Drainage structures such as culverts, head walls, manholes, inlets and catch basins shall be constructed in accordance with Sections 3 and 12, Division 5, of the above state specifications. Drainage shall be constructed according to approved practice and design. The velocity shall be greater than two feet and less than three feet per second. For those velocities there shall be provided a well-grassed ditch. Where velocities of flow are between three feet and four feet per second, the ditches are to be sodded. Where velocities are between five feet and seven feet per second, they shall be made of cobbled stone (nine inches of stone minimum on three-inch gravel bed) or bituminous paving (six inches thick on four-inch gravel bed). Where velocities of flow exceed seven feet per second or where change of direction of flow will cause erosion, then the bottom and the banks of the ditches shall be protected with a suitable material such as riprap or concrete bag material installed in accordance with Section 15, Division 2, of the above state specifications. The sides of the ditch shall be sloped at three to one wherever practical and not steeper than 1 1/2 to one. The ditch cross section shall be designed by use of the Manning formula.