FLOWING WATER HYDRAULICS AND OPEN CHANNEL FLOW by hmv21438

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```									FLOWING WATER: HYDRAULICS AND OPEN CHANNEL
FLOW

FLOW SETTINGS OF INTEREST:

•Overland flow, interflow throughflow
•Groundwater flow
•Open channel flow (rivers, canals etc)
•Pipe flow
Flow
•Flow through lakes
•Flow through wetlands
•Tidal flow (wetlands)
flow (c rrents a es
•Ocean flo (currents, waves etc)

Each of these are complex physical settings, and prediction of the flow
regime will be equally complex. We use the laws of physics, along with
simplifications, to make estimates of the flow regime.
VELOCITY SCALES

There            fl       i      fi t    t f diff      t
Th are many flow regimes of interest for different purposes.
In this class we will focus on flow in river channels.
OPEN CHANNEL FLOW:
How deep and how fast water will flow in an open channel
Basic question: how deep and fast will the river flow at your
point of interest in the watershed for various recurrence
interval storms [ie, Q(100); Q (2)]?

Point of interest
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4. Sub-critical            i i l fl
4 S b i i l vs super-critical flow
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5 Estimating flow velocity, depth: Manning s equation
5.                 velocity         Manning’s
6. Beyond Manning’s Equation: Backwater programs (HEC-RAS)
7. Hydrodynamic modeling: 1-d, 2-d, 3-d: unsteady flow models
Principle of “Continuity”
Applied to rivers: Flow rate
(cfs) is the same at each cross-
section
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4. Sub-critical            i i l fl
4 S b i i l vs super-critical flow
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5 Estimating flow velocity, depth: Manning s equation
5.                 velocity         Manning’s
6. Beyond Manning’s Equation: Backwater programs (HEC-RAS)
7. Hydrodynamic modeling: 1-d, 2-d, 3-d: unsteady flow models
Steady vs Unsteady flow: does the flow rate (at a specific location
g            )
change over time?)
But we often assume a steady rate of flow to analyse a specific
example                     100-year
problem: For example, we know that the 100 year flow event will
include a complete hydrograph (ie, flow changes over time) but we
may only be interested in the flow properties at the peak (maximum
flow t ) f th t       t
fl rate) of that event.
During this rain event, the flow in
the river will vary over the duration
we may choose to just determine
hat     flo               elocit
what the flow depth and velocity
will be in the channel for the Peak
flow rate
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4 S b i i l vs super-critical flow
4. Sub-critical            i i l fl
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5.                 velocity
5 Estimating flow velocity, depth: Manning equation
Uniform vs
Non-uniform:
Variation of
Flow along the
Channel
Uniform flow: constant depth along the channel
Non-uniform flow: water is deeper in the pool, and shallow in the rapid
Open Channel Flow
y         p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow
i l l i         fil
vertical velocity profile
4. Flow in real rivers

Bernoulli equation

Specific energy
- critical depth
- froude number

Flow equation, Manning equation
Laminar flow virtually never occurs in nature. Only a lab phenomenon
Because of the inherent “roughness of natural channels, virtually all
rivers experience “turbulent” flow
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4 S b i i l vs super-critical flow
4. Sub-critical            i i l fl
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5.                 velocity
5 Estimating flow velocity, depth: Manning equation
Energy Concepts in flowing water:

•   The water at any location in the watershed has a fixed amount of
“energy” associated with it.
Potential                        i t d ith th f t th t h
• P t ti l energy: energy associated with the fact that it has
been raised (by evaporation) to a point above MSL (think of a
ball at the top, vs bottom of a set of stairs).
• Energy based on the depth of water in the channel (higher
depth has more energy.
• Energy based on the fact that it is flowing (just like a moving
car has energy, since it takes counter energy to stop it.

•   Energy is neither created or destroyed as the water moves down
the channel. Some of the energy forms described above are
converted to heat (very slight, and hardly measurable) as a result
of friction on the channel banks and bed, and by internal
turbulence (as the water molecules rub past each other)
How is energy lost in streamflow?
Subcritical   Super-critical         Subcritical

Hydraulic jump
Most of the flow in rivers is “subcritical” flow, again because of the
roughness of the channels.
• We do see supercritical flow in some concrete channels
•We see it in natural rivers in flow over drops, boulders etc.
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4 S b i i l vs super-critical flow
4. Sub-critical            i i l fl
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5.                 velocity
5 Estimating flow velocity, depth: Manning equation
Flow in Natural Channels
Spiral or helical flow
occurs at river
bends/meanders.
bends/meanders The
centrifugal force pushes
the water out and down,
d                  h f
and creates somewhat of a
“corkscrew” motion. This
y
and scour on the outside
of the meander bend, and
causes the meander to
grow over time.
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4 S b i i l vs super-critical flow
4. Sub-critical            i i l fl
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5.                 velocity
5 Estimating flow velocity, depth: Manning equation
One-dimensional
(downstream) flow:

assume that the forces pushing
the water downhill (gravity) are
balanced by the resistance to
flow (shear stress) exerted by the
(             )         y
channel bed and banks and the
internal shear of the water
flowing past itself.

We can assume that the flow
velocity (V) will be directly
ti l t th l
proportional to the slope andd
to the roughness
Manning’s equation is applicable to steady, uniform flow conditions
Estimating values of Manning’s “n”: roughness coefficient
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4. Sub-critical            i i l fl
4 S b i i l vs super-critical flow
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5 Estimating flow velocity, depth: Manning s equation
5.                 velocity         Manning’s
6. Beyond Manning’s Equation: Backwater programs (HEC-RAS)
7. Hydrodynamic modeling: 1-d, 2-d, 3-d: unsteady flow models
Open Channel Flow: how do we do this in real life?
1.   Not surprisingly, we have computer programs that help us do this. There are a
companies
number available from different software companies, but the most common one is
produced by the US Army COE in the Davis HEC center.
2.   It is called HEC-RAS (for River Analysis System). It used to be called HEC-2.
3.   It is often used in conjunction with HEC-HMS (old HEC-1):
j                          (             )
1. We first use HEC-HMS to tell us what the flow RATE will be at a given
location along a river, in response to a certain rainfall event.
2. Then we use HEC-HMS to tell us how deep and how fast the water will be
flowing at our “point of interest”
4.   We have to provide the computer program with information about our river
channel system (just like we did for our watershed in HEC-HMS). We need to
i i          hi f       i
give it enough information to solve M i
l Mannings equation: channel area (A), wetted
i     h      l     (A)    d
perimeter (P), slope (S) and roughness (n) and the flow rate (Q):
1. Channel cross-section data: we used surveyed cross section data to tell the
P).
program what our river looks like at a specific location (A and P) We provide
multiple cross-sections along a river reach to characterize changes in the
channel shape at various locations.
2                                                                   cross-section
2. The model computes channel slope (S) by comparing one cross section with
the ones above and below it.
3. We tell the program how “rough” the channel is (with Mannings “n”)
5.   Using this data, the program computes the depth and velocity of flow at each XS.
Open Channel Flow
y        p
1. Why is it important?
2. Some definitions
3. Hydraulic processes: “types of flow”
1.
2. Uniform vs nonuniform
3. Laminar vs turbulent flow (vertical velocity profile)
4. Sub-critical            i i l fl
4 S b i i l vs super-critical flow
Specific energy
p
- critical depth
- froude number
4. Flow in Actual Channels
5 Estimating flow velocity, depth: Manning s equation
5.                 velocity         Manning’s
6. Beyond Manning’s Equation: Backwater programs (HEC-RAS)
7. Hydrodynamic modeling: 1-d, 2-d, 3-d: unsteady flow models

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