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Advancements in Civil Engineering & Technology

Engineering Feasibility Study of a Civil-Scale Pressurized Flexible Barrier Under Storm Surge Loading

Sigrid A1*, Peter T2, Alexander N1, Gianni S2 and Herman B2

1Department of Civil and Environmental Engineering, USA

2Department of Civil Engineering, Belgium

*Corresponding author: Sigrid A, Department of Civil and Environmental Engineering, USA

Submission: September 23, 2019;Published: October 02, 2019

DOI: 10.31031/ACET.2019.03.000570

ISSN: 2639-0574
Volume3 Issue4

Abstract

Conventional coastal structures (i.e. seawalls) obscure real estate sightlines, inhibit commercial and recreational activities and have high environmental and economic costs. Their long-life spans and lack of adaptability pose risks in the context of changing storm surge behavior. In this paper we demonstrate a proof of concept for a pressurized flexible barrier that overcomes these disadvantages and effectively shields civil infrastructure along shorelines from the impact of storms. The research objective is to demonstrate that pressurized civil-scale flexible barriers can resist asymmetrical unidirectional loading from storm surge events (including extreme wave break loading). Made of rubber-coated nylon structural textiles such barriers can be designed and constructed for a shorter lifespan to account for projected yet uncertain changes in storm behavior. They can also be stowed and deployed as needed and thus not hinder human activities. Our study employs a combination of experimental and numerical approaches. The preliminary study establishes the flexible barrier as novel paradigm for coastal flood defense technologies and promotes large-scale flexible pressurized systems that resist extreme loading.

Introduction and Context

The purpose of coastal structures is to shield civil infrastructure along shorelines from the impact of storms. Conventional coastal structures (such as seawalls) function due to their large rigid mass. However, they obscure real estate sightlines, inhibit commercial and recreational activities and have high environmental and economic costs [1-4]. Their long-life spans and lack of adaptability pose risks in the context of changing and uncertain future storm behavior. Natural coastal buffer strategies such as marshes and mangrove forests [5-9], can be unreliable due to human activities. Flexible hydraulic barriers have only been studied at smaller scales (such as hydraulic) [10] or under different loading conditions (such as submerged barriers not subjected to wave break loading) [11]. What is pursued in this paper is novel: to establish the engineering feasibility of a civil-scale flexible pressurized storm surge barrier (PSSB) that responds to the dynamic and static loading of storm surges. This is challenging due to the unprecedented large scale of the PSSB and the unknown interaction between the non-linear structure and extreme nature of the storm surge loading. The proposed PSSB (Figure 1) consists of an air and watertight structural textile clamped to a reinforced concrete foundation with skirt walls. The PSSB, subjected to external loading on one side of the barrier only, is compartmentalized along its length to avoid catastrophic failure and to facilitate operation and maintenance. When deflated, the PSSB is folded and stowed while remaining permanently clamped to its foundation. The PSSB is deployed using air compressors in the event of an impending storm surge. We conducted preliminary numerical and experimental studies to evaluate the engineering feasibility of such PSSBs.

Methodology and Discussion of Results

To develop an initial understanding of the effects of the dynamic and hydrostatic storm surge loading, we conducted experiments on a scale model in a wave flume tank (Figure 2). In these experiments, a 0.3m diameter styrene-butadine rubber barrier was mounted in a wave flume tank with dimensions 30.0m x 1.0m x 1.5m. Two water depths were chosen for the test matrix: d=0.157m and d=0.24m. The initial internal pressure in the barrier model varied from p= {0.8,1.0,1.2 to 1.4kPa}. Wave heights were varied from 0.01 to 0.03m (for d=0.157m) and from 0.01 to 0.04m (for d=0.24m). The wave period was held constant at 1s. Our study showed that, in general, the maximum displacement of the barrier’s crown decreases with increasing internal pressure and that this displacement increases with increasing water depth and wave height. High correlations were found between the internal pressure variations and the water depth and wave height. No overtopping or resonance was observed. Due to the inherent scaling effects of the experiments, the results of the experimental study only have qualitative value because these experimental studies do not satisfy similarity of both the incompressible fluid flow (i.e. Froude criterion) and of the compressible air flow within the PSSB (i.e. Cauchy criterion). For that reason, no further experimental approach was adopted.

Figure 1: Transverse cross-section of the proposed PSSB.


The numerical studies adopted a two-dimensional (2D) approach as most of the loading is carried to the foundation along the circumferential direction of the PSSB. The barrier's cross-section is modeled using a lumped-mass (LM) cable model and subjected to internal pressure of 475kPa. In the LM method, the cross-section is discretized into 𝑛 segments and their masses are concentrated at 𝑛 + 1 nodes. The nodes are assumed to be connected by linear elastic springs. The tension in each spring is calculated from the thickness T, elastic modulus 𝐸, current length 𝐿, and unstretched length 𝐿_R. A value of 𝐿 < 𝐿_R indicates that an element is slack and carries no force. The barrier’s shape and the storm surge loading are coupled since changes in the form also induce changes in the loading. Due to this fluid-structure interaction, we adopted an extension to the Dynamic Relaxation process to carry out these non-linear analyses [10]. The storm surge loads for the barrier were initially approximated as quasi-static wave break loads and hydrostatic loads based on values for Hurricane Sandy recorded along the East Coast of the USA [12] (Figure 2). The material is a 0.02m thick rubber coated textile (E=5700kN/m), used in practice for submerged flexible pressurized hydraulic dams [11]. Establishing the preliminary engineering design feasibility of PSSBs to storm surge loading is complicated as no prescriptive design guidelines for such structures exist. The Japanese Industrial Standard (JIS) for the design and maintenance of pneumatic membranes subjected to aquatic loads [13] describes design considerations but gives little guidance on performance criteria besides strength. Based on the results of the numerical study and following the limited JIS standard criteria, the proposed barrier membrane can be designed as a 20mm-thick rubber-coated nylon textile (tensile strength 970kN/m after fatigue loading, ageing and relaxation from pre-stresses). Since the textile must be replaced every 25 years due to the material’s lifespan, the design of the PSSB can be updated to accommodate any future changes in storm behavior. The results of our study showed the barrier’s internal forces at the foundation to be comparable to values for large submerged flexible dams, on the order of 700kN/m [11]. We designed the reinforced concrete foundation to resist the barrier’s internal forces while considering failure modes due to floating, overturning, large settlement, and exceeding the soil bearing capacity. To prevent the possibility of piping and sliding, skirt walls were attached to the reinforced concrete foundation. The dimensions of the foundation and the skirt walls (Figure 1) in clay and dense sand conditions were found to be comparable to those of submerged hydraulic dams [11].

Figure 2: Side view styrene-butadine rubber scale model in wave flume tank (a), Numerical model of transverse section of PSSB (b).


Conclusion

The conceptual engineering design of the PSSB based on our preliminary numerical and experimental studies indicates that such a barrier is feasible. However, time-varying hydrodynamic loads are thought to induce a larger response in the barrier compared with the quasi-static storm surge loads used in the preliminary 2D numerical studies. In addition, hydrodynamic forces vary spatially along the length of the barrier and could cause folds and high stress concentrations in the membrane. To address these phenomena, a three-dimensional fluid-structure interaction model is needed. Additionally, the spatially and temporally varying loads depend upon projected changes in future storm behavior. This preliminary study provides a proof of concept for a novel paradigm for coastal flood defense technology.

Acknowledgment

This material is based upon work supported by the Princeton Environmental Institute at Princeton University.

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© 2019 Makiko Kobayashi. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and build upon your work non-commercially.



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