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Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference Kitakyushu, Japan, May 26 ­31, 2002 Copyright © 2002 by The International Society of Offshore and Polar Engineers ISBN 1-880653-58-3 (Set); ISSN 1098-6189 (Set)

Environmental and Safety Considerations for Design of a Perforated Seawall

T.L. Yip, Dao-Hua Zhang and Allen T. Chwang

Department of Mechanical Engineering The University of Hong Kong Pokfulam Road, Hong Kong

ABSTRACT

Perforated seawalls are designed to calm down the turbulent waves in Victoria Harbour of Hong Kong. Most accounts are given to hydraulic performance and structural issues to minimize the wave reflection and enhance the structural stability. With an ever-increasing array of environmental laws, regulations and requirements, more efforts are made to maintain the compliance. Therefore, a perforated seawall must provide a safe and healthful environment for the personnel working on and around it. Experience has shown that the establishments on environmental and safety considerations at a design stage can offer significant benefits to the consequent operations and maintenance. Early planning can make compliance with legal and technical requirements achievable without unnecessary cost and/or schedule impacts. As a standard procedure to the establishment of an environmental management system (EMS), environmental aspects are identified first and then evaluated. Considerations cover over-topping, noise, water circulation, and floating debris. This work can be considered as a technical guidance for the eco-design of perforated seawalls.

the harbour is the major source of waves. Those waves can stay for a long time in the sheltered harbour and in turn cause hazards to navigating and moored vessels and erosion of unprotected shores. The wave reflected from the seawall causes disturbed water near the seawall, that can promote deep scour holes immediately offshore of the seawall. The disturbed surface can be hazardous to marine activities near the seawall. The scour may excavate the supporting sand from under the seawall and in turn compromise its stability (Whitehouse, 1998, Ch. 7.6). A typical scour profile in front of a vertical wall is shown in Figure 1. The conventional solid vertical seawalls present another problem. Owing to the reflection of the vertical surface, the resultant stationary waves create an oscillation of twice the height of the incident wave, which can result in a large mass of water overtopping the seawall. Therefore, overtopping is another common problem associated with vertical seawalls. Overtopping may result in water trapping and erosion behind the seawall and structural instability. Overtopping also endangers the workers working on and around the seawall. With a prime emphasis on the wave dissipation, the perforated seawall is a multi-purpose structure. It may also be a solution to the scour formation and overtopping. The structure consists of a concrete endwall with a hollow wave chamber separated from the exposure of the sea by a perforated front-wall (Figure 2). The perforated seawall dissipates the wave energy and results in an appreciably smaller oscillation at the end-wall than that at the conventional vertical seawall. Meanwhile, the wave reflection is significantly reduced. The basic mechanism of wave dissipation by perforated seawalls is to transfer the wave energy into kinetic energy of the flow which is dissipated through the perforations. It is well known that a system of stationary waves is formed at the solid seawall together with incident waves. Flow of maximum horizontal velocity is observed at the nodes of stationary waves. If the perforated front-wall is placed at the nodes, most wave energy is dissipated. The theory behind can be found from Chwang and Dong (1984) and Yip and Chwang (2000). The wave is the primary consideration of a perforated seawall design.

KEY WORDS: Perforated seawall, environmental management, safety, water waves, harbour, port INTRODUCTION

Seawalls are built along coastlines to protect the shore from erosion and subsequent damages. Seawalls are designed to form a clear boundary dividing sea and land. Vertical seawalls are the most popular ones, while a few of sloped seawalls are constructed. Vertical seawalls are preferred for their low cost, small space, and ease for berthing. The primary consideration for a seawall design is the stability in the surrounding conditions, as water waves attack seawalls over years and induce severe stress on the structures. The reflection coefficient of a vertical (or nearly vertical) seawall is close to unity. Minor surface irregularities have little reduction on the reflection. The superposition of incident and reflected waves from the seawall becomes steep waves, which deteriorate the vessel motion, especially for small vessels. In Hong Kong, the marine traffic within

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The width of the chamber, the thickness of the perforated front-wall, and the perforations are parameters related to the design wave. The effectiveness of a perforated seawall depends solely upon the combination of the ratio of perforations, front-wall thickness and the chamber width. The perforations may be horizontal or vertical slots, or circular or rectangular holes. In addition to the wave reflection, a perforated seawall may also reduce the run-up and overtopping, and their induced forces. The hydraulic design of perforated seawalls was systemically discussed by Tsinker (1996, Ch. 10.3). The establishment of design criteria is the first important step in identifying the characteristics of perforated seawalls. Considerations should cover various operational, functional, and navigational requirements, environmental and site conditions, and physical and regulatory constraints. General design considerations for marine facilities were outlined by Gaythwaite (1990). The primary function of perforated seawall structures is to dissipate the wave energy and to enhance the structural suitability to port activities such as ship berthing and cargo handling. The seawall installation, maintenance and impact to the environment should also be assessed during the development of seawall models. In the development phase of the engineering design, special attention should be paid to materials and structures of seawalls, in order to reduce the cost for engineering maintenance. The details that facilitate engineering maintenance such as the inspection access, maintenance easement, replacement of structure members, procedure/schedule of maintenance, etc. need to be incorporated in the engineering design. A wide range of perforated seawall systems may be used to dissipate the wave energy at some coastal locations such as banks along inner harbours and seawalls along port boundaries. The design of a seawall is not simple. Current engineering considerations of seawalls often include hydraulic performances, geo-technical conditions, installation, operation, maintenance, failure modes and materials. Such coastal development may adversely impact the coastal and marine environment. Safety is another constraint that must be considered by different design methods. Systematic, but unified, methods are not available. Engineers and scientists have to play an active role in informing the policy makers and resource managers as well as in educating the general public of the adverse environmental impact of unregulated, non-sustainable forms of a coastal development. The water wave is a phenomenon on the free surface and the peak wave load may cause the front wall of a perforated seawall (Figure 2) slide or overturn. To avoid the sliding, the wave chamber normally has a bottom slab. To increase the overturning stability, sometimes the water depth inside a wave chamber is reduced, particularly for deep-water applications. Such a reduction of water depth is determined based on the design wave condition. Yip and Chwang (2000) suggested an alternative to reinforce the overturning stability of a seawall by installing a submerged horizontal plate inside the wave chamber. The environmental legislation is always the first aspect in ISO 14001 and other environmental management systems. The primary purpose of the environmental regulations and associated measures is to promote the sustainable use of natural resources. This study addresses the approaches and methods required to achieve the goal of a sustainable development, which is characterized by an improved quality of life and an improved ecological environment. In the first stage of the set-up of an environmental management system, the root causes of environmental problems have to be identified and effective solutions to be provided accordingly if the problems are shortlisted by the prescribed criteria. Of course, engineers should be able to systematically improve the degraded ecosystem, reduce the marine

pollution, prevent the soil erosion and sustain the economic development in coastal areas. Any good environmental study should cover two main aspects: (1) the present environmental conditions and (2) the potential environmental impact due to the coastal development. Associated with seawalls, the main environmental aspects considered are the wave related characteristics (i.e., tides, currents, waves, and sediment transport), the water quality (i.e., sources and ambient levels of pollution) and the marine ecology. The potential environmental impact of seawall construction, like other forms of development, is assessed from two time scales: (1) short-term effect ­ during the construction, and (2) long-term effect ­ after the construction completed.

ENVIRONMENTAL ASPECTS

An environmental management system (e.g. ISO 14001 in Figure 3) requires the identification of environmental aspects and their impacts. In general, environmental aspects have links with the requirements of legislation, regulations and policy. Engineering designers should identify all the environmental aspects of the seawall design at an early stage. The major environmental aspects may include: (1) environmental benefits for the seawall installation (traditional usage, economic activities); (2) environmental elements likely to be affected (nearshore fisheries, coastal erosion); (3) potential losses or damage to natural habitats; (4) visual impacts to the landscape (channel width, tourism); (5) mitigation measures to minimize the environmental disturbance and nuisance during construction (noise); (6) measures to control debris (e.g., empty barrels, litter); and (7) associated port facilities (e.g., housing, roads, sewage).

Existing Conditions

Wave Climate and Water Quality The design of different seawalls should vary according to the characteristics of coastal areas where the seawall will be located. Victoria Harbour of Hong Kong is quoted as an example here. Hong Kong is strategically located, both in relation to China and the neighbouring Asian countries. It lies at the mouth of the Pearl River Delta and at the centre of the Asia-Pacific Rim, a region where economy is growing at a phenomenal pace. The east part of Hong Kong waters is oceanic while the western one is estuary. Therefore, there are two different forms of trans-boundary maritime transport: (1) the large ocean-going vessels from the Pacific Ocean and (2) the smaller, coastal and river trade craft from the Pearl River. The main hydrodynamic factors affecting a seawall design are tides, currents and waves. The wave climate is very mild and waves may reach 1.5 m in height with 3 to 5 seconds of periods (Chwang, 1998). The normal tidal range in Hong Kong waters is between one and two metres, depending on the relative influence of the moon and sun. The tidal pattern is complex due to the relative effects of the diurnal and semi-diurnal components. The basic pattern during flood tides is for oceanic water to flow north into Mirs Bay and west through Lei Yue Mun into Victoria Harbour and through Kap Shui Mun and the Ma Wan Channel. This flow is reversed during the ebb tide. Maximum tidal currents generally range from 0.5 to 1.5 knots, peaking up to five knots in narrow channels (Hong Kong 2000). There are some sources of pollution for local waters, namely, harbours, power plants and stormwater outfalls. Each of these sources is known

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to contribute specific pollutants to the surrounding marine environment. The perforated seawall is required to have a superior wave-absorbing efficiency while responding to a relatively wide range of frequency of incident waves from different directions. In Victoria Harbour of Hong Kong, because of the irregular and multi-directional nature of shipgenerated waves, and of the combined effects of short ship waves and wind waves, adequate seawall designs developed in the study need to be able to trap a significant portion of oblique incident waves of various frequencies. The harbour wave climate, characterized by parameters of wave steepness, joint distribution of wave heights and periods, the wave directions, wave grouping and the spectral shapes, should be fully integrated in the developed wave absorbing seawall model specifications. Marine Ecology Ecology is defined as the inter-relationship between the living organisms or biotic components of an ecosystem and its non-living or abiotic components. Each component of the ecosystem is influenced by the others, and the alternation of any single component ultimately affects all others. Every ecosystem requires an input of energy, the only significant source of energy in living environments is the sun. This energy is used by producers in the ecosystem (e.g. bacteria, phytoplankton and large plants) to produce their own food.

transmission of light. Consequently, plant productivity is reduced because plants obtain all of their energy from light. If plant productivity is greatly reduced, the aquatic productivity may be diminished correspondingly. Turbidity has also a strong adverse effect on the oxygen uptake by aquatic organisms.

Long-Term Environmental Considerations perforated seawalls)

Floating Refuse

(special for

Floating rubbish imposes serious cleaning problems to seawalls of complicated structures. There are many perforations on the front wall. Although the perforations can be of different geometry and arrangement, there is a possibility for some floating stuff to block those perforations, and such blockage may deteriorate the hydrodynamic performance of the seawall. Waste materials trapped by wave chambers or other wave dissipating structures may reduce the designed wave absorbing efficiency and cause considerable environmental problems. Especially, the trapped waste materials may create an environment for undesired bacteria and animals. Those living organisms will modify the salinity in the surrounding waters and deteriorate the original eco-system. To reduce the long-term cleaning cost, the seawall must satisfy the general requirements on structure cleaning. Laboratory and prototype tests can be conducted with (scaled) floating and buoyant objects under the design and extreme conditions. The size of perforations should be large enough for those objects to flow back properly, even after they have entered the chamber. Therefore, those refuses will not fill up the chamber over a short period. There is usually a considerable amount of floating refuse in a busy port and harbour, like in Hong Kong. In 1988, there were hospital debris, industrial and household refuse, and other solid waste spread over many miles of the coastline of the New Jersey shore (Schmeltz, 1992). Unpleasant odour may be produced from accumulated refuse. The scaled prototype structure can be tested in a wave flume to assess its ability of trapping floating debris carried by incident waves. Floating debris samples of various characteristic sizes and materials are placed in front of the model and driven by fans under design wave and hydraulic conditions. The effect of natural wind on mobilizing debris can be emulated by fans, which generate airflow of different speeds towards the structure. From laboratory observation of Chwang (1998), it is expected that floating debris may not generate severe maintenance problems to the proposed structure. To be more precise, for circular perforations of up to 700 mm diameter, the trapping of floating refuse should not be a signification problem to a perforated seawall (Chwang, 1998). Of course, regular clean-up is desired to ensure the chamber to be clear of excessive refuse. Further observation and field assessment are recommended under onsite prototype conditions. Without rubbish accumulation in front of seawalls, the perforated seawall certainly has a good sight. Noise Noise is another issue associated with the perforated seawall. The wave chamber can be considered as a resonator. Under certain condition of wind blowing to the seawall or some oscillation put on the top slab, the sound may be resonated in the chamber, which results in a noise pollution. When waves slap on the top slab of the wave chamber with high impact, a roar will be generated. Noise can also be generated from site traffic and structural vibrations. The testing of noise

Short-Term Environmental Impacts (for general seawalls)

Water Quality Reclamation and dredging processes are usually included in seawall constructions. Relatively high turbidity is expected to be created in the vicinity of the developing area during the reclamation and dredging operations. The reclamation area is usually surrounded by barriers to prevent the dumped materials from causing external effects. However, the release of suspended sediments due to land reclamation is expected to be controlled and thus causes minimal turbidity. The dredging bottom sediment increases turbidity and makes possible the release of dissolved or absorbed chemicals into the water. Some pollutants are expected to be released into the marine water column and subsequently cause a drop in the dissolved oxygen level, which would create lethal conditions for some marine biota. Moreover, the suspended sediment and the associated pollutants may affect the harbour facilities, which are within the distance where the released sediments and pollutants may spread. Based on the marine structure layout and soil investigation, the volume and type of dredged materials can be identified. Dredging activities are expected to give rise to increased concentrations of suspended sediments within the dredging site according to the capacity of the dredging method used (i.e., dredging rate), the dredged materials (i.e., type, density, particle size, and settling velocity), prevailing tidal currents (i.e., speed and direction), the intensity of turbulence due to currents and waves, and the available water body (i.e., salinity and depths). Numerical models are usually used to predict the effect of suspended sediments and associated pollution on the surrounding activities. To prevent dispersion of soil particles in the sea water during seawall construction, a silt curtain (screen) has been used in Hong Kong (CED, 2000, p.12). Marine Ecology and Fisheries The marine ecology and fisheries will be adversely affected by the dredging and the disposal of dredged materials for land reclamation. The turbidity associated with dredging and reclamation reduces the

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generation can be performed with the floating refuse in a wave flume at the same time. Likelihood of noise due to wave-structure interactions can be assessed by comparing sound levels between the vertical solid wall and the perforated seawall model under different testing conditions. Sound level can be measured by a sound level meter in front of the model and inside the wave chamber. Chwang (1998) reported a laboratory test. Outside the porous wall, the perforated seawall generates similar sound level to that due to a solid wall. Sound variations to both models range from 4 dB to 5 dB while interacting with incident waves. Inside the wave chamber, however, the sound pulse can reach 7 dB when waves break in the chamber. In comparison with the background sound level, which is about 45 dB, wave induced noise around the model is relatively weak. Laboratory tests on noise indicate that the perforated seawall should not generate serious noise problems during daytime to the regional sound level around installation sites. Noise from the wave chamber, however, may become noticeable locally near the seawall structure, especially around maintenance manholes on the top slab.

Berthing Safety

The rectangular section of perforated seawalls is utilized as quay wall for cargo handling and berthing, and the suitability to normal port activities is satisfactory. Special requirements on seawall structures from the top slab to the wave absorbing section and to the under-water base structure need to be adequately considered to ensure harbour functions. In comparison with the environmental loading, loads due to vessel berthing, on land cargo handling and filled reclamation are important loads to the perforated seawalls. Figure 4 shows a vessel berthing closely at a perforated seawall. Sawaragi (1995, pp. 399-404) showed that, for short waves, the magnitudes of roll and sway of a berthing vessel with a low-reflection seawall become smaller than those with a solid wall, while that of heave is larger with low-reflection seawall. However, the relation becomes the reverse, if the incident wave period is 1.2-1.5 times larger than the natural periods of roll, sway and heave. Therefore, a perforated seawall is NOT always more effective than a solid quay wall in term of vessel motions. Besides, the relative reduction of vessel motions by the perforated seawall is small when the incident wave height is small (Table 1).

SAFETY CONSIDERATIONS

Similar to environmental considerations, the choice of safety factors should be project-specific and therefore based on a number of specific considerations related to a particular project. Considerations should cover: (1) normal operating conditions; (2) extreme operating conditions (e.g., under typhoon attacks); and (3) failure and emergency situations. All of these considerations determine the confidence level the engineering designer tries to achieve.

Vessel Safety

Wave reflections from solid seawalls can cause serious problems to vessel activities. Perforated seawalls reduce the wave reflection substantially and hence reduce the wave disturbance in the surrounding areas. Isaacson & Qu (1990) and Yip et al. (2001) found that a harbour with perforated seawall along its boundary is of less water oscillation even at resonance conditions. However, it is expected that, if only a portion of the boundary is modified, there is a change of energy distribution and thus the wave oscillation in some areas becomes severe. The modification from an existing solid seawall to a perforated seawall is a promising approach to improve the safety condition for vessel activities. Vessel activities within a harbour consist of navigation, mooring, berthing and cargo handling. All vessel activities are affected by the motion (especially for swing) induced by waves. Vessel safety is an important factor for Hong Kong to install perforated seawalls along her harbour. Hong Kong has operated one of the busiest container ports in the world, while the import and export trade is one of Hong Kong's major areas of economic activity. Most cargoes of the import and export trade of Hong Kong are shipped by sea through containers. More than 17% of Hong Kong's annual container throughput is handled by derrick barges in mid-stream operations (Figure 5), which involve the loading and unloading of cargoes from ships moored at buoys or anchorages in the harbour (MD, 2000). Although the perforated seawall is a safer option, special care should be taken if various seawalls are built along the shores. As aforementioned, such combination results in uneven change of oscillation amplitude over the harbour. Physical and/or numerical modelling are desired in such cases. Visual and noise impacts should also be assessed in physical model testing and engineering design.

Overtopping, Access and Maintenance

Although the perforated front wall weakens the overtopping in comparison with the solid seawall, necessary attention still has to be paid to in a proper design. Excess overtopping causes structural damage in a long run and endangers the pedestrians and vehicles nearby. A crest wall at the waterfront may help to reduce the overtopping. Human beings may accidentally fall or be trapped in the chamber. The perforations should be large enough for them to swim or climb out of the chamber. And there should be a platform to assist an individual to temporarily stay inside the chamber. The platform should be built slightly over the high tide level and large enough for an adult. A ladder from the platform into the water is a necessary aid for an individual to climb up to the platform. Due to the high porosity requirement, the structural strength of the front wall is significantly weaker than that of a solid wall. Hence, if the perforated seawall is used for berthing of vessels, additional attention should be paid and the strength of the front wall should be checked against the design berthing force. For safety considerations, the whole seawall structure should be stable even without the perforated front wall. Thus, the structure will be stable, when the front wall fails accidentally and can be replaced later. Seawall durability and maintenance cost should be reasonable. Maintenance, including structural cleaning and engineering maintenance, should be addressed in engineering designs of structures.

CONCLUSIONS

Advantages of perforated seawalls include: (1) Dissipation of wave energy; (2) Partial reduction of the destructive force due to incoming waves on the shore; and (3) Great reduction of erosion at the base of the structure.

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The general considerations regarding the environmental feasibility of seawall construction are given and measures to minimize the adverse impact on the environment are recommended. Mitigation of the environmental impact is also discussed.

ACKNOWLEDGEMENTS

This research was sponsored by the Hong Kong Research Grants Council under Grants HKU 7068/00E and NSFC/HKU 8. Dr. T.L. Yip was supported financially by a Post-Doctoral Fellowship for the Area of Excellence in Harbour and Coastal Environment Studies.

REFERENCES

CED (2000) Environmental Report 2000, Hong Kong: Civil Engineering Department, Hong Kong SAR Government, 27 p. Chwang, A.T. (1998) Study of ship waves in Victoria Harbour, Invited Lecture, Proceedings of the 3rd International Conference on Fluid Mechanics, Beijing, China, pp 9-16. Chwang, A.T. and Dong, Z.N. (1984) Wave trapping due to a porous plate, Proceedings of the 15th ONR Symposium on Naval Hydrodynamics, pp 407-414. Gaythwaite, J.W. (1990) Design of Marine Facilities for the Berthing, Mooring, and Repair of Vessels, New York: Van Nostrand Reinhold, 525 p. Hong Kong 2000, Hong Kong: The Hong Kong Special Administrative Region Government. Isaacson, M. and Qu, S.Q. (1990) Waves in a harbour with partially reflecting boundaries, Coastal Engineering, Vol 14, Iss 3, pp 193214. MD (2000) Port of Hong Kong Handbook 2000, Hong Kong: Marine Department, Hong Kong SAR Government, 104 p. Sawaragi, T. (1995) Coastal Engineering ­ Waves, Beaches, Wavestructure Interactions, The Netherlands: Elsevier, 479 p. Schmeltz, E.J. (1992) Design of a mechanical refuse barrier, Proceedings of Coastal Engineering Practice '92, ASCE, Long Beach, California, pp. 680-696. Tsinker, G.P. (1996) Handbook of Port and Harbour Engineering, New York: Chapman & Hall, 1054 p. Whitehouse, R. (1998) Scour at Marine Structures, London: Thomas Telford, 198 p. Yip, T.L. and Chwang, A.T. (2000) Perforated wall breakwater with an internal horizontal plate, Journal of Engineering Mechanics, ASCE, Vol 126, No 5, pp 533-538. Yip, T.L., Sahoo, T. and Chwang, A.T. (2001) Wave oscillation in a circular harbour with porous wall, Journal of Applied Mechanics, ASME, Vol 68, Iss 4, pp 603-607.

Figure 1.

Scour profile in front of a vertical wall for relatively fine suspended sand

Figure 2.

Schematic diagram of a perforated seawall Initial Environmental Review

Environmental Review Continual Improvement Implementation & Operation

Environmental Policy

Roll and Sway Heave

T < (1.2-1.5) Tnatural Small Similar

T > (1.2-1.5) Tnatural Large Small

Checking & Corrective

Planning

Table 1.

Summary of Sawaragi's (1995) findings: a scenario with a perforated seawall against the other one without it.

Figure 3.

Environmental management system ­ the ISO 14001 approach

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Roll

Heave

Sway Crane operator Derrick barge

Figure 4.

Reduction of vessel motion by the perforated seawall

Figure 5. Cargo handling

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