4. PRÓTESIS OCULARES
4.2. HISTORIA Y EVOLUCIÓN DE LAS PRÓTESIS OCULARES
5.4.1 Basic Concept
Spur dikes are river training structures constructed along the banks of rivers and flood dikes to deflect or repel the flow for the purpose of training the course of the river channel and to protect the banks from scouring by inducing siltation in the area.
A spur dike is a river structure with the following functions:
Increases the flow roughness and reduces the flow velocity around the riverbank.
Redirects river flow away from the riverbank.
Corollary to the above functions, installation of spur dikes has the following purposes:
Prevents bank erosion and damage to revetment.
Deepens water depth for navigation.
Figure 5-13 Example of Spur dikes used to protect outer River Bank
Source: CIRIA, 2007
Figure 5-14 Example of Spur dikes used with Bridge Design
Source: PNG DoW, 1987
5.4.2 Types of Spur Dike
Spur dikes can be broadly classified into permeable and impermeable/semi-permeable. The permeability of spurs is defined simply as the percentage of the spur surface area facing the streamflow that is open.
Permeable type – the spur dike of this type is made of piles and frames, preferably in series. Its purpose is to reduce the river flow velocity at the immediate downstream of the spur dike and induce sedimentation. In cases where piles cannot be driven due to the presence of boulders on the riverbed, crib frame, skeleton works or concrete block type shall be used.
Impermeable/semi-permeable type - This type of spur dike is made of masonry (impermeable) or concrete blocks and loose boulder (semi-permeable), preferably in series. Its purpose is to divert the river flow direction away from the riverbank. These types of spur dikes can be further classified:
Overflow type – the main purpose is to reduce the river flow velocity. This type of spur dike can be considered as a series of spur dikes.
Non-overflow type – its main purpose is to change the river flow direction away from the riverbank.
Impermeable spurs provide more positive flow control but cause more scour at the toe of the spur and, when submerged, cause erosion of the streambank. High permeability spurs are suitable for use where only small reductions in flow velocities are necessary as on mild bends but can be used for more positive flow control where it can be assumed that clogging with small debris will occur and bed load transport is large. Permeable spurs may be susceptible to damage from large debris.
Figure 5-15 Example Permeable Spur Dike
Source: PNG DoW, 1987
5.4.3 Spur Dikes vs Revetments
The choice between the adoption of spur dikes and revetments is not always straight forward in riverbank protection. Also, in some situations it may be appropriate to adopt a combination of the two, where the spur dike can provide protection for the revetment. Some considerations are provided below.
Revetments:
Typically used where it is intended to protect the riverbank in its existing position.
Where it is necessary to re-instate the riverbank before protecting it, then a revetment may be appropriate.
Spur dike:
If it is necessary to re-instate the riverbank, but re-instatement is expensive, then a spur dike might be appropriate. This will result in gradual re-alignment of the riverbank.
If the cost of a continual revetment is expensive, then a spur dike, or small spur dikes, may in some situations be more economical.
Typically used on wide, shallow rivers, rather than narrow deep channels.
May not be appropriate where the variation in water level from low flow to flood level is large.
Can be useful for navigational rivers, where they can assist in defining the navigational channel.
5.4.4 Design Criteria
5.4.4.1 Design Water Level
The design flood level and the ordinary water level during the rainy season shall be considered in the design of a spur dike. These should be indicated on design plans.
The design water level needs to be calculated based on the hydraulic methods presented in Section 4, and allowing for the reduced cross sectional area presented by the spur dikes. It should generally be assumed that the cross sectional area is the portion of the channel clear of any spur dikes, and that there is no effective flow within the spur dike field.
5.4.4.2 Design Velocity
The design velocity, used in the sizing of any protection measures for the spur dike, needs to be increased for the local velocity acting on the spur dike. It is recommended to adopt a design velocity of 2 times the cross sectional average velocity for the design of the spur dike (following Maynard, 1978).
5.4.4.3 Orientation
Permeable retarders are designed to provide flood retardance near the streambank, and this is typically achieved regardless of orientation. Therefore, for construction cost purposes, the cheapest alternative is to typically construct the permeable retarder spur dike perpendicular to the bank.
As identified in HEC23 (2009), there is no clear consensus of orientation of impermeable/ semi-permeable spurs. Spur orientation at approximately 0 degrees (perpendicular)has the effect of forcing the main flow current (thalweg) farther from the concave bank than spurs oriented in an upstream or downstream direction. Therefore, more positive flow control is achieved with spurs oriented approximately normal to the channel bank (HEC23, 2009).
As per HEC23 (2009), it is recommended that the spur furthest upstream be angled downstream to provide a smoother transition of the flow lines near the bank and to minimize scour at the nose of the leading spur. Ideally, this first spur dike should be located upstream of the most severe scouring area, to ensure that it remains during larger flows. Subsequent spurs downstream should generally all be set normal to the bank line to minimize construction costs.
5.4.4.4 Height
The height of impermeable spur dikes should not exceed the top of the banks.
Otherwise, erosion can occur at the overbank end of the spur dike. Where it does not exceed this, the following shall also apply:
The height of a non-overflow type spur dike should be at the level of the design flood.
The height of overflow type spur dike shall be the maximum of:
- 10% to 40% of the distance reckoned from the average riverbed to the design flood level.
- 0.5 to 1.0 m above the ordinary water level during rainy season.
Permeable spurs, and in particular those constructed of light wire fence, should be designed to a height that will allow heavy debris to pass over the top.
5.4.4.5 Top Width/ Crest Width
Usually, the top width or crest width of impermeable spur dikes ranges from 1 to 3 m.
5.4.4.6 Slopes
A spur dike should slope from the bank to the river, to prevent overtopping occurring at a low point on the spur dike. The longitudinal slope of the spurdike should be 1V:20H to 1V:100H toward the center of the river.
The side slopes shall depend on the quality of the subsoil, groundwater flow and the type of structure. Slopes are typically between 1V: 1H and 1V:2H on the upstream side and 1V: 1H and 1V:2H on the downstream side.
5.4.4.7 Length
Spur dikes should have lengths up to 10% to 15%of the width of the river or channel but not to exceed 100 m.
The river flow capacity should be examined when the length of the spur dike is more than 10% of the river width (distance of left to right bank); or when the spur dike is to be constructed in a narrow river, since this could affect the opposite bank and considerably reduce the river flow capacity.
Figure 5-16 Dimensions of Spur Dike – Impermeable Overflow Type
5.4.4.8 Spacing
The spacing of spur dikes is related to the length of the spur dike, the angle of the spur dike, permeability and the degree of curvature of the bend.
As a general rule of thumb, the spacing for semi-impermeable (up to 35%
permeable) or impermeable spur dikes should be less than 2 times its effective length at flow attack zones and 2 to 4 times at straight sections of channel.
The effective length is the length from the desired bankline to the tip of a spur.
Where it is proposed to protect the bankline it its existing position, then the effective length will be the same as the length of the spur. Where the spur dikes are expected to result in an increase in the bankline, then the effective length is the length from the planned bankline to the end of the spur. This is demonstrated by
“L” in Figure 5-7.
Permeable spurs should be spaced closer together. Based on the procedures identified in HEC23, for a 75% permeable spur, the spacing should be approximate 70% of that for an impermeable spur.
A more detailed procedure for determining the spacing of spur dikes is provided in HEC23, in Design Guidance 2.12, Section 2.2.7.
Figure 5-17 Effective Length of a Spur Dike
Source: HEC23, 2009
5.4.4.9 Embedment Depth
For concrete and stone masonry type spur dike, a minimum embedment depth of 0.5 m is recommended.
For gabion-type, boulder type and concrete block type spur dikes, only a provision of about 0.2 m layer of gravel before placement of the main body is sufficient.
Piles supporting permeable structures can also be protected against undermining by driving piling to depths below the estimated scour. Round piling are recommended because they minimize scour at their base.
Extending the facing material of permeable spurs below the streambed also significantly reduces scour. If the retarder spur or retarder/deflector spur
performs as designed, retardance and diversion of the flow within the length of the structure may make it unnecessary to extend the facing material the full depth of anticipated scour except at the nose.
5.4.4.10 Slope Protection
Impermeable spur dikes will require protection of the slope. Furthermore, if the spur dike is expected to overtop during design flows, then the crest will also require protection. Typical protection for spur dikes includes gabions, gabion mattresses, concrete blocks and rip rap. The design of these can be adopted as per revetments, which is detailed in Section 5.5.4.2.
5.4.4.11 Impact Loading
The structural design of any permeable spur dikes will need to be able to resist dynamic and hydraulic loads based on the bankfull condition. An appropriate design debris loading conditions will need to be selected by the designer. The proposed log debris loading condition for bridges in Volume 5 could be adopted as an initial estimate. However, this should be based on review and judgment by the designer.
5.4.4.12 Toe Protection Works
Toe protection should be provided to prevent collapse of the spur dike due to riverbed degradation or scouring. Riprap or gabion can be used for toe protection work. The design of these can be based on the approach for revetments, which is detailed in Section 5.5.6. However, the methods identified in Section 5.5.6 should be adjusted to account for the scour estimated for a transverse structure. This is identified in Annex A.
When the spur dike is not orientated at a right angle to the bank, then Figure 5-18 should be used to adjust the estimated scour depth calculated in Annex A.
Figure 5-18 Scour Adjustment for Spur Orientation
Source: HEC23, 2009
Figure 5-19 Toe Protection Works for Spur Dike
5.4.4.13 Shape of Spur dikes
In general, straight spurs should be used for most bank protection. Straight spurs are more easily installed and maintained and require less material.
The shape of permeable spur dike will depend on the material adopted.
For impermeable and semi-permeable spur dikes, they should be straight with a rounded nose, as identified in Figure 5-20.
Figure 5-20 Shape of Spur Dike
Source: HEC23, 2009
5.4.4.14 Base Protection
The base of spur dike is the joint to the bank or to the revetment usually prone to damage and outflanking. Therefore, the gap between the base and bank shall be filled up by adequate materials, such as riprap and gabion.
5.5 Revetments
5.5.1 Basic Concept
Revetments are flood control structures constructed along river banks subjected to direct attack of the river flow and along levee slopes for protection against erosion, scouring, riverbed degradation and wave wash. They are used in many situations where the riverbank is to be protected in its existing location.
A revetment should be designed based on the existing site conditions, such as river flow velocity and direction, embankment material, topographical, morphological, and geological conditions of the riverbank, etc. Further, the revetment should be designed to withstand the lateral forces due to high velocity flow, when located in flow attack zone, on a weak geological condition of riverbank, and with poor embankment materials.
It is important to note that most flexible revetments (riprap, gabion mattress (spread type), concrete blocks) do not provide resistance against geotechnical instability, such as slumping failure in saturated streambanks and embankments (HEC-23, 2009).
Typical applications of revetments include:
Along meander bends of the river, to prevent scouring.
At downstream and upstream of hydraulic and other related structures where turbulent flow usually occurs.
Alongside slopes of irrigation canals to prevent loss of water due to percolations.
Figure 5-21 Location of Revetment at River Bend
5.5.1.1 Types of Revetment
Rigid (concrete slab)
Flexible (riprap, quarry stones)
Revetment may range from rigid to flexible. Concrete slab-on-grade is an example of rigid while riprap and quarry stones are an example of flexible. Rigid revetments tend to be more massive but are generally unable to accommodate
settlement or adjustments of the underlying materials. Flexible revetments are constructed with lighter individual units that can tolerate varying amounts of displacement and shifting.
5.5.1.2 Components of a Revetment
The typical components of a revetment include:
Slope covering work: directly covers and protects the bank slope from direct attack from flood water, boulders and floating debris.
Foundation work: constructed at the toe of the slope that supports the slope covering works.
Foot protection work: constructed to prevent scouring in front of the foundation work and outflow of material from the back of the slope covering work.
Shoulder beam work: headwall installed at the shoulder of the revetment to prevent damage.
Backfilling material: materials which are backfilled to the slope covering work to prevent residual water pressure underneath the slope covering work.
Filter material/cloth: installed behind the backfilling material to prevent the coming out of fine materials underneath the revetment due to flow forces or the residual water pressure.
Crest work: protect the crest of the slope covering work.
Key: installed at the end portion of the crest work to protect it against erosion at the back of the revetment.
Crest protection work: installed at the end portion of the key to join the crest and the original ground in order to protect against erosion at the back of the revetment.
The components of revetment are illustrated in Figure 5-22 and Figure 5-23.
Figure 5-22 Components of a Revetment
Figure 5-23 Components of a Revetment Cross-Section
5.5.1.3 Planning & Considerations
During planning and design stage, the following are some general considerations:
Alignment of revetment shall be as smooth as possible, preferably following the alignment of the existing bank.
Where revetments are used to provide scour or erosion protection, they should be designed to have as little impact on hydraulic performance of the river as possible.
Where the rate of erosion is unpredictable, or future erosion is expected, it may be suitable to set the revetment back from the edge of the river. However, it is important that the toe of the revetment is designed to accommodate future movement of the river.
The type of the revetment shall be determined based on the estimated external forces (velocity of flood flow) and the characteristics of river, as well as economic and environmental aspects of alternative options. This should be undertaken early on within the design process.
Foot protection works shall be considered based on external forces.
Transition structure (end protection works) of the revetment to the original bank shall be provided. The end of the revetment should run as smoothly as possible into the natural channel to avoid scouring and turbulence.
The revetment should start at a stable, fixed point on the bank and continues downstream to another stable location or to some point below which the river can safely be left uncontrolled.
On a meandering river, the revetment will effectively stop the protected bend from migrating. This may have subsequent impacts outside of the protected
bend as the rest of the meandering river changes to accommodate this.
Therefore, revetments cannot be considered in isolation.
5.5.2 Estimation of Design Velocity
The design velocity is the effective velocity acting on the revetment, and is not equal to the average cross sectional velocity as determined in Section 4.
The cross section average velocity should first be estimated using the procedures as outlined in Section 4. Note that this should be assessed at all cross sections along the revetment. The highest velocity should be adopted in most cases, as for construction purposes it is simpler to adopt a uniform protection measure.
The cross section average velocity then needs to be adjusted to the design velocity, which represents a point approximate 20% up the slope from the toe of the revetment. The following provides a simplified relationship for estimating this, based on HEC23 (2009).
Equation 5-1
𝑉𝑉𝑑𝑑𝑑𝑑𝑑𝑑 =∝ 𝑉𝑉𝑎𝑎𝑎𝑎𝑎𝑎 where:
Vdes = design velocity
Vavg = cross section average velocity
α = velocity adjustment factor, which can be determined based on:
For natural channels:
∝= 1.74 − 0.52log (𝑅𝑅𝐶𝐶⁄ ) 𝑊𝑊
∝= 1 for RC/W > 26
For trapezoidal channels:
∝= 1.71 − 0.78log (𝑅𝑅𝐶𝐶⁄ ) 𝑊𝑊 ∝= 1 for RC/W > 8
where:
RC = radius of bend
W = width of river/ channel
The velocity adjustment factor for natural channels is also provided in Figure 5-24.
Figure 5-24 Velocity Adjustment Factor
5.5.3 Slope Protection Works
There are many types of slope covering work, with some of these shown in Table 5-5. It provides an indication of typical constraints and considerations, but certain slope protection works may be applicable outside of the ranges indicated. The type of slope covering work at the site shall be selected based on the design velocity, slope, availability of construction materials near the site, ease of construction works and economy, etc. When there are constraints due to the required boulder stones during flood and the slope of the bank, a combination of the slope covering works shall be considered.
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
0 5 10 15 20 25 30
Velocity Factor (α)
Rc/W
Table 5-5 Overview of Different Slope Protection Works & Considerations
1. Sodded Riverbank with Pile
Fence 2.0 Milder than 1:2 Not applicable for places near roads and houses
Diameter and length of wooden pile shall be determined considering past construction records.
Note that this is not a common technique used for revetments.
2. Dry Boulder Riprap 3.0 to 4.0 Milder than 1:2 Diameter of boulder shall be determined using Table 5-7.
Height of generally less than 3 to 5 m.
3. Grouted Riprap (Spread
Type) 5.0 Milder than 1:1.5 Use Class “A” boulders for grouted riprap and loose boulder apron.
4. Grouted Riprap (Wall Type) 5.0 1:1.5 to 1:0.5 Use class “A” boulder for grouted riprap.
5. Gabion (Mattress or Spread
Type) 5.0 Milder than 1:1.5 Not advisable in rivers affected by saline water intrusion.
Not applicable in rivers where diameter of boulders present is greater than 20 cm.
6. Gabion (Pile-up type) –
Gabion Wall 6.5 1:1.5 to 1:0.5 Not advisable in rivers affected by saline water intrusion.
Not applicable in rivers where diameter of boulders present is greater than 20 cm.
7. Rubble Concrete (spread
type) Milder than 1:1.5
type) Milder than 1:1.5