• This method considers the whole dam as a single block. It is used for final design of very high dams that extend well beyond zone V. It can also be used with an accuracy of 2 to 4% on the safe side; for preliminary designs to obtain the area of the maximum section of the dam. 

• The dam designed by a single step method has a straight downstream face. When extended it  intersects the upstream face at the headwater surface.  Consider the sketch given:

L = 10-15% of h1
H10 = 2L (when earthquake is considered)
= 3L (when earthquake is not considered)
H6 = 1.33L

• When designing (analyzing) a dam in the single step method, the dam is considered as a single block; and dam dimensions are determined in such a way that rules of Zone IV are satisfied.

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• Gravity dam section relationship in single step method of design

Comparison of Single step and Multiple step design of gravity dam

• Dams of smaller height can be designed economically by the Multiple step method.

• High dams beyond zone IV are designed by Single step method so that convex curvature of downstream face and excessive flat slope of upstream face are avoided. It may be economical to increase the concrete strength through the use of more expensive material, so that even a high dam designed by dividing it into only four zones, thus eliminating zone V and VI.