Assessing the Effects of Ground Sill Height on Fish Habitat Diversity: A Case Study of Tengqiao Creek

Wild Streams in Taiwan are characterized by steep channel slopes, rapid flow velocities, short channel lengths, and small watershed areas. These streams are highly susceptible to severe compound flood and sediment disasters triggered by typhoons and intense rainfall. To safeguard human life and property in surrounding areas, river management has historically emphasized engineering safety and disaster mitigation. Consequently, ground sills were widely installed to stabilize riverbeds and reduce channel erosion, while the potential ecological impacts of such engineering measures were largely overlooked, often resulting in severe degradation of river ecosystems. With the extensive installation of ground sills, river environments have become increasingly homogenized. The height configuration of the structures can alter hydraulic conditions and longitudinal connectivity, as well as modify the natural diversity of flow types (e.g., shallow flow, riffles, runs, and deep pools), leading to a decline in biodiversity. As a result, aquatic ecosystems and fish habitats have been significantly disturbed, causing degradation of river ecological functions. This has prompted efforts to restore and rehabilitate rivers whose ecological functions have been degraded by human interventions.

The object of investigation in this study was the Tengqiao Creek and the Two-Dimensional Habitat Diversity Construction Model was applied to simulate and investigate changes in fish habitat types and the spatial distribution under different discharges and varying heights of ground sills. Habitat diversity was quantified using the Shannon index as the Habitat Diversity Index (HDI). The results indicate that, under all discharge conditions and ground sill heights, habitat types 1(shallow pool) and 2 (medium pool) account for the largest proportion of habitat area within the study reach. When discharge is lower than 0.04 cms and the ground sill height is below 0.5 m, habitat type 3 (deep pool) does not occur; the area of habitat type 3 increases with increasing structure height. The area proportions of habitat types 4 (slow riffle), 5 (fast riffle), and 6 (run) are considerably smaller than those of types 1 and 2, with type 4 occupying a relatively larger area than types 5 and 6. When the ground sill height exceeds 0.75 m, the areas of habitat types 4, 5, and 6 are significantly smaller than those observed under structure heights below 0.75 m. Across all flow conditions, the habitat diversity index increases with increasing discharge but exhibits a decreasing trend as the height of ground sills increases. The findings of this study provide valuable references for future river restoration and ground sill design.