Structural Interventions to Combat Flooding

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5-Day Training WorkshopOn

“Flood Mitigation”

Title: Structural Interventions toCombat Flooding

On: February 16, 2010

At: Islamabad

Organized By: NDMA-UNDP

Sequence of Lecture

• Objective of Flood Protection;

• How Flood Protection is achieved;

• Structural Interventions.

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What is Flood Protection?

long-term structural measures that physically stopsome or all flood water from entering a designatedarea;

It dose not necessarily mean complete protectionfrom flood; OR

Provision of controlled flooding and drainage.

Objective

improved social and economicactivity within a designated areaduring and after a flood event.

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How Flood Protection Can Be Achieved?

Through structural interventions;

Through non-structural interventions.

Structural Interventions

Flood Embankments/Bunds;

Retired Bund;

Marginal Bund;

Spur/Groyne;

Stud;

Guide Bank;

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Structural Interventions

Pitched Island;

Gabion Retaining Walls;

Submerged Sills;

Flood Diversion/Dispersion Structures

Structural Interventions

Flood Embankments/Bunds:

¾ They are low height earthen embankments;

¾ Extending, generally parallel to the river channel;

¾ Designed to protect the area behind them fromover ow o oo s;

¾ Aligned on high ridge of natural banks of a river;

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Structural Interventions

Flood Embankments/Bunds:

¾ Where land is high;

¾ Suitable construction material is available;

¾ Inhabited areas, settlements and properties alonge r ver an are e ou s e e un ;

¾ They can be stone pitched also.

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Structural Interventions

Retired Embankments/Bunds

¾ They are built at a distance from the river ridgebehind the existing flood embankment/bund;

¾ Act as second line of defence or;

ep acemen o a amage oo em an men un ;

¾ Designed parameters same as that of Flood bund.

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Structural Interventions

Marginal Bunds:

¾ They are built to contain river spill generated by raising ofwater level at a barrage, bridge etc;

¾ Due to back water effect, at locations, the Marginal Bund couldextend upto 20 miles (32 km) in length;

¾ Start from 500 ft (150 m) u/s of the gate-line of barrage;

¾ At locations they are stone pitched.

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Structural Interventions

Spur:

¾ Spurs are structures placed transverse to the riverflow;

¾ Extend from the bank into the river;

¾Widely used for the purpose of river training; and

¾ To serve one or more the following functions:

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Structural Interventions

Spur: (Functions)

- Training the river along a desired course by

attracting, deflecting and holding the flow in a

channel.

- Creating a zone with the object of silting up the

.

- Protecting the river bank by keeping the flow away

from it.

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Structural Interventions

Types of Spurs

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Structural Interventions

Stud:

¾ Studs are short spurs;

¾ Used as protection against spill flow causing erosionalong a river bank of a flood embankment;

¾ Placed in series at suitable locations in the s ill flowchannel along the bank;

¾ Reduce erosion by deflecting the high velocitycurrents away from the eroding bank.

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Structural Interventions

Guide Banks:

¾ A river generally flows in a wide khadir;

¾ But it is necessary to restrict its course to remain flowingcentrally through the barrage, weir or a bridge placed across it;

¾ To achieve the above purpose, guide banks are placed in pairs

¾ There should be no spurs projecting from the guide bank as thespurs produce swirls.

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Structural Interventions

Pitched Island:

¾ Used a s a river training measure for its ability tocause redistribution of velocity and tractive force;

¾ The tractive force near a pitched island begins toincrease rapidly, with the result that deep scour

eg ns o orm roun e s an an gra ua y raws

the main river channel towards it and holds itpermanently;

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Structural Interventions

Pitched Island:

¾ Used singly or in series for;

-Correcting oblique approach u/s of weirs, barrages,

bridges by training the river to be axial.

-Altering the river flow pattern and stablizing its

course.

-Redistributing harmful concentration of flow forrelieving attack on guide banks, marginal bunds and

river bends.

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Structural Interventions

Gabion Retaining Walls:

¾ Used in hilly areas where there is high gradient andhigh velocity flows and practically it is not possibleto provide stone protection against bank erosion;

¾ In such areas stones are provided in stonecrates/gabions in a retaining wall format;

¾ Gabions are well suited for retaining walls due totheir flexibility;

¾ Designed as gravity walls.

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Structural Interventions

Submerged Sills:

¾ When a mid-channel bela in the river has grown overthe years, it bifurcates the river channel into twoseparate channels flowing around the bela;

¾ One of these two meandering channels may flowclose along the bank and cause bank erosion;

¾ For controlling of such bank erosion a series ofsubmerged sills across the channel may be placed.

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Structural Interventions

Delay Action Dams:

¾ Used for taming the flood waters of hill torrents;

¾ Main purpose is to delay the downstream flow offlood water to first ensure maximum retardation ofd/s flood water.

Water Diversion/Dispersion structures:

¾ Used for taming the flood waters of hill torrents.

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End of Presentation

Thank You