Drainage Design Manual - the City of San Luis Obispo

Manning’s Roughness Values are provided in Table 5-2. Table 5-3 provides guidance onManning values to use for managed natural areas along stream courses with variousdensities of vegetation. Roughness values corresponding to a mature creek system shallbe used for design. Table 5-3 can also be used to select plant spacings for restoration andenhancement planting, and for thinning vegetation for increased flood conveyance.Values of the Roughness Coefficient “n”Type of Channel & DescriptionMannings“n”(Normal)A. Constructed Channelsa. Earth, straight and uniform1. Clean, recently completed 0.0182. Gravel, uniform section, clean 0.0253. With short grass, few weeds 0.027b. Earth, winding and sluggish1. No vegetation 0.0252. Grass, some weeds 0.0303. Dense weeds or aquatic plants in deep channels 0.0354. Earth bottom and rubble sides 0.0305. Stony bottom and weedy banks 0.0356. Cobble bottom and clean sides 0.040c. Rock Lined1. Smooth and uniform 0.0352. Jagged and irregular 0.040d. Channels not maintained, weeds and brush uncut1. Dense weeds, high as flow depth 0.0802. Clean bottom, brush on sides 0.0503. Same, highest stage of flow 0.0704. Dense brush, high stage 0.100B. Natural StreamsB-1 Minor streams (top width at flood stage