Post on 14-Jan-2016
description
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
CIV 276 – DISEÑO DE OBRAS HIDRAULICAS
2
Desagües y drenajes
Objeto
Evitar el exceso de humedad en la obra básica.
Componentes
Desagües: facilitan el escurrimiento de las aguas superficiales.
• Alcantarillas: permiten el paso de aguas a través del
terraplén.
• Cunetas: canales abiertos para recolectar el agua superficial
proveniente de la calzada, banquina, taludes y cuenca
interceptada por el terraplén.
Drenajes: facilitan el escurrimiento de las aguas no superficiales.
3
Método Racional para la determinación de los caudales
a servir
Valores típicos de E:
0,15 : Terreno llano, permeable y boscoso.
0,50: Terreno ondulado con pasto o cultivo.
0,95: Pavimento.
360
** REMQ
Q = caudal a desaguar = m3/s
M = área de cuenca = Ha
R = intensidad = mm/h (a determinar)
E = coeficiente escorrentía (función de
las características de la cuenca)
Cuenca de un curso de agua en una sección
La totalidad de la superficie topográfica
drenada por el curso de agua y sus afluentes
aguas arriba de la sección
4
Cuencas de cunetas y alcantarillas
60
70
80
80
90
90
100 100 A B
C D
E
F
G
talw
eg
ABCD = Cuenca de la
alcantarilla.
AEFD = Cuenca de la
cuneta en DF.
GBCF = Cuenca de la
cuneta en FC.
Cuneta
Camino
Cuneta Alcantarilla
5
Dimensionado de alcantarillas
Fórmula de Talbot : 4 3* MCA
A = sección de la alcantarilla = m2
M = área de la cuenca = Ha
C = coeficiente de cuenca
• Conductos cerrados que dan continuidad al escurrimiento a través
del terraplén de un camino.
• Las secciones más comunes son las circulares, semicirculares,
rectangular, etc.
• Diámetro mínimo para evitar obstrucciones = 0,60m.
• Pendiente entre 0,5% y 2%.
• Las cabeceras retienen el talud del terraplén, encauzan la corriente
de agua y protegen el talud de socavaciones.
Valores típicos de C:
0,04 : Terreno llano.
0,10: Terreno ondulado.
0,18: Terreno montañoso.
6
Control de salida Control de entrada
Alcantarillas
ALGUNAS VISTAS
Weirs
Weirs: Weirs are elevated structures in open channels that are used to measure flow and/or control outflow elevations from basins and channels.
There are two types of weirs in common use:
Sharp-crested weirs and the broad-crested weirs.
The sharp-crested weirs are commonly used in irrigation practice
Sharp-Crested Weirs
Sharp-crested or thin plate, weirs consist of a plastic or metal plate that is set vertically across the width of the channel.
The main types of sharp-crested weirs are rectangular, V-notches and the Cipolletti or the Trapezoidal weirs.
The amount of discharge flowing through the opening is non-linearly related to the width of the opening and the depth of the water level in the approach section above the height of the weir crest.
Sharp Crested Weirs Contd.
Weirs can be classified as being contracted or suppressed depending on whether or not the nappe is constrained by the edges of the channel.
If the nappe is open to the atmosphere at the edges, it is said to be contracted because the flow contracts as it passes through the flow section and the width of the nappe is slightly less than the width of the weir crest (see figure).
If the sides of the channel are also the sides of the weir opening, the streamlines of flow are parallel to the walls of the channel and there is no contraction of flow.
Figure : Rectangular Weirs
(a) Suppressed Weir
(b) Unsuppressed Weir
(Contracted)
Sharp Crested Weirs Contd.
In this case, the weir is said to be
suppressed. Some type of air vent
must be installed in a suppressed weir
so air at atmospheric pressure is free to
circulate beneath the nappe. (See
Figure 6.2 for suppressed and
unsuppressed weirs).
Sharp Crested Weirs Contd.
The discharge, Q (m3/s) over a rectangular suppressed weir
can be derived as:
Q C g b Hd2
32 11 5. .................................( )
Where: Cd is the discharge coefficient, b is the width of the weir crest, m (see
Figure 6.2 above) and H is the head of water (m) above weir crest.
According to Rouse (1946) and Blevins( 1984),
………………..(2)
Where: Hw is the height of the crest of the weir above the bottom of the
channel.
CH
Hd
w
0 611 0 075. .
Weirs Contd
This equation is valid when H/Hw <5, and is approximated up to
H/Hw = 10. If H/Hw < 0.4, Cd can be approximated as 0. 62 and
equation (1) reduces to:
Q = 1.83 b H1.5 ………. (3)
This equation is normally used to compute flow over a rectangular suppressed
weir over the usual operating range. It is recommended that the upstream
head, H be measured between 4H and 5H upstream of the weir.
For the unsuppressed (contracted) weir, the air beneath the nappe is in contact
with the atmosphere and venting is not necessary. The effect of side
contractions is to reduce the effective width of the nappe by 0.1 H and that flow
rate over the weir, Q is estimated as:
Q = 1.83 (b – 0.2 H) H1.5 ………………… (4)
This equation is acceptable as long as b is longer than 3 H
Cipoletti Weir
A type of contracted weir which is
related to the rectangular sharp-crested
weir is the Cipoletti weir (see Figure 6.3
below) which has a trapezoidal cross-
section with side slopes 1:4 (H:V). The
advantage of a Cipolletti weir is that
corrections for end contractions are not
necessary.
Cipolletti Weir Contd.
The discharge formula can be written as:
Q = 1.859 b H1.5 …………….. (5)
Where: b is the bottom width of the Cipolletti weir. The
minimum head on standard rectangular and Cipolletti weirs is 6
mm and at heads less than 6 mm, the nappe does not spring free
of the crest.
Figure 6.3: A Trapezoidal of Cipolletti Weir
Example
A weir is be installed to measure flows
in the range of 0.5 to 1.0 m3/s. If the
maximum depth of water that can be
accommodated at the weir is 1 m and
the width of the channel is 4 m,
determine the height of a suppressed
weir that should be used to measure the
flow rate.
Solution to Example
The flow over the weir is shown in the Figure 6.4 below. The height of water is
Hw and the flow rate is Q. The height of water over the crest of the weir, H is
given by:
H = 1 – Hw
Assuming that H/Hw , 0.4, then Q is related to H by equation (3), where:
Q = 1.83 b H 1.5
Figure 6.4: Weir Flow
Solution to Example
Concluded
Taking b = 0.4 m, Q = 1m3/s (the maximum flow rate will give the
maximum head, H), then:
The height of the weir, Hw is therefore given by:
Hw = 1 – 0.265 = 0.735 m
And H/Hw = 0.265/0.735 = 0.36
The initial assumption that H/Hw < 0.4 is therefore validated, and
the height of the weir should be 0.735 m.
m
b
QH 265.0
483.1
1
83.1
67.05.1/1
V-Notch Weir
A V-notch weir is a sharp-crested weir that has a V-shaped
opening instead of a rectangular-shaped opening. These weirs,
also called triangular weirs, are typically used instead of
rectangular weirs under low-flow conditions ( mainly < 0.28 m3/s),
where rectangular weirs tend to be less accurate. It can be
derived that the flow rate, Q over the weir is given by:
Q C g Hd8
152
2
2 5tan ( ) .
V-Notch Weirs Contd.
Parshall Flume
Although weirs are the simplest structures for measuring the discharge in open channels, the high head losses caused by weirs and the tendency for suspended particles to accumulate behind weirs may be important limitations.
The Parshall flume provides an alternative to the weir for measuring flow rates in open channels where high head losses and sediment accumulation are of concern.
Such cases include flow measurement in irrigation channels.
The Parshall flume (see Figures 6.7 and 6.8 below) consists of a converging section that causes critical flow conditions, followed by a steep throat section that provides for a transition to supercritical flow.
Parshall Flume
Parshall Flumes
Parshall Flume Contd.
The unique relationship between the depth of
flow and the flow rate under critical flow
conditions is the basic principle on which the
Parshall flume operates.
The transition from supercritical flow to
subcritical flow at the exit of the flume usually
occurs via a hydraulic jump, but under high
tail water conditions the jump is sometimes
submerged.
Parshall Flume Contd
Within the flume structure, water depths are measured at two locations, one in the converging section, Ha and the other at the throat section, Hb. The flow depth in the throat section is measured relative to the bottom of the converging section as illustrated in the figure below.
If the hydraulic jump at the exit of the Parshall flume is not submerged, then the discharge through the flume is related to the measured flow depth in the converging section, Ha by the empirical discharge relations given in Table 6.2, where Q is the discharge in ft3/s, W is the width of the throat in ft, and Ha is measured in ft.
Parshall Flume Contd
Submergence of the hydraulic jump is determined by the ratio of the flow depth in the throat, Hb, to the flow depth in the converging section, Ha, and critical values for the Hb/Ha are given in Table 6.3.
Whenever, the ratio exceeds the critical values in the table, the hydraulic jump is submerged and the discharge is reduced from the values given by the equations in Table 6.2.
Corrections to the theoretical flow rates as a function of Ha and the percentage of submergence, Hb/Ha are given in the Figures 6.8 and 6.9 below for throat widths of 1 ft and 10 ft.
Parshall Flumes Contd.
Parshall Flumes Contd.
Flow corrections for the 1 ft flume are applied to larger flumes by multiplying the correction for the 1 ft flume by a factor corresponding to the flume size given in Table 6.4.
Similarly, flow corrections for flume sizes greater than 10 ft. are applied to larger flumes by multiplying the correction for the 10 ft flume by a factor corresponding to the flume size given in Table 6.5.
Parshall flumes do not reliably measure flow rates when the submergence ratio, Hb/Ha exceeds 0.95.
Parshall Flume Correction
Tables For Parshall Flume
Correction
Example
Example : Flow is being measured by
a Parshall flume that has a throat width
of 2 ft. Determine the flow rate through
the flume when the water depth in the
converging section is 2.00 ft and the
depth in the throat section is 1.70ft.
Solution to Example
From the given data: W = 2 ft, Ha = 2 ft, and Hb = 1.7 ft.
According to Table 6.2, Q is given by:
In this case: Hb/Ha = 1.7/2 = 0.85
Therefore, according to Table 6.3, the flow is submerged. Figure
6.8 gives the flow rate correction for a 1 ft flume as 2ft3/s, and
Table 6.4 gives the correction factor for a 2 ft flume as 1.8. The
flow rate correction, dQ for a 2 ft flume is therefore given by:
DQ = 2 x 1.8 = 3.6 ft3/s
And the flow rate through the Parshall flume is Q – dQ, where Q
– dQ = 23.4 – 3.6
= 29.8 ft3/s
Q W H ft sa
W x 4 4 2 2 2341522 1522 2 3
0 226 0 026. .. .
( ) ( ) . /