
stage recorders, like that shown at right. These
data are compiled to create statistical databases
on runoff and channel flow.
Flow Data
• Gauging stations usually
record data on channel
width, depth and velocity
during various flow stages
• These data can be used to
calculate the quantity of
flow, Q
• If sufficient data exists, a
stage record can be
constructed for this site
which relates Q to flow
velocity, depth, and width
• The hydrograph is a graphical plot of Q versus
time at a given point along the stream or river.
It is influenced by a number of factors,
including interflow.
Impacts of Land Use
and Impermeable
Surfaces
• Changes in land use
and vegetation affect
runoff by increasing
the peak flow,
causing erosion of
bed and banks
• Hard, impermeable
surfaces such as
pavement and roofs
tend to reduce the
time to concentration
Runoff
Coefficients
Terrasets caused by compaction of
grazing cattle hooves
Slopes cleared of vegetation for grazing
• The runoff coefficient
depends on ground
cover, land use, and
antecedent moisture
• The time-toconcentration
depends on slope,
permeability of the
ground surface, and
distance to an
adjacent watercourse
Lag Time
• Lag time
describes the
time interval
between the
center of mass
of rainfall and
the runoff
• The lag time
diminishes
with increasing
impermeable
surfaces
• The lag time describes the interval between
the centroid of the precipitation and the
centroid of flow in the hydrograph