Stormwater runoff from lots or adjacent properties shall not be carried over cut or fill slopes steeper than 5:1. Such runoff shall be provided for as follows:
(A) As required by § 162.140(B), whenever feasible, drainage shall be retained onsite within percolation areas or drywells identified on the grading plan and as approved by the Director of Public Works.
(B) If division (A) of this section is determined by the Director of Public Works not to be feasible, each lot shall be graded so that stormwater will drain away from the structure at a grade of 2% and from the backyard through the side yard and front yard with a grade of ½% minimum directly to an abutting street or approved drainage facility, without flowing across other lots or cut and fill slopes. Where the velocity of the flow is found to be erosive, an improved drainage device shall be required.
(C) When the provisions of divisions (A) and (B) of this section are not feasible as determined by the Director of Public Works, stormwater shall be collected along the top of banks or at the rear of the graded lots by means of improved gutters or interceptor drain and carried to properly sized improved outfalls or area drains or devices which shall not be allowed to drain across the surface of sidewalks or parkways.
(D) Connecting downdrains between the interceptor drains and/or terrace ditches shall be constructed of poured portland cement concrete or airblown mortar, both reinforced with wire mesh, and of sufficient depth (a minimum of 18 inches deep) to allow for an unimpeded flow when terraces are crossed. If pipe downdrains of concrete are used, anchor lugs or collars may be required. Pipe specifications shall be approved by the Director of Public Works. Special design features shall be provided as required for abrupt changes of direction.
(E) The discharge from any downdrain, ditch, or pipe shall be controlled so as to prevent erosion of the adjacent ground. Velocities shall be reduced by means of adequately sized aprons of rock, grouted rip rap, or box type energy dissipaters. Rip rap sizing shall be designed based upon flow velocities and rip rap shall be placed in a manner so as not to create other erosion problems.
(F) Runoff volumes shall be computed using reasonable estimates of infiltration and evaporation of rainfall. Infiltration shall be assessed based in infiltration capacity of soil types expected to be encountered during project development a well as the percentage covered by impervious or nearly impervious surfaces. Conditions assumed for determining infiltration rates shall be consistent with absorption capacity of the soil being satisfied by prior irrigation or storm activity. Actual time of concentration shall be used to determine rain fall rates used in design.
(G) Surface drainage shall not be carried across a lot or parcel within three feet of a structure, without the use of an improved drainage device.
(Ord. 1449, passed 10-19-05)