The new construction of a rapid built auxiliary spillway made from polymer materials for a low-head hydro system of a small reservoir is proposed. The objective of the study is to develop the methodology for calculation of the main parameters of the auxiliary spillway which are defined on the basis of hydraulic calculations assuming the pass of a peak design flood discharge higher than the rated probability. Basic data for calculation were the parameters of ΔQ and Í. The calculation sequence is the following. First the preliminary width of the closing gap bottom (b), is set. Then the coefficients of speed (fi) , and compression (e), are determined according to the formulas. The coefficient taking into account the spatiality of the flow Ê is calculated. Substituting the found values of the coefficients (fi), (e) , Ê and basic data ΔQ and Í into the end formula the width of the closing gap can be estimated in the first approximation (âi) . Subsequent calculation is repeated before the values of the width in the end approximation will be varying from the previous one less than 5-10 %. Thereby, based on the conducted calculations the width at the bottom of the closing gap is b=3.82 m, which can provide the rated discharge ΔQ=11.5 m³/sec at the inlet pressure head Í=1 m. If the length of the artificial closing gap is exceeded Lï>10 Í, the hydraulic calculation is made as for a short-channel assuming the head loss.
