Focus

This subproject focuses on the qualitative and quantitative description of water exchange of estuaries and coastal peatlands with their coastal forefields. Despite water, salt and heat, solutes and particulate organic matter are exchanged. The development and extension of state-of-the-art quantification methods for exchange flows and their relation to mixing with special focus on surface fluxes will be investigated. Furthermore, these methods will be tested intensively against idealized and realistic three-dimensional numerical model simulations.

State of the art

The forefield of estuaries/coastal peatlands is influenced by several aspects such as wind-driven currents, vertical mixing as well as wave-driven near-shore circulation. Additionally, from the nearshore/coast, i.e. by exchange flows in terms of river plumes or dense gravity currents from estuaries. Due to external forces, the complex coastal system can change water properties rapidly and therefore also the exchange flow with adjacent estuaries/peatlands. For example the density gradient between forefield and estuary/peatland could change and therefore the direction of the exchange flow.

Quantification of estuarine exchange flows is typically done by computing two-layer Knudsen bulk values, i.e. one value for inflowing and outflowing properties in terms of volume fluxes and representative salinities, respectively. These values are computed using an isohaline analysis framework. Recent studies have shown that these bulk values are closely linked to estuarine mixing and can be used to exactly compute the mixing inside the estuary.

In this subproject we will extend the above framework to include surface fluxes, i.e. precipitation and evaporation, as these are very important quantities for coastal peatlands and are mandatory to describe inverse estuaries and have been neglected so far.

Work package

This project includes several work packages. First, the extension and generalization of the quantification method of exchange flows has to be carried out. This includes numerical convergence, multi-layer exchanges and extension to temperature and therefore heat exchange. Second, the inclusion of surface fluxes into existing theory has to be carried out.

Third, the application of the new methods on idealized and realistic model simulations have to be conducted as well as the creation of numerical setups. Furthermore, extreme events will be investigated.

Major exchanges of coastal peatlands can be linked to storm surges where high water levels and waves can cause floods of coastal peatlands and can change the coastal forefield, i.e. due to redistribution of sediments. Using the example of the study site NSG „Heiligensee & Hütelmoor“ we study the exceptional flooding of January 2019 to investigate the regime shift of a fresh water peatland to a regime of salt water.