Assessing Large-Scale Circulation and Local Island Hydrodynamics in a Shallow Lake

Abstract

Shallow lakes are systems where wind-driven circulation affects water quality and sediment transport. This study investigates how bathymetry, lake planform, and artificial islands influence hydrodynamics in Lake Markermeer, a wind-driven, non-tidal shallow lake (~ 4m depth). Using Delft3D-FLOW hydrodynamic depth-averaged simulations, this study examines the effects of depth variation, shoreline complexity, and the presence and shape of islands on horizontal circulation patterns and vorticity generation. In this study, vorticity is considered as a proxy for fluid mixing, which, together with horizontal circulation, plays an important role in determining water quality.

A multi-step modeling approach was used: (1) idealized simulations assessed bathymetric and shoreline effects on horizontal circulation and vorticity; (2) an artificial island (Marker Wadden) was placed in the lake to evaluate its impact. A high-resolution numerical model quantified local vorticity for different island shapes, which were generated using Fourier transformation applied to the island boundaries.

Bathymetry plays the dominant role in large-scale circulation, with depth variations producing pressure gradients that drive gyre formation. In particular, depths over 4 meters are responsible for creating gyres in Lake Markermeer, while lakes with uniform depth show little horizontal flow. Lake planform has limited influence, mainly affecting local vorticity near shorelines. Although limited in influence, an irregularly shaped lake like Lake Markermeer can have vorticity values up to 70% higher than a circular lake of similar size.

Artificial islands influence local hydrodynamics but have minimal effect on large-scale horizontal circulation within the basin. Circular islands lead to more uniform and predictable beach erosion, while irregularly shaped islands induce flow separation and vortices that enhance mixing but increase the risk of localized and less predictable erosion. For optimal erosion control, an elliptical island shape aligned with the dominant large-scale circulation is recommended.

These findings provide insights for lake management, demonstrating that bathymetric modifications are more effective than shoreline modifications for influencing horizontal circulation. The results suggest that in artificial island design, streamlined shapes are preferable for erosion control, while complex shapes enhance local mixing, promoting nutrient distribution and improving water quality. The methodology developed in this study is applicable to other shallow lakes worldwide, providing a structured approach for hydrodynamic assessments in lake management.