MAXIMUM VELOCITIES FOR GRASSED WATERWAYS
Cover Slope Permissible Velocity Easily Eroded
Range 2/ 1/ Erosion-Resistant Soils
Soils
% (ft./sec) (ft./sec)
Kentucky bluegrass 0-5 7 5
Tall fescue 5-10 6 4
Smooth brome over 10 5 3
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Grass mixtures 2/0-5 5 4
Reed canary 5-10 6 3
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Redtop 3/
Red fescue 0-5 3.5 2.5
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1/ Use velocities exceeding five feet per second only where good cover and proper maintenance can be obtained.
2/ Do not use slopes steeper than ten percent except for vegetated side slopes in combination with a stone, concrete or highly resistant vegetative center section.
3/ Do not use on slopes steeper than five percent except for vegetated side slopes in combination with a stone, concrete or highly resistant vegetative center section.
DRAINAGE FIELD DITCHES
Drainage field ditches are shallow graded ditches with flat side slopes which do not interfere with tillage operations. Generally, the side slopes range from 8:1 to 15:1. The purpose of drainage field ditches is to collect water from depressional or nearly flat areas within a field and remove it to a stable outlet.
Generally, erosive velocities will not be a problem because of the low gradient of fields in which drainage field ditches are used and because of the shallow side slopes. Maximum velocities shall be limited to 2.5 feet/second unless on-site studies show that higher velocities will not result in erosive conditions.
MAXIMUM VELOCITIES FOR VEGETATED STREAM CHANNELS
Drainage Areas Less Than One Square Mile
The maximum permissible design velocity shall be based on site conditions and shall be such as to result in stability of the ditch bottoms and side slopes. Maximum permissible velocities will be computed using bank-full stage or ten-year frequency stage, whichever is lower. The following table will be used as maximum velocity for all drainage main or lateral designs. Vegetation will be established immediately after construction.
SUBSOIL TEXTURE: MAXIMUM VELOCITY 4/ (ft./sec.)
Sand and sandy loam (noncolloidal) 2.5
Silt loam (also high-lime clay) 3.0
Sandy clay loam 3.5
Clay loam 4.0
Stiff clay, fine gravel and graded loam to gravels 5.0
Graded silt to cobbles (colloidal) 5.5
Shale, hardpan, coarse gravel 6.0
Drainage Areas Greater Than One Square Mile
Channel velocities for newly constructed channels with drainage areas in excess of one square mile shall meet special stability requirements contained in U.S. Soil Conservation Service Technical Guide (Technical Release 25, Planning and Design of Open Channels).
4/ Channels that cannot be designed to meet the maximum velocity limitation must be stabilized with materials other than vegetation. Such materials include crushed rock, concrete, gabions, etc.
(Ord. 61-1987. Passed 9-14-87.)