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HomeMy WebLinkAboutAppendix B: Stearns Cty Ord.... Appendix B Stearns County - Erosion Control and Storm Water Ordinances, Jan. 2004 Erosion and Sediment Control Standards The purpose of this Section is to prevent or reduce, to the most practicable extent, erosion and sedimentation and their associated effects and to provide for the protection of public waters as well as natural and artificial water storage and retention areas within the County. An erosion and sediment control plan shall be submitted to and approved by the Department prior to construction of a new plat or a commercial or industrial facility, or when the Department detennines an erosion and sediment control plan is necessary due to potential impacts of construction on the property or surrounding properties. 7.5.1 General Standards Proper erosion and sediment control practices shall be followed within the County as described in this Section. All land disturbing activities, whether or not a pennit is required, shall be subject to the following perfonnance standards: A. No land owner, operator, contractor or applicant shall cause or conduct any land disturbing activity which causes erosion or sedimentation, damages water or soil resources or creates off-site impacts. B. All development shall confonn to the natural limitations presented by the topography and soil types in order to minimize soil erosion and sedimentation. C. Land disturbing activities shall only occur in increments of workable size such that adequate erosion and sediment controls can be provided throughout all phases of a development. The smallest practical area ofland shall be exposed or otherwise disturbed at anyone period of time. D. Every applicant for a construction site pennit, interim use pennit, provisional use pennit, alteration pennit, conditional use pennit or subdivision approval shall, at a minimum, adhere to erosion control measure standards and specifications contained in the MPCA publication "Protecting Water Quality in Urban Areas"; or successor publication. 7 .5.2 Specific Standards A. No land disturbing activity shall result inactive gully erosion or create negative off-site impacts. B. No land disturbing activity shall result in an increase in channel erosion in any watercourse, whether pennanent or intennittent, at any time during or following development. C. No land disturbing activity shall result in the creation of unstable slopes which persist after the completion of the development. D. Pennanent or temporary soil stabilization shall be applied to disturbed areas (areas where vegetation has been removed or where cuts have been made), as soon as possible, but not to exceed fourteen (14) days after a substantial portion of rough grading has been conducted unless an extension is granted by the Director. Soil stabilization measures shall be selected to be appropriate for the time of year, site conditions and estimated duration of use. E. An erosion and sediment control plan, when required by the Department shall include the following: (1) Location map. An 11 'x17" map locating the site in relation to the surrounding area. (2) Indicate north. Show the direction of north in relation to the site. (3) Scale. Indicate scale in relation to the actual size of the site, usually in feet per inch. (4) Benchmark. Show the established elevation affixed to a permanent object which can be used to check grade. (5) Plan preparer. Indicate the name and phone number of the individual or agency responsible for preparation of the plan. (6) Contact person. Give the name and phone number of the individual responsible for plan implementation. (7) Existing contours. Show existing two foot contours of the site extending at least 200 feet beyond the property boundaries. (8) Final contours. Show all proposed changes to the existing contours due to land disturbance. (9) Existing vegetation. Indicate existing woods, tree lines, cultivated areas, grasslhay fields, CRP, wetlands and other vegetative types. (10) Disturbed area. Identify the disturbed acreage for each drainage area. Include roads and lot clearing. (11) Utilities. Show the locations of stonn sewer, sanitary sewer, water supply, electrical and other utilities in the area ofthe proposed development. (12) Location ofBMP. Indicate the location of erosion and sediment control practices proposed for the site. (13) Implementation schedule. Outline the proposed order ofland clearing, road installation and other aspects of construction. (14) Critical erosion areas. Identify areas susceptible to erosion during and after construction. Critical erosion areas are areas which are prone to accelerated erosion, areas which have slopes of 12% or greater, areas oflong, continuous slopes or areas which contain erosive soils. (15) Limits of disturbed areas. Show the extent of the areas which must be disturbed to accommodate the proposed construction. (16) Sediment pond. Show the location of any temporary pond to be used to collect sediment during construction. (17) Adjacent areas. Describe neighboring areas which could be affected by land disturbance. (18) Temporary erosion control plan. Indicate how erosion on the site will be temporarily controlled until permanent erosion control can be implemented (seeding and mulching rates, sod installation, etc.) F. Soil stockpiles shall be stabilized or protected with sediment trapping measures to prevent soil loss. G. A permanent vegetative cover shall be established on disturbed areas not otherwise permanently stabilized. H. Properties adjacent to the site of a land disturbance shall be protected rrom sediment deposition. 1. Sediment basins and traps, perimeter dikes (for diversion), sediment barriers (silt fences) and other measures intended to trap sediment on- site shall be constructed prior to or concurrent with any grading and shall be functional before upslope land disturbance takes place. Earthen structures such as dams, dikes and diversions shall be seeded and mulched within fourteen (14) days of installation. J. Storm water runoff rrom drainage areas with more than five (5) acres of disturbed area must pass through a temporary sediment trapping basin or other suitable sediment trapping facility. K. Cut and fill slopes shall be designed and constructed in a manner which will minimize erosion. Slopes which will not be vegetated within one (1) year of construction shall be provided with additional slope stabilizing measures until the problem is corrected. Slopes that are found to be eroding excessively shall immediately be provided with additional slope stabilizing measures until the problem is corrected. L. Properties and waterways downstream rrom development sites shall be protected rrom erosion due to increases in the volume, velocity and peak flow rate of storm water runoff. M. All on-site storm water conveyance channels shall be designed and constructed to withstand the expected velocity of flow rrom a 1 O-yea~ rrequency storm without eroding. N. Rip-rap shall be placed at culvert outfalls in accordance with applicable MnDOT standard specifications. O. All stonn sewer inlets which are made operable during construction shall be protected so that sediment laden water will not enter the conveyance system without first being filtered or otherwise treated to remove sediment. P. Construction vehicles and other equipment shall be kept out of watercourses to the maximum extent possible. Q. Wherever construction vehicle access routes intersect paved public roads, provisions, such as rock construction entrances, shall be made to minimize the transport of sediment by runoff or vehicle tracking onto the paved surfaces. R. All temporary erosion and sediment control measures shall be properly disposed of within thirty (30) days after final site stabilization is achieved or after the temporary measures are no longer needed, unless otherwise authorized by the Department. S. All temporary and permanent erosion and sediment control practices shall be maintained and repaired as needed to assure continued performance of their intended functions. Storm Water Management The purpose of this Section is to prevent or reduce, to the most practicable extent, the effect or impacts of storm water runoff and to provide for the protection of public waters and natural and artificial water storage and retention areas within the County. Further, this Section clarifies the performance standards as they pertain to standards and specifications for conservation practices and storm water planning activities. 7.21.1 Performance Standards Proper storm water management practices shall be followed within the County as described in this Section. A. General Standards. (1) The need for storm water management facilities may be reduced by incorporating or restoring the use of natural topography and features, such as wetlands, ponds, natural swales and depressions to the degree that they can accommodate the additional flow of water without compromising the integrity of the natural feature. When development density, topographic features, soil or vegetation conditions are such that natural features are not sufficient to handle storm water runoff, various types of constructed facilities, such as diversions, settling basins, skimming devices, dikes, waterways and ponds may be used. (2) Soil laden runoff shall be treated before it is allowed to enter any water body. Preference shall be given to designs using surface drainage, vegetation and infiltration rather than buried pipes, manmade materials and facilities. (3) Storm water rate control. When required by the Director, a storm water management plan shall include the design of all storm water management facilities necessary to manage increased runoff so that the 2-year, 10-year and 100-year storm peak discharge rates from the property boundary do not exceed pre-development conditions and so that accelerated channel erosion on and off-site will not occur as a result of the proposed land disturbing or development acti vi ty. (4) The minimum design capacity of all drainage systems shall accommodate the runoff from a ten (10) year storm event. All drainage systems and facilities shall be designed to withstand the runoff from the critical one hundred (100) year event or accumulative antecedent conditions without damage to the system or facility, downstream areas and without significant risk to human health and safety. ~(1) The applicant or their successors shall be responsible for the installation and maintenance of any temporary or permanent measures identified in the storm water management plan. At the time of completion of the development, those structures, measures and systems constituting the storm water runoff facility may be permanently maintained by the Township after official acceptance by the Township Board, by a legally organized homeowner's association, by a watershed district after official acceptance by the district or by other means acceptable to the County. -1 91· An application for a storm water management plan shall be accompanied by a financial guarantee in the form of a letter of credit, cash deposit or bond in favor of the County equal to one hundred twenty-five (125) percent of site grading and erosion/sediment control costs necessary to ensure the satisfactory installation, completion and maintenance of the measures as required in the storm water management plan. ~ 0') Storm water volume control. For protection of downstream water bodies and properties, post development runoff volumes shall not exceed pre-development conditions. For protection of downstream water bodies, the most current Best Management Practices (BMPs) shall be employed to reduce the general impacts of runoff volume and rates. Development resulting in the creation of impervious surfaces must explicitly address the use of BMPs to limit the loss of pervious areas, BMPs to be evaluated shall include, but not be limited to, vegetated swales, pond outlets perched above ground water levels, roof drainage to pervious areas, depressed casual storage areas, minimization of the number and width of parking stalls, "rural section" roads and road width minimizatïon and mitigation of disturbed soils. l\. /(4) Prevention of downstream nuisance and damage. When conditions do not permit post-development runoff volumes to be equal to pre-development conditions, the applicant shall provide a study which demonstrates that increased storm water runoff volumes above pre-development conditions will not adversely affect downstream properties or water resources. The study shall include, at a minimum, an evaluation of downstream landlocked lakes and ponds and lakes and ponds with inadequate outlets where flood levels could be increased by added runoff volume; and downstream drainage paths that are not public waters that are inadequate to convey added runoff volumes as evidenced by (a) a lack of public easements or access to the drainage facility; (b) existing channel erosion prior to discharge to public waters; (c) undersized drainage systems or drainage systems in disrepair; and (d) other downstream drainage conditions that would tend to create nuisance conditions or safety problems. \ Û ø5 Storm water management facilities must be designed, installed and maintained consistent with the most current Best Management Practices. \\ (},Ø}Developments shall be planned and conducted in a manner that will minimize the extent of disturbed area, runoff velocities, erosion potential and both reduce and delay runoff volumes. Disturbed areas shall be stabilized and protected and facilities or V methods used to retain sediment on site. \ (J1)All storm water management facilities shall be designed to minimize the need for maintenance, to provide access for maintenance purposes and to be structurally sound. All storm water management facilities shall have a plan of operation, maintenance and clean-out that assures continued effective removal of sediment carried in storm water runoff. It shall be the responsibility of the applicant to obtain any necessary easements or other property interests to allow access to the storm water management facilities for inspection and maintenance purposes. B. Specific Standards for Storm Water Conveyance and Rate Control Facilities. (1) All storm water management calculations submitted to the Department for review as part of a storm water management plan shall include sufficient information for the Department to evaluate the changes to the storm water drainage characteristics within the watershed areas affected by the proposed land disturbing activity. The applicant shall include calculations which clearly show the effects of this development on the peak rate of discharge, the time of concentration, channel velocities and other potential drainage impacts to water and soil resources both on and off the development site. The Director may require the applicant to provide any additional information, calculations or data needed to complete the review of a storm water management plan. (2) The storm water calculations submitted for review shall be based upon standard hydrological and hydraulic analysis methods that are acceptable to the Director. Calculations that are based upon unproven methodologies or apply proven methodologies incorrectly shall be determined by the County to be unacceptable and shall be returned to the applicant for correction and be resubmitted. (3) Acceptable hydrological methods and procedures to determine peak runoff discharge rates and runoff volumes for all development, except for street and highway pavement drainage systems, shall be the standard methods of the Natural Resources Conservation Service SCS TR 55 and the SCS TR 20 Methods as defined in the current Hydrology Guide for Minnesota. (4) Precipitation events for the Rational Method shall be for the two (2), ten (10) and one hundred (100) year twenty-four (24) hour frequency storm events using the US. Weather Bureau Technical Paper No. 40 rainfall intensity duration curves for a Type II rainfall distribution. (5) Acceptable hydrological methods and procedures to determine peak runoff discharge rates for street and highway pavement drainage systems, inlet capacities and piped storm sewer systems shall be based upon the Rational Method as defined in the current Minnesota Department of Transportation Drainage Manual. (6) Where development site drainage discharges to an existing roadway, ditch, stonn sewer or other public facility, the applicant shall provide, as part of the calculations, all survey, utility or other topographic data of the existing condition needed for the Director to detennine that the proposed development does not impact or degrade any critical roadway element or negatively impact the safety, maintenance or function of the public facility. (7) Drainage areas. Stonn water management plans shall show existing and proposed drainage areas used for stonn water analysis, including off-site portions of subwatersheds that are partly located on the property for which the plan is being prepared. Where drainage areas include runoff from off-site areas, those areas may be shown and measured from maps at larger scales (e.g. United States Geological Survey Quadrangle Maps) if better mapping is not reasonably available. In all drainage areas, the direction of flow for each area and the travel path used for detennining the Time of Concentration shall be shown. No direct entries for detennining the Time of Concentration shall be allowed without prior approval by the Director. (8) Runoff curve numbers (RCN). Stonn water management plans shall include a detailed breakdown of existing and proposed runoff curve numbers. (9) Soil types. Stonn water management plans shall identify the boundaries of the soil types present on-site and their hydrologic classification and acreage. (10) Pre-settlement conditions. The following runoff curve numbers shall be use to analyze pre-settlement conditions: Hydrologic Soil Group Runoff Curve Number: A-30 B-55 C-71 D-77 (11) For evaluation of post-development runoff, drained hydric soils shall be assumed to revert to an undrained condition unless the applicant demonstrates that publicly owned and maintained drainage facilities will be adequate to maintain the drained condition. (12) Impervious coverage. Stonn water management calculations shall list the new impervious area created in each subwatershed and shall include the assumptions and calculations used for detennining impervious areas, such as house pad, driveway and outbuildings. (13) Runoff calculations. The applicant shall provide calculations for two (2), ten (10) and one hundred (100) year peak discharge rates for each subwatershed comparing pre-settlement conditions and proposed post-development conditions. (14) Where pre-settlement conditions indicate no runoff, the infiltration capacity required elsewhere in this Ordinance may be used to demonstrate compliance with a no runoff requirement for the stonn frequency and duration being considered. (15) Stonn water management plans shall show preexisting drains and tile lines. Stonn water facilities shall be designed assuming that tile lines will no longer function unless an easement is supplied for future maintenance and the applicant demonstrates that the tile line has design capacity and service condition that makes it a suitable component of the stonn water management system. (16) Stonn water management plans shall include an evaluation of landlocked lakes and ponds in the design analysis and demonstrate that runoff from a full year of runoff in the 100 year wet year will not exceed the predicted 100 year flood level. (17) Stonn water management plans shall identify the location of conveyance systems and clearly identify all dimensions, cross sections and outlet elevations. (18) Stonn water management plans shall include the locations of all property lines, lot lines, section lines and adjacent plats. (19) Stonnwater management plans shall contain infonnation which clearly identifies all elevations and grades for streets, ditches, ponds, wetlands, lakes, pipe inverts and pipe outlets. C. Specific Standards for Wet Detention Basins. (1) All wet detention basins shall be designed and constructed in accordance with the W.W.Walker Method (1987) described in the Best Management Practices. (2) Designs for wet detention basins shall include, but not be limited to, calculations for estimated inflow and outflow, pennanent and temporary storage volumes, mean depth, outlet design, downstream stabilization, emergency spillways, basin profiles and basin cross sections. (3) In the LI, GI and CR zoning districts, skimmers shall be included on the outlet of wet detention basins. Construction details of the skimmers shall be shown on the construction plans for the wet detention basin. (4) Ground water sensitivity. Wet detention basins located in areas identified as being highly susceptible to ground water contamination, except ground water discharge areas, shall be designed so that the bottom of the basin is located at least three feet above the seasonal high ground water elevation and/or bedrock and be lined with two (2) feet of soil having a penneability less than five (5) minutes per inch. D. Specific Standards for Volume Control. (1) Infiltration practices for control of storm water runoff volume shall be capable of infiltrating one-half (112) inch of runoff from all new impervious surfaces within the development within seventy two (72) hours. (2) Infiltration volume and facility sizes shall be calculated using the appropriate hydrologic soil group calculation and saturation infiltration rate from the table below. Documented site specific infiltration or hydraulic conductivity measurements may be used in place of the values in the following table, if approved by the Director. The goals of these BMPs are to minimize the amount of directly connected impervious surface created, to preserve the infiltration capacity of the soil and to incorporate practices into the design which are capable of allowing the infiltration of one~ half (1/2) inch of runoff from impervious surfaces within seventy- two (72) hours. Soil Group Infiltration Rate (in/hr) Soil Texture A. 0.3 sandy, loamy sand or sand loam B. 0.15 silt loam or loam C. 0.07 sandy clay loam D. 0.03 clay loam, silty clay loam, silty clay or clay Source: Urban Hydrology for Small Watersheds (SCS, June 1986) (3) Infiltration areas shall be limited to the horizontal areas subject to prolonged wetting. (4) Areas of permanent pools tend to lose infiltration capacity over time and shall not be accepted as an infiltration practice. (5) New constructed storm water outfalls to any public waters must provide for filtering or settling of suspended solids and skimming of surface debris before discharge. E. Specific Standards for Areas of Moderate or High Susceptibility to Ground Water Contamination. (1) Infiltration areas shall have either natural undisturbed soil or be lined with at least two feet of soil with a permeability of five (5) minutes per inch or slower as defined in the Ground Water Protection Plan. (2) Infiltration areas shall be at least three feet above the seasonal high ground water elevation and/or bedrock. F. Compliance with storm water management plans. If an applicant does not implement the requirements of a storm water management plan, the Director may issue a restoration order as outlined in Section 4.23 of this Ordinance.