The commissioner, in establishing such encroachment lines, shall base their location on the boundaries of the area which would be inundated by a flood similar in size to one or more recorded floods which have caused extensive damages in such area or on a size of flood computed by accepted methods applicable generally throughout the state or a region thereof. The determination of the size of the flood and the boundaries of the inundated area shall take into consideration the effects of probable future developments. The position of the lines may vary from the boundaries of the inundated area so as to minimize the area of land to be regulated when a portion of the inundated area does not contribute to the flood-carrying capacity of the waterway. The position of the lines shall, insofar as practical, equitably affect riparian properties and interests depending upon existing topography and shall be interdependent throughout the reaches of the waterway, and shall conform with the requirements of the federal government imposed as conditions for the construction of flood control projects. When the existing waterway, because of natural or man-made constrictions, is such that such lines cannot be established by standard engineering methods, a channel may be adopted, whereby the removal of such constrictions may be anticipated so that reasonable lines can be established by methods applicable to the state generally. When the flood boundary falls along the channel banks, the lines shall be placed at the top of the bank.