Attachment B

 

 

 

Urban Lot Drainage and Retaining Walls

January 4, 2006

 

1.         For all subdivisions with lots less than 15,000 square feet in size and not requiring a site plan, a lot grading plan must be approved by the County’s Program Authority prior to the issuance of a building permit for a new residence.  The lot grading plan must satisfactorily demonstrate compliance with all Erosion and Sediment Control requirements and drainage conveyed across the lot(s).  This requirement can be satisfied with either a grading plan for all the subdivision lots (“overlot grading plan”) or with individual lot grading plans submitted at the time of each building permit.  If lot grading is shown with an overlot grading plan, an “Agreement in Lieu of a Plan” will be allowed for building permits, provided the general drainage patterns and grading matches that shown on the overlot grading plan.  The overlot grading plan may be revised at any time by the subdivision developer or individual lot owners, provided all work can be accomplished within their property lines or within available easements.  For building permits where there is no approved overlot grading plan, an “Agreement in Lieu of a Plan” will not be allowed and an individual lot grading plan will be required to satisfy County Water Protection Ordinance requirements.     

 

An individual lot grading plan must demonstrate the following to the satisfaction of the County’s Program Authority:

a.       Consistency with Erosion and Sediment Control requirements.  (Note: this is not a change, but simply states the expectation that the plan must satisfy the current requirements.)

b.      All concentrated runoff leaving the lot must be conveyed in vegetated swales or underground drainage structures in a manner that does not result in flooding of buildings or erosion as a result of the lot grading.   For the purpose of this requirement, flows from roof downspouts will be considered concentrated flows if not adequately dispersed before reaching the property line.   The builder/developer will not be considered to have a presumptive right to alter the existing drainage patterns in a manner that affects adjoining lots or increases the runoff flow rates directed at downstream properties.  (Note: This specifies MS-8 of the Erosion and Sediment Control requirements is addressed either through vegetated swales or storm sewer.)

c.       All concentrated runoff coming onto the property must be conveyed across the property in vegetated swales or underground drainage structures in a manner that assures flooding of buildings or erosion will not occur.   The builder will not be considered to have a presumptive right to alter drainage patterns in a way that adversely affects upstream property owners. (Note: This specified MS-8 of the Erosion and Sediment Control requirements is addressed either through vegetated swales or storm sewer.)

d.      Overland relief must be assured in the event that drainage structures do not function.  Overland relief will be considered satisfied if buildings are designed to have finished floors at least 1’ above low points for any drainage area.  With dams and similar impoundments, this should be measured from the top of dam. 

e.       The Program Authority may allow other drainage structures (e.g. riprap ditches) where it has been determined this change will not significantly impact usable yards (e.g. cobblestone swale next to a driveway could be considered acceptable), where slopes are too steep for vegetated swales (e.g. steeper than 33% grades), or where the change would better mitigate impacts on adjoining properties (e.g. matches offsite drainage structure).      (Note:  This is to recognize that riprap and other drainage channels may be allowed and provide guidance for when it might be considered acceptable.)

 

An overlot grading plan must demonstrate the following to the satisfaction of the County’s Program Authority: 

a.                   All concentrated runoff is conveyed across lots using vegetated swales or underground drainage structures in a manner that does not result in flooding of buildings or erosion as a result of the grading.    For the purpose of this requirement, flows from roof downspouts will be considered concentrated flows if not adequately dispersed before reaching the property line. (Note: This specifies MS-8 of the Erosion and Sediment Control requirements is addressed either through vegetated swales or storm sewer.) 

b.                  Overland relief is assured in the event that drainage structures do not function.  Overland relief will be considered satisfied if buildings are designed to have finished floors at least 1’ above low points for any drainage area which includes the house.  With dams and similar impoundments, this should be measured from the top of dam. 

c.                   The Program Authority may allow other drainage structures (e.g. riprap ditches) where it has been determined this change will not significantly impact usable yards (e.g. cobblestone swale next to a driveway), where slopes are too steep for vegetated swales (e.g. steeper than 25% grades), or where the change would better mitigate impacts on adjoining properties (e.g. matches offsite drainage structure).     

 

2.                  Public drainage across lots must be in storm sewers except open drainageways may be allowed more than 50’ from any building site.   If storm sewer is used across lots, easement widths must be sufficient to allow excavation with 1:1 side slopes on the trench, sufficient room on one side of the trench to stockpile excavated materials, sufficient room on the opposite side of the trench to allow for movement of materials, and adequate room for a backhoe boom to swing.. Fences, walls, driveways, and other uses are not allowed within the easements, except where a “hold harmless” clause is included in the easement agreement.   

 

3.         Retaining walls higher than 4’ (top of face to ground on downhill side) must be designed by a professional engineer to assure long-term stability.   Retaining walls built using a VDOT standard or a pre-engineered product that includes certification are not required to provide a separate professional engineer’s certification provided the building contractor provides an affidavit the wall was constructed per the standard.  Retaining walls higher than 4’ in usable yards or places where the public might walk must include railing similar in design to what is required for elevated decks.  In circumstances where it is questionable whether a railing is required, the County Engineer will make the determination.  The builder must provide evidence of the ability to maintain any retaining wall which could not be maintained without the use of adjoining property.     

 

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