  We investigate of a special dam optimal location at the Volga river in area
of the Akhtuba left sleeve beginning (7 \, km to the south of the Volga
Hydroelectric Power Station dam). We claim that a new water-retaining dam can
resolve the key problem of the Volga-Akhtuba floodplain related to insufficient
water amount during the spring flooding due to the overregulation of the Lower
Volga. By using a numerical integration of Saint-Vacant equations we study the
water dynamics across the northern part of the Volga-Akhtuba floodplain with
taking into account its actual topography. As the result we found an amount of
water $V_A$ passing to the Akhtuba during spring period for a given water flow
through the Volga Hydroelectric Power Station (so-called hydrograph which
characterises the water flow per unit of time). By varying the location of the
water-retaining dam $ x_d, y_d $ we obtained various values of $V_A (x_d, y_d)
$ as well as various flow spatial structure on the territory during the flood
period. Gradient descent method provide us the dam coordinated with the maximum
value of ${V_A}$. Such approach to the dam location choice let us to find the
best solution, that the value $V_A$ increases by a factor of 2. Our analysis
demonstrate a good potential of the numerical simulations in the field of
hydraulic works.
