Predicting friction factor in herbaceous emergent wetlands

Over 53% of all wetlands in the US have been lost since the mid-1780s; to counteract wetland losses, wetland land area is being replaced through wetland restoration and mitigation. Development of the target wetland hydroperiod is critical to restoration success. For wetlands in which outflow is a component of the water budget, such as in riparian wetlands, surface water stage is controlled all or in part by the hydraulic resistance within the wetland, requiring accurate simulation of hydraulic resistance due to vegetation. Hydraulic models that consider vegetation rely on an accurate determination of a resistance parameter such as a friction factor or drag coefficient. At low Reynolds numbers typical of flows in wetlands, hydraulic resistance is orders of magnitude higher than fully turbulent flows and resistance parameters are functions of the flow regime as well as the vegetation density and structure. The exact relationship between hydraulic resistance, flow regime, and vegetation properties at the low-Reynolds number flows remains unclear. Prior research has typically involved laboratory studies of flow through idealized, individual stems. However, emergent wetland vegetation frequently grows as clumps. The goals of this research were to investigate the effect of clumping vegetation on flow resistance and to develop a prediction equation for use in wetland design. A 6-m by 1-m by 0.4-m recirculating flume was planted with mature common rush, Juncus effusus, a common emergent wetland plant. Three different flow rates (3, 4, and 5 L/s) and three different tailgate heights (0, 2.5, and 5 cm) were used to simulate a range of flow conditions. Plant spacing and clump diameter were varied (20 and 25 cm, 8 and 12 cm, respectively). Friction factors ranged from 9 to 40 and decreased with increasing plant density. Non-dimensional parameters determined through Buckingham Pi analysis were used in a regression analysis to develop a prediction model. Results of the regression analysis showed that the fraction of vegetated occupied area was most significant factor in determining friction factor.