Roughness Advisor

Karen Fisher10 June 2004

Conveyance Estimation System (CES) Launch

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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

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Discharge in m3/sec

Stag

e in

m

CG (Normal roughness) CG (Min roughness) CG (Max roughness)

Measured Data Lotter DCM Pavlovskii DCM

0.0

0.5

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0 5 10 15

Distance in (m)

Stag

e in

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Bank marker