@EliasMeseret The Elevation difference b/n the full supply level of any Dam and the bed level of the spillway is Zero,that means the right and left side of the dam so called "spillway structure " starts its function When the height of the dam,the required dam design reservoir capacity is
@EliasMeseret ...achieved ,No one can reduce the height of the dam When the spillway of the dam is fixed (started to built ) ,as we have known already it's started and completed . In addition to this,the main difference b/n Earthen dam and Gravity dam are Overtopping , strength & durability1/n
@EliasMeseret ...rather than Construction of metheodology & construction material but with the same purpose . So that the remaining height of the dam is so called "Free board" of the dam to be built ,it doesn't has any relationship with capacity & purposes of the dam ,GRED.