LECTURE 11 Flashcards
a weir structure, is limited to a maximum drop of 3 m and it is not a favorable structure where temporary spillway storage is desired to obtain a large reduction in the discharge at or d/s from the structure.
drop structure
PGCS means
Permanent Gully Control Structures
Three basic permanent structures, generally employed in stabilizing gullies are:
- Drop spillway
- Drop inlet spillway
- Chute spillway
The main functions of PGCS are:
a) To halt the advance of over-fall at gully head,
b) To stabilize the grade so that a gully can be changed to vegetative waterway.
Two primary causes of failure of permanent structures are:
a) Insufficient hydraulic capacity and
b) Insufficient provision for energy dissipation.
Basic data needed for the planning for design:
- Topographic map of the contributing watershed and the adjoining downstream area,
- Information on soil.
- Information on rainfall.
- Existing land use pattern.
The design procedure of a PGCS may be divided into three phases:
- Hydrologic design
- Hydraulic design
- Structural design
- It involves the determination of the design runoff rates and volumes which the
structure is expected to handle. - Prediction of design peak runoff rates and flood volumes includes the study of the
factors influencing the runoff characteristics of rainfall and watershed. 3. It is designed to handle runoff from the heaviest rain expected once in 25 to 50 years
or more, depending upon the estimated life of structure. - For the design of spillway for flood protection structures like drop inlet spillway
information on total runoff volume, inflow-outflow, reservoir stage and storage
characteristics are important. - Flood routing procedure is employed in designing the spillways of drop inlet
structures. - The Rational method of predicting peak runoff rate can be employed for designing
drop structures and chute spillways.
Hydrologic design
- It involves the study of the requirements of the dimensions of the structure, in order
to handle the estimated peak runoff through drop and chute structures. - It also involves the study of the effects of flow on the upstream and downstream
reaches of the channel and the dissipation of the kinetic energy produced by the drop
in the water surface elevation.
Hydraulic design
- It provides the required strength and stability to the component parts of the
structure. It involves the analysis of the various forces acting on the structure. The
forces are:These forces disturb the equilibrium of the structure and give rise to
internal stress, which should be effectively resisted by the material with which the
structure is constructed.
a. The water pressure (static and dynamic) which acts on the structure.
b. The forces developed due to the outflow over the structure.
c. The effect of water flow underneath the structure (seepage, sub-surface flow). - These forces disturb the equilibrium of the structure and give rise to internal stress,
which should be effectively resisted by the material with which the structure is
constructed. - The structure must be stable under the action of the external forces and be able to
withstand the sliding forces resulting from its own weight.
Structural design
Basi components of PGCS divided into three groups:
a. Inlet
b. Conduit
c. Outlet
Water enters the structures through the ____, which may be in the form of a box or
weir in a wall.
inlet
receives the water from the inlet and conducts it through
the structure. It restricts the water to a definite channel. The ______ may be closed in the
form of a box channel or it may be open as in a rectangular channel.
conduit
Its function is to discharge the water into the channel below at a safe velocity. The
________ should provide for the dissipation of kinetic energy of the discharge within the
confines of the structure.
outlet
It acts as a front wall against runoff flow in the drop spillway. It is constructed across the gully width.
head wall
It is the extended portion of head wall into the gully sides. It permits stable fill and prevents piping (due to seepage) around the
structure. Its main function is to provide structural strength against sliding of
the structure and also to prevent the flow of water from the sides of the drop
spillway.
Head wall extension
These are constructed at the rear end of the structure with some
inclination, usually at 45o from the vertical. These walls are extended up to the
gully sides and perform the function of preventing the flow backward into the
space left between gully wall and side wall of the structure. They provide
stability to the fill and protect the gully banks and surface.
wing walls
These are constructed to provide structural strength against
sliding of the structure. They increase frictional resistance of the structure
which opposes the force causing the slide.
cut-off walls
Prevent undercutting of apron.
Toe walls
These are constructed in the side along the gully walls. They guide the water and protect the fill against erosion. The function of the _______ is
to prevent splashing of water over the gully banks and also to confine the
water flow within the apron.
side walls
These are the elevated portion of rear end of the apron. Its main
function is to obstruct the water from directly moving into the channel below.
They also raise the tail water level to create hydraulic jump and to dissipate the
energy of the flowing water.
end sills
These are constructed in the apron section. They are constructed lengthwise parallel to the side walls. The sills are useful to make
the apron stable.
Longitudinal Sills
It is one of the main downstream components of the straight drop
spillway as it receives the gully flow with high velocity and changes the flow
regime so as to minimize the soil erosion on the downstream channel.
Apron
used to control the velocity of runoff in a channel by allowing the
water to fall from higher elevation to much lower elevation.
drop structure
The main three purposes of
drop spillways are as follows:
- To provide a transition between a broad or flat waterways and ditch or gully section.
- To raise the flow line to allow formation of sufficient soil depth for vegetative growth
where bottom of the gully is at risk. - To raise the flow line of the waterway so as to provide drainage in case of wet
waterways.
In hydraulic design, the design of drop structures is done using the following steps:
- Inlet design
- Outlet design
- Design of component parts
It involves the determination of strength and stability of different parts of the structure.
Structural design
It involves the estimation of design runoff rate and flood volume which the structures have
to handle safely. Runoff rate is calculated by the rational method.
Hydrologic design
The various components of an earthen dam are:
- Shell, Upstream Fill, Downstream Fill or Shoulder
- Upstream blanket
- Drainage filter
- cut off wall
- riprap
- Core Wall, Membrane or Core
- toe drain
- transition filter
types of earther dam (based on method of construction)
(a) Rolled Fill Earthen Dams:
(b) Hydraulic Fill Earthen Dam:
types of earther dam (based on the mechanical characteristics of earth materials used in making the section of dam)
(a) Homogeneous Earthen Dams:
(b) Zoned Earthen Dams:
(c) Diaphragm Earthen Dam:
In this type of dams, successive layers of moistened or damp
soils are placed one above the other. Each layer not exceeding 20 cm in thickness is properly
consolidated at optimum moisture content maintained by sprinkling water. It is compacted
by a mechanical roller and only then the next layer is laid.
Rolled Fill Earthen Dams:
In this type of dams, the construction, excavation and
transportation of the earth are done by hydraulic methods. Outer edges of the embankments
are kept slightly higher than the middle portion of each layer. During construction, a
mixture of excavated materials in slurry condition is pumped and discharged at the edges.
Hydraulic Fill Earthen Dam:
It is composed of one kind of material (excluding slope
protection). The material used must be sufficiently impervious to provide an adequate water
barrier, and the slopes must be moderately flat for stability and ease of maintenance
Homogeneous Earthen Dams:
It contains a central impervious core, surrounded by zones of
more pervious material, called shells. These pervious zones or shells support and protect the
impervious core
Zoned Earthen Dams:
This type of dam (Fig. 11.4) is a modified form of
homogenous dam which is constructed with pervious materials, with a thin impervious
diaphragm in the central part to prevent seepage of water. The thin impervious diaphragm
may be made of impervious clayey soil, cement concrete or masonry or any impervious
material. The diaphragm can be constructed in the central portion or on the upstream face of
the dam. The main difference in zoned and diaphragm type of dams depends on the
thickness of the impervious core or diaphragm. The thickness of the diaphragm is not more
than 10 m.
Diaphragm Earthen Dam:
These components of the earthen
dam are constructed with pervious or semi-pervious materials upstream or
downstream of the core. The upstream fill is called the upstream shell and the
downstream portion is the downstream shell.
Shell, Upstream Fill, Downstream Fill or Shoulder:
It is a layer of impervious material laid on the upstream side of an
earthen dam where the substratum is pervious, to reduce seepage and increase the
path of flow. The blanket decreases both the seepage flow and excess pressure on the
downstream side of the dam. A natural blanket is a cover of naturally occurring soil
material of low permeability.
Upstream Blanket:
It is a blanket of pervious material constructed at the foundation to
the downstream side of an earthen dam, to permit the discharge of seepage and
minimize the possibility of piping failure.
Drainage Filter:
It is a wall, collar or other structure intended to reduce
percolation of water through porous strata. It is provided in or on the foundations.
Cutoff Wall or Cutoff:
Broken stones or rock pieces are placed on the slopes of embankment
particularly the upstream side for protecting the slope against the action of water,
mainly wave action and erosion.
Riprap:
It is a centrally provided fairly impervious wall in the
dam. It checks the flow of water through the dam section. It may be of compacted
puddled clay, masonry, or concrete built inside the dam.
Core Wall, Membrane or Core:
It is a drain constructed at the downstream slope of an earthen dam to
collect and drain away the seepage water collected by the drain filters.
Toe Drain:
It is a component of an earthen dam section which is provided with
core and consists of an intermediate grade of material placed between the core and
the shells to serve as a filter and prevent lateral movement of fine material from the
core.
Transition Filter:
Components of Drop Inlet Spillway
Anti-seep Collars:
Cradle to the Conduit:
Emergency Spillway:
Stone Pitching:
Filters:
These are provided on the conduit pipe and are constructed using
concrete masonry. These are necessary for control of seepages and prevention of
failure due to piping. The total length of the seepage collars should be nearly 30 per
cent of the total length of seepage and to get this length two or more seepage collars
are provided. The anti-seep collars shall be placed within the saturated zone. The
normal saturation zone may be determined by projecting a line at a slope of
4 horizontal to 1 vertical from the point where the normal water elevation touches the
upstream slope of the fill to a point where this line intersects the invert of the pipe
barrel. All soil fill located below this line may be assumed as saturated.
Anti-seep Collars:
To prevent uneven settlement and to develop hoop stress in
the concrete pipes a cradle of masonry or concrete is provided to the conduit.
Concrete pipes withstand more loads when hoop stress is developed than otherwise.
Cradle to the Conduit:
If the runoff exceeds the design runoff, there is a danger of
overtopping of the embankment and failure of the structure. To prevent such an
occurrence, an emergency spillway is located on the embankment at the convenient
location. The emergency spillway leads to downstream of the structure. The channel
of the emergency spillway is protected with grass or stone pitching. The flood routing
procedure gives the elevation at which the emergency spillway is to be located.
Emergency Spillway:
______ is recommended on the upstream side on the
embankment and downstream side beyond the outlet to prevent soil erosion.
Stone Pitching:
Sand and gravel filters are provided to help drainage and prevent piping.
filters
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