The construction, maintenance or existence within the City of any unprotected, unbarricaded, open or dangerous excavations, holes, pits or wells, or of any excavations, holes or pits which constitute a danger or menace to the public health, safety or welfare, are hereby prohibited. However, this section shall not prevent any excavation under a permit issued pursuant to this chapter or the Building Code where such excavations are properly protected and warning signs posted in such manner as may be approved by the Building Inspector. Nor shall this section apply to lakes, streams or other natural bodies of water, or to ditches, streams, reservoirs or other major bodies of water created or existing by authority of any government agency.