Geomorphic impact of historical slate mining activity on gravel-bed streams
published: Sep 1, 2016
ArtNo. ESP022006003004, Price: 29.00 €
Individual sediment inputs of coarse material may affect downstream channel geometry, morphology, and grain sizes of bed sediments. This study deals with the geomorphic interactions of artificial sediment inputs in the forms of large slate dumps and adjacent gravel-bed mountain streams (Nízký Jeseník Mts, Czech Republic). The dumps were composed of coarse slate slabs up to 500 mm in diameter, whereas local natural streams usually contained cobbles and gravels in plane beds and riffles, and much finer sand, silt, and clay fractions in pools. The parameters of bankfull width, and bed grain sizes, here represented by mean boulder diameter as an arithmetic mean of five largest bed particles, were investigated in short intervals within four longitudinal profiles of first- to fourth- order streams influenced by historical slate mining. Coarse sediment supply from investigated dumps in low-order channels i) often transformed channel-reach morphologies (pool-riffles to plane beds), ii) significantly altered adjacent and downstream calibre of bed sediments, and iii) decreased the bankfull width near the points of active sediment inputs. Fluvial transport processes were influenced especially in case of ordinary flows close to bankfull discharges, although it is likely the flow competence of low-order channels was not able to transport coarsest material in both affected and unaffected channel-reaches. Measured largest particle diameters exceeded up to 11 times the flow competence calculated for the bankfull flow in those reaches directly supplied by dumps. By contrast, unaffected pool-riffles showed the coarsest bed particles are only 2.5–5 times larger in comparison with the critical transported diameter in case of bankfull discharge. It implies that a significant increase in channel-bed stability occurred in the channel-reaches with artificial sediment supply from dumps. Nevertheless, some high-magnitude flood events were capable to move almost all bed sediment fractions, as it was indicated by a grain-size coarsening in the reaches downstream of the dumps, and observed imbrication of large boulders in studied low-order channels. Minimal alterations were found in the fourth-order channel owing to the shorter contact zone with adjacent dump and higher transport capacity of this stream. By increased transport capacity of a stream, the significance of slate dumps as sediment inputs for evaluated supply limited or transport-balanced streams decreased.