2009-07-24_GENERAL DOCUMENTS - M2009076 (2)

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BMP: DRIVEWAY AND PARKING AREA STABILIZATION METHODS: As soon as driveways and parking areas are graded, pave them or cover them with gravel to prevent soil erosion. Spread a 4" layer of 1/2" to %" gravel over these areas. Size culverts to handle the peak flow during a heavy storm. Generally, the county engineer or public works department will size the culvert necessary for installation in roadside ditches along public roads. Where sizing requirements are not provided, as a rule-of-thumb, use a culvert with twice the cross-sectional area of the channel draining into it. The extra capacity is needed because of flow efficiency loss at the culvert's inlet. If you must construct a driveway and culvert across a roadside ditch, it is recommended to have a civil engineer design the culvert and its outlet protector. Erosion frequently occurs at culvert entrances. It is Good best to install a culvert with its inlet flush to the embankment. The edge of the pipe should be rounded or flared to improve flow into it. Place 441*r AY rocks or sandbags around the inlet to prevent 0 scour. As flow volumes and velocities increase, outlet protection is usually needed. An outlet protector, such as a rock apron, is a device for ru'rop ptmpw- absorbing the energy of water discharging from pipe or channel. Outlet protectors should be used: Below culverts. ? Below sediment trap outlets. ? Where a steep or paved channel drains into an unlined or natural drainage way. a Poor MAY 6W*V"v Alwa dfp?tND The following method should be used to construct? a rock apron outlet protector. (Note: The following description applies to a ditch or culvert discharging at the side of a drainage way.) The ditch or culvert should end at the top of the bank of this drainage way if you are constructing a driveway and culvert across a natural drainage way. A. Place a 3" layer of sand or gravel in a trapezoidal-shaped apron from the ditch or culvert to the middle of the drainage way. The apron width at a ditch should be as wide as the ditch. The apron width at a culvert should be 3 `= r times the culvert's diameter. The apron should flare out at a ratio of 1' sideways for each 2' of length until the apron is 5 times the width of the ditch or culvert. B. Place a layer of 4" to 6" rock on the top of the apron. The top of the finished apron should be at the same level as the inflowing ditch or culvert and should 13