Educational Basic Civil Engineering

Method of preventing dampness

1. By providing DPC ( Damp proof course )
   
2. By surface treatment i.e. by providing damp proof paint
   
3. By integral water proofing method
   
4. By special devices i.e. by providing chajjas & by providing cavity walls etc
   

Corbels

This is provided in internal side of roofs

• For decoration
  
• For preventing dampness
  

DPC ( Damp proof course )

It is continuous layer of impervious material

 

• For internal wall we only provide horizontal DPC ( 175 kg/cm2 standard pressure for bitumen )
  
• Three layers of bitumen is provided
  
• You should provide a mortar layer before DPC
  

Types of DPC

There are two types of DPC

1. Flexible DPC : It is DPC when load doesn’t crack
   

e.g. Polythene and Bitumen


Three layers

1. Bitumen mastic: Bitumen mix with fine sand
   
2. Bitumen felt: It is available in the form of rolled sheets
   
3. Hard laid bitumen
   
4. Metal sheets
   

e.g. Lead, copper, aluminum is provided with mortar, to avoid rusting.


Rigid DPC: It is DPC when loaded; it cracks e.g. Rich cement concrete 1:2:4

 

Design, Maintenance, Types & Components of Hydraulic Structures

Definition of Hydraulic Structures:

Hydraulic structures are anything that can be used to divert, restrict, stop, or otherwise manage the natural flow of water. They can be made from materials ranging from large rock and concrete to obscure items such as wooden timbers or tree trunks.


A dam, for instance, is a type of hydraulic structure used to hold water in a reservoir as potential energy, just as a weir is a type of hydraulic structure which can be used to pool water for irrigation, establish control of the bed (grade control) or, as a new innovative technique, to divert flow away from eroding banks or into diversion channels for flood control.


OR


A hydraulic structure is a structure submerged or partially submerged in any body of water, which disrupts the natural flow of water. They can be used to divert, disrupt or completely stop the flow. An example of a hydraulic structure would be a dam, which slows the normal flow rate of river in order to power turbines.A hydraulic structure can be built in rivers, a sea, or any body of water where there is a need for a change in the natural flow of water.


Rate "Hydraulic Structures Course " Help us know How we are doing??

 

Canal Headwork - Types & Locations



Definition:

Any hydraulic structure which supplies water to the off taking canal. Diversion head-work provides an obstruction across a river, so that the water level is raised and water is diverted to the channel at required level. The increase water level helps the flow of water by gravity and results in increasing the commanded area and reducing the water fluctuations in the river.


Diversion head-work may serve as silt regulator into the channel. Due to the obstruction, the velocity of the river decreases and silt settles at the bed. Clear water with permissible percentage of silt is allowed to flow through the regulator into the channel.


To prevent the direct transfer of flood water into the channel.


Functions of a Headwork

A headwork serves the following purposes

• A headwork raises the water level in the river
• It regulates the intake of water into the canal
• It also controls the entry of silt into the canal
• A headwork can also store water for small periods of time.
• Reduces fluctuations in the level of supply in river

Types of Headworks

1. Storage headwork
2. Diversion headwork

Component parts of Diversion Headwork

Types of Diversion head works

1. Temporary:
2. Spurs Bunds
3. Permanent

Components

1. Weir or Barrage
2. Divide Wall
3. Fish Ladder
4. Approach Canal
5. Silt prevention device
6. Canal head regulator
7. River training works

 

Location of Headworks

1. Rocky Stage
2. Sub mountainous or boulder stage: boulder or gravel
3. Alluvial plan

Rocky stage:

River steep slope, high velocity


Advantages:

1. Good foundation at shallow depth
2. Comparatively silt free water for turbines
3. High head for hydro-electric work

Disadvantages:

1. Long ---- length of canal. In reach soil is good for agriculture.
2. More cross damage works
3. More falls (ground steep gradient - lined to permit high velocity)
4. Costly head regulator excluding shingle
5. Frequent repairs of the weirs.

Sub mountainous or boulder stage: boulder or gravel

Advantages:

1. Less training works
2. Suitable soil for irrigation available
3. Availability of construction material locally.
4. Falls can be utilized for power generation

Disadvantages:

1. It has a strong sub-soil flow as a result
2. Reduce in storage and damage floor downstream
3. More percolation loss from canal
4. More x-drainage works
5. Less demand of water at head reaches (more idle length of canal)

Alluvial plan:

1. x- section of river alluvial sand silt
2. Bed slope small, velocity gentle
3. No idle length of canal
4. less x- drainage works
5. Comparatively less sub soil flow

Disadvantages:

1. Cost of headwork is more due to poor foundation
2. More river training works
3. Problem of silt in canal

 

Design of Barrage - How to - Criteria for Design of barrage



How to Design Barrage - Design of Barrage Criteria

The Barrage and the Head Regulators of feeder channels and appurtenant structures will be designed on the basis of standard design criteria established for other barrages and allied structures, already constructed on the Indus River and its tributaries.


The design criteria, including formulae, coefficients and constants will be used in all hydraulic designs as applicable.


Factors affecting the Design of Barrage are as follows:

1. Estimation of Design Flood
2. Hydraulic Units
3. Width of Barrage
4. Afflux
5. Tail Water Rating Curve
6. Crest Levels
7. Discharges through a Barrage (Free Flow Conditions)
8. Discharge through a Barrage (Submerged Flow Conditions)
   1. Fane's Curve
   2. Gibson's Curve

Estimation of Design Flood

Basis of Estimation

The design flood for any given return period is usually estimated by the frequency analysis method. Appropriate type of frequency distribution will be selected from among the following:

1. Pearson & Log Pearson Type III distributions
2. Gumbel's Extreme Value distributions
3. Normal & Log Normal distributions

{loadposition articlemid}


It is pertinent to point out that Log Pearson Type III distribution has been adopted by United States Federal Agencies whereas Gumbel distribution has generally been found to be suitable for most of the streams in Pakistan including river Indus and its tributaries.

 

Design Return Period

A return period of 100 years is generally adopted in the design of important and costly barrage structures where possible consequences of failure are very serious. Accordingly, the estimation of design flood will be carried out for various return periods of 100 years, 200 and 500 years subject to Client's concurrence. However, the actual recorded peak flood discharge will be reviewed for design if it exceeds the discharge calculated for the concerned return period.


Hydraulic Units

The dimensions and units of properties used in solving hydraulic problems are expressed in three fundamental quantities of Mass (M), Length (L), and time (T). All analyses and designs will be carried out in the Foot-Pound-Second system of units and conversion to S.I Units will be made only of important results as necessary.


Width of Barrage

Three considerations govern the width of a barrage. They are the design flood, the Lacey design width and the looseness factor. It is generally thought that by limiting the waterway, the shoal formation upstream can be eliminated. However, it increases the intensity of discharge and consequently the section of the structure becomes heavier with excessive gate heights and cost increases, though the length of the structure is reduced.


The design flood is discussed in section 2.2 and the other two considerations are discussed in the following sections.


Lacey's Design Width

The Lacey's Design or Stable width for single channel is expressed as:

W = 2.67 v Q


Where Q is the Design Discharge in cusecs (ft3/sec).


The Barrage is designed for a width exceeding W, partly to accommodate the floodplain discharge and partly to take advantage of the dispersion of the channel flow induced by the obstruction caused by the barrage itself.


The Looseness Factor

The ratio of actual width to the regime width is the "looseness factor", the third parameter affecting the barrage width. The values used have varied from 1.9 to 0.9, the larger factor being applied in the earlier design. Generally it varies from 1.1 to 1.5. From the performance of these structures, a feeling arises in certain quarters that with high Looseness Factor, there is a tendency for shoal formation upstream of the structures, which causes damages and maintenance problems. The Consultants will use the most appropriate looseness factor to provide reasonable flexibility keeping the ill effects to the minimum.


Afflux

The rise in maximum flood level of the river upstream of the barrage as a result of its construction is defined as Afflux. Afflux, though confined in the beginning to a short length of the river above the barrage, extends gradually very far up till the final slope of the river upstream of the barrage is established.


In the design of barrages/weirs founded on alluvial sands, the afflux is limited to between 3 and 4 feet - more commonly 3 feet. The amount of afflux will determine the top levels of guide banks and their lengths, and the top levels and sections of flood protection bunds. It will govern the dynamic action, as greater the afflux or fall of levels from upstream to downstream the greater will be the action. It will also control the depth and location of the standing wave. By providing a high afflux the width of the barrage can be narrowed but the cost of training works will go up and the risk of failure by out flanking will increase. Selection and adoption of a realistic medium value is imperative.

 

Tail Water Rating Curve

Tail water rating curve for the barrages will be established through analysis of gauge discharge data. The proposed tail water levels for new designs will be established by subtracting the designed retrogression values from the existing average tail water levels.


Crest Levels

Fixation of crest level is clearly related with the permissible looseness factor and the discharge intensity in terms of discharge per foot of the overflow section of the barrage. After considering all the relevant factors and the experience on similar structures the crest levels will be fixed in order to pass the design flood at the normal pond level with all the gates fully open.


Discharges through a Barrage (Free Flow Conditions)

The discharge through a Barrage under free flow conditions shall be obtained from the following formula:


Q = C. L . H3/2 .......(1)

Where,


Q = discharge in cusecs


C = Coefficient of Discharge


L = Clear waterway of the Barrage (ft)


H = Total Head causing the flow in ft


The value of C is generally taken as 3.09, but may approach a maximum value of 3.8 for modular weir operation (Gibson). However to design a new barrage it will be determined by physical model studies.


Discharge through a Barrage (Submerged Flow Conditions)

The flow over the weir is modular when it is independent of variations in downstream water level. For this to occur, the downstream energy head over crest (E2) must not rise beyond eighty (80) percent of the upstream energy head over crest (E1). The ratio (E2/E1) is the "modular ratio" and the "modular limit" is the value (E2/E1 = 0.80) of the modular ratio at which flow ceases to be free.


Fane's Curve

For submerged (non - modular) flow the discharge coefficient in equation (1) above should be multiplied by a reduction factor. The reduction factor depends on the modular ratio (E2/E1) and the values of reduction factor (Cr) given in the table below are from Fane's curve (Ref: 2.3) which is applicable to weirs having upstream ramp and sloping downstream with slope 2H:1V or flatter:


"E2/E1 "


Value of "Cr"


0.80


0.99


0.85


0.99


0.90


0.98


0.92


0.96


0.94


0.90


0.95


0.84


0.96


0.77


0.97


0.71


0.98


0.61


The submerged discharge is given by the equation:


Q = 3.09. Cr.b .E11.5

Gibson Curve

Q = C'bE1.5

Where:


Q = submerged discharge over crest (cusecs)


C' = submerged discharge coefficient


B = width of weir (ft)


E1 = upstream energy head above crest


= h1 + v12/2g (ft)

For submerged discharges the free flow discharge coefficient (C=3.80) is multiplied by a reduction factor (C'/C). The coefficient factor depends on the modular ratio (h/E), where his downstream depth of flow above crest. The values of reduction factor "C'/C" given in the table below are from Gibson curve applicable to the broad crested weirs:


h/E


C'/C


C'


0.70


0.86


3.27


0.80


0.78


2.96


0.90


0.62


2.36


0.95


0.44


1.67

 

Canal Head Regulator



Structure at the head of canal taking off from a reservoir may consist of nu ber of spans separated by piers and operated by gates.


Regulators are normally aligned at 90° to the weir. upto 10" are considered preferable for smooth entry into canal. These are used for diversion of flow. Silt reduces carriage capacity of flow.


Types of regulators in canals

1. Still pond regulation:
2. Open flow regulation
3. Silt control devices

1. Still pond regulation:

• Canal draws water from still pond
• Water in excess of canal requirements is not allowed to escape under the sluice gates.
• Velocity of water in the pocket is very much reduced; silt is deposited in the pocket
• When the silt has a level about 1/2 to 1m below the crest level of Head Regulator, supply in the canal is shut off and sluice gates are opened to scour the deposited silt.

Head Regulator


2. Open flow regulation

• Sluice gates are opened and allow excess of the canal requirement
• Top water passes into the canal
• Bottom water maintain certain velocity in the pocket to keep the silt to remain in suspension
• Canal is not closed for scouring the silt.

3. Silt control devices

• Silt control at head works:
• Entry of silt to canal can be controlled by:
• Providing a divide wall to:
• Create a trap or pocket
• Create scouring capacity of under sluices
• By concentrating the currents towards them
• Paving the bottom the approach channel to reduce disturbance because due to disturbance sediment remains in suspension

 

Installing silt excluders

• Making entry of clear top water by:
• Providing raised sill in the canal
• Lower sill level of scouring sluices
• Wide head regulator reduces velocity of water at intake
• Smooth entry to avoid unsteady flow
• Handling careful the regulation of weir
• Disturbance is kept at minimum in weirs

Silt excluder:

• Silt is excluded from water entering the canal, constructed in the bed infront of head regulator - excludes silt from water entering the canal
• Designed such that the top and bottom layers of flow are separated with the least possible disturbance
• Top water to canal - bottom, silt laden through under sluices
• No of tunnels resting on the floor of the pocket of different lengths
• The tunnel near th head regulator being of same length as that of the width of head regulator - tunnel of different length.
• Capacity of tunnel is about 20% of canal discharge
• Minimum velocity 2 to 3 m/s to avoid deposition in tunnel is kept the same as sill level of head regulator
• From discharge and scouring velocity the total waterway required for under water tunnels can be determined،
• Silt extractor or silt ejector:
• Device by which the silt, after it has entered the canal is extracted or thrown out.
• Constructed on the canal some distance away from head regulator
• Horizontal diaphragm above the canal bed
• Canal bed slightly depressed below the diaphragm 0.5 to 2.8m
• Under diaphragm, tunnel which extent the highly silted bottom water tunnel.
• There should be no disturbance of flow at the entry.
• Sediment - laden are diverted by curved vanes
• Forwards the escape chamber: steep slope to escape channel is provided.
• The streamlined vane passage accelerate the flow through them, thus avoiding deposition (decreasing section area increases the flow velocity)
• The tunnel discharge by gate at the outlet end (escape channel)

Location:

• If near head regulator, silt will be in suspension
• If too far away than result in silting of canal.