I don't see a 30 foot tall dam failing in the clay we have here. 30 foot tall dam with 3 to 1 slopes and a 12 foot wide top is 192 foot thick at the base. A lot matters as to acres drained to pool size and type of soil. Some places do require an engineered dam. Size of drain pipe and emergency spillway as well as amount of free board all play into design as well as amount of spread acres the lake has. A deep gully needs different free board and drain system than a lake that spreads to half again it's size to control big rains. We had a 5 inch rain on a 18 acre lake that drains 250 acres with no problem. Went out the emergency spillway maybe 18 inches deep for a few hours. You don't want a spillway that has water standing on it all the time, these will cut out faster than a dry spillway with a good stand of grass. Also the emergency spillway is always cut into virgin soil which helps control scalping it and diverted on the lower part so it won't cut the base of the dam out. I agree you have to take a lot into consideration when building a lake or pond but common sense and experience goes a long way.