Post on 16-Sep-2020
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Roughness Advisor
Karen Fisher10 June 2004
Conveyance Estimation System (CES) Launch
ENVIRONMENTAGENCY Conveyance Estimation System
How is roughness quantified?
• Historically described with Manning’s n– Barnes (1967), Chow (1959), Cowan (1956), Hicks &
Mason (1993/8)
• The Manning n-value: – is a resistance parameter – “all-encompassing”
• Roughness Advisor has adopted a unit roughness value:
– to inspire confidence / familiarity– User Consultative Group
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What is a unit roughness?
• The unit roughness nl is the roughness due to an identifiable segment of boundary friction
[dimensions same as traditional n]
• The total unit roughness is composed of up to three component roughness values:
– ground material– vegetation and / or – irregularities
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• It highlights the largest component contribution
How is unit roughness calculated?
- assume loss mechanisms are mutually independent
- equivalent to summing velocity head energy losses, hence n is squared before summation
- more complex methods lack sufficient data
Why?222irregvegsubl nnnn ++=
• nl is calculated from root sum of the squares of the components:
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Δy
pebbles grass
222nonegrasspeb irrvegsubl nnnn ++= 036.003.002.0 22 =+=
grasspebln
Example calculation • Consider a section with both pebbles and grass
at a particular point in the cross-section:
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What is a roughness zone?
• A roughness zone is a plan area of a region characterised by similar unit roughness values
F1 F4C11
C1F1 F42
C2F3 F44
C1F3 F2 F43
F1F2
F3
C1C2
1 3 42 F4
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Each zone has a type• A roughness zone type is a physical portion of the
channel cross-section i.e.: – bed – bank– floodplain
• Roughness zone types are used in the software to encourage user awareness of the expected roughness coverage (they have no influence on the calculation)
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What is the Roughness Advisor?• A tool within the CES software
• Assists the user in defining roughness zones for the conveyance calculation
• Includes an extensive database of both roughness values and photographs for the different roughness components (based on over 700 references)
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Natural: 12 Morhpotypes
Human intervention: Grass, trees, hedges, shrubs, crops: wheat, sorghum, sunflower
• Vegetation
Natural: Bedrock, cobbles, gravel, sand, silt, clay, peat, earth
Man-made: sheet piling, wood piling, hazel hurdles, gabion, rip-rap, concrete
• Substrate
Natural: Pools, riffles, tree stumps, debris deposits, floodplain undulations, boulders
Human intervention: groynes, obstructions, trash, ridges on ploughed fields
• Irregularity
Typical component roughness
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Min Mean Max1 None 0.000 0.000 0.0002 Free-floating plants typically algae or duckweeds 0.010 0.030 0.0403 Filamentous algae attached shallow nutrient rich waters 0.000 0.015 0.0504 Mosses attached to bed or banks 0.000 0.0305 Trailing bank-side plants 0.000 0.050 0.1006 Emergent reeds, rushes, flag and large grasses 0.020 0.150 0.2007 Floating-leaved typically water lillies in deeper slower
waters0.030 0.100 0.140
8 Emergent broad-leaved rooted plants, water cress or water parsnip
0.050 0.150 0.50+
9 Submerged broad-leaved, pondweeds 0.020 0.100 0.20010 Submerged fine-leaved, shallow rivers, chalk streams,
water crowfoot0.020 0.300 0.45+
11 Submerged fine-leaved, medium depth rivers, regular management
0.021 0.100 0.249
12 Submerged fine-leaved, medium to deep rivers, some management
0.010 0.080 0.120
Unit roughnessAquatic Vegetation : 12 MorphotypesUpper Exp. Lower
Submerged vsemergent
RA Vegetation morphotypes
Broad vs fine leaved
Rooted vs free-floating e.g.
algae
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How does nl compare to the traditional Manning n?• Smaller
• Improvement on Chow (1959). Includes interdisciplinary advice & knowledge on river roughness
• Component roughness values easy to understand & interpret
• Depth variation
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Can I enter my own roughness?
• Yes - manual over-writing is allowed
• Values should be equivalent unit roughness values, not “all-encompassing” Manning’s n
• The RA suggested roughness values are a guide in the absence of calibration data. As with all 1D models / backwater calculations, these values can and should be altered in the calibration process.
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• It enables the user to implement cutting regimes [e.g. 75% in July] and illustrates the expected natural regrowth.
• It provides advice on the expected vegetation morphotypes in the absence of other data.
• It includes information on expected seasonal growth / cover for the vegetation and the corresponding unit roughness values.
Roughness Advisor features...
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RHS advice
Accessing RHS advice
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Example: River Heathcote
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Discharge in m3/sec
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CG (Normal roughness) CG (Min roughness) CG (Max roughness)
Measured Data Lotter DCM Pavlovskii DCM
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Distance in (m)
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Bank marker