Wetland Network Design for Mitigation of Saltwater Intrusion by Replenishing Freshwater in an Estuary

Abstract

In this paper, a wetland network is designed to mitigate saltwater intrusion based on the relationship between river discharge and salinity in Modaomen waterway (MDMW), the Pearl River estuary (PRE) of southern China. The designed network consists of existing and expanded wetlands, river channels and their tributaries. The freshwater stored in wetlands can be diverted into river channels to adjust salinity gradients when saltwater intrusion reaches the predefined locations. In the MDMW, two exponential regression models are established between the freshwater discharge and the saltwater intrusion length at both high water slack (HWS) and low water slack (LWS), respectively. The wetland network can effectively mitigate the saltwater intrusion when the freshwater is diverted from wetlands into river channels according to the regressive models. The method of wetland network can save over 50% more freshwater (varying from 46.1 to 56.4% at HWS and from 53.4 to 61.8% at LWS) than the emergent water allocation schedule (EWAS) implemented in 2005 to protect against saltwater intrusion in the same area. Wetland network design facilitates water resource management in the PRE and can also be easily generalized to other estuaries.Abbreviations: EWAS, emergent water allocation schedule; HWS, high water slack; LWS, low water slack; MDMW, Modaomen waterway; PRE, Pearl River estuary 1036Considering the water saving and the mitigation of saltwater intrusion, how to allocate the upstream freshwater in the estuarine and coastal regions is another important question. Poulter et al. [4] presented a network-based method to identify the major shoreline entry points of saltwater to the drainage river networks. These points are useful in the managing and monitoring of saltwater. The network-based method is nearly natural and cost-effective [38]. With the application of graph theory to network analysis, wetland network approaches are widely used in watershed storm water management [39, 40] and regional drought and flood control [38]. This methodology proves to be effective and has few negative effects on surrounding ecosystems. Typically, there are a number of wetlands which are connected by tributaries/river channels in an estuary. These wetlands include marsh, lakes, and reservoirs that form a network, together with river channels, to regulate hydrological processes [38][39][40][41][42][43][44]. In a network, the freshwater can be diverted into river channels from wetlands. The increased river discharge then decreases the range of saltwater intrusion along the river [26][27][28][29][30]. A new equilibrium can be established between water and salinity in river channels [24].The objectives of this paper are as follows: (i) to investigate the relationship between the freshwater discharge and the saltwater intrusion length; (ii) to design a wetland network to mitigate saltwater intrusion; and (iii) to compare water allocations between network method and EWAS and evaluate the performance of wetland networks in different scenarios.

A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands

“…2, Table 2), multiple channel deepening projects have resulted in seawater intrusion (Duberstein and Kitchens 2007). Similar impacts are expected due to dredging of deepwater channels in the Yangtze and Pearl River Deltas of China (Zhang et al 2011(Zhang et al , 2012.…”

Wetland Network Design for Mitigation of Saltwater Intrusion by Transferring Tidal Discharge

“…It will lead to salinization of soil when saltwater is pumped into wetland network, but the salinity can be carried away by low saltwater during low tide. The salinity at the same site is lower at low tides than at high tides 23, 62.…”

“…Totally, the wetland network increases 55 river segments (about 5.63 km), and the storage capacity of wetland network is up to 5.56 × 10 6 m 3 . It is cost‐effective to transform land use types from other types to wetlands 62. All the wetlands in the network can be used to store saltwater.…”

Analytical model captures intratidal variation in salinity in a convergent, well-mixed estuary

“…the West River, the North River, and the East River) in the upper drainage basin. The annual river discharge, with 80 % occurring in the wet season, empties into the South China Sea via eight outlets (Zhao, 1990). The Lingding bay is created by the inflows of fresh water from the Pearl River through four major discharge outlets, namely Humen, Jiaomen, Hongqimen, and Hengmen.…”