Stratification and its consequences in two constructed urban stormwater wetlands

Sherif S Ahmed; Wenming Zhang; Mark R Loewen; David Z Zhu; Tadros R Ghobrial; Khizar Mahmood; Bert van Duin

doi:10.1016/j.scitotenv.2023.162179

Stratification and its consequences in two constructed urban stormwater wetlands

Sci Total Environ. 2023 May 10:872:162179. doi: 10.1016/j.scitotenv.2023.162179. Epub 2023 Feb 13.

Authors

Sherif S Ahmed¹, Wenming Zhang¹, Mark R Loewen¹, David Z Zhu², Tadros R Ghobrial³, Khizar Mahmood⁴, Bert van Duin⁴

Affiliations

¹ Dept. of Civil and Environmental Engineering, University of Alberta, T6G 1H9, Canada.
² Dept. of Civil and Environmental Engineering, University of Alberta, T6G 1H9, Canada; School of Civil and Environmental Engineering, Ningbo University, Zhejiang 315211, China. Electronic address: dzhu@ualberta.ca.
³ Department of Civil and Water Engineering, Laval University, Quebec, QC G1V 0A6, Canada.
⁴ Water Resources, City of Calgary, Mail Code #403, PO Box 2100, Station M, Calgary, AB T2P 2M5, Canada.

PMID: 36791861
DOI: 10.1016/j.scitotenv.2023.162179

Abstract

Stratification in constructed urban stormwater wetlands is one of the fundamental physical processes that affect hydrodynamics, transport and fate of stormwater pollutants. Adverse effects of stratification include decreasing pollutant retention capacity, causing the water at lower depths to become anoxic, degrading water quality and increasing stress on the downstream aquatic communities. The current study reports on a comprehensive field monitoring program of stratification and hydrodynamics in two ice-free seasons (May - October) in two constructed urban stormwater wetlands in Calgary, Canada, with different inlet, outlet, morphometric and vegetation designs. Despite their small sizes of 0.5 and 1.2 ha and shallow water depths of 0.8 m, stratification was strong and persistent in the wetlands. The response of stratification and mixing to atmospheric forcings (e.g., air temperature, atmospheric instability, rainfall depth, wind speed) and the impact of design characteristics (inlet/outlet design, water depth, surface area and aquatic vegetation) were examined and discussed. Thermal stratification, defined as a vertical temperature gradient >1 °C/m, was found to be significantly higher (up to ten times) near the inlets and last longer (up to twice) than in the main cells and the outlet basins due to the relatively cold summer inflows. The wetland with twice the permanent water volume and surface area and half the length-to-width ratio had denser submerged aquatic vegetation, higher (by up to 2 °C) water temperature and more severe (up to eight times) thermal stratification. Strong densimetric stratification and low wind stress on the water surface caused hypoxic conditions near the bed, potentially adversely affecting water quality and downstream aquatic communities.

Keywords: Aquatic vegetation; Hydrodynamics; Physical processes; Stormwater; Stratification; Water quality; Wetlands.