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Erosion and Sediment Pollution Control Program Manual.pdf

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TABLE 3.4 - Recommended Maximum Spacing of Ditch Relief Culverts (18” dia. CMP)<br />

For Permanent Access Roads<br />

Road Grade<br />

Percent<br />

Gravels,<br />

S<strong>and</strong>y<br />

Gravels,<br />

Aggregate<br />

Surfacing<br />

Silty Gravels,<br />

Clayey<br />

Gravels<br />

Soil Type in Ditch<br />

Plastic <strong>and</strong><br />

Nonplastic<br />

Inorganic<br />

Clays<br />

363-2134-008 / March 31, 2012 / Page 32<br />

Inorganic<br />

Silts, Silty or<br />

Clayey S<strong>and</strong>s<br />

S<strong>and</strong>s, Silty<br />

S<strong>and</strong>s, <strong>and</strong><br />

Gravelly<br />

S<strong>and</strong>s<br />

Culvert Spacing Feet*<br />

2 390 315 245 170 95<br />

4 335 275 210 145 85<br />

6 285 230 180 125 75<br />

8 240 195 150 105 65<br />

10 200 160 125 90 55<br />

12 160 130 105 75 45<br />

14 135 110 85 60 35<br />

Adapted from USDA Forest Service<br />

*Culvert spacing may be adjusted slightly to take advantage of natural drainage courses.<br />

USDA Forest Service<br />

STANDARD CONSTRUCTION DETAIL #3-10<br />

Ditch Relief Culvert<br />

Minimum diameter for any culvert is 12”; otherwise culvert shall be sized for anticipated peak<br />

flow. Place culvert so bottom is at same level as bottom of ditch or adjoining slope. Culverts<br />

shall be placed with a slope of 2 to 4%. Lower end shall be at least 2” below upper end.<br />

Extend culvert 12” beyond base of road fill on both sides. Firmly pack fill around culvert,<br />

especially the bottom half.<br />

Provide suitable outlet protection* <strong>and</strong>, where appropriate, inlet protection.<br />

Inspect culvert weekly: remove any flow obstructions <strong>and</strong> make necessary repairs immediately.<br />

NOTE: This detail may be used for ditch relief culverts <strong>and</strong> for crossings of roadside ditches. It<br />

is not appropriate for stream crossings.<br />

* For steep slope ( > 2H:1V) outfalls, a minimum 20 foot long R- 5 apron is recommended for<br />

temporary access roads where the recommended culvert spacing is used. For permanent<br />

access roads, a minimum R-6 rock size is recommended.

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