Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

mentation value, obtained as the ratio of volume R silted during one year, to the maximum volume V

0 of the reservoir. By determining further the storage coefficient as the ratio of average reservoir volume V during the period under consideration to the average annual runoff W, as well as the average concentration с of inflow into the reservoir, the relationship between the above mentioned factors is defined in the form with the help of correlation analysis. Relationships of this type are likely to retain their validity, provided that the records are reliable enough and the correlation coefficients are acceptable. This validity is, however, restricted to the range of values defined by the records and to truly identical conditions, but once these restrictions are observed, they may be suited for predicting sedimentation of reservoirs. Experience gained during the operation of reservoirs in Poland — together with relevant observations abroad — suggests the importance of allowing also for the sediment volumes produced by bank-erosion within the reservoir itself. In the case of reservoirs of lake character the volume of sediment material originating from shore erosion may approximate the order of magnitude of sediment transported by the feeding river and allowed for in sediment estimates. Quite obviously, the results of estimates based exclusively on the sediment balance, may under such circumstances be entirely misleading. Operational experience gained with reservoirs in Poland is utilized also by B. Wisiewsky [12] in deriving the sedimentation index as the ratio of annual average silting of a reservoir to the annual average sedimentation of the catchment area above the dam site. This number appears to be actually applicable for defining trend-like relationships, since in principle the characteristics particular to a given catchment are reflected thereby. For this method to be successful a reliable method of observation must be developed for determining the average sedimentation of the catchment area. The objective of similar general geological description is served also by the denudation index introduced by Raniger, who classified surface erosion into nine categories. The index indicates in tons/sq.km/year units the total amount of solids transported by water into the reservoir. An additional characteristic value is the settling efficiency of the reservoir, denoting the percentage of suspended material deposited during one year in the reservoir, related to the total volume of suspended sediment transported by the river. Reliable information for determining the rate of sedimentation of reservoirs is supplied also by the observation results described by B. Georgiev [4], who obtained his data with reservoirs in Bulgaria. These data are used for studying the regularities of changes in unit weight. (13) 76