USER

GU

IDE

Guidelines for the Design ofRiver Bank Stability andProtection using RIP-RAP

Prepared byAssociate Professor R. J. Keller

www.toolkit.net.au/riprap

Document History

Date Author Revision Description of Change July 2004 R. J. Keller 1.0.0 Original draft January 2005 Keirnan Fowler 1.1.0 Toolkit version, added tutorial

Copyright Notice

CRC for Catchment Hydrology, Australia 2005

Legal Information

To the extent permitted by law, the CRC for Catchment Hydrology (including its employees and consultants) accepts no responsibility and excludes all liability whatsoever in respect of any persons use or reliance on this publication or any part of it

Acknowledgements

These Design Guidelines, in part, reproduce material originally prepared by Ian Drummond and Associates (now Earth Tech Pty Ltd.). The permission of Dr. John Tilleard to freely use this material is appreciated.

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Contents

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RIP-RAP User Guide

CONTENTS

1 Introduction...........................................................1 1.1 The user guide ...................................................................................................................1 1.2 Software............................................................................................................................1

1.2.1 Overview ......................................................................................................1 1.2.2 Audience ......................................................................................................2 1.2.3 RIPRAP theory summary ..................................................................................2 1.2.4 Limitations ....................................................................................................2

1.3 Key references ...................................................................................................................2

2 Installation ............................................................3 2.1 Technical specifications.......................................................................................................3 2.2 Licence agreement .............................................................................................................3 2.3 Installation.........................................................................................................................3

3 RIPRAP Tutorial ......................................................4 3.1 Getting started...................................................................................................................4 3.2 The tutorial scenario ...........................................................................................................4 3.3 Inputting data ....................................................................................................................5 3.4 Running RIPRAP..................................................................................................................6 3.5 Results...............................................................................................................................6 3.6 Further reading ..................................................................................................................8

4 Using RIPRAP.........................................................9 4.1 Inputs................................................................................................................................9

4.1.1 Energy slope .................................................................................................9 4.1.2 Bank angle .................................................................................................10 4.1.3 Rock specific gravity .....................................................................................10 4.1.4 Rock angle of repose ...................................................................................11 4.1.5 Maximum depth ..........................................................................................11 4.1.6 Depth of interest ..........................................................................................11 4.1.7 Factor of safety............................................................................................12

4.2 Outputs...........................................................................................................................12

5 Notes on the rip-rap technique............................14

RIPRAP User Guide

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5.1 Introduction to the rip-rap technique.................................................................................. 14 5.2 Further design considerations............................................................................................ 15

5.2.1 Extent of bank protection ............................................................................. 16 Length of bank to be protected ..................................................................... 16 Proportion of bank height to be protected...................................................... 16

5.2.2 Allowance for scour at toe of rip-rap ............................................................. 17 5.2.3 Specification for rock quality and grading and thickness of layer ...................... 17 5.2.4 Details of filters ........................................................................................... 18

Geotextile filters .......................................................................................... 18

Appendix A - RIPRAP Theory .........................................20

Appendix B - References...............................................23

Contents

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iii

TABLE OF FIGURES Figure 3-1: The tutorial scenario .........................................................................................................5 Figure 3-2: The input table with tutorial input values .............................................................................6 Figure 3-3: The output table with calculated tutorial values....................................................................7 Figure 3-4: The output graph based on calculated tutorial values...........................................................7 Figure 4-5: Ratio of local to reach-averaged energy gradient as function of bend geometry....................10 Figure 4-6: Angle of repose of dumped rip-rap. (Source: Simons and Senturk, 1977) ...........................11 Figure 4-7: RIPRAP output table ........................................................................................................13 Figure 4-8: RIPRAP output graph.......................................................................................................13 Figure 5-9: A typical RIPRAP scenario, showing various design considerations. ......................................15

Figure A-1: Forces on a rip-rap particle .............................................................................................20

TABLE OF TABLES Table 4-1: Commonly specified bank angles for rip-rap ......................................................................10 Table 4-2: Specific gravities of typical rock types used for rip-rap .........................................................11 Table 4-3: Typical values of the Factor of Safety. ................................................................................12 Table 5-4: Suggested rock gradation.................................................................................................18 Table 5-5: Advantages and disadvantages of geotextile and granular filters ..........................................19

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Introduction

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1 Introduction RIPRAP is a design tool developed for use in the hydraulic design of rip-rap bank protection. Rip-rap is the term given to loose rock armour, usually obtained by quarrying, which can be employed to provide protection to actively eroding or potentially eroding banks in rivers and channels.

1.1 The user guide These guidelines primarily address the program RIPRAP and its proper use in the hydraulic design of rip-rap for specified flow conditions.

A tutorial is provided in Section 3 as a step-by-step introduction to RIPRAP, while Section 4 provides a more detailed program description.

It is emphasised very strongly that hydraulic design (the purpose of this program) is only one aspect of the correct application and design of rip-rap protection. It is clearly not possible to provide a complete treatise on the other issues of importance. However, design notes on these other issues are provided in Section 5.2. These notes are intended for guidance only and not as a set of prescribed rules.

Section 5 also contains an introduction to the rip-rap technique.

In Appendix A, the hydraulic theory underpinning the program is presented.

1.2 Software

1.2.1 Overview RIPRAP is written as an EXCEL spreadsheet-based program. The internal calculations are run by macros.

Inputs to the program include:

Site details, ie. bank angle, local energy slope, maximum depth

Rock details, ie. specific gravity, angle of repose

Factor of safety

Program output is a table of design values of median rip-rap diameter for a range of depths and bank angles. A graph of the table data is also provided.

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Note The data or datasets used to run significant or important models should be archived together with the results of the model. If the model results are challenged, then the model can be re-run with the original dataset.

1.2.2 Audience The program RIPRAP is designed for use by engineers in Catchment Management Authorities, Local Government Organisations, and consulting practices who are involved in river and stream rehabilitation and restoration studies or projects.

1.2.3 RIPRAP theory summary The theory is based on a fundamental force balance, requiring that, at the point of incipient particle motion, the disturbing forces on the particle (drag force exerted by the flow and weight component resolved down the slope) are balanced by the restoring forces (the product of the weight component resolved perpendicular to the bank and the tan of the natural angle of repose of the rip-rap material). Appendix A gives a detailed description and explanation of the hydraulic theory underpinning the RIPRAP program.

1.2.4 Limitations It is important to emphasise that hydraulic design is only one aspect of correct implementation of rip-rap. Indeed, rip-rap often fails for reasons other than inadequacies in its specific hydraulic design. Such failures are often related to poor understanding of the prevailing site conditions such as hydrology, overall stream morphology, floodplain and channel hydraulics, and foundation conditions. In all cases it is important to access local knowledge and experience with other chutes on the same stream or under similar conditions.

1.3 Key references The design procedure is essentially a development and simplification of the group of methods based on tractive force analysis, and particularly the so-called Factor of Safety method developed by Simons and co-workers (Simons and Senturk, 1977).

These Design Guidelines, in part, reproduce material originally prepared by Ian Drummond and Associates (now Earth Tech Pty Ltd.). The permission of Dr. John Tilleard to freely use this material is appreciated.

Installation

3

2 Installation

2.1 Technical specifications The software is written as a Microsoft Excel 2003 workbook, and will run on any computer with a compatible version of Excel installed. Earlier versions of Excel (eg Excell 97) are known to have problems with certain aspects of the program. The embedded macros are programmed in Visual Basic for Applications.

2.2 Licence agreement A licence agreement is part of the installation procedure. You must acknowledge that you have read, understood and agree to be bound by the software licence agreement to be able to proceed with the installation.

2.3 Installation Download the RIPRAP installer from the Toolkit website. Unzip the files and run the RIPRAP setup program.

When running RIPRAP you must choose the enable macros option if prompted to do so. The macro security level on your computer must be set to Medium or Low in order for RIPRAP to run properly.

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3 RIPRAP Tutorial This tutorial aims to provide the user with a brief introduction to RIPRAP. For more information on any aspect of the tutorial, please refer to the more extensive sections of the manual.

3.1 Getting started Start RIPRAP from the menu Start > Programs > Toolkit > Riprap > Riprap, or from the shortcut menu on the Desktop.

You must choose enable macros if prompted to do so. If you encounter problems, it may be because your macro security level is too high. Change it through Tools>Macro>Security (while in Excel).

For more information on technical specifications or other aspects of installation, see Section 2.

3.2 The tutorial scenario The tutorial will use a hypothetical scenario to introduce the key concepts of RIPRAP. The main characteristics of the scenario are shown in Figure 3-1 below.

RIPRAP Tutorial

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Figure 3-1: The tutorial scenario

3.3 Inputting data Type in the following input data:

Energy slope

Enter 0.001

This is the local hydraulic energy gradient. For information on how to derive this value from a reach-averaged value, see Section 4.1.1.

Bank angle

Enter 31

This is the angle (in ) of the finished rip-rap surface to the horizontal.

Rock specific gravity

Enter 2.65

This is the density of the composite rock relative to the density of water.

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Rock angle of repose

Enter 40

This is the natural angle (in ) of repose of the rock being used as rip-rap. See Section 4.1.4 for more information.

Maximum depth

Enter 5

This is the maximum depth (in m) for which data is required.

Depth of interest

Leave this blank

RIPRAP outputs at 10 equally spaced depths, varying between 0 and the maximum. If you have an additional depth for which you require information, enter it here.

Factor of safety

Enter 1.2

This is a user-defined variable that accounts for such things as uncertainty, high consequences of failure, or other site-specific factors. See Section 4.1.7 for more information.

Figure 3-2 shows the input table with the tutorial values entered.

Figure 3-2: The input table with tutorial input values

3.4 Running RIPRAP To run the program, press Calculate. Values will appear in the output table.

If you have entered an incompatible value, you will receive a warning message.

3.5 Results The maximum safe bank angle appears at the bottom of the input table. This is calculated by RIPRAP and is a function of the inputs angle of repose and factor of safety.

RIPRAP Tutorial

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The output table should look like that shown in Figure 3-3. The output table values are median diameters for the rip-rap rocks.

Figure 3-3: The output table with calculated tutorial values

Click the Rip-rap graph tab to view the results as a graph. The graph is displayed in Figure 3-4 below.

Figure 3-4: The output graph based on calculated tutorial values

The use of a table of results rather than a single design value is deliberate. It is intended to force the user to consider the sensitivities of the results to the chosen design parameters. In particular, the table shows a strong sensitivity of the results to the design bank angle when the design bank angle is close to the natural angle of repose of the rip-rap.

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3.6 Further reading Please refer to:

Section 4 for more information on RIPRAP inputs, outputs and assumptions

Section 5 for more information on the rip-rap technique and various design considerations beyond what is covered above

Appendix A for the theory that underpins the RIPRAP program

Using RIPRAP

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4 Using RIPRAP

4.1 Inputs The following is a detailed description of each RIPRAP input variable.

4.1.1 Energy slope This is the local hydraulic energy gradient.

The value adopted for this parameter is crucial to the accuracy of the computed required rip-rap size. Indeed, the theoretical development in the appendix indicates that computed D50 values are directly proportional to the adopted energy gradient. The description of the hydraulic conditions, which dictate the forces on the rip-rap particle, is fully embodied in the value adopted.

The adopted value must represent the local energy gradient adjacent to the rip-rap. The local slope will vary significantly from the reach-averaged energy gradient at constrictions, bridges, other in-stream structures, and at channel bends. Only in straight channels of reasonably prismatic cross-section will the local energy gradient approximate the reach-averaged value. The following comments provide some guidance on appropriate values to be adopted.

On channel bends, the multiplying factor, Se/Sa, to be applied to the reach-averaged energy gradient may be estimated as a function of the ratio of the outside bend radius of curvature to the channel width from Figure 4-5, adapted from US Soil Conservation Service (1971).

At bridges and constrictions the local energy slope needs to be estimated through detailed hydraulic analysis. Backwater analyses with closely spaced cross-sections and detailed bridge waterway computations will prove helpful.

For the nose of groynes and at bridge abutments, work by Maynard (1978) suggests that a design energy gradient of 4 times the reach-averaged value in the channel is appropriate.

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Figure 4-5: Ratio of local to reach-averaged energy gradient as function of bend geometry.

4.1.2 Bank angle This is the angle (in degrees) of the finished rip-rap surface to the horizontal.

Commonly specified bank angles are presented in Table 4-1 below.

Table 4-1: Commonly specified bank angles for rip-rap

Bank slope Corresponding bank angle ()1.5 H : 1 V 34 2.0 H : 1 V 30 2.5 H : 1 V 22 3.0 H : 1 V 18.5

4.1.3 Rock specific gravity This is the density of the composite rock relative to the density of water.

Quartz-based granite typically has a specific gravity of about 2.65. Table 2 gives a guide to relative densities, but it is recommended that, for major projects, actual densities should be measured.

Using RIPRAP

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Table 4-2: Specific gravities of typical rock types used for rip-rap

Rock Type Relative Density Sandstone 2.1 2.4 Granite 2.5 3.1 (typically 2.65) Limestone (crystalline) 2.6 Basalt 2.7 3.2

4.1.4 Rock angle of repose This is the natural angle of repose of the rock being used as rip-rap.

The natural angle of repose of rock typically varies between about 300 and 430, depending on rock size and shape. Figure 4-6, sourced from Simons and Senturk (1977), provides a good estimate. It is evident from this figure that for angular rock of size greater than 100mm, the natural angle of repose is 41-420, covering most design situations.

Figure 4-6: Angle of repose of dumped rip-rap. (Source: Simons and Senturk, 1977)

4.1.5 Maximum depth This is the maximum depth for which data is required.

RIPRAP splits the maximum depth into ten equal intervals and provides output at each interval of depth.

The generation of depth-dependant rip-rap size permits the specification of different rip-rap stone sizes at different depths, which may be appropriate on very large projects where cost is a substantial issue.

4.1.6 Depth of interest This is a user-specified depth for which output data is required.

In the output table, this depth appears along with the ten of equal interval (see above).

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4.1.7 Factor of safety This variable relies on the judgement and experience of the designer for suitable application. Its technical definition is the ratio of the restoring force to the disturbing force on a rip-rap particle (see Appendix A, equation A-3 for more information).

The following factors will influence this variable:

The consequence of failure

The return period and likely duration of the design flood

The reliability of estimates of the design flood

The reliability of the estimate of the local energy gradient

The quality, consistency, and grading of available rock

The reliability of placement techniques

The severity and mode of failure of existing bank erosion

The likelihood of turbulence or high velocity eddies

The likelihood of the rip-rap being stabilised by vegetation within a time period much less than the return period of the flood

Other uncertainties in the analysis

The most important of these factors is the consequence of failure in terms of subsequent erosion, loss of habitat, and potential threat to other assets such as bridges.

As a guide, a factor of safety of 1.5 is appropriate for most major projects.

As a guide for inexperienced users, values for factor of safety are suggested in Table 4-3 below.

Table 4-3: Typical values of the Factor of Safety.

Value Comment

1.1 Minimum value to be adopted

1.2 Typical protection of eroding banks in a rural environment

1.3 For eroding banks in a rural environment where additional uncertainties exist or where assets are threatened.

1.5 Where failure would threaten a major asset or cause major loss, or where major uncertainties exist in the appropriate values of input parameters.

The relatively low factors of safety reflect the generally conservative assumptions built into the rock sizing procedures. The recommended values should lead to design conditions in which there is no significant rock movement under the design flow condition

4.2 Outputs The program generates both tabular and graphical output. An output table is shown in Figure 4-7.

Using RIPRAP

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Figure 4-7: RIPRAP output table

The table comprises the design values of median rip-rap diameter for a range of bank angles and a range of depths. The ranges include the actual design bank angle (27 in the above case) and design depth (2.5m) as specified in the input table.

There is a maximum allowable bank angle, which is a function of the specified angle of repose and the selected factor of safety. Rock sizes for angles greater than the maximum allowable are not shown.

Note Use of a table of results rather than a single design value is deliberate. It is intended to force the user to consider the sensitivities of the results to the chosen design parameters. In particular, the table shows a strong sensitivity of the results to the design bank angle when the design bank angle is close to the natural angle of repose of the rip-rap.

In a normal design case, the user will re-run the program several times after altering one or more of the input parameters. This procedure is greatly simplified by the input table appearing directly above the output table on the screen.

The graphical output is generated directly from the output table. It comprises a graph of median rip-rap size as a function of depth for each bank angle. An example is shown in Figure 4-8.

Figure 4-8: RIPRAP output graph

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5 Notes on the rip-rap technique

5.1 Introduction to the rip-rap technique Rip-rap is the term given to loose rock armour, usually obtained by quarrying. It is widely used for bank protection. Useful engineering qualities of rip-rap include:

General ease of placing can be placed underwater

Flexibility

High hydraulic roughness to attenuate waves and currents

Low maintenance requirements and convenience of repair

Durability

Rip-rap is normally placed by machine in the dry, although it can be tidied or even placed by hand, to improve its packing density or to key larger stones into the underlayer or subsoil. As with granular underlayers, rip-rap can become segregated into different sized zones if dumped into place. The aim of machine placing is always to release the material as close as possible to its final position. Later spreading, by bulldozer or otherwise, may increase breakage, segregation, and surface roughness.

A major advantage of rip-rap protection is that it is very flexible. As a result, damage tends to occur gradually and, as stones move relative to each other, is, to some extent, self-healing. This allows maintenance work to be undertaken on a routine basis in contrast to rigid protection systems, such as concrete blocks, which require immediate repairs to prevent widespread progressive failure once localised failure occurs. It is, nevertheless, important that maintenance is carried out, since the misaligned surface where stones have been lost or where deformation has occurred will generally experience higher than average hydrodynamic forces.

Although the concept of rock rip-rap is simple, proper hydraulic design is very important to ensure that the river geometry and rock size are matched with the expected flow conditions such that the rock remains stable under all expected flow conditions. In addition, appropriate rip-rap design requires that a number of other issues be adequately addressed. In particular:

The grading of sizes within the rock rip-rap minimises the presence of voids within the protective layer and minimises the area of individual rocks exposed to forces from the flow

Notes on the rip-rap technique

15

A filter layer is provided where necessary to prevent bank material washing out through the protective rip-rap layer

The rock rip-rap extends a distance upstream and downstream which is appropriate to the level of security to be achieved and the overall cost of the protection

The rock rip-rap covers a proportion of the bank height which is appropriate to the level of security to be achieved and the overall cost of the protection

The rock rip-rap extends below estimated scour depth

The rock is of suitable quality

5.2 Further design considerations Hydraulic design, which is covered by the RIPRAP program, is only one aspect of overall riprap design. Considerations must be made of a number of additional factors, such as:

Extent of bank protection (including length of bank to be protected and proportion of bank height to be protected)

Allowance for scour at toe of rip-rap

Specification for rock quality and grading and thickness of layer

Details of filters

The following section considers each of these factors.

Reference is also made to Figure 5-9 below throughout this section.

Figure 5-9: A typical RIPRAP scenario, showing various design considerations.

These notes are intended as a guide only.

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5.2.1 Extent of bank protection There are no universal rules to determine the extent of bank protection appropriate to a particular site and design case. Factors to be considered include the cost of protection, acceptable degree of risk, and consequences of failure. The following paragraphs provide some guidance in this assessment.

Length of bank to be protected

A site inspection and an understanding of the mechanisms causing erosion will assist in determining the appropriate length of bank to be treated. Aerial photographs will provide guidance in understanding the temporal development of the river alignment and, together with local knowledge, will often assist in determining the history of erosion at the site. Such a history is often a valuable indicator of likely future developments.

Flow lines and corresponding points of attack will vary significantly with the flow level. In a meandering stream, the main current lines tend to straighten, and the point of attack on a bend will tend to move downstream, with increasing flow. Braided streams are less predictable.

Erosion on the outside of bends will also move downstream with time. For this reason, it is desirable to continue erosion protection downstream beyond the limit of existing erosion. Generous location of the downstream limit is essential to successful rip-rap protection. As a guide for the treatment of major meander developments, rip-rap protection should be provided for a distance of twice the bank-to-bank distance downstream of the existing erosion.

The upstream limit of erosion protection is generally easier to locate. As a guide for the treatment of major meander developments, rip-rap protection should be provided for a distance upstream of the existing erosion equal to the bank-to-bank distance.

Proportion of bank height to be protected

It is generally not necessary to extend rip-rap protection to the top of the bank unless dictated by unusual site constraints. Such constraints may include the presence of strong overbank flows, upper bank erosion by strong wave action or prolonged high flows, and severe consequences of rip-rap failure.

As a general rule, protection of the lower two thirds of a bank (as shown in Figure 5-9) is usually considered to offer optimum protection. The upper one third of the bank can be treated, as appropriate, with less resistant and less expensive techniques such as vegetation.

However, this rule should always be reviewed in the light of local knowledge and conditions. For example, conditions where a river has a very high bank such that bank-full flows are reached less than once a year, may justify a reduction in the height of protection. Conversely, a low bank, relative to the level of the annual flood, may need full protection.

An understanding of the mode of failure also assists in this assessment. For example, if the bank failure mechanism is through undermining of the toe and subsequent collapse, protection of the toe is crucial. On the other hand, if fretting at high levels is primarily responsible for erosion, it is more important to provide protection at that water level.

In designing extensive rip-rap works, it may be desirable to consider, in addition, the stream longitudinal profile. This gives the designer the additional options of ensuring that the height of rip-rap protection represents an adequate proportion of the average bank height, or allowing the rip-rap height to vary along the reach to reflect variations in the water surface profile.

Notes on the rip-rap technique

17

5.2.2 Allowance for scour at toe of rip-rap Many failures of rip-rap occur through undermining of the toe of the rip-rap by scour of the stream bed during high flow events. Two methods of allowing for this are (see also Figure 5-9):

1 Extend the rip-rap protection below the estimated scour level by placing rip-rap material in an excavated trench

2 Provide extra rip-rap material at the toe of the bank which can drop down and provide protection should scour occur.

Use of the second alternative requires care. The response of the rip-rap to settling is unpredictable. In particular, if a graded rock is used as is recommended the finer material will be susceptible to loss during settlement. Accordingly, it is recommended that allowance should be made for at least 50% loss of rock.

In a particular design situation, the likely severity of bed scour needs to be assessed. Although this assessment should be technically based on techniques for estimating scour depth, the particular design situation is also of great importance. The following comments are pertinent:

In meandering gravel bed rivers, allowance for scour on the outside of bends is generally made by ensuring generous provision of rock at the toe of the protection works. Additional scour depths beyond the deep holes typical of this situation are likely to be reasonably small.

In sand bed streams, scour depths can be several metres in magnitude, particularly if the channel is steep. Allowance for scour is an essential component of successful design.

Deep scour may occur at constrictions, groynes, bridge abutments, and other areas of flow constriction and flow disturbance. Standard texts on scour should be consulted to estimate the extent.

The depth of scour increases with increasing slope and depth of flow and with decreasing bed material size.

Where scour is estimated to be very severe, in-channel scour control techniques may be considered as an alternative means of ensuring rip-rap integrity.

5.2.3 Specification for rock quality and grading and thickness of layer

Rock should be hard, tough, and durable. It should have a crushing strength of at least 25Mpa. The rock should be free of defined cleavage planes and should not be adversely affected by repeated wetting and drying.

The rock should be predominantly angular in shape with not more than 25% of rocks, distributed through the gradation, having a length more than twice the breadth or thickness. No rock should have a length exceeding 2.5 times its breadth or thickness.

Where rock fails to meet this specification, it may still be considered at the designers discretion, provided allowance is made in the design for its shortcomings.

Rock to meet the necessary size and strength criteria will normally be won from a hard rock quarry by drilling and blasting. A hydraulic rock breaker mounted on a hydraulic excavator provides an excellent means of producing rock to design size specifications.

Rock should not be single sized, but, instead, should be a well-graded mixture designed to ensure that all interstices between large rocks are filled with rock of progressively smaller size. This has the effect of ensuring that no significant voids occur in the rock blanket through which underlying material can be washed out. Additionally, it helps to create an interlocking mass of rock, which is highly stable.

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Experience suggests a rock gradation such as that summarised in Table 5-4. When specifying rock gradation to field staff and contractors, it is helpful to transform this grading by weight into an equivalent grading by number.

Table 5-4: Suggested rock gradation

Equivalent spherical diameter

Percent by weight of rock of smaller size

1.5 2.0 D50 100%` D50 50%

0.3 0.4 D50 10% - 20%

The thickness of rock rip-rap protection should be at least twice the median rip-rap diameter or equal to the largest rocks in the rip-rap mixture, whichever is the greater.

5.2.4 Details of filters Filter materials may be necessary to stabilise rip-rap protection over fine material. The filter layer prevents bed material being washed through residual interstices in the rock layer (see Figure 5-9 for filter layer position). Normally, a filter layer is only necessary where the underlying material is largely non-cohesive such as uniform sand or silt, high groundwater levels create large pore pressures, or an unusually high factor of safety is required. If one or more of these conditions prevails, the need for a filter layer can be further assessed from the following required criteria:

For stability

5material)bank (D

rap)rip(

85

15 D

and

25material)bank (D

rap)rip(

50

50 D

For permeability

5material)bank (D

rap)rip(

15

15 D

If the relationship between bed material grading and chute rock grading is outside these limits, the need for a filter layer becomes more paramount.

A granular filter layer can be designed by applying the above relationships twice once between the bed material and the filter layer and once between the filter layer and the chute rock.

Geotextile filters

In those conditions that require provision of a filter layer between the rip-rap and the parent material, geotextile can be an alternative to a filter layer.

However, caution is recommended. Several failures and partial failures have occurred involving rock sliding on the filter cloth. This occurs where the friction between the rock and the filter cloth is less than the internal friction of the rock mix.

Notes on the rip-rap technique

19

The most vulnerable design cases are those for which the hydraulic forces are relatively small, permitting the bank angle to be steep. For designs where a flat batter is required to ensure rip-rap stability against hydraulic forces, the risk of failure by sliding of the rock on the filter cloth is diminished.

Accordingly, care must be taken to ensure maximum resistance between the rip-rap and the cloth. This can be achieved by avoiding preparation of the bank to a smooth and even batter, not stretching the cloth tightly over the underlying bank, and avoiding cloths with low friction surfaces.

A comparison of the advantages and disadvantages of using geotextile as an alternative to granular filters is presented in Table 5-5, after Hemphill and Bramley (1989).

Table 5-5: Advantages and disadvantages of geotextile and granular filters

Geotextile Granular material

Adva

ntag

es

Cost

In-plane tensile strength

Limited thickness

Self-healing in some circumstances

Generally very durable

Deformable retains good surface contact above and below

Relatively easy to repair

Dis

adva

ntag

es

Some uncertainty over long-term behaviour

Edges must be carefully protected

Easy to damage, difficult to repair

Careful design and installation needed to accommodate settlement of uneven foundation

Careful control needed to achieve specified grading sand thickness

Compaction difficult on steep side slopes

Control of construction difficult underwater.

RIPRAP User Guide

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Appendix A - RIPRAP Theory

The theoretical development is based on determining the balance between the disturbing and restoring forces on a rip-rap particle on a sloping bank.

The force balance is developed using Figure A-1.

Figure A-1: Forces on a rip-rap particle

The disturbing force, RD, is the resultant of the drag force on the particle, FD, and weight of the particle, WS, resolved down the bank slope, . This force is expressed as:

[ ] 2222222 sinsin SSDSD aWFWR +=+= (A-1) where

a is the effective particle area exposed to flow drag

S is the shear stress on the bank

The restoring force, RR, is given by the product of the component of the particle weight normal to the bank and the tan of the natural angle of repose, . This force is expressed as:

tancosSR WR = (A-2) The factor of safety is then defined as the ratio of restoring force to disturbing force:

RIPRAP Theory

21

2222 sin

tancos

SS

S

D

RS

aW

WRRF

+== (A-3)

Now, on a flat bed at the threshold of particle motion, = 0 and FS = 1 by definition. The determination of the critical shear stress at the threshold of motion on a flat bed is given by:

( )

constant150

=s

c

SD

(A-4)

where

c is the critical shear stress

is the specific weight of water

D50 is the particle size for which 50% of the sample is finer

Ss is the relative density of the bed material

Equation (A-4) is correct provided the particle Reynolds Number is above a certain value, typically taken to be about 400. This corresponds to a particle size of about 6mm (Henderson 1966). In all practical rip-rap designs, the rock size will always be larger than this value, so the use of the equation is justified.

The magnitude of the constant is the subject of some difference of opinion in the literature. The classic work of Shields (Henderson 1966) produced a value of 0.056, although other studies have yielded lower values. For the present study, a more conservative value of 0.047 is chosen, consistent with the work of Meyer-Peter and Muller (1948) and Yalin and Karahan (1979).

Thus, Equation (A-4) may be written as:

( )1047.0 50 = sC SD (A-5)

An alternative expression for c may be obtained from Equation (A-3) with = 0 and FS=1. Under these conditions, Equation (A-3) devolves to:

a

WSC

tan= (A-6)

Equating the right hand sides of Equations (A-5) and (A-6) and algebraic manipulation yields:

( )1047.0

tan

50 =

s

S

SDW

a

(A-7)

Now, on the bank of a trapezoidal channel, the shear stress may be expressed as (after Chow 1959):

SyS 75.0 = (A-8)

where

S is the local gradient of the total energy line.

Substituting Equations (A-7) and (A-8) into Equation (A-3) yields:

( )

2

50

22

1047.0tan75.0

sin

tancos

+

=

s

SS

SS

SDySWW

WF

(A-9)

RIPRAP User Guide

22

Simplification of Equation (A-8) by dividing the top and bottom lines by WS and rearranging yields:

( )

2250

sintancos

1047.0tan75.0

=

S

s

F

SyS

D (A-10)

It is evident from the form of Equation (A-10) that a solution is only possible if

sintancos SF

. Thus, the limiting bank slope is given by:

=

SF

tantan 1max (A-11)

Equations (A-10) and (A-11) are solved by the program RIPRAP.

References

23

Appendix B - References Carter, A. C. (1953). Critical Tractive Forces on Channel Side Slopes. U. S. Bureau of

Reclamation, Hydraulic Laboratory Report Hyd-366, February

Chow, V. T. (1959). Open Channel Hydraulics, McGraw-Hill, New York

Hemphill, R. W., and Bramley, M. E. (1989). Protection of River and Canal Banks, Butterworths, London

Henderson, F. M. (1966). Open Channel Flow, Macmillan, New York

Meyer-Peter, E., and Muller, R. (1948): Formulas for Bed-Load Transport. Proceedings of the 2nd Congress of the International Association for Hydraulics Research, IAHR, Stockholm, Sweden, June

Simons, D. B. and Senturk, F. (1977): Sediment Transport Technology, Water Resources Publications, Fort Collins, Colorado

Yalim, M. Selim, and Karahan, E. (1979): Inception of Sediment Transport. Journal of the Hydraulics Division, ASCE, Vol. 105, No. HY11, Proc. Paper 14975, November, pp 1433-1443.

1 Introduction1.1 The user guide1.2 Software1.2.1 Overview1.2.2 Audience1.2.3 RIPRAP theory summary1.2.4 Limitations

1.3 Key references

2 Installation2.1 Technical specifications2.2 Licence agreement2.3 Installation

3 RIPRAP Tutorial3.1 Getting started3.2 The tutorial scenario3.3 Inputting data3.4 Running RIPRAP3.5 Results3.6 Further reading

4 Using RIPRAP4.1 Inputs4.1.1 Energy slope4.1.2 Bank angle4.1.3 Rock specific gravity4.1.4 Rock angle of repose4.1.5 Maximum depth4.1.6 Depth of interest4.1.7 Factor of safety

4.2 Outputs

5 Notes on the rip-rap technique5.1 Introduction to the rip-rap technique5.2 Further design considerations5.2.1 Extent of bank protectionLength of bank to be protectedProportion of bank height to be protected

5.2.2 Allowance for scour at toe of rip-rap5.2.3 Specification for rock quality and grading and thickness of layer5.2.4 Details of filters

Geotextile filtersAppendix A - RIPRAP TheoryAppendix B - References