# Some Basic Aspects of CHANNEL HYDRAULICS - PowerPoint PPT Presentation

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## Some Basic Aspects of CHANNEL HYDRAULICS

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### The volume of water that passes by any given point along a ... these man-made channels can aggrade and fill with sediment, diminishing their design capacity ... – PowerPoint PPT presentation

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Title: Some Basic Aspects of CHANNEL HYDRAULICS

1
Part 2
• Some Basic Aspects of CHANNEL HYDRAULICS

2
• 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).

3
MANNINGS EQUATION for Open Channel Flow (1889)
• Where
•           Q Flow Rate, (ft3/s)
•           v Velocity, (ft/s)
•           A Flow Area, (ft2)
•           n Mannings Roughness Coefficient
•           S Channel Slope, (ft/ft)

4
• 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

5
Mannings 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

6
• Trapezoidal channels are commonly excavated for
flood control because they have predictable
characteristics
and fill with sediment, diminishing their design
capacity

7
• 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.

8
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

9
• 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.

10
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

11
Runoff Coefficients
• The runoff coefficient depends on ground cover,
land use, and antecedent moisture
• The time-to-concentration depends on slope,
permeability of the ground surface, and distance

Terrasets caused by compaction of grazing cattle
hooves
Slopes cleared of vegetation for grazing
12
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

13
• The lag time describes the interval between the
centroid of the precipitation and the centroid of
flow in the hydrograph