DRAFT - Design Procedure Manual for Groynes
Design procedure manual for groynes...
Design Procedure Manual for Groynes The main objective of this manual is to provide an overview on Coastal Engineering and detailed guidelines on how to design Groynes structures for coastal protection.
1. Basic Concept Groynes are coastal structures with a principle function of trapping longshore sediment transport and eventually reducing erosion along the shore. These structures play a significant role in the change of beach alignment as it is constructed mostly perpendicular or slightly oblique to the shoreline, and extending into the sea. 1.1
In order to understand the technical terms as regards to beach or coastal structures, the figure below provides a visual definition of terms to be discussed in this manual. This is to avoid the differences and varied complicated dialogue that have occurred over the years in the history of coastal engineering. Furthermore, a glossary of terms in Appendix A was gathered to establish a common vocabulary for this manual.
Figure 1.0 Coastal Zone (Reference: https://www.taskutark.ee/m/rannaprotsessid)
According to the United States Army Corps of Engineers (USACE), the beach and nearshore zone of a coast is the region where the forces of the sea react against the land. The physical system within this region is composed primarily of the motion of the sea, which supplies energy to the system, and the shore, which absorbs this energy. Because the shoreline is the intersection of the air, land, and water, the physical interactions which occur in this region are unique, very complex, and difficult to fully understand. As a consequence, a large part of the understanding of the beach and nearshore physical system is simply descriptive in nature. These coastal zone will play an important role in the understanding of the design of groynes. 1.2
Functions and Uses
Groynes are barrier-type structures that extend from the backshore into the littoral zone. They are generally constructed in series, referred to as a groyne field or system, along the entire length of beach to be protected. The basic purposes of a groyne are to modify the longshore movement of sand and to either accumulate sand on the shore or retard sand losses. Figure 2.0 presents a visual definition of Groynes dimensions to be utilized in this manual.
Figure 2.0 Dimensions of Groynes (Plan and Profile View)
According to Fleming (2004), a well-designed Groyne can have the following functions and uses:
Arrest or slow down the alongshore drift of material on a coastline and, by building up the volume of material in the groyne bays, stabilize the foreshore and protect the coastline Reduce the impact of changes in shoreline orientation Deflect strong tidal currents away from the shoreline Help to hold material on a beach that has no natural supply and has been artificially nourished Control seasonal shifts of material alongshore within a bay
Reduce the long-term erosive effect of wave activity in an area of coastal defence by accumulating beach material in front of hard beachheads such as sea walls, revetments and cliffs. This requires an adequate supply of material moving alongshore; Improve the extent and quality of an amenity beach; Increase the depth of beach material cover to an otherwise erodible seabed.
Types of Groynes
Groynes can be classified into different types according to its permeability, structure or material used, lateral cross sections, and horizontal shape. In this manual, we will focus on the classification of groynes based on its function as permeable or non-permeable with the corresponding materials used, and based on the lateral cross-sections (See table 1.0). Type Name Permeable
Rubble mound and Concrete block
Composed of rocks and concrete blocks (with deformed blocks) placed randomly or orderly. One type uses piles to connect and form a series of blocks.
Concrete piles are arranged in two lines, and medium size rocks are filled inside them. The transmissivity is very small -- close to nonpermeable type.
Stone piled, stone pitching
Rocks are filled inside frame made of reinforced concrete. (Very seldom used recently) Fill with rock, and rubble on the surface. The rock-fill type has a front surface slope greater than 1:1, and the rubble type has a front surface slope less than 1:1.
Concrete block filled type
Filled with concrete blocks with abnormal shape. There are types with porous spaces made inside horizontal blocks, and blocks are connected by piles going through these spaces to form a series.
Placed concrete type
Majority used on land portion of groyne.
Well, caisson, cellular block type
Double sheet piles type
Two pointed sheet piles are driven in the sand, and a mixture of soil, sand and gravel is filled inside.
Pointed steel piles are driven in a row
A majority of structures placed at steep slope beach have wells on the surfaces facing ocean. The other structures belong to the composite breakwater types.
Table 1.0 Structural Types of Groynes (Reference: Design Manual for Coastal Facilities JSCE 2000)
With front surface perpendicular or at a slope greater than 1:1
The slope of the front surface is smaller than 1:1
Stone piled type, concrete block fill type, caisson type, cellular block type, well type, rock fill type etc. Stone pitched type, rubble mound type, concrete block type Combination of the above types
Table 2.0 Type of Groynes based on Lateral Cross sections (Reference: Design Manual for Coastal Facilities JSCE 2000) Permeable type of groyne structure has a reduced capability of trapping longshore sediment transport. On the other hand, non-permeable type offers a near perfect wall for trapping of sediments transported along the shore. The efficiency of non-permeable type of groynes could be easily enhanced based on its design. Longshore sediment transport can be controlled based on the design length with respect to the littoral zone of the coastal area and based on the design crown height. It is still important to take note that in a permeable groyne, reflected waves and flow along the groynes are much smaller. This effect results to a lesser scouring on the foundation and downdrift erosion is less serious. All in all, the effect still varies from the beach types and existing conditions where the structure is to be erected. This includes taking into account the placement water depth, wave force, bed material, geology, seabed, topography, cost, constructability, environmental and beach utilization. As for the type of groynes based on lateral cross sections, the same considerations are applied and is usually based after choosing the type of structure for the groynes.
2. Design Criteria Groynes have been around a long time and many parameters on its design exist but most only provide a few rules of thumb. No systematic methods or functional design under a wide range of structural shapes, waves, and sediment transport conditions presently exist. 2.1
In designing a coastal structure such as groynes, the initial objective is to establish or estimate the sought-after alignment of the beach after the construction of the groynes. It is also important to remember that the main element in the groyne design pertains to the understanding of longshore sediment transport. The longshore sediment transport or longshore drift is the geological process that pertains to the transportation of sediments such as clay, silt, sand and shingle along a coast parallel to the shoreline. This drift is dependent on oblique incoming wind direction and occurs at the surf zone of the coast as seen in Figure 1.0.
A flowchart is presented in Fig. 3.0, summarizing the systematic process of designing a groyne. The eventual alignment of the beach depends on the plane shape and spacing of the groynes. Various elements or key factors affects the efficiency of the groynes in terms of trapping longshore sediment transport.
Confirmation of design conditions • • •
Analysis of sediment transport mechanism
Placement purpose Protection degree Protection area
Selection of groyne type
Wave, flow and sediment transport Shoreline change
Decision on the basic type
Permeable or Nonpermeable type Lateral Cross section
Decision on structure Comparison of each structural type
basic cross section structure of each part structure of groyne head selection of material
Construction cost evaluation
• • • •
Length Interval or spacing Direction Crown
Construction schedule and construction planning
Figure 3.0 Flowchart for the Design of a Groyne (JSCE Design Manual for Coastal Facilities 2000)
The groyne’s efficiency of trapping longshore sediments is greatly dependent on its design length. With this, the length of the groyne will be dependent on the desired alignment of the beach through estimating of the longshore drift. As per site condition of the beach, an engineer should still carefully take note that a greater length of the groyne leads to a higher rate of erosion on the downdrift side of the groyne. 2.3
Similar to the design length, the crown height of the groyne also plays a role in the efficiency of longshore sediment trapping. USACE classifies groyne’s into three sections with varying height and as a reference to its shore location. Figure 4.0 shows the three sections of groynes i.e., Horizontal Shore Section (HSS), Intermediate Sloped Section (ISS), and Outer Section (OS). 1. Horizontal Shore Section – Mainly extends towards the landside of the beach for anchoring and prevention of flanking. As the highest section of the whole groyne, HSS is usually at the height of the maximum high water plus the height of the normal wave uprush. Conversely, this section or just a part of it could also be designed to have a lower height than the existing berm of the beach to allow overtopping of sediments during periods of high tide.
Figure 4.0 Three Mechanisms for Creating Rip Currents between Groynes (USACE Shore Protection Manual)
2. Intermediate Sloped Section – located between the HSS and OS, this section is considered as the transition part of the groyne in terms of height. The slope of this section should ideally be parallel to the slope of the beach profile natural foreshore. The lower end elevation of this section is usually dependent on the construction methods used, the degree to which it is desirable to obstruct movement of the littoral material, or the requirements of swimmer or boaters. 3. Outer Section – this section is composed of all the parts of the groyne extending seaward from the ISS. The crown height for the outermost section is decided on the basis of longshore sediment trapping efficiency and structural stability. In the case where an allowable overtopping is considered, the lower limit of the crown height can be lowered and set to the high-water level (H.W.L.); the crown height is set as the wave run-up height plus the mean water level (M.W.L.). The crown height of a groyne depends on the purpose of constructing the coastal structure. Some groynes are constructed ideally for overtopping with a design limiting the downdrift erosion to let longshore sediments pass through. Most groynes are constructed for trapping of these sediments and are thus designed with a crown height higher than the mean water level (M.W.L.). In consideration with the type of structure of the groyne and the construction method, a value of one (1) meter is usually used. 2.4
According to the Japan Society of Civil Engineers (JSCE), the crown width of the groyne has minimal influences on the its sediment trapping efficiency. With this, crown width is design for the stability of the whole structure with respect to the type of groynes to be constructed. For wave-dissipating concrete blocks, crown width is equal to the width of two or three rows of concrete blocks. Otherwise, the crown width depends on stability computation, construction methodology, and other recreational purposes to be considered.
Beach Alignment Determination of the eventual beach alignment prior to the construction of the groynes plays a crucial role in the whole design process of the groyne system. It is considered as the projected orientation of the shoreline along the proposed groynes. The first step in designing groynes is to determine the nearshore direction of the predominant wave approach and then assume a beach alignment perpendicular to that direction (USACE, 1984). The figure below provides three aspects of beach alignment to be considered in the design of groynes:
Figure 5.0 Three Cases of Groyne-Adjusted Shoreline (USACE Shore Protection Manual)
Spacing or Placement Intervals
The design on interval placing of groynes greatly influences the amount of erosion and accretion in the adjusted shoreline along the intermediate shoreline and must be in consideration with the design length of the groyne. Consequently, if the distance between the groynes is widened, the advance and retreat amount increases. The groyne spacing (Sg) must be set to ensure coastal conservation and to maintain beach width needed to protect the environment. Historical data or existing condition of the beach must be considered in the design of groyne spacing. JSCE sets a general standard of 20m to 30m.
3. Basic Principles Basco, on his research, stated the modern and basic rules and principle for the functional design of groynes: 1. If cross-shore sediment transport process is dominant, consider nearshore breakwater systems first 2. Conservation of mass for transport of sediment longshore and cross-shore means groynes neither create nor destroy sediments. 3. To avoid erosion of adjacent beaches, always include a beach fill in the design 4. Agree on minimum, dry beach width, Ymin for upland protection during storm events as a measured to judge success. 5. Begin with Sg/L = 2-3 where Sg is the longshore spacing and Yg is the effective length of the groyne from its outer section tip to the design shoreline for beach fill at the time of construction. 6. Use a modern, numerical simulation model (e.g. GENESIS) to estimate shoreline change around single groynes and groyne fields. 7. Use a cross-shore, sediment transport model (e.g. SBEACH) to estimate the minimum, dry beach width, Ymin during storm events. 8. Bypassing, structure permeability and the balance between net and gross longshore transport rates are the three key factors in the functional design. Use model simulations to iterate the final design and to meet the Ymin criterion. 9. Consider tapered ends, alternate planforms and cross sections to minimize impacts on adjacent beaches. 10. Establish a field monitoring effort to determine the adjacent beach impacts if the project is successful. 11. Establish a trigger mechanism for decisions to provide modification or removal if adjacent beach impacts fount not acceptable