Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Effectiveness of restoring a saltmarsh in the eroded steep coast
Yue Yin1, Qing Zhang2, Xiaotao Du2

1.Water Conservancy Branch II, Shanghai Water Engineering Design and Research Institution, Shanghai, China

2.Planning & Scientific Research Branch, Shanghai Water Engineering Design and Research Institution, Shanghai, China

ecology and safety, while knowledge about how saltmarsh can be created,developed and restored under difficult circumstances are shared.

ABSTRACT
A two-stage saltmarsh restoration approach is proposed. The eroded
steep coast will be protected by lifting the seabed level in two steps,
firstly slowing down the flow velocity and initially increasing the bottom
elevation, and secondly establishing a tidal flat for saltmarsh
development. The approach is implemented in a typical eroded coast in
Hangzhou Bay and the effects of the engineering design are simulated
by numerical model TELEMAC. It is shown that after the project iscompleted, the previous erosion state in the local area is radically alteredand a tidal flat of 90m width can be restored. The effectiveness of theproject is discussed.

In this study, the authors proposed a two-stage saltmarsh restoration
approach that can be implemented in the eroded coast, especially for
those coasts which have no tide flat in front of the dike but only deep
water. For a coast like this, erosion protection is always urgent, however,
the regular approach such as widening and reinforcing the existing
structure, building huge spur dikes and breakwaters is costly and notecological. To verify the feasibility of author's idea, an intended
saltmarsh restoration project in Hangzhou Bay is chosen as a case study.
By analyzing the local hydrodynamics and sea bottom erosion trend, therestoration engineering designs have been worked out and their impact
are numerically simulated.

KEYWORDS: saltmarsh; tidal flat; restoration; erosion; breakwater;
deposition; numerical model.

PROJECT OVERVIEW AND NATURAL CONDITIONSINTRODUCTION

The intended saltmarsh restoration project is located at the middle coastof Fengxian district, Shanghai, where suffering the most serious
shoreline erosion in Hangzhou bay(Li, Liu, Song and Zhao, 2019). The
basic data of the restoration site are collected and analysedcomprehensively.

Saltmarshes are of importance in the coastal ecosystem that delivers richecological services including providing habitats of coastal life, purifying
water quality and protecting the coastline from erosion (Allen and Pye1992; Möller, Spencer, French, Leggett and Dixon, 1990; Möller,Kudella, Rupprecht, Spencer, Paul, Van Wesenbeech and Schimmels,2014). Nowadays, with the enhancement of people's awareness ofenvironmental protection, saltmarsh restoration has attracted more andmore attention and many coastal countries have begun to put effort into
saltmarsh restoration projects. In 2016, more than 340 acres of coastal
wetland area have been saved and reestablished in Braddock byconstructing a barrier beach in the form of the breakwater with naturebased features, where the interior wetland erosion was reduced and thenatural littoral drift processes for lake sediment was restored (Silva,Wilcox and Polzer 2021). The Hamilton Wetlands restoration is anothersuccessful project in the US. More than 5.9 million cubic yards ofdredged material were deposited to increase the tidal flat elevation from
1 to 2 feet and provide a suitable environment for marsh vegetation togrow (Styles, Perkey, Smith, Bryant, Gailani and Boyd, 2020). Saltmarsh
restorations are also achieved in Europe, where the mud motor has been
proved as an efficient and natural process to spread sediment to nearby
salt marshes in the Wadeen Sea (Baptist, Gerkema, Van Prooijen, VanMaren, Van Regteren, Schulz and Van Puijenbroek, 2019). Above
engineering practices show people’s great efforts of combining coastal

Site Location

Hangzhou Bay is a funnel-shaped inlet of the East China Sea, located in
the middle of Shanghai and Zhejiang province. The Bay is famous for its
world’s largest tidal transportation, up to 9 meters high and travelling up
to 40km per hour (Li, Pan, Chanson and Pan, 2019). Due to the stronghydrodynamics and special terrain topography, the north coast ofHangzhou Bay is facing more serious erosion than the south (Chen, Liuand Zhang,1990). In recent years, land reclamation activities accelerated
the economic development of Shanghai, but there is no denying that thechange of shoreline leads to the direct exposure of some embankments
to the deep-water environment (Li, Ye, Wang, He and Shao, 2019).
Together with the strong tidal flow, coast erosion has become the biggest
threat to the urban flooding defense in this region.

Bathymetry

The construction site is a typical silty coast, which was suffering seriousbank erosion in recent years. According to the topographic survey, the0m depth contour is only 10m far from the toe of the dike. Although this2867