THE SHARE OF RUNOFF FROM DRAINAGES IN THE SIZE OF FLOOD DISCHARGE

The aim of the study was to estimate the values of maximum runoff unit -yields q(dmax) (l/s/ha) with the periodicity of occurrence 1% and max imum flood values of surface runoff unit-yields q(1997) in Some catchm ents. Because there are no direct measurements of drainage runoffs dur ing floods, processed lines of exceeding of drainage runoffs of the ne arest stations in eastern Bohemia from the 60s and 70s with similar pr ecipitation data and slope conditions like in the upper parts of flood -affected catchment were used. For the share of drainage runoffs in ma ximum flood surface runoff unit-yields in percentage (Tab. IV) the val ues of runoffs in heavy-textured soils which form the studied catchmen ts were used. The values for medium-heavy soils allow more complex vie w. The values of drainage runoffs were reduced in the ratio of drained areas and whole areas of catchments (line 1) and were further reduced for medium-textured soils for the case of glutting of the charge cour ses of downcome (line 2). These values were compared with the culminat ion values of surface runoff unit-yields in selected catchments from f lood-affected regions in 1997 and from the flood on the brook Bradava in 1925. The values of shares in Tab. IV were not reduced for the time shift of culmination of drainage runoff against culmination of surfac e runoff. It is evident that in medium-heavy loam soils the share of d rainage runoffs may be great, after reduction on glutting of drains, h owever, the share falls below 7%. In heavy-textured soils: of which th e given catchments are built, it is maximum 2.9% but for Ostravice and Bradava the share of drainage runoff in culmination discharge in stre ams is lower than 1%. Drainage rainoffs are not the most decisive comp onent, with a great probability, in formation of flood discharges.