Meta analysis of Manning's coefficient for saltmarsh and mangroves to support broad scale assessment

The natural protection offered by saltmarsh and mangrove wetlands is increasingly recognised and there is widespread interest in understanding this phenomenon to assist nature-based coastal interventions. Roughness plays a fundamental role in shallow-water hydrodynamic process and hence is a critical parameter for simulating flow over coastal vegetation and the resulting dynamics of surge and wave attenuation. This study provides a comprehensive review of our understanding of the Manning’s coefficient (T/L 1/3 ) in saltmarsh and mangroves. The literature was screened using a rigorous selection procedure to identify studies relevant to saltmarsh and mangrove hydrodynamics and bed roughness. Considering all the numerical, laboratory and field studies that used Manning’s coefficient in saltmarsh and mangrove ecosystems, 36 papers were identified and assessed. The results indicate an interquartile range of Manning’s coefficient (s/m 1/3 ) from 0.04 to 0.08 for saltmarshes and 0.10 to 0.14 for mangroves, providing indicative ‘global’ estimates for these two coastal wetland types. Geographic location, tidal range and temperature showed no statistical relationship, supporting the use of global indicative values. However, statistical testing reveals that the Manning’s coefficient for saltmarshes and mangroves do show significant cross-shore variability with increasing values from the seaward to landward direction. This should be considered and further assessed, especially in more detailed studies. Hence, this analysis provides an indicative ‘global’ constant estimate of Manning’s coefficient for saltmarshes and mangroves, as well as suggestions for more systematic future research on wetland roughness. • A global review of Manning’s number in saltmarsh and mangrove wetlands is presented. • Global assessment of supporting nature based solutions restoring coastal wetlands • Indicative Manning’s values: 0.04–0.08 for saltmarsh and 0.10–0.14 for mangroves. • No statistical link found between Manning’s number and location, tide or temperature. • Cross-shore variability of roughness is significant and needs further investigation.