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Chapter 6Factors controlling geomorphic

response to large floods.Drs. Nanette C. Kingma

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Factors controlling geomorphic response to large floods.

Large floods; T≥ 50 yr;The reponse is controlled by:

Drainage basin factors (I)River channel factors (II)

(or called Network, Basin and Channel factors).

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I. Drainage basin factors.

Controls on the river that are external to the channel and floodplain ; including;

climatological factors (mainly rainfall regime);hydrology (mainly peak discharge, contributing area, and hydrograph reponse)basin morphometry;sediment load;vegetation;soils;bedrock.

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II. River channel factors.

Internal controls on the river resulting from physical characteristics of the channel and flow, including:

channel gradient;channel and floodplain geometry;channel morphology;bank cohesion.

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Ad 1.2. Hydrologic factors:

Contributing Drainage Area:extremely variable:arid/semi-arid often CDA minor portion of drainage basin; If small contributing area producing large flood

large amount of rainfall; high intensity; short duration flash flood.humid regions: widespread rainfall over entire drainage basin.

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Ad 1.2. Hydrologic factors:

Peak Discharge & Flash Flood Magnitude Index:

Q peak absolute not so important; more important ratio Q max peak / Q mean annual discharge the morphology of many rivers is adjusted to most commonly experienced flow (bankfull) for humid regions; in contrast with semi-arid regions adjustment often to the less frequent large floods;

Flash flood indexFlash flood index developed in the US.

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AD.I.2 Hydrologic factors:FFMI

High Flash Flood Magnitude Index (FFMI)indicates the likelihood of streams experiencing major geomorphic changes in large floods.

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Ad I.2 : Hydrologic factors:FFMI

Results of index “checked” with observations of large floods whether or not major geomorphic changes occurred.

Result: also other factors play a role: topography, vegetation / lithology / basin morphometry !!

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Example; basin study after hurricane Camille.

Downstream valley widening related to upstream debris avalanches;

high supply of coarse bed load (1)steep gradients (2)basin shape(3)

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Upstream - down stream effects.

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Ad I.3 Basin morphometry.

Basin shape influences hydrograph.(see chapter 2; equidimensional versus elongated

Ruggedness number: drainage density * relief;

Basin magnitude;high first order stream frequency;sparse vegetation ; thin soilsMost important factors in explaining variation in peak discharges; when similar climatic regions concerned.

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Drainage basin morphometry

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II. River channel factors

1.1 Channel gradient & bedload:channel bank and floodplain erosion is facilitated by high velocity flows carrying coarse bedload materials that can be used as abbrasive tools)

Lithology + Climate⇒control on sediment sizeLithology ⇒control on channel gradient; resistant rock often steeper gradients.

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Channel geometry

Plays important role in determining geomorphic effect of large floods.

Increased flood discharges in bedrock channel accomodated by dramatic depth increasse +veloclity increase ⇒ results in macro turbulent flow phenomena.Enormous capacity to transport large sediment amounts; e.g. large bouldersresults finally in significant erosion of channel wall & floor by macro turbulent flow!!

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Channel geometry

Often no effect due to high boundary resistance of bedrock channel walls.

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Check list

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Temporal ordering of geomorphic effects.

Responses to floods of equal magnitudeand frequency vary greatly depending on temporal ordering of events !!

1954 flood dramatic redistribution of channel gravel and erosion of fine grained terraces;

1974 flood only minor redistribution of fluvial sediments.

From: Flood Geomorphology

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Conclusions.

It’s difficlult to make prediction of expected channel response.

Basin-, network and Channel factors interact.Man’s influence excluded (influence on sediment load, discharge, channel gradient, channel geometry etc.)

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Major trends.

Major trend:i.e. important variables:

1. Climate: intensity - duration frequency intensity rainfall;

2. River hydrology: factors relating to flashiness of hydrograph;

3. Basin morphometry: Factors having an effect on the flashiness of hydrograph.

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Streams experiencing major geomorphic change.

Streams experiencing major geomorphic change ( and less dependent on climate) are characterised by:1. Flashy hydrograph; 2. High channel gradient;(relation to lithology ;

tectonics)3. Abundant coarse load;4. Relatively low bank cohesion5. Channel cross sections enabling deep, high

velocity flows where macro turbulence can be initiated and maintained.

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Important factors controlling channel and floodplain response to large magnitude floods.

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Geomorphic work.

Two different perceptions exist;1. Measured as the amount of transported

load;2. Modification of landforms also called

geomorphic effectiveness.

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Future studies.

Increased effort is needed to determine the true recurrence interval of large floods.Multivariate statistical techniques may be necessarry to understand the complex interactions of channel response to large floods.