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   14.4.3   General Erosion and Sedimentation Control Standards
      All persons conducting land-disturbing activities shall take all reasonable measures to protect all public and private property from damage by such activities. Any and all state and/or federal standards apply and must be met. In situations where state and/or federal rules conflict with the provisions of this section, the more restrictive shall apply. All land-disturbing activities, except for those exempted by Section 3.13.1 of this Ordinance, shall meet the following standards:
      (A)   Conveyance Channels
         (1)   Stream banks and drainageway channels downstream from any land- disturbing activity shall be protected from increased degradation by accelerated erosion caused by increased velocity of runoff from the land-disturbing activity. The land-disturbing activity shall be planned and conducted such that the velocity of stormwater runoff in the receiving watercourse at the point of discharge resulting from a ten (10)-year storm after development shall not exceed the greater of:
            (a)   The velocity as determined from Table 14.4-1 below; or
            (b)   The velocity in the receiving watercourse determined for the ten (10)-year storm prior to development.
TABLE 14.4-1: MAXIMUM PERMITTED VELOCITIES FOR STORMWATER DISCHARGES
Material
Feet per Second
Meters per Second*
TABLE 14.4-1: MAXIMUM PERMITTED VELOCITIES FOR STORMWATER DISCHARGES
Material
Feet per Second
Meters per Second*
Fine sand (noncolloidal)
2.5
0.8
Sandy loam (noncolloidal)
2.5
0.8
Silt loam (noncolloidal)
3.0
0.9
Ordinary firm loam
3.5
1.1
Fine gravel
5.0
1.5
Stiff clay (very colloidal)
5.0
1.5
Graded, loam to cobbles (noncolloidal)
5.0
1.5
Graded, silt to cobbles (colloidal)
5.5
1.7
Alluvial silts (noncolloidal)
3.5
1.1
Alluvial silts (colloidal)
5.0
1.5
Coarse gravel (noncolloidal)
6.0
1.8
Cobbles and shingles
5.5
1.7
Shales and hard pans
6.0
1.8
* These values are for channels with straight alignment. For sinuous channels: multiply allowable velocity by 0.95 for slightly sinuous channels; by 0.9 for moderately sinuous channels; and by 0.8 for highly sinuous channels.
 
         (2)   If the requirements of paragraph (1) above cannot be met, then the channel below the discharge point shall be designed and constructed to withstand the expected velocity. Any channel stabilization activities involving stream bank stabilization may not be approved without the required state and/or federal permits.
         (3)   Alternative and innovative measures to satisfy the intent of this paragraph (A), applied either alone or in combination, may be used if there are no objectionable secondary consequences and the applicant can demonstrate to the Stormwater Manager that such measures have the potential to keep stormwater discharge velocities from creating an erosion problem in the receiving watercourse. Some alternative measures include:
            (a)   Avoiding increases in surface runoff volume and velocity by including measures to promote infiltration to compensate for increased runoff from areas rendered impervious;
            (b)   Avoiding increases in stormwater discharge velocities by using vegetated or roughened swales or waterways in lieu of closed drains and high-velocity paved sections;
            (c)   Providing energy dissipators, which may range from simple rip-rapped sections to complex structures, at the outlets of storm drainage facilities to reduce flow velocities to the point of discharge; and/or
            (d)   If the watercourse velocity is greater than 2.0 F.P.S., improving cross-sections and installing erosion-resistant linings.
      (B)   Slopes Left Exposed
         Slopes left exposed shall be planted or otherwise provided with ground cover, devices, or structures sufficient to restrain erosion within fifteen (15) calendar days after completion of any phase of grading.
      (C)   Design Requirements
         Erosion and sedimentation control measures, structures, and devices shall be planned, designed, and constructed to provide protection from the calculated maximum peak rate of runoff from the ten (10)-year storm. Runoff rates shall be calculated in accordance with Cary's Standard Specifications and Details Manual. Calculations shall be submitted with the plans and shall be drafted and organized in a neat and readable manner.
      (D)   Protective Cover Required
         All uncovered areas shall be provided with protective cover unless the Planning Director has granted an extension of time, for good cause shown, upon written request of the developer or landowner. This cover shall be installed within fifteen (15) working days (exclusive of days where seedbed preparation is not possible due to weather as determined by the Stormwater Manager) or ninety (90) calendar days following completion of any phase of grading, whichever period is shorter. Ground cover is not required on cleared land forming the future basin of a planned reservoir.
      (E)   Activity Near Watercourse
         All land-disturbing activities near a lake or natural watercourse shall provide erosion and sedimentation control measures. An undisturbed buffer may be used as an erosion/ sedimentation control measure, provided that the undisturbed zone shall be of sufficient width to confine visible siltation within the twenty-five percent (25%) of the undisturbed zone nearer to the land-disturbing activity. This requirement shall not apply to land-disturbing activities for the construction of facilities to be located on, over, or under a lake or natural watercourse.
      (F)   Protection During Construction
         During construction, soil erosion and sedimentation control practices and devices shall be employed that are sufficient to retain the sediment generated by the land-disturbing activity within the boundaries of the tract during construction upon and development of said tract in accordance with plans approved by Cary.
      (G)   Limits on Single-Unit Residential Lot Grading
         Grading and Erosion Control Plans submitted as part of a development plan associated with residential development that has a gross density of three (3) units per acre or less are limited to clearing and disturbing only the land area required to install the infrastructure to serve the development (e.g., sewer and water lines, roads, storm water devices, greenways, and similar infrastructure). Until a building permit is issued for a lot, each lot must remain undisturbed, with the exception of portions of a lot impacted by the installation of infrastructure. Exceptions to grade upon a lot prior to the issuance of a building permit may be granted as a modification to the original plan approved by Cary, if there is sufficient justification (e.g., problematic drainage issues and/or severe topographic issues).
      (H)   Limit on Grading Area for Medium Density Residential Developments
         Development plans containing residential development with a gross density not exceeding eight (8) units per acre shall not grade more than twenty-five (25) acres per phase or section of development. Each twenty-five (25) acre phase or section of development must be stabilized and seeded meeting the requirements of this ordinance prior to grading an additional phase or section of development. Exceptions to this requirement may be granted as a modification to the original plan approved by Cary if compliance is not practicable due to physical site constraints, such as topography, presence of stream buffers or other natural features, or lot dimensions; or due to presence of existing development or infrastructure.
      (I)   Installation of Required Landscape After Grading
         See Section 7.2.3, Requirements for Perimeter Buffers and Landscape Areas, for the specific requirements.
      (J)   Compliance with Town Manual
         In addition to the standards set forth in this section, all soil erosion and sedimentation control measures shall conform to the applicable minimum standards set forth in Cary's Standard Specifications and Details Manual.