PLANNING AND DESIGN OF FLOOD CONTROL STRUCTURES REVETMENT

Grecile Christopher R. Damo Engineer IV, UPMO-Flood Control Management Office

Introduction: For the first five (5) years of its existence, the Project Management Office - Flood Control and Sabo Engineering Center (PMO-FCSEC) conducted several site survey and investigation of the different flood control structures of the Regional and District Engineering Offices of DPWH. The team analyzed both the good and damaged structures. These structures were compiled in the database together with the plans and typical designs (if available). Based from the available data and information gathered from the RO/DEO, manuals were formulated and hoping in the near future DPWH can come up with a more effective and efficient structures for flood control and river works. Revetment, by definition, it is a flood control structure built to protect the riverbank from collapsing due to erosion, scouring and/or riverbed degradation. But why revetment structures always damage? If these structures that we planned and designed are always damaged why we always stick on the same plan and design? This lecture material will be used to enhance the capability of DPWH in flood control structures and river works system. Problem Analysis Revetment is frequently damaged. First we have to understand the different phenomena acting on the river and the possible (based on the analysis of damaged structures) source of damage:

Scouring Revetment has insufficient depth of foundation There were no countermeasures against scouring Poor quality of construction

Direct water attack

Inconsistency in the design of adjoining (extension) revetment Improper alignment Did not consider additional countermeasures (i.e. spur dikes) No provision of end protection works

Residual Hydraulic Pressure or Water table

No provision of weep holes No provision of gravel or filter cloth in the weep holes Improper use of weep holes

Degradation/Aggradation Collapse due to dredging or quarrying activities Foundation depth was not based on the natural riverbed (before

aggradation) Based on our statistics on the published Damaged Structures Profile (December 2002) we have the following breakdown as shown below, but this does not represent the whole DPWH, only selected good samples of bad sites (note: we intend not to humiliate DPWH Engineers, for we are also part of DPWH but we intend to remedy the problem): Statistics on Damaged Structures Type of Damages to Revetment

Statistics on Damaged Structures

Revetment

Bridge

groundsill

spurdike

Type of Damages to Revetment

Scouring at

foundation

Slope erosion

Passive Force

Others

Planning for Revetment Structures Before the selection of type of revetment to be used we must first plan for the structure. Revetment structure can be planned if the existing discharge capacity of the river is just enough to contain the flood discharge every year and theres no record of overflow for the past 10 years or so and there are no remarkable riverbed variation (less than 40 cm in the past 10 years). For urgent countermeasure for riverbank erosion the required stretch of the revetment is less thank 1 km. The following should be followed as procedures:

Identify the target area the length of the project and the degree of importance.

Establish the whole stretch of the river improvement

Conduct cross-section, topo and establish the control point for stake out.

Observe and study the present situation (Riverbed gradient, Representative Grain Size Diameter, Surveyed Scouring Depth and tendency of the riverbed variation).

Delineate and calculate the catchment area and calculate the discharge capacity using the cross-section survey The discharge corresponding to the experienced maximum flood

generally recommended as the design discharge, in order to avoid similar disaster.

In case where the elevation of the beneficiary is quite high, the bigger design discharge as compared with the experienced maximum flood shall be considered.

For cases other than bank erosion, such as scouring or riverbed degradation and/or damages of revetment, a thorough study is needed. The appropriate river improvement plan shall be established. Designing for Revetment Structures Revetment structures shall consist of the following:

Slope covering works

Foundation Works

Foot protection works The following should be considered in the designing of the revetment

The alignment shall be as smooth as possible

Structural type of the revetment shall be determined based on the estimated external forces (velocity of flood flow) and the characteristics of the river.

Foot protection works shall be considered based on the external forces

Transition structure (end-protection works) of the revetment to the original bank shall be provided.

Parts of the revetment

Flow Direction Foot Protection

(Apron)

Transition Works (Gabion Mattress)

Slope Covering

End Protection Foundation Work

(Cut-off Walls)

Partition

Foot Protection

Backfill Materials

Shoulder Beam (Head Wall)

Slope Covering

Lean Concrete

Foundation

Filter Cloth

Definition

a. Slope Covering = Directly covers and protect the bank slope from erosion, direct attack from boulders and floating debris.

b. Foundation = Constructed at the toe of the slope that supports the slope covering.

c. Foot Protection = Constructed to prevent scouring in front of the foundation work and escape of material from the bank of the slope covering work.

d. Crest work = For revetment that frequently overtopped during flood, it protects the crest or top of the slope covering works.

e. Key = Installed at the end portion of the crest work to protect it against erosion at the back of the revetment.

f. Crest protection = Installed at the end portion of the key to join the crest and the original ground for protection against erosion at the back of the revetment.

g. Shoulder beam = A headwall that is installed at the shoulder of the revetment to prevent damage.

h. Partition = Installed between regular construction sections, to prevent the damage of the revetment from spreading.

i. End protection = installed at the upstream and downstream end of the slope covering to prevent undermining of materials behind the structure.

j. Transition Work = Installed between the natural bank and the end protection for a smooth transition. It can be constructed at the upstream or downstream.

k. Backfill Materials = Composed of suitable materials which are backfilled to the slope.

l. Filter Material/Cloth = Installed behind the slope covering for prevention of underneath materials from escaping due to residual hydraulic pressure.

Crest Protection

Key

Crest

Trial Design Flood Level After computing and setting the Design Discharge we have to set up the trial Design Flood Level. The design flood level of the proposed stretch can be established after the determination of the alignment of the revetment and the cross-section of the river. The trial design flood level shall be assumed as the projected line of the maximum value of the experienced maximum flood level plotted together with the riverbed gradient. Maximum Scouring Depth/Deepest Riverbed Level Bank erosion is attributable primarily to scouring of toe and sides of the banks during the periods of floods. Forecasting the deepest riverbed in the future is an important factor in determining the foundation work of the revetment. The deepest riverbed can be calculated using scouring depth analysis which will be based on the average riverbed level. Factors Contributing to Scouring:

a) Changes in average riverbed elevation Channel excavation/dredging lowers the average be elevation, and the bed elevation in scoured areas becomes lower accordingly. There are also cases where the reduction in sediment transport from the upstream destroys the sediment balance, resulting in a lower bed elevation.

b) Variation of Rivers Cross-section

There are two (2) reasons why the river cross-section directly influences scouring. One is that the change in river width from wide to narrow causes the increase in water depth. The other is that a curved or meandering river causes the flow to move toward one side of the channel, resulting in bank scouring.

c) Structures

A structure located in the path of flowing water increases the velocity of flow around the structure and cause local scouring.

d) Sand bar induced scouring Sand bars are alluvial deposits in the river, which cause an obstruction to flow. Since the height of sand bars is roughly equal to water depth, the amount of bar-induced scouring becomes large if the influence of bars is similar to curving or meandering.

The manual focuses on the scouring depth due to the meandering river (bend) and sand bar

Estimation Method for Maximum Scouring Depth The scouring depth is measured from the average riverbed level. Principally,

maximum scouring depth (Z) at the proposed structure site is estimated as the

larger value between the computed maximum scouring depth (Zc) and the

surveyed maximum scouring depth (Zs).

- Calculated maximum scouring depth (Zc) is an empirical value that considers the relationship among the width of a waterway, depth, the riverbed material, and the radius of the curve, etc.

- Surveyed maximum scouring depth (Zs) is the deepest riverbed determined from actual field survey (cross-sectional survey)

Scouring phenomena occur along the entire river stretch with different effects for straight line and bend or curve waterway. The primary factors that contribute to scouring based on the alignment of river are:

Straight-line waterway : sand bar height Curve waterway : bend of river alignment

a) Maximum scouring depth for straight waterway:

Since maximum scouring depth for this case is influenced by the height of sandbars, the calculations will be based on the conditions and existence of sandbars developed at the site. The following legend will be used:

b = River Width (meters)

Hd = Average Water Depth (meters)

Hs = Height of Sandbar

dr = Representative Riverbed Material

dH

b = Ratio of River Width and Average Water Depth

Case 1: 10dH

bor rd 0.2mm

In case that the riverbed is formed by fine sands (around 0.2mm or less)

and the ratio of river width and average water depth

dH

bis 10 or less, the

sand bar is not developed. Therefore, the surveyed maximum scouring

depth (Zs) is the maximum scouring depth (Z).

Cross-section Survey

Zs = Z

Average Riverbed

Surveyed maximum scouring depth

(Zs)

Hd

b

DFL

Case 2: 10dH

bor rd 2cm

When the ratio of

dH

b exceeds 10 and the riverbeds composition is

gravel, sand bar is generally formed. In this case, maximum scouring

depth (Zc) should be determined. Afterwards, compare with the surveyed

maximum scouring depth (Zs). The maximum scouring depth (Z) will be the larger value.

The maximum scouring depth is estimated as follows:

1) Calculate the ratio of width of waterway and average water depth dH

b

2) Calculate ratio of average water depth to the typical riverbed materials r

d

d

H

3) Using the graph shown below identify the ratio d

s

H

H

Hd

Cross-section Survey

b

DFL

Average Riverbed

Height of Sand bar (Hs)

Surveyed maximum

scouring depth (Zs)

Computed maximum

scouring depth (Zc)

0 10 20 30 40 50 60 70 80 90 100 0

1.0

2.0

3.0

H d /d R - 5~20 move to sand bar of plural rows

H d /d R = 25

H d /d R = 50 H S /H d

H d /d R < 100 sand bar of plural rows

H d /d R > 500

b/H d

8 8 9

7 6

8.7 7

5.7

H d /d R = 100 H d /d R = 200

H d /d R = 300

Relationship of H s / H d ~ b/h d

4) Calculate the water depth at the maximum scoured portion (Hmax) using the formula

))((8.01max dd

s HH

HH

5) Calculate the maximum scouring depth (Zc)

dHHZc max

6) Compare the resulting (calculated) maximum scouring depth (Zc) to the

cross sections (surveyed) maximum scouring depth (Zs). Use the larger value.

Case 3: 10dH

band rdmm2.0 2cm

When the ratio of

dH

b exceeds 10 and the riverbed is formed by coarse

sand and medium sand, fish scale sand bars are generally developed. In this case, height of bar becomes higher due to integration of sand bars.

The maximum scouring depth is estimated as follows:

1) Calculate the ratio of width of waterway and average water depth dH

b

2) Calculate ratio of average water depth to the typical riverbed materials r

d

d

H

3) Using the graph shown below identify the ratio d

s

H

H

0 10 20 30 40 50 60 70 80 90 100 0

1.0

2.0

3.0

H d /d R - 5~20 move to sand bar of plural rows

H d /d R = 25

H d /d R = 50 H S /H d

H d /d R < 100 sand bar of plural rows

H d /d R > 500

b/H d

8 8 9

7 6

8.7 7

5.7

H d /d R = 100 H d /d R = 200

H d /d R = 300

Relationship of H s / H d ~ b/h d

4) Calculate the water depth at the maximum scoured portion (Hmax) using the formula

))((8.01max dd

s HH

HH

5) Calculate the maximum scouring depth (Zc)

)(5.1 max dHHZc

6) Compare the resulting (calculated) maximum scouring depth (Zc) to the

cross sections (surveyed) maximum scouring depth (Zs). Use the larger value.

b) Maximum scouring depth of the curved waterway:

Calculated maximum scouring depth (Zc) of the curve waterway is calculated using the ratio of width (b) and the curve radius (r) of waterway. I) The ratio of water depth at maximum scouring and average water

depth dH

Hmax can be acquired using the graph below and plot the

ratio of width (b) and curve radius (r) of the waterway

II) Calculate the water depth at the maximum scoured portion

maxH Value of dH

Hmax acquired above x dH

III) Calculate the maximum scouring depth (Zc)

dc HHZ max

IV) Compare the scouring depth at the cross-section survey and the computed value. Use the larger value to be the maximum scouring depth.

0

1

2

3

4

5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

b

r

R

Relationship of Hmax / Hd ~ b/r

( b/r )

r = R - b/2

(Hm

ax / H

d)

Height of Revetment The height of revetment is based on the longitudinal profile and cross sectional profiles, and shall be determine as follows:

1) Crest Elevation of Revetment The crest elevation shall be principally same as the elevation of bank shoulder (the ground elevation of bank) or the crest elevation of dike and shall have a longitudinal gradient corresponding to the riverbed gradient. a) Draw average line of the ground elevations of the bank, with the

gradient corresponding to the riverbed gradient. b) In case of revetment on a dike, draw the average line of the shoulder

elevations of the dike, with the longitudinal gradient corresponding to the gradient of the dike crest.

But in case the improvement stretch is less than 100 m, crest of revetment may be level.

2) Depth of Top of Foundation

The depth of the foundation shall be placed deeper than 1.0 meter from the maximum scouring depth, principally. If it is difficult to calculate the maximum scouring depth, it shall be placed deeper than 1.0 meter from the deepest riverbed. The process of determination of the top elevation of the top of foundation is as follows: a) Plot the elevations of 1 meter deeper than the maximum scouring

level/deepest riverbed level at the respective cross-sections in the longitudinal profile.

b) Draw the circumscribed line of the lowest elevation of the above, with

the same longitudinal gradient of the top of slope covering.

Average River bed

DFL

Scouring depth

Heig

ht

of R

evetm

ent

d>

1.0

m

Z

In case there is difficulty in identifying the top of foundation due to extreme scouring or riverbed degradation, use steel sheet pile as foundation or toe protection works. The following can be considered for the identification and setting of elevation of the foundation.

i) The elevation of the foundation work shall be set at the maximum scouring depth, and the minimum foot protection work shall be installed.

ii) The elevation of the foundation shall be set above the maximum scouring depth, and the foot protection shall be installed to cope with the scouring.

Top of Foundation

Foot Protection

Existing Riverbed

Bed elevation at maximum scouring

0.5 1.5 m

Existing Riverbed

Bed elevation at maximum scouring

Top of Foundation

Foundation

Foot Protection

iii) The elevation of the foundation shall be set above the maximum scouring depth and the foundation work by sheet pile and foot protection works shall be applied in order to cope with scouring.

iv) In cases where it is difficult to secure the adequate depth of embedment for the foundation work such as high ordinary water level, tidal river, etc, cantilever sheet piles shall be installed as foundation works.

0.5 1.5 m

Existing Riverbed

Bed elevation at maximum scouring

Top of Foundation

Foot Protection

Sheet Pile

Overflow Water Level Tide Level

Top of Foundation

Sheet Pile

Bed elevation at maximum scouring

Slope and Berm Arrangement After the determination of the height of the revetment, slope and berm arrangement shall be planned based on the following:

(1) Slope

a) The slope of the protection work shall be gentle as much as possible for stability purposes and shall be based on the natural slope of the adjacent bank.

b) The slope shall be 2:1 (H:V) or milder. c) In case of rapid flow stretches wherein floodwater includes a large

quantity of boulders or gravels, the slope shall not necessarily gentle and shall be determined considering safety against the flood frequency.

d) In case of joint portion with rock-strewn slope, the slope of revetment shall be gradually changed to smoothly connect with the latter.

e) For the retaining wall type revetment, a maximum slope of 0.3:1 shall be observed in consideration to stability and the resulting residual hydraulic pressure.

(2) Berm Arrangement

a) If the height of revetment is more than 5.0 meters, berm (banquette)

must be provided and is so designed in order to separate the revetment into segments, and in consideration of site condition (geography and geology).

b) Berm shall be at least 1.0 meter in width for maintenance, stability and patrol of the river purposes.

c) For a single berm revetment, the berm location shall be just above the ordinary water level whenever possible.

Berm is considered when H > 5.0 meters

H

Berm

(banquette)

Alignment The alignment of the revetment should be as smooth as possible considering the direction of flow, natural bank alignment nearby, condition of bend, scouring portion, etc. In case of the joint portion with the rock-strewn slope, however, the alignment shall not be unnaturally smooth. Cross-section profiles In accordance with the above dimensions, the cross-section profiles at the sites shall be prepared and evaluated considering the river conditions. If there are any difficulties in the profiles, the slope, the berm arrangement and the alignment of the revetment shall be revised to cope with the difficulties. Establishing the Design Flood Level Based on the arranged cross section profiles, the design flood levels of respective sections shall be determined.

a) The representative cross-sections shall be selected as follows:

Condition of Stretch Representative Cross-section

Straight and almost uniform One (1) section with the smallest capacity

Non-uniform stretch including meandering

At least three (3) sections with the smallest discharge capacities.

b) For water level calculation at the design flood for each representative

cross section using uniform flow calculation.

c) Plotting the above results to the longitudinal profile.

d) Drawing the circumscribed line of calculated water level with a gradient corresponding to the riverbed gradient.

e) Adjustment of uneven design flood levels of the respective cross

sections from the circumscribed line Velocity The velocity of flow is an indispensable factor in the selection of the types of revetment. The mean velocity derived in the uniform flow calculations is not equal to the velocity of flow in front of the revetment. The velocity is actually influenced by the effects of sand wave, bend and foot protection work. For designing the revetment, it is recommended to correct the mean velocity to the design velocity of the revetment. It is necessary to apply the correction to the mean velocity derived from the uniform flow calculation. In cases there will be no correction, it is recommended that the maximum value in the mean velocities of the representative cross-sections at the design flood is adopted as the design velocity of the revetment at the proposed site

Correction of the mean velocity for design

The design velocity DV is estimated based on the average value of the mean velocities of the representative cross sections maveV at the design flood, as follows:

maveD VV

Where:

DV : Design Velocity (m/s)

maveV : Average value of the mean velocities of the

representative cross sections at the design flood : Correction Coefficient

can be calculated using the following conditions:

i. Straight stretch without foot protection work Considering the decrease of the stream area due to sand bar, the correction coefficient can be calculated as

dH

Z

21

Equation 1

note that 0.2 Z Maximum scouring depth (m)

dH Average design water depth (m)

ii. Bend area without foot protection works

For inner bank of the bend: r

B

21 Equation 2

Outer bank of the bend dH

Z

r

B

221

Equation 3

Where: Correction Coefficient

(Segment 1: 2 and Segment 2 and 3: 6.1

B River width (m) r Radius of the bend (m) Z Maximum scouring depth (m)

dH Average design water depth (m)

r

B

iii. With foot protection works In case of structure with adequate foot protection works (crest width of 2 m or more), either bend or straight, the correction coefficient calculated from

equations 1, 2 and 3 (noted here as 1 ) will be further multiplied as

follows:

If 0.1l

w

H

b 19.0 Equation 4

If 0.1l

w

H

b 10.1 Equation 5

Design Flood Level

lH dH

Z

wb

Average Riverbed

Foot Protection

Selection of Slope Covering There are many types of slope covering works which can be used with consideration to the design velocity, slope, availability of construction materials near the site, ease of construction, most economical, etc. In case where measures are required for the boulder stones during flood and the slope of the bank is constrained, combination of the slope covering works shall be considered.

Type of Revetment Allowable

Design Velocity

(m/s)

Slope (H:V)

Remarks

Sodded Riverbank with Pile Fence

4.0

Milder than 2:1

- Not applicable for places near roads and houses. - Diameter and length of wooden pile shall be determined

considering the past construction records. - For Type II, diameter of fill boulder shall be determined using

the table for the Minimum Diameter of Boulder (Riprap Type) for foot protection.

Dry Boulder Riprap

3.0

Milder than 2:1

- Diameter of boulder shall be determined using the table for Diameter for Dry Boulder Riprap.

- Height shall not exceed 3.0 meters. - Crest protection works shall be provided in case overflow is

frequent. - Boulders and reinforcing materials shall be interlocked with

each other. Maintenance activity shall be carried out after the flood.

- In case the revetment is submerged during the flood , the crest of the revetment shall be protected.

- Filter cloth or any equivalent materials shall be included in the design to prevent backfill materials from sucking out.

Grouted Riprap (Spread Type)

5.0

Milder than 2:1

- Use Class A boulders for grouted riprap and loose boulder apron.

Grouted Riprap (Wall Type)

1.5:1 to

0.5:1

- Use Class A boulders for grouted riprap.

Gabion (Piled-up Type)

6.5

1.5:1 to

0.5:1

- Not advisable in rivers affected by saline water intrusion - Not applicable in rivers where diameter of boulders present is

greater than 20 cm.

Gabion (Spread Type)

5.0

Milder than 1.5:1

- Not advisable in rivers affected by saline water intrusion - Not applicable in rivers where diameter of boulders present is

greater than 20 cm.

Rubble Concrete (spread Type)

Milder than 1.5:1

Rubble Concrete (Wall Type)

Milder than 1:1

Reinforced Concrete

- A minimum thickness of 20 cm

Sheet Pile

N.A.

- In cases where ordinary water level is very high. - Design procedure and sample design computation for steel

sheet pile is provided in Annex A of the Manual on Design of Flood control Structures January 2005 Publication (Light Green Book)

Diameter of Boulder for the Dry Boulder Riprap

Water Depth

Design Velocity (m/s)

1.0

2.0

3.0

4.0

5.0

1.0 20 20 20 60 -

2.0 20 20 20 30 70

3.0 20 20 20 30 50

4.0 20 20 20 20 40

5.0 20 20 20 20 40

Selection of Type of Foundation The foundation of the revetment has to support the slope covering properly with consideration and analysis of scouring during flood. The type of foundation works shall be selected in accordance with the conditions of the foundation and the type of slope protection. The following conditions for foundation are applied: Ordinary Foundation - Direct type Weak Foundation - Pile or sheet pile type In addition, the sheet pile type foundation is also used in case where scouring of the bed is severe and the dewatering will be difficult during the construction. Appurtenance Works The appurtenance works of the revetment are installed in order to protect the crest, the upstream and downstream end of the slope covering from erosion and to prevent the outflow or sucking out of materials or soils of the banks. The following shall be considered in the design:

1) Backfill materials For the rigid type revetment, backfilling materials shall be installed in order to reduce the force of the residual hydraulic pressure to the covering works and to make the structure stable. For the permeable type revetment such as wooden fence type and gabion mattress type, the backfill materials is not necessary. The backfilling materials shall be with high permeability, such as crushed gravel, etc. Thickness of the backfill materials shall be 30 40 cm for wall type and 15 20 cm for pitching or lining type. For the residual water pressure, weep holes are provided to let water from the land side pass through the structure, however these weep holes should have filter cloth or equivalent materials to prevent backfill materials from escaping.

2) Outflow preventing Materials For permeable type revetment, filter cloth or its equivalent shall be installed. No need to include in the design for impermeable type of revetment. However, it is still needed in the weep holes.

3) Crest Protection Works

If the overflow frequency is very high due to inadequate flow capacity, the crest of the revetment shall be protected. The width of the crest shall be more than 1.0 meter

4) End Protection Works The end protection work is indispensable to the rigid structure type revetments The end protection shall cover the extent of the covering work and crest work. The thickness of the end protection work shall be from the surface of revetment up to backfill material. The thickness of the end protection shall be more than 50 cm.

Crest Armouring Crest

Key

1,000 mm (min)

Shoulder Beam

1,500 2,000 mm

Crest Armouring Crest

500 (min)

5) Partition Works In case the length of the rigid type revetment is more than 50 m, a partition shall be installed in order to prevent the damage of the revetment from spreading. The structure of the partition works shall be the same as the end protection works.

6) Transition Works A transition work to the natural bank is installed in order to connect the revetment and the natural bank smoothly and to prevent erosion at the upstream and downstream sides of the revetment from spreading behind the revetment. A transition work shall be flexible type like gabion mattress and the like. The fitting angle to the natural bank shall be 30 degrees or less at the upstream side and 45 degrees or less at the downstream side. However the fitting angle shall be determined based on the present condition of the bank.

45o 30

o

Foot Protection Works The foot protection work is a structure used to secure the safety of the foundation work from the effect of scouring. The basic requirements for the foot protection work are as follows:

- Sufficient weight against the flow forces. - Sufficient width to prevent scouring in front of the revetment. - Durability - Flexibility

1) Type of Foot Protection Work

The type of foot protection shall be determined based on the river condition and the most economical but effective structures. Types: Riprap Type/Boulder Type

For Riprap the minimum diameter for different velocities is as follows

Design Velocity Diameter (cm)

2 -

3 30

4 50

5 80

6 120

Gabion Type

For Gabion filling boulders in cm

Water Depth

Design Velocities

1.0 2.0 3.0 4.0 5.0 6.0

1.0 5-15 5-15 5-15 10-20 - -

2.0 5-15 5-15 5-15 5-15 15-20 -

3.0 5-15 5-15 5-15 5-15 15-20 15-20

4.0 5-15 5-15 5-15 5-15 5-15 15-20

5.0 5-15 5-15 5-15 5-15 5-15 15-20

6.0 5-15 5-15 5-15 5-15 5-15 15-20

Wooden Stockade

Water Depth

Design Velocities

1.0 2.0 3.0 4.0 5.0 6.0

1.0 5 5 10 30 - -

2.0 5 5 10 15 35 65

3.0 5 5 10 15 25 45

4.0 5 5 5 15 25 40

5.0 5 5 5 10 20 35

6.0 5 5 5 10 20 30

Gabion Box

Concrete Block Pile-up Type / Concrete Block Disorder Pile-up Type

1 10 1000.1

1.0

10.0

100.0W

(t)

We

igh

t o

f c

on

cre

te b

loc

k

V d (m/s) Velocity of flow

Single unit

Disorder pile up

Order pile up

B

Ln at least 2.0 meters

Z

2) Elevation of Foot Protection Works The top elevation of foot protection shall be set at the same elevation of that foundation work of the revetment. In order to prevent scouring, the top elevation of foot protection work is also set above the top of foundation of the revetment. In this case, the bottom elevation of foot protection shall be set at the same elevation as the top of foundation work. In case the thickness of the foot protection work is more than 1.0 meter, the bottom elevation of the foot protection shall be set at the same elevation with the bottom of the foundation.

3) Width of Foot Protection works The foot protection work requires a sufficient width that will prevent scouring of riverbed in front of the foundation of the revetment. The width of foot protection shall consider that a flat width of at least 2.0 meters will be left after the scouring. The computation of the width of the foot protection works is as follows:

sin

ZLB n

Where:

nL Flat width in front of revetment (at least 2.0 meters)

Slope at the scouring (assume 30 degrees) Z Height between the foot protection and the scoured bed

References:

Manual on Planning and Design Manual on Design of Flood Control Structures Technical Standards and Guidelines on Planning and Design of Flood Control

Volume 1