Introduction
1
Stability of channel bed and banks
2
100
80 r e n i f e g a t n e c r e P
60
40
20 A
0
0·002
Clay
B 0·006
Fine
0·02 0·06 0·2 Grain size: mm
Silt Medium Coarse
Fine
0·6
2·0
Sand Medium Coarse
Gravel
τ
′
n φ
τ =
+ c ′
a σ ′ t
φ′
c ′
σ′
W s
F W n
W
α
α
α
Before
Before
After
After
(a) Shallow failure shallow bank angle usually in non-cohesive banks failure nearly parallel to slope at α = φ′ water seepage from bank can substantially reduce stable α vegetation will normally help stabilise against failure
(b) Planar failure steep or vertical bank angle frequently (but not always) in non-cohesive banks water table/channel water level usually low relative to total bank height
>0·3 m
m 0 · 1 >
Before
Before
After
After
(c) Planar/slab failure steep or near vertical banks deep tension cracks failure occurs by sliding and/or toppling failure more likely if crack fills with water little affected by groundwater table
(d) Rotational failure in homogeneous material usually on moderately high or steep banks usually in cohesive material tension cracks reduce stability particularly when water-filled significantly affected by position of water table failure may extend beyond toe, see also type (e)
Water table
Tension crack
Bulging Erosion
Before
Before
After
After
(e) Rotational failure with weak zone failure surface dictated by position of weak zone see also comments for type (d)
Desiccation
(f) Massive rotational failure/landslide erosion of river bank threatens stability of whole valley side very large volume of slipped material tension cracks up valley side, bulging above toe, or noticeable movement are signs of potential failure
Cohesive
Cohesive Sand/ gravel
Sand/ gravel
Before
Before
After
After
(g) Failure of composite bank (in tension) occurs only where upper cohesive layer overlies erodible sand/gravel failure by tension of lower part of overhanging block
(h) Failure of composite bank (as beam) occurs as type (g) failure with upper soil in tension, followed by rotation after failure block usually remains intact with vegetation towards river failure can also be by shear
HWL MWL 1st stage bed level Maximum predicted scour (a)
HWL
MWL 1st stage bed level
Maximum predicted scour (b)
HWL MWL 1st stage bed level
Maximum predicted scour 2nd stage position of riprap (c)
F lo w
Topsoil and seeded
Stone root
Spur
MWL Stone fill Typical section
Streamwise flow direction (primary flow) Secondary flows
Secondary current
Contours of primary flow velocity
Lower velocity
Higher velocity
Bank shear stress
Bank shear stress
Bed shear stress
Lower velocity Higher velocity (a)
(b)
(c)
(d)
w F l o v U d
w F l o
Q U A
Mean cross-sectional velocity / A U = Q Vertical velocity profile
Transversal stern wave
Return currents
Front wave
Water depression
Bank
Boat direction
Boat
35° Bank Secondary stern waves
Secondary bow waves Plan
Undisturbed level
Water elevation (exaggerated)
LW
Wind direction B
Bank under consideration (a)
LW
B
45°
Wind direction
Bank under consideration (b)
LW
Bank under consideration Wind direction B
(c)
WIND WAVE DESIGN
Obtain data on watercourse layout, width, length of reach
Obtain value of wind speed U 10 at 10 m above mean water level from local records; if not possible, assume U 10 = 18 m/s for sheltered UK sites and U 10 = 25 m/s for exposed UK sites
Calculate fetch F from Figure 2.12 (approximate method) or refer to Yarde et al . (1996) for more accurate estimation
Calculate significant wave height H s and period T z using Equations (2.9) and (2.10)
Calculate maximum wave height H i using Equation (2.11)
Use H i and T z to calculate stable size of revetment: Equations (2.12) and (2.13) for riprap Equations (2.14) and (2.13) for solid concrete blocks Use manufacturers' literature for other revetment types
(a)
(b)
(c)
(d)
(e)
(f)
Smooth bank
(g)
Rougher bank
(h)
A
B Point bar Point bar
C
A B Thalweg Typical bed contours
Outer bank
Section A–A
Inner bank
C
Inner bank
Section B–B Inflexion point
Outer bank
Section C–C
0·41
0·65
0·76
0·88
1·0
1·0
0·76
0·41
1·0
Outer bank 0·85
(a)
Inner bank
0·43 0·57
0·78 0·9 0·71 0·85
1·0
(b)
Recirculating surface vortex
Flow direction
Horseshoe vortex Notch
Design procedure
3
CONCEPTUAL DESIGN
Assessment of stability (Morgan et al ., 1998) Specialist surveys for beds
Unstable channel
Stable channel
Collection of historic data (floods, previous protection, failures)
Identification of types and causes of bank failure (Morgan et al . 1998 and Section 2.1) Identification of causes of bed failure (usually proximity of structures — Section 2.4.1)
Consequences of erosion (Morgan and Section 2.2)
Definition of objectives (Morgan and Section 3.1)
et al .,
et al .,
1998
1998
Accessibility and construction issues (Chapter 6)
Choice of strategy (Morgan et al ., 1998 and Section 3.2)
Light hydraulic load: U ≤ 1m/s; H *≤0·15 m and straight channel or gently meandering
Intermediate conditions
Bioengineering and/or management practices (Morgan et al ., 1998) Hemphill and Bramley (1989)
Heavy hydraulic load: U ≥2·5 m/s; H * ≥ 0·5 m or need for instant protection or proximity of hydraulic structures or sharp bends
Biotechnical or structural solutions
* Take H as significant wave height (Section 2.3.2) Less common practice
OUTLINE DESIGN
OUTLINE DESIGN
Obtain detailed design data: bank and bed geometry flow rates and velocities water levels soil characteristics existing or previous revetments and practices history of erosion flora and fauna accessibility/construction issues local structures
Insufficient hydraulic data
Hydrographic surveys
Complex soil conditions
Geotechnical tests
Define predominant hydraulic loadings: current attack (Section 2.3.1) waves (Section 2.3.2) tidal flow (Section 2.3.3) high turbulence (Section 2.4.1) flow around bends (Section 2.4.2)
Consider the importance of: ecological impact appearance feasibility of regular maintenance cost vandalism potential
Assess need for filter layer or geotextile (Chapter 5, Figure 5.4 up to Step 4) and feasibility of installation (Chapter 6)
Compare alternative solutions: Tables 3.1 and 3.2 in Section 3.2, Chapter 4, Tables 4.7 and 4.8 in Section 4.5, Appendix 1 (Data sheets); also Chapter 2
Complex geometry and combination of loads; low risk acceptance
Choose solution(s)
DETAILED ENGINEERING
Laboratory tests and/or field trials
DETAILED ENGINEERING
DESIGN OF COVER LAYER 1.
Check stability against wind waves (Section 2.3.2)
2.
Hydraulic loads identified in Outline Design
Yes
Case C Yes
Use formulae for high turbulence (Section 2.4.1)
3.
High turbulence ?
Are there boat waves bigger than wind waves ?
No
Case A No
Case C Yes
Use formulae for boat waves (Section 2.3.2) and current flow (Chapter 4); Check design for tidal flows (Section 2.3.3) and flow around bends (Section 2.4.2) if applicable
Use formulae for high turbulence (Section 2.4.1)
High turbulence ?
Case B No
Use formulae for current flow (Chapter 4); check design for tidal flows (Section 2.3.3) and flow around bends (Section 2.4.2) if applicable
Case A: adopt as the stable size for the chosen revetment the sum of the sizes obtained for stability against currents and against boat waves (Equation (2.26)) Case B: adopt as the stable size for the chosen revetment the sum of the sizes obtained for stability against currents and against wind waves (Equation (2.27)) Case C: adopt as the stable size for the chosen revetment the biggest of the sizes determined in Steps 1 and 2
DESIGN OF FILTER LAYER(S) Initial design of granular filter or geotextile (Chapter 5; Figure 5.4) For detail design follow manufacturers' guidance
Specification of cover layer materials (Chapter 4, Appendices 1, 2, 3 or 4 Specification of filter layer (Appendices 2, 5)
Edge and toe detailing (Appendix 6)
Specification of construction procedures (Chapter 6)
Produce cost analysis for whole life of structure (capital and maintenance costs)
Define maintenance procedures (Chapter 7)
Types of revetment and design formulae
4
Quarry stone, hand placed to smooth profile, wedged with smaller stones
Pitching continuous or rigid toe Bedding as necessary Underlayer Dressed stone Toothed undersurface
0·5 or 1·0 m
, 3 · 0 , 0 · , 2 1 · 5
m · 0 r 6 o · 0 , 5 0 · 4
1·0 or 2·0 m
Typical dimensions shown
(a)
2 · 0 o r 3 · 0 m
m 1 · 0 m · 0 r 6 o · 0 , 5 0 · , 4 3 · 0
0·15, 0·17, 0·20,0·23, 0·25 or 0·30 m
m 1 · 0
Typical dimensions shown (b)
0·65 m
2·0, 3·0, 4·0 or 5·0 m (c)
(a)
Geomat
Grass cover
Soil
(b)
Geoweb channel lining system with concrete infill below NWL
Geoweb channel lining system with topsoil infill and seeded above NWL
NWL
Nonwoven geotextile
J-pins hooked over cell walls
Native soil (a)
Geoweb channel lining system with concrete infill below HWL topsoil and seed above HWL Geoweb gravity wall
HWL Nonwoven geotextile High strength polyester tendons J-pins hooked over tendons (b)
Planted rolls Capping unit
Sheet piling
(a)
Concrete block protection HWL LWL Potential failure surface
Ground anchor Sheet piling (b)
Capping unit
Dead man anchor
Potential failure surface Sheet piling (c)
Use of granular filters and geotextiles
5
Water and soil particles
Soil fines
Water (and few fines) Water (and few fines)
G r a f i l t e n u l a r r
(a)
Geotextile (b)
FILTER DESIGN
Identify soil properties: (see Section 2.1) type of soil grading curve
Step 1
Determine revetment permeability (low, high) (Table 4.7, Section 4.5)
Are granular materials easy/economical to find? AND Are differential loads and waves not significant? AND Is there space for granular filter (of several mm, minimum)? AND Is turbulence level low (TI in Section 2.4.1 ≤0·2)? AND Is filter to be built in the dry or under light hydraulic attack?
No
Usually choose GEOTEXTILE Section 5.2.2
Initial design of geotextile (Section 5.3.2): determine maximum O 90 (Equations (5.7) to (5.10)) check required permeability k g (Equation (5.6))
Check need for composite filter (Section 5.3.3): is soil susceptible to downslope migration? is granular layer needed for protection of geotextile?
Step 2
Yes Step 3
Usually choose GRANULAR FILTER Section 5.2.1
Step 4
Design granular Step 5 filter using Equations (5.1) to (5.5), Section 5.3.1
No
Yes Design composite filter (Section 5.3.3 and specialist advice)
Final geotextile design (manufacturers' advice)
Step 6
Construction issues
6
Maintenance procedures
7
Worked examples
8
100 90 80 70 r e n i f
e g a t n e c r e P
60 50 40 30 20 10 0
0·002
Clay
0·006
Fine
0·02 0·06 0·2 Grain size: mm
Silt Medium Coarse
Fine
0·6
2·0
Sand Medium Coarse
Gravel
100 90 80 70 r e n i f
e g a t n e c r e P
60 50 40 30 20 10 0
0·002
Clay
0·006
Fine
0·02 0·06 0·2 Grain size: mm
Silt Medium Coarse
Fine
0·6
2·0
Sand Medium Coarse
Gravel
m 1
1
m
1 m
m 1
m 1 1 m
Lifting eyes
0·9 m
×
0·9 m × 0·9 m
Lid
2 m 6 m
Diaphragms End panel
Side panel
0 m 2 · 0
Sack gabion
m 5 6 · 0
2·0; 3·0; 4·0; 5·0 m Metal stakes or staples
Armorloc data Gross area/block Open area Mannings n
m m 5 · 1 0 3
0·09 m 2 20% 0·040
t
403 mm
Concrete specification Density Sulphate resistance Freeze thaw test
2200 kg/m3 BS 5328: 1991 Class 2 No visible effect
Type
Thickness t
Armorloc
90 mm
Wt. of block 12·8 kg
Wt. of No.of block/m2 blocks/m2 150 kg
11
A
B C
450 2 2 2
*A
AR
1
2
*AB
ARB
D
DA
DC
EA
EC
225 *AT
ART
AL
ALB
Where blocks are holed for articulation suffix/H2 will apply (block width only)
2 2 2
E
Plain concrete blocks for hand laying available in types marked * only
ALT
Dycel 100, 125 and 150
Dycel 101 and 151
Dycel 220
Density 2·3
Column height
t/m2
m2 /t
15 20 25 30 35 40
0·312 0·416 0·521 0·628 0·710 0·811
3·21 2·40 1·92 1·59 1·41 1·23
Slight deviation in dimension and weight possible
m 4 1 1 · 0
m 9 1 4 · 0
0·469 m
Grasscel 80
Grasscel 100
0 4 4
Grasscel 125
0 0 4
330
0 4 4
600
400
b
a
16
a
6 0 4
406
b
16
6 0 4
6 1
406
3 0 1
3 0 1
a–b
a–b
H
Cable ducts
Cellular block
L
B
L
B
Dycel 100, 125 and 150 Dycel 101 and 151 Dycel 220
450 2 2 2
*A
AR
*AB
ARB
*AT
ART
AL
ALB
Where blocks are holed for articulation suffix/H2 will apply (block width only)
ALT
1
2
D
DA
DC
EA
EC
225 2 2 2
E
Plain concrete blocks for hand laying available in types marked * only
Soil Mesh Concrete Sand Sub-base Grasscrete GC2 150 mm deep
Armater (polyester honeycomb mattress) Topsoil fill
Stone fill
LWL
Expanded Collapsed
B A H
L B
m 1 ·0 0· 5 –
A – Patented Terramesh® unit of double twist hexagonal mesh type 8 zinc coated and PVC coated wire dia. 2·7 mm (3·7 mm o/d).
×
10
B – Diaphragm made with double twist hexagonal mesh type 8 × 10 zinc coated and PVC coated wire dia. 2·7 mm (3·7 mm o/d).
2
4
3 1
1. Reinforcing geotextile 2. Steel mesh mesh form work 3. Vegetation Vegeta tion geotextile 4. Hydroseeding
Standard cell 244 mm × 203 mm
Cell depth: mm 3 0 2 2 0 1
2 5 1
6 7
Erosamat placed on prepared slope and filled with topsoil then seeded
Anchorage trench
Steel pins Normal river level
Thick nonwoven geotextile filter fabric
Cut-off walls
d s d s
(a)
(b)
Falling aprons
d s
d s
(c)
(d)
Embedded toe
d s
d s
(e)
(f)
Toe trench d
2d
d s
(g)
(h)
Other types
Anchor
Boards King post (~3·5 m long; every 1·5 to 2·5 m) (j)
(i)
45
°
90
°
(a)
(b)
(c)
Free-standing wall
Flow
Slope if required (d)
(e)
(f)
