gully control measures drop spillway 2011

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Gully Control Measures

Prevention of formation of a gully is easier and better than

Gully Control Measures

Prevention of formation of a gully is easier and better than controlling the existing gullySoil conservationists often confront with problems of controlling further development of established gullies. Gully formation & development

Runoff is the prime causeC t l f ff f th d i fi t tControl of runoff from the drainage area –first step

Three waysRetention of runoff in the gullied areaDiversion of runoff around gullied areaConveyance of runoff through the gully

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1. Retention of runoff in the gullied area

Possible through good crop management and applicable conservation practices

Contouring, strip cropping, bunding, terracing etcWhere contour bunds are used,

runoff is greatly reduced.

On cultivated areas, small and medium sized gullies can also be reclaimed

by placing a series of earth fills across the gully.

2. Diversion of runoff around gullied area

Complete elimination of runoff from the gullied area – Most Co p ete e at o o u o o t e gu ed a ea osteffective control How

by diverting runoff from the gully, causing it to flow at a non- erosive velocity to a suitable outlet.

Terraces and diversion ditches used for diverting runoff from its natural outlet. Terraces are very effective in the control of y

small gullies on cultivated fields or even medium size shallow gullies.

If the slope above a gully is too steep for terracing, or if the drainage area is pasture or woodland,

diversion ditches may be used to keep the runoff out of gully.

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3.Conveyance of runoff through the gully

If it is not possible to either retain or divert theIf it is not possible to either retain or divert the runoff,

runoff must be conveyed through the gully itself.

Possible only if vegetation can be established in the gullies, or if soil conservation structure are built at critical points to give primary controlgive primary control.

Vegetati e meas re

Measures to Control Gully

Vegetative measure — anti-erosion crops are grown that not only stabilize

the gully but a supplementary income is made through crop

― But generally, alone these are not very effective so to get a better control, they must be supplemented either by a temporary or a permanent gully control structure.

Temporary gully control structures.Permanent gully control structures.

- has a life of 3 to 8 years & pretty effective where the amount of runoff is not too large

Used when erosion control program requires bigger structure

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Vegetative measures

1. Changing gully into grassed waterway:

Small and medium size gullies can be converted into grassed waterways.In practice, gully is shaped and suitable species of grasses are grownChannel cross – section should be broad and flat,

to keep water spread uniform over a wide area

Used to control over fall in gullies with less

Vegetative measures

2.Sod flumes

than 3 m head and 10 ha area.Prevents further waterfall erosion by providing a protected surface over which the runoff may flow into the gullySlope varies with

soil type, size of watershed,

30 cm

Sod

3 m

height of overfall and type of sod used

To maintain a non– erosion velocity, flume should be enough wide. Maximum depth of flow over the flume should not exceed 30 cm

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3.Sod checks

) S d i h k

Vegetative measures

a) Sod strip checksBest adapted to small gullies that has small to medium sized watershedsStrips are laid across gully channel. Strips should have a minimum width of 30 cm and should extend up to gully sides at least 15cmStrip spacing usually varies from 1.5 to 2.0 m.

SoddedSoddedstrips

Low sodded earthfill

30 cm

1.5 to2.0 m

Elevation

Vegetative measure

b. Sod Checks:They can be used as substitute temporary gully control structure in small and medium gulliesAlready growing sods are cut along with soil mass and combined together to form earth fill damsMax. Height = 45 cm, U/s side slope = 3:1, and D/s side slope = 4:1

15 – 30 cm

3:1u/s

45 cm

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4. Plantation

Tree, shrubs etc. are usually used to stabilize severely eroded and

Vegetative measures

Tree, shrubs etc. are usually used to stabilize severely eroded and gullied area. Generally gullied area is fenced and trees are grownSpacing between plants = 1 × 1 m or 1.2 × 1.2 mMaximum 2 × 2 m

Temporary Gully Control Structure

Made of locally available materials and have a life of 3 to 8 yrsMade of locally available materials and have a life of 3 to 8 yrsWoven wire check damBrush damLoose rock damPlank or slab damLog damg

Basic purposes they serve are,―Retain more water as well as soil for proper plant growth.—Prevent channel erosion until sufficient vegetation is

established in u/s.

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Design criteria

The overall height of a temporary check shouldn’t ordinarily be


The overall height of a temporary check shouldn t ordinarily be more than 75 cm. An effective height of about 30 cm is usually considered sufficient. Also, sufficient freeboard id necessary.Life of the check dams under ordinary conditions should be in between 3 to 8 years.Spillway capacity of check dams is generally designed to handle peak runoff that may be expected over 5 to 10 year periodpeak runoff that may be expected over 5 to 10 year period.

Contd

Since the purpose of check dams in gully control is to eliminateSince the purpose of check dams in gully control is to eliminategrade in the channel, check dams theoretically should be spacedin such a way that the crest elevation of one will be same as thebottom elevation of the adjacent dam up-streamAs an integral part of most of checks dams, an apron or platformof sufficient length and width must be provided at the down-stream end to catch the water falling over the top and to conductstream end to catch the water falling over the top and to conductit safely without scouring.

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1.Woven Wire Dams

These dams are used in gullies of moderate slope and small


These dams are used in gullies of moderate slope and small drainage area.They help in the establishment of vegetation for permanent control of erosionThe dam is commonly built in half-moon shape with the open end up-stream.

Contd

The amount of curvature is arbitrary but an off set equal to 1/6th of the


The amount of curvature is arbitrary but an off-set equal to 1/6th of the width of gully at the dam site is optimum.To construct a woven-wire dam a row of posts is set along the curve of the proposed dam at about 1.2 m intervals and 60-90 cm deepHeavy gauge woven wire is placed against the post with the lower part set in a trench (15-20 cm deep) so that 25-30 cm projects above the ground surface along the spillway interval.g g p yRock, brush or sod may be placed for approximately up to a length of 1.2 m to form the apron.For sealing the structure, straw, fine brush or similar material should be placed against the wire on the upstream side to the height of spillway crest.

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2.Brush Dams

Th h d t b ild b t l t t bl f ll t f h k


They are cheap and easy to build, but least stable of all types of check dams. They are best suited for gullies with small drainage area.The center of the dam is kept lower than the ends to allow water to flow over the dam rather than around it.For a distance of 3-4.5 m along the site of structure, sides and bottom of the gully are covered with thin layer of straw or similar fine mulch.Brushes are then packed closely together over the mulch to about one halfBrushes are then packed closely together over the mulch to about one half of the proposed height of dam.Several rows of stakes are then driven crosswise in the gully, with rows 60 cm apart, and stakes 30-60 cm apart in the rows.

Contd

Heavy galvanized wire is used to fasten the stakes in a row as well as to


Heavy galvanized wire is used to fasten the stakes in a row, as well as to firmly compress the brushes in places.Sometimes large stones are also placed on top of brush to keep it compressed and in close contact with the bottom of the gully.The major weakness is the difficulty of preventing the leaks and constant attention is required to plug openings of appropriate size with straw as they develop.

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3.Loose Rock Dam

Loose rock dams are suitable for gullies have small to medium size


Loose rock dams are suitable for gullies have small to medium size drainage area.They are used in areas where stones or rocks of appreciable size and suitable quality are available. Flat stones are the best choice for dam making as they can be laid in such a way that the entire structure is keyed together.If round or irregular shed stones are used, structure is generally encased g , g yin woven-wire so as to prevent outside stones from being washed away.If the rocks are small, they should be enclosed in a cage of woven-wire.To construct the dam, a trench is first made across the gully to a depth of about 30 cm

Contd

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This forms the base of the dam on which the stones are laid in rows and


This forms the base of the dam on which the stones are laid in rows and are brought to the required height. The center of the dam is kept lower than the sides to form spillway.To serve as an apron, several large flat rocks may be countersunk below the spillway, extending about 1 m down-stream from the base of the dam.

4. Plank or Slab Dam

These dams are suitable in areas where timber is plentiful and can be


These dams are suitable in areas where timber is plentiful and can be constructed with much less labor as compared to other types of temporary structures. They can generally be used in gullies with larger drainage area.The planks are placed across the gully to form the dam.If the planks are mot close fitting, straw or grass may be used for sealing purposesA suitable opening for the spillway notch is made over the headwall. On the up-stream face, a well tempered earth fill is made.On the down-stream, the apron may be made of loose rock, brush, sod or planks.

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5. Log Dam


They are similar to plank or slab dams. However, they are used.

Permanent Structures

Permanent structures, built of masonry, reinforced concrete or earth, are efficient supplemental control measures in soil and water conservation

They are helpful in situations where vegetative measures or temporary structures fail to serve the purpose of (alone are inadeq ate to) controlling the concentration of r noffare inadequate to) controlling the concentration of runoff or reclaim a gully

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Main functions of permanent structures

Permanent structures

Main functions of permanent structures

To halt the advance of overfall at gully headTo stabilize the grade so that a gully can be changed to a vegetated waterway

Permanent structures are costly and thus usedWhen the vegetative measures and temporary structures fail to serve the purpose

Permanent structures are generally used in medium to large gullies with medium to large drainage area

Spillway types

Three basic permanent structures employed in stabilizing


Three basic permanent structures employed in stabilizing gullies

Drop spillwayDrop-inlet spillwayChute spillwayp y

Each of these is adaptable to specific site conditions

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Design Requirements of Permanent Structures

A gully control structure must not only have sufficient capacity to pass the


g y y p y pdesign discharge, but the kinetic energy of the discharge must also be dissipated within the confines of the structures, in a manner and to a degree that will protect both the structure and the downstream channel from damage.

Primary causes of failure of permanent structures are:Insufficient hydraulic capacity.Insufficient provision for energy dissipation.

Soil conser ation str ct res are designed to performSoil conservation structures are designed to perform Water conservation ( for irrigation, industrial and municipal uses and for fish culture etc. )Soil erosion controlSediment controlFlood controlDrainage

Planning for design

These structures must be designed after careful investigations of


These structures must be designed after careful investigations of Various factors influencing the characteristics of runoff approaching the structure (with reference to specific site conditions)The d/s flow characteristics Other specific requirements

There are no standard solutions, which can be applied to all the problems encountered in the fieldThe design should include an analysis of all the factors affecting the work

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Basic data requirements

Topographic map of the contributing watershed and the


Topographic map of the contributing watershed and the adjoining downstream areaInformation on soilInformation on rainfall Existing land use pattern

In the absence of adequate data, a safe design requires conservation assumptions

Design procedure

The design may be divided into three phases:


The design may be divided into three phases:

Hydrologic designHydraulic designStructural design

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Hydrologic design

Determination of the design runoff rates and flood volumes which


ete at o o t e des g u o ates a d ood vo u es w cthe structure is expected to handle

Prediction of design peak runoff rates and flood volumes includes the study of the factors influencing the runoff.

They include the characteristics of rainfall and watershed. The principal rainfall characteristic is the amount, intensity p p , yand distribution pattern during the year.

These structures are designed to handle runoff from the heaviest rains that may be expected once in 25 to 50 years or more, depending upon the estimated life of structure.

Hydrologic design

For the design of spillway for flood protection structures like


For the design of spillway for flood protection structures like drop inlet spillway information on total volume of runoff and the inflow-outflow and reservoir stage and storage characteristics are important.

Flood routing procedures are employed in designing he spillway of drop inlet structuresof drop inlet structures.

The rational method of predicting peak runoff rates can be employed for designing drop structures and chute spillways.

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Hydraulic design

Deals with determination of dimensions of the structure which can


Deals with determination of dimensions of the structure which can handle

the estimated peak runoff through drop and chute structures and the design outflow and storage capacity in case of drop inlet structures

It also involves the study of the effect of flow on Upstream and downstream reaches of the channel Dissipation of the kinetic energy liberated by the drop in the water surface elevation

Structural designStructural design provides the required strength and stability to


St uctu a des g p ov des t e equ ed st e gt a d stab ty tothe component parts of the structure

It involves the analysis of various forces acting on the structure.The water pressure (static and dynamic) which acts on the structureForces developed due to the overflow over the structureThe effect of water flow underneath the structureThe effect of water flow underneath the structure

e.g. seepage, subsurface flow.

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.

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Basic Components

Inlet


InletWater enters the structure through the inlet, which may be in the form of a box or weir in a wall

ConduitReceives the water from the inlet and conduits it h h hthrough the structure

It restricts the water to a definite channelIt may be closed in the form of a box channel or open as in a rectangular channel

Basic Components

Outlet


OutletDischarge the water into the channel below at a safe velocity

Should provide for the dissipation of kinetic energy of the discharge within the confines of the structure

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Drop spillway

It is a weir structure in which flow passes through the weir


It is a weir structure, in which flow passes through the weir opening, fall or drops on an approximately level apron or stilling basin and then passes into the downstream channel

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Uses of drop spillway

To control gradient in either natural or constructed channels

Drop Spillway

To control gradient in either natural or constructed channels

To control tail-water at the outlet of a spillway or conduit

To serve as reservoir spillway where the total drop is relatively low

To serve as inlet and outlet structures for tile drainage system in conjunction with gradient control

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Material

For most soil conditions drop spillways may be built of any of the

Drop Spillway

For most soil conditions, drop spillways may be built of any of the construction materials adopted for use in hydraulic structures.

It may be concrete, masonry, concrete blocks etcReinforced concrete is most widely used and has been very satisfactory for long – life and low annual cost.

In case a number of structures are involved, the selection of i l h ld b b dmaterial should be based on

Required life span of structures Annual cost comparison, which includes maintenance, for built of different available material

Advantages

Stability

Drop Spillway

Stab tyIt is very stable and likelihood of serious structural damage is remote

Non – clogging of weirRectangular weir, which is used in this case, is less susceptible to clogging

by debris.Low maintenance costEase & economy of construction

Relatively easy to constructRelatively easy to constructStandardization

They may be standardized which result in savings in engineering and constructional costs.

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Disadvantages ( Limitations )

Drop Spillway

Maximum drop 3 mIf used for more than 3 m drop, they may be more costlier than other structures

It is not a favorable structure where it is desired to use temporary spillway storage to obtain a large reduction in the discharge at orspillway storage to obtain a large reduction in the discharge at or d/s from the structure

Components and Functions

Head wall extension

Drop Spillway

Permits stable fill and prevents piping ( due to seepage around the structure)

Wing wallsProvides stability to fill and gives protection to gully banks and surface

Cut – off wallP t i i d thPrevents piping under the structure besides reducing uplift and preventing sliding

Toe wallPrevents undercutting of apron

Side wall Guides the water and protects the fill against erosion

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Hydraulic Design

Hydrologic Determinations

Drop Spillway

Hydrologic Determinations

Peak rate of runoff.Inflow hydrograph of runoff.

Selection of the frequency of design flood flow for a particular drop spillway should be based on the following factors:drop spillway should be based on the following factors:

Life of the structure ( intended ) Probable extent of damage if the spillway fails due to lack of discharge capacity.Relative size and cost of structure

Design Approach

Generally two cases are encountered

Drop Spillway

Generally two cases are encountered

Discharge capacity, Q and total drop, F are Known

F, L and h are known

To find Q of spillway operating withTo find length, L, and depth, h , of the weir to provide adequate discharge capacity and freeboard

To find Q of spillway operating with adequate freeboard or at maximum capacity

In either case flow may be free or submerged

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Aeration

Drop Spillway

Free flow occurs when the weir is adequately aerated Prevent formation of excessive negative pressure below nappe

which, in turn, would causes fluctuations of head, instability of flow and increased load on headwall

Air vent

Nappe

Size of air vent can be estimated by the equation

Drop Spillway

Size of air vent can be estimated by the equation

Where, A = required area of aeration hole (s), sq.cmL = length of weir, mHe= sp. energy head producing flow through weir, m

3.64

1.640.121A HeL p=

e p gy p g g ,P = differential pressure between atmosphere and

pressure under the nappe, m of water.

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The discharge capacity of an aerated, rectangular weir without submergence is

Drop Spillway

Q = cumecs; H = head on weir, m ;Va= mean velocity of approach, m/sec

23 / 2( )

2aVQ C L Hg

= +

Total energy head, which causes the flow over the weir

C = discharge coefficient =1.77The section at which the total energy head i.e H + Va2 /2g

It should be 3H or more u/s from the weirIt should not be so far u/s that the energy losses between the chosen section and weir will affect the design. Normally (3H – 5H)

Freeboard

It is the vertical distance from the maximum water surface

Drop Spillway

It is the vertical distance from the maximum water surface elevation on u/s of head wall to the top of headwall extension

It is a factor provided against possible occurrence of conditions, not anticipated during design and protection against overtopping by wave actionagainst overtopping by wave action

2

,2

aVTotal weir depth h f Hg

= + +

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Design for free flow condition

Case I

Drop Spillway

Case IKnown : Q, F, free flow condition.To find : combination of L & h for desired Q and minimum freeboard.

(1.1 0.03 )Q FUse L +

Obtain relationship between L & h.

3 / 2.Use L

C h=

Example

Drop Spillway

Given Q = 10 cumec and F = 2.5 m for free flow condition L = 4 m.

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Required Length Practical Depth Calculated

Drop Spillway

qdischarge

gLength

pdischarge

10 2.35 3 2 12.7810 3.28 4 1.6 12.1910 3.61 4 1.5 11.0710 3.8 4 1.45 10.5210 4.0 4 1.4 9.98

Solution

232

2 / 3 1 0 .5 2( ) 1 .32 1 .7 7 4

aV QHg C L

⎛ ⎞+ = = =⎜ ⎟×⎝ ⎠

2

( ) 1.45 1.3 0.152

aVf h Hg

= − + = − =

Design of component parts

Height of transverse sill (s)

Drop Spillway

Height of transverse sill (s)

Height of headwall (HB) HB = F + s

23 22

dc QS where dc L g= =

Height of headwall extension (HE)HE = F + s+ h

Minimum length of headwall extension (E)E = 3h + 0.6 or 1.5 F whichever is more.

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Minimum length of apron L

Drop Spillway

Minimum length of apron, LB

Whichever is greater

Height of side wall & wing wall at the junction.( 0.13)2 , { }

2BLJ h or F h s +

= + + −

[2.3 0.50] 2[ 0.3 ],BhL F or F s h Jf

= + + + + −

Whichever is greater

Depth of cutoff wall (c) and toe wall (T).

2

1.65( 0.4 0.75)4

s FC T + += =

ExampleHeight of transverse sill (s)

Drop Spillway

Height of headwall (HB)

2 233 2 210.12 0.874 9.81

0.87 0.442 2

Qdc L gdcS

= = =×

= = =

HB = F + s = 2.5 + 0.44 = 2.94Height of headwall extension (HE)

HE = F + s+ h = 2.5 + 0.44 + 1.48 = 4.42

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Minimum length of headwall extension (E)

Drop Spillway

Minimum length of headwall extension (E)E = 3h + 0.6 = 3 x 1.48 + 0.6 = 5.06or 1.5 F = 1.5 x 2.5 = 3.75

Minimum length of apron, LB1.482.3 0.50 2.5 2.3 0.50 4.672.5B

hL Ff

⎡ ⎤ ⎡ ⎤= + = + =⎢ ⎥ ⎢ ⎥⎣ ⎦⎣ ⎦

Height of side wall & wing wall at the junction( 0.13)2 , { }

2(4.67 0.13)2 1.48 2.96 2.5 1.48 0.44 2.035

22.96

BLJ h or F h s

J or

J

+= + + −

+⎧ ⎫= × = + + − =⎨ ⎬⎩ ⎭

∴ =

Depth of cutoff wall (c) and toe wall (T)

Drop Spillway

Depth of cutoff wall (c) and toe wall (T).

1.65( 0.4 0.75)4

1.65(0.44 0.4 2.5 0.75) 0.94

s FC T + += =

+ × += =

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A drop inlet or shaft spillway is one in which the water enters

Drop Inlet Spillway

d op et o s a t sp way s o e w c t e wate e te sthrough a horizontally positioned circular or rectangular boxtype riser or inlet and flows to some type of outlet protectionthrough a circular (horizontal or near horizontal) conduit.

The inlet of spillway may be of plain concrete, reinforcedconcrete blocks, masonry or pipe.

The conduit may be reinforced concrete or clay tile, orcorrugated or smooth metal pipe having water tight joints.

Fig.:- Drop inlet Spillway

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Drop Inlet Spillway

It is used where head is over 3 m.It may also be used in connection with relatively low head suchas road culvert.It can be used as principal spillway for farm ponds and debrisbasins, grade stabilization, disposal system at lower end, roadwayt t fl d ti t t d f t i l t fstructure, flood prevention structure and surface water inlet for

drainage or irrigation.For high heads it requires less material than drop spillway.

A chute spillway is a steeply open channel which leads the water from the

Chute Spillway

A chute spillway is a steeply open channel, which leads the water from thereservoir to the downstream channel.

The concrete chute, is particularly adopted to high over fall, where a full flowstructure is required and where site conditions do not permit the use of adetention type structure.

It is usually more economical than drop structure when large capacities arerequired.q

It must be placed on compacted fill.

There is considerable danger of undermining of the structure due to rodents.

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Fig.:- Chute Spillway

Reinforced concrete is the most widely used and safest

Chute Spillway

Reinforced concrete is the most widely used and safestmaterial for large chutes.

It can be used to control the gradient in either natural orconstructed channel.

It can serve as spillway for flood prevention, waterconservation and sediment collection structure.

gully control measures drop spillway 2011

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