Vízügyi Közlemények, 1962 (44. évfolyam)

4. füzet - IX. Könyvismertetés

(26)* HYDROLOGIC ASPECTS OF STAGE CONTROL OF LAKES By K. Szesztay (For the Hungarian text see pp. 167.) The water level of lakes is most frequently controlled by gated sluices, which regulate the outflow of excess waters. When selecting the design discharges for the hydraulic dimensioning of the sluices and the other water conveying installations, it should be remembered that level fluctuations decrease first rapidly then at a gradually slower rate as the discharging capacity is increased (Figs. 1 and 3). One of the basic data for the calculations is the discharge Q™

a

x, which corresponds to the maximum possible natural rise in stage, and which can be determined by subjecting the monthly natural variations in stage to a probability analysis (Fig. 2). Under meteorological conditions prevailing in Hungary, discharges around the lower quarter— third of the range between the constant, uniform outflow Q^ and the discharge Q"

t\

a

x

, represent mostly the economical solution. The effectiveness of stage control depends, besides the discharging capacity of the regulating structures, to a great extent also on the method of operation adopted for them. Information in this respect can be obtained from the results of comparative estimates carried out on the basis of monthly water balances of Lake Balaton for the period 1921 to 1958 (Figs. 4 to 8). The effectiveness of gate operation can be materially improved by long-range forecasts concerning the natural rise in stage during the winter- and spring period (Fig. 9). Successful stage control with outlet sluices is impossible unless the average water surplus (L) attains at least 40 to 50 per cent of the sum (P+L), where P is the long-term average evaporation from the watersurface (Fig. 10). Complete surface control under meteorological conditions similar to those in Hungary can only be achieved by the simultaneous provision of outlet sluices and the possibility for artificial water supply to the lake (Fig. 12). For selecting the rate of supply q

m, information can be gained from the relationship between the extent and duration, by, respectively for which the stages drop below the specified lowest limit, and the value of q

m (Fig. 11). In some instances the ratio P : (P + L), characterizing the trend of the water balance can be reduced by increasing, or reducing the area of the lake, and thereby the conditions of stage control can be influenced favourably (Figs. 13 and 14, Table I). For smaller water surfaces (not exceeding a few sq.km-s) the use of chemicals reducing evaporation can also be considered for regulating waterhousehold conditions (Fig. 15). In view of the present stage of investigations in this field the application of this method in Hungary is not justified economically, unless the value of water saved thereby is in excess of 3 to 4 Ft/cu.m. Where it is intended to use water of a lake for supply purposes in any significant extent, the effects of the planned withdrawal on stage fluctuations should be examined. The data characteristic for stage fluctuations increase slowly at first, later at an increasing rale, as the discharge Q

v to be withdrawn continuously and uniformly is increased (Figs. 16 and 17). As far as the distribution within the year of the withdrawn volume is concerned, desirable for stage fluctuations is if greater volumes are withdrawn in periods having a positive water balance, i.e., in the winter months (Fig. 18). Translated by Z. Szilvássy struct, eng.