(a) Sheet and Rill Erosion. To control pollution of public waters by soil sediment from accelerated sheet and rill erosion on development areas, the responsible person shall:
(1) Construct and maintain sediment basins sized in accordance with the United States Soil Conservation Service handbook, Water Management and Sediment Control for Urbanizing Areas;
(2) Apply and maintain a level of management and conservation practices such that the predicted average annual soil loss, accumulative monthly in accordance with the procedures in the United States Soil Conservation Service handbook, Water Management and Sediment Control for Urbanizing Areas, is less than fifteen tons per acre the first year commencing from the time of initial earth disturbance, ten tons per acre the second year, and five tons per acre for any other year of the development process. The management and conservation practices shall be designed, applied and maintained so that the entire development area and any part thereof are protected from accelerated erosion in accordance with the stated criteria; or
(3) Use other methods to control sediment pollution, including, but not limited to, a combination of paragraphs (a)(1) and (2) hereof, provided those methods are acceptable to the approving agency.
(b) Concentrated Water Erosion. To control pollution of public waters by soil sediment from accelerated erosion in drainage ways and grassed waterways and in streams and ditches disturbed or modified in conjunction with the development process, on a development area, the responsible person shall:
(1) Design, construct and maintain concentrated water flow channels such that the velocity of flow does not exceed the permissible velocities listed in Appendix A following the text of this chapter;
(2) Design, construct and maintain sediment basins sized in accordance with the United States Soil Conservation Service handbook Water Management and Sediment Control for Urbanizing Areas; or
(3) Use other methods to control sediment pollution; this may include, but is not limited to, a combination of paragraphs (b)(1) and (2) of this standard, provided those methods are acceptable to the approving agency.
(c) Sloughing, Landsliding and Dumping. To control sediment pollution of public waters caused by sloughing, landsliding, or dumping of earth material, or placing of earth material into such proximity that it may readily slough, slide or erode into public waters by natural forces, no person shall:
(1) Dump or place earth material into public waters or into such proximity that it may readily slough, slide or erode into public waters unless such dumping or placing is authorized by the approving agency for such purposes as, but not limited to, constructing bridges, culverts, erosion control structures and other in-stream or channel bank improvement works; or
(2) Grade, excavate, fill or impose a load upon any soil or slope known to be prone to slipping or landsliding, thereby causing it to become unstable, unless qualified engineering assistance has been employed to explore the stability problems and to make recommendations to correct, eliminate or adequately address the problems. Grading, excavating, filling or construction shall commence only after the approving agency has reviewed and approved the exploratory work and recommendations and only in accordance with the approved recommendations.
(d) Stream Channel and Flood Plain Erosion. To control pollution of public waters by soil sediment from accelerated stream channel erosion and to control flood plain erosion caused by accelerated storm water runoff from development areas, the increased peak rates and volumes of runoff shall be controlled such that:
(1) The peak rate of runoff from the critical storm and all more frequent storms occurring on the development area does not exceed the peak rate of runoff from a one-year frequency, twenty-four hour storm occurring on the same area under pre-development conditions.
(2) Storms of less frequent occurrence (longer return periods) than the critical storm, up to the 100-year storm, have peak runoff rates no greater than the peak runoff rates from equivalent size storms under pre-development conditions. Consideration of the one, two, five, ten, twenty-five, fifty and 100-year storms will be considered adequate in designing and developing to meet this standard.
(e) Determination of Critical Storm. The critical storm for a specific development area is determined as follows:
(1) Determine by appropriate hydrologic methods the total volume of runoff from a one-year frequency, twenty-four hour storm occurring on the development area before and after development.
(2) From the volumes determined in paragraph (e)(1) hereof, determine the percentage of increase in volume of runoff due to development, and, using this percentage, select the twenty-four hour critical storm from this table:
IF THE PERCENTAGE OF INCREASE IN VOLUME OF RUNOFF IS:
Equal To Or And The Critical Storm For Discharge
Greater Than Less Than Limitation Will Be
- 10 1 year
10 20 2 year
20 50 5 year
50 100 10 year
100 250 25 year
250 500 50 year
500 - 100 year
(f) Methods of Controlling Peak Runoff. Methods for controlling increases in storm water runoff peaks and volumes may include, but are not limited to:
(1) Retarding flow velocities by increasing friction, for example, grassed road ditches rather than paved street gutters where practical (low density development areas, access roads, etc.); discharging roof water to vegetated areas; or grass and rock lined drainage channels;
(2) Grading and construction of terraces and diversions to slow runoff and use of grade control structures to provide a level of control in flow paths and stream gradients;
(3) Induced infiltration of increased storm water runoff into the soil where practical; for example, constructing special infiltration areas where soils are suitable; retaining topsoil for all areas to be revegetated; or providing good infiltration areas with proper emergency overflow facilities; and
(4) Provisions for detention and retention; for example, permanent ponds and lakes with storm water basins provided with proper drainage, multiple use areas for storm water detention and recreation, wildlife, transportation, fire protection, aesthetics or subsurface storage areas.
(Ord. 61-1987. Passed 9-14-87.)