Table 2. Manning's n values recommended by HEC-RAS.   Type of channel and description Minimum Normal Maximum A. Natural streams 1. Main channels a. Clean, straight, full stage, no rifts or deep pools 0.025 0.030 0.033 b. Same as above, but more stones and weeds 0.030 0.035 0.040 c. Clean, winding, some pools and shoals 0.033 0.040 0.045 d. Same as above, but some weeds and stones 0.035 0.045 0.050 e. Same as above, lower stages, more ineffective slopes     and sections 0.040 0.048 0.055 f. Same as "d" but more stones 0.045 0.050 0.060 g. Sluggish reaches, weedy, deep pools 0.050 0.070 0.080 h. Very weedy reaches, deep pools, or floodways with     heavy stands of timber and brush 0.070 0.100 0.150 2. Flood Plains a. Pasture no brush   1. Short grass 0.025 0.030 0.035 2. High grass 0.030 0.035 0.050 b. Cultivated areas   1. No crop 0.020 0.030 0.040 2. Mature row crops 0.025 0.035 0.045 3. Mature field crops 0.030 0.040 0.050 c. Brush   1. Scattered brush, heavy weeds 0.035 0.050 0.070 2. Light brush and trees, in winter 0.035 0.050 0.060 3. Light brush and trees, in summer 0.040 0.060 0.080 4. Medium dense brush, in winter 0.045 0.070 0.110 5. Medium dense brush, in summer 0.070 0.100 0.160 d. Trees   1. Cleared land with tree stumps, no sprouts 0.030 0.040 0.050 2. Same as above but heavy sprouts 0.050 0.060 0.080 3. Heavy stand of timber, few down trees, little     undergrowth, flood stage below branches 0.080 0.100 0.120 4. Same as above but with flood stage reaching     branches 0.100 0.120 0.160 5. Dense willows, summer straight 0.110 0.150 0.200 3. Mountain streams, no vegetation in channel, banks usually     steep, with trees and brush on bank submerged a. Bottom: gravels, cobbles, and few boulders 0.030 0.040 0.050 b. Bottom: cobbles with large boulders 0.040 0.050 0.070   Type of channel and description Minimum Normal Maximum B. Lined or built-up channels 1. Concrete a. Trowel finish 0.011 0.013 0.015 b. Float Finish 0.013 0.015 0.016 c. Finished with gravel bottom 0.015 0.017 0.020 d. Unfinished 0.014 0.017 0.020 e. Granite, good section 0.016 0.019 0.023 f. Gunite, wavy section 0.018 0.022 0.025 g. On good excavated rock 0.017 0.020   h. On irregular excavated rock 0.022 0.027   2. Concrete bottom float finished with sides of: a. Dressed stone in mortar 0.015 0.017 0.020 b. Random stone in morter 0.017 0.020 0.024 c. Cement rubble masonry, plastered 0.016 0.020 0.024 d. Cement rubble masonry 0.020 0.025 0.030 e. Dry rubble on riprap 0.020 0.030 0.035 3. Gravel bottom with sides of: a. Formed concrete 0.017 0.020 0.025 b. Random stone in mortar 0.020 0.023 0.026 c. Dry rubble rip rap 0.023 0.029 0.036 4. Brick a. Glazed 0.011 0.013 0.015 b. In cement mortar 0.012 0.015 0.018 5. Smooth steel surface a. Smooth steel surface 0.011 0.012 0.014 b. Corrugated metal 0.021 0.025 0.030 6. Asphalt a. Smooth 0.013 0.013   b. Rough 0.016 0.016   7. Vegetal lining 0.030   0.500   Type of channel and description Minimum Normal Maximum C. Excavated or dredged channels 1. Earth, straight and uniform a. Clean recently completed 0.016 0.018 0.020 b. Clean after weathering 0.018 0.022 0.025 c. Gravel, uniform section, clean 0.022 0.025 0.030 d. With short grass, few weeds 0.022 0.027 0.033 2. Earth, winding and sluggish a. No vegetation 0.023 0.025 0.030 b. Grass, some weeds 0.025 0.030 0.033 c. Dense weeds or aquatic plants in deep channels 0.030 0.035 0.040 d. Earth bottom and rubble side 0.028 0.030 0.035 e. Stony bottom and weedy banks 0.025 0.035 0.040 f. Cobble bottom and clean sides 0.030 0.040 0.050 3. Dragline-excavated or dredged a. No excavation 0.025 0.028 0.033 b. Light brush on banks 0.035 0.050 0.060 4. Rock cuts a. Smooth and uniform 0.025 0.035 0.040 b. Jagged and irregular 0.035 0.040 0.050 5. Channel not maintained, weeds and brush a. Clean bottom, brush on sides 0.040 0.050 0.080 b. Same as above, highest stage of flow 0.045 0.070 0.110 c. Dense weeds, high as flow depth 0.050 0.080 0.120 d. Dense brush, high stage 0.080 0.100 0.140