Hydraulics of Structures

1
Hydraulics of Structures
2
What are structures?

• Structures in this context are simply something
  placed in the channel to either measure or
  control flow.
• Example A principle spillway is used as part of
  a dam design to control the rate at which water
  is discharged from a reservoir.
• Include both inlet and outlet control devices.
• Control devices can operate as
• Open channel flow in which the flow has a free
  surface or
• Pipe flow in which the flow is in a closed
  conduit under pressure.

3
Most basic principle of hydraulics of structures

• As head on a structure increases, the flow that
  is discharged through the structure increases.
• Figure 5.1 (Haan et al., 1994) shows the
  head-discharge relationships for several flow
  control structures.

4
Weirs

• At its most basic, just an obstruction placed in
  a channel that constricts flow as it goes over a
  crest.
• The crest is the edge of the weir over which the
  water flows.
• As the water level (head) over the crest
  increases, the flow rate increases dramatically.
• Two basic types of weirs
• sharp crested
• broad crested

5
Sharp Crested Weirs

• A sharp crested weir is defined by a thin crest
  over which the water springs free as it leaves
  the upstream face of the weir.
• Flow over a weir is also called the nappe.
• Sharp crested weirs are generally constructed of
  sheet metal or similar thin material.

6
Sharp Crested Weir
7
Sharp Crested Weirs

• Can have several shapes
• Triangular (or v-notch)
• Rectangular
• Trapezoidal
• Classified by the shape of its notch.
• V-notch weirs have greater control under low flow
  conditions.
• Rectangular weirs have larger capacity but are
  less sensitive for flow measurement.

8
Sharp Crested Weirs-General
h
H
dh
z
9

• Making the assumption that the velocity head at
  the upstream point will be much smaller than the
  velocity head as the flow goes over the weir we
  assume v12/2g is negligible and

10
Integrating this from h 0 to h H gives
11
Rectangular Weirs
A rectangular weir that spans the full width of
the channel is known as a suppressed weir.
12

• Hydraulic head (H) for weirs is simply the height
  of the water surface above the weir crest,
  measured at a point upstream so that the
  influence of the velocity head can be ignored.
• L is the length of the weir.
• The coefficient of discharge (C) is dependent
  upon units and of the weir shape.
• For a suppressed weir with H/h lt 0.4 (where h is
  the height of the weir) C 3.33 can be used.
• For 0.4 lt H/h lt 10, C 3.27 0.4 H/h

13
A rectangular weir that does not span the whole
channel is called a weir with end contractions .
The effective length of the weir will be less
than the actual weir length due to contraction of
the flow jet caused by the sidewalls.
Where N is the number of contractions and L is
the measured length of the crest.
14
Triangular (v-notch ) weirs

• Used to measure flow in low flow conditions.

15

• For Q 90, K 2.5 (typically), tan (Q/2) 1
  therefore,

16

• Note Your handout with Figure 12.28 presents
  the equation for a v-notch weir as

with
17
Broad Crested Weirs
H
W
Where L is the width of the weir.
18
Broad Crested Weirs

• Broad crested weirs support the flow in the
  longitudinal direction (direction of flow).
• They are used where sharp-crested weirs may have
  maintenance problems.
• The nappe of a broad crested weir does not spring
  free.

19
Roadway Overtopping
Where Qo overtopping flowrate Cd - overtopping
discharge coefficient L length of roadway
crest HWr upstream depth
Cd ktCr Cr discharge coefficient kt
submergence factor Figure 5.7
20
Orifices

• An orifice is simply an opening through which
  flow occurs.
• They can be used to
• Control flow as in a drop inlet
• Measure the flow through a pipe.

21

• The discharge equation for orifice flow is

Where C is the orifice coefficient (0.6 for
sharp edges, 0.98 for rounded edges). A is the
cross-sectional area of the orifice in ft2 g is
the gravitational constant H is the head on the
orifice
22

• At low heads, orifices can act as weirs.
• Calculate the discharge using the suppressed weir
  equation where L is equal to the circumference of
  the pipe.
• Calculate the discharge using the orifice
  equation.
• The lower discharge will be the actual discharge.

23
Pipes as Flow Control Devices
H
H
Energy Grade Line
D
0.6D
L
Elbow and Transition
24
(No Transcript)
25
Head Loss Coefficients

• Ke is the entrance head loss coefficient and is
  typically given a value of 1.0 for circular
  inlets.
• Kb is the bend head loss coefficient and is
  typically given a value of 0.5 for circular
  risers connected to round conduits.
• For risers with rectangular inlets, the bend head
  losses and entrance head losses are typically
  combined to a term Ke where values of Ke can be
  found in Table 5.3 and

26
Head Loss Coefficients

• Kc is the head loss coefficient due to friction.
  
• Values for Kc are given in Tables 5.1 and 5.2 for
  circular and square pipes.
• Kc is multiplied by L, the entire length of the
  pipe, including the riser.

27

• Frequently, when the drop inlet is the same size
  as the remainder of the pipe, orifice flow will
  control and the pipe will never flow full.
• If it is desirable to have the pipe flowing full,
  it may be necessary to increase the size of the
  drop inlet.

28
Using Flow Control Structures as Spillways

• A given drop inlet spillway can have a variety of
  discharge relationships, given the head.
• At the lowest stages the riser acts as a weir.
• As the level of the reservoir rises, water
  flowing in from all sides of the inlet interferes
  so that the inlet begins to act as an orifice.
• As the level continues to rise, the outlet
  eventually begins to flow full and pipe flow
  prevails.
• A stage-discharge curve is developed by plotting
  Q vs. H for each of the three relationships. The
  minimum flow for a given head is the actual
  discharge used.

29
Rockfill Outlets as Controls
ROCKFILL
HYDRAULIC PROFILE
dh
h1
have
h2
dl
30
Rockfill Outlets

• Advantages
• Abundant
• Generally available
• Usually inexpensive
• Relative permanence

31
Rockfill Outlets

• Major expenses
• Grading
• Transporting
• Placing stone

32
Rockfill Outlets

• Used for
• Protective channel linings and breakwaters
• Add stability to dams
• Provide energy dissipation zones for reservoir
  outlets
• Flow control structure

33
Modified Darcy-Weisbach Equation
34
Rockfill as Control Structure Model
Reynolds Number Equation
Friction factor
35
Friction Factor-Reynolds Number Relationship
36
h2 have Relationships