(a) The edge of any excavation made under this article shall not he nearer than two times the proposed depth of the excavation to the line of any public or private property other than that owned or controlled by the person making such excavation. The excavation may be carried to within ten feet of any such public or private property, provided the banks be supported in a manner satisfactory to the Board and that a substantial fence be erected along the top of the bank, when deemed necessary by the Board.
(b) In no case shall the depth of any excavation made under this article exceed the depth of the foundation of any pier, abutment, wall or support of any bridge or trestle across a water course upon which such excavation is made, with in Two Hundred (200) feet upstream or Five Hundred (500) feet downstream from such excavation.
Whenever, in the opinion of the Board, it shall be dangerous to the public welfare and safety to permit any such excavation to approach within five hundred (500) feet of the foundation of any pier, abutment, wall or support of any such bridge or trestle the Board may increase the distance within which such excavation may approach such pier, abutment, wall or support to such distance as it may deem necessary.
(c) In portions of the Arroyo Seco or the Los Angeles River where one or both banks is or are protected by walls, no excavation opposite along any wall, shall be made to a depth lower than three feet above the level of the bottom of the foundation of that portion of any wall opposite such excavation.