DRAFT - Design Procedure Manual for Jetties
A draft design procedure manual for jetties...
Design Procedure Manual for Jetties The main objective of this manual is to provide an overview on Coastal Engineering and detailed guidelines on how to design Jetty structures for coastal protection.
1. Basic Concept Jetties are quite similar to groyne and breakwater structures in terms of structural design but is considered much larger and is built to stabilize an inlet, canal mouth or other coastal feature while also trapping longshore sediments. They are long and narrow structures that stretch from the shore into the sea that could also serve as protection for entrance channels from wave action and cross-currents. Jetties are usually built in pairs, one on either side of an entrance of a bay or river channel. They could also be constructed and used in combination with a breakwater. A single jetty may also be used, located on the updrift side of the entrance. However, disadvantages occur when the navigation channel is unconfined and migration might occur.
Figure 1.0 Parallel Jetty Built in an Entrance Bay (Reference: USACE Coastal Engineering Manual) 1.1
Types of Jetty
The different factors that could affect the type of jetty and design of the structure include: physical characteristics of the site area and its exposure to wind, waves, currents, tides, and most especially, the purpose of the structure. Rickson and Rodolf (1950) lists and explains some general types of jetties:
1. Random stone – a rubble-mound structure is in fact a long mound of random stone. This type is adaptable to any depth, may be placed on any kind of bottom, and absorbs the wave energy with little reflected wave action. This type requires relatively large amounts of material. If not carried high enough, storm waves may sweep entirely over the jetty and cause a secondary wave action in the protected area, and if the voids between the stone are too large a considerable portion of the wave energy may pass through the structure. 2. Stone and concrete type – a combination of rubble stone and concrete. This type ranges from a rubble-mound structure, in which the voids in the upper portion of the rubble are filled with concrete, to massive concrete superstructure on rubble-mound substructure. The mound is used either as a foundation for a high concrete superstructure or as the main structure surmounted by a concrete cap with vertical, stepped, or inclined face. This type requires less material, and is used where the foundation is soft or subject to scour. 3. Caisson Type – is built with reinforced concrete, floated into position, settled upon a prepared foundation, and occasionally, parapet walls are added. This type is suitable for depths up to 11 meters. Foundations are either rubble stone alone or piling and rubble stone. Riprap-or-heavy stone is used alongside to prevent scour, to provide resistance against sliding, and to prevent weaving under wave action. On sand bottom, considerable riprap is required. Similar to groynes, jetties could also be designed considering different classifications based on cross sectional shape, structure type, and planar shape. (See figures from previous chapter) 2. Design Criteria 2.1
• Tide Levels • Wave • Terrain • Return Period
Dimension of Jetty
• Purpose and Function • Classification •Cross section shape •Structure •Planar shape • Length • Height
• Wave Tide Conditions • Ground Conditions • Weight calculations • Prevention from scouring • Vertical Bearing
Figure 2.0 Overview of the Design Process of Jetties
Preparation of Plans
The basis of the design for a coastal structure will be dependent on the tide level, wave, terrain and the adopted return period for the structure. The condition varies relevant to the purpose and function of the jetty. Most jetties are constructed for stabilizing of canal mouth and minimize coastal sediment transport. Required tide level data includes Design High Tide Level (DHTL), High Water Level (HWL), Mean High Water Level (MHWL), and Mean Low Water Level (MLWL) based from historic data, analyses, and master plans for the proposed site. The design wave depends on comprehensive analysis of the existing condition on the proposed project location. Waves to analyze should be based on a 30 year to 50 year return period. For offshore waves, wave height and period should be determined by extreme statistical analysis using maximum wave observed in a long time (in principle, use data of more than 30 years). In case of insufficient available data, perform numerical simulation, wave forecasting and hindcasting. It is important to take note to consider irregularity of waves by numerical simulation and appropriate wave transformation in shallow water. 2.3
Since most jetties are constructed for stabilizing canal mouths and inlets, the tip of the Jetty should be situated roughly on the shoreline extending to the sea. It should be long enough to effectively influence the wave actions, sediment transports and current. The direction of the length of jetty should be set in the same direction as normal with the river channel. 2.4
A study conducted by CTI Engineering Co., Ltd., classifies three sections of jetty structures the same way with groyne structures thru varying heights for efficiency and stability (See previous chapter for reference and figures): Section 1 – Horizontal Shore Section (HSS) Section 2 – Intermediate Sloped Section (ISS) Section 3 – Outer Section (OS) According to CTI, the height of the section 1 is often set at a height such as to prevent wave overtopping. This will be based on the wave run-up height for a corresponding wave return period of Highest High Water Level (HHWL). The height for section 2 which has an inclined feature is often parallel to the beach cross section. Lastly, the height for section 3 is often from High Water Level (HWL) to the height needed to prevent wave overtopping. This is also calculated through the wave run-up height under the conditions of design wave period and annual maximum wave. Wave run-up height ≤ Crown Height
According to the CTI, the crown width of the jetty has no influence in controlling coastal sediment transport. Therefore, the crown width should be set based on the constraints of construction, aesthetics, materials to be used and stability of the whole structure. 2.6
Stability Analysis Analysis of the stability of a jetty will require determination of the allowable weight of the materials to be used e.g., Gabions. The necessary weight is evaluated in the Hudson formula: 𝑀= M 𝜌𝑟 𝐻𝑖 𝐾𝐷 𝑆𝑟 𝜌𝑜 ∝
: : : : : : :
𝜌𝑟 𝐻𝑖 3 𝐾𝐷 (𝑆𝑟 − 1)3 𝑐𝑜𝑡 ∝
Necessary weight (t) Density of the material (t/m3) Height of jetty surface area affected by wave height (m) Material specific coefficient Specific gravity to the seawater of material ( 𝜌𝑟 /𝜌𝑜 ) Density of seawater (1.03 t/m3) Angle of Slope
A detailed structural calculation considering both normal and seismic condition is required. The following water level and safety factor for vertical bearing capacity should be considered in the design: Normal Condition Seismic Condition
Required Safety Factor 3.0 2.0
Water Level 0.00 (MSL) 0.00 (MSL)
Table 1.0 Water Level and Required Safety Factor (Reference: FRIMP Design Report – Tagoloan) Attached in Annex A is a sample calculation for the Stability of Jetty against bearing. 2.7
Spacing or Placement Intervals
Spacing between dual jetties should be determined with consideration of tidal processes, wave protection requirements, river flood discharge requirements, and safe navigation requirements. Jetty spacing for tidal concerns depends on the tidal prism volume or the actual tidal flow exchange through the inlet.