Soils on River Embankments
Embankments or levees are constructed along river banks to prevent flooding. Vegetation on a levee is expected to prevent soil erosion and provide suitable conditions to support an active biotope. River banks fostering these fragile ecosystems have been artificially constructed using anthropic soils mixed with various types of artifacts. A research site was located along the Rokkaku-gawa River in the Saga prefecture that flows into Ariake Bay. The constructed levee reaches three meters in height and functions to prevent seawater intrusion into agricultural fields and to provide protection from floods brought on by heavy rain along the course of the meander of the river. Soils for the embankment were transported from the surrounding mountains. The top of the levee is sealed with asphalt pavement to provide a traffic route for levee management. Artifacts in the soil profile were common in a higher section of the levee, resulting in higher pH. Mixing of different soil materials to establish the levee stable was also observed especially at the central section. Soil disturbance was not detected in the soil profile at the bottom of the levee, and soils here had relatively low pH values. Cogon grass (Imperata cylindrica), a native plant species, covers the slopes of embankments under natural conditions. However, goldenrod (Solidago canadensis), an invasive foreign species, was growing well at the study site. High values of pH on the upper sections of the levee provide suitable growing conditions for goldenrod. The relationship between the growth rate of invasive foreign species and soil pH is an excellent example of the effect of artificial soils on vegetation communities.
KeywordsRiver banks Embankment Foreign vegetation Soil erosion
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