Comparison of blockage factors in modelling the resistance of channels containing submerged macrophytes

The proportion of a river channel containing vegetation (termed the blockage factor) has historically been determined in three ways: cross-sectional, surface area and volumetric. The first two versions are two-dimensional measures. Meanwhile, the three-dimensional volumetric version is biased towards deeper sections of a reach. A fourth version of the blockage factor is proposed that does not have such limitations: the multi-cross-sectional blockage factor. Between five and nine cross-sections were sampled to determine the four blockage-factor versions for 35 river sites containing the clumped submergent macrophyte, Ranunculus subgenus Batrachium (water-crowfoot). The ability of these four measures to act as predictors of vegetative Channel resistance was then assessed. The vegetated proportion of individual cross-sections was poorly related to the vegetation resistance of a channel reach, primarily due to the high spatial variability of patch-forming macrophytes. The weighted median of all cross-sectional blockage factors measured at each site produced the strongest relationship with vegetation resistance, though this was not significantly better than the volumetric or surface area versions. The resistance model using the surface-area blockage factor gave a very high residual between predicted and calculated resistance for a mat-forming macrophyte species, and this model is unlikely to hold for conditions other than baseflow. Likewise, the volumetric version is not expected to hold for sites that have more depth variability than those measured in this study. Copyright (c) 2005 John Wiley & Sons, Ltd.