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The effect of valley constrictions and tributary junctions on flood inundation and sediment delivery in the Fitzroy River basin; the Medway Creek example Chris Thompson, Jacky Croke & David Purvis-Smith Outline Introduction

SLIDE 1

The effect of valley constrictions and tributary junctions on flood inundation and sediment delivery in the Fitzroy River basin; the Medway Creek example

Chris Thompson, Jacky Croke & David Purvis-Smith

SLIDE 2

Outline

Introduction – Constrictions & Tributary junctions
Study area – Medway Catchment
Methods & Analysis

– Hydrology & flood scenarios – Radionuclide Cs-137 analysis

Calibration

– October 2005 event – January 2008 event

SLIDE 3

Valley Constrictions

SLIDE 4

Tributary junctions

Are the basic building blocks of river networks,
Their potential to affect channel morphology depends on

relative size,

Sites of discontinuity in longitudinal sediment grain sizing,
Sites of discontinuity in smooth longitudinal river profiles,
Potential sites for increased floodplain sedimentation

SLIDE 5

Medway Catchment

Study area

SLIDE 6

Medway Geology

Drummond Land type:

– Dissected ranges of Permian (250-300 Ma) Colinlea Sandstone

Soils:

– Dispersible sodic soils

SLIDE 7

Lower Medway Creek

SLIDE 8

SLIDE 9

Floodplain surveys

SLIDE 10

Hydrology

Analysis of flow &

rainfall records (1972-85) indicate 3 flood scenarios:

1. Nogoa dominant, 2. Medway dominant, and 3. Both on

1980 1977

SLIDE 11

Nogoa dominant Medway dominant Both on

Modelled scenarios

For 5 year floods

SLIDE 12

Core 9 Vcore 3 Core 11

Medway Floodplain Core Cs-137

Vcore 6 Core 2 Core 13

SLIDE 13

Oct/Nov 2005 Flood – Scenario 1

SLIDE 14

Nogoa Catchment Total Rainfall 16-20 Jan 2008

Source: DPI&E, Emerald

SLIDE 15

SLIDE 16

SLIDE 17

Flood Jan 2008

6.5m deep 7.4m deep 9.8m deep

20 Jan Landsat TM 21 Jan MODIS

Source: ACRES, Geoscience Aust

24 Jan MODIS

Peaked at Rutland homestead on 17/18th

SLIDE 18

Sedimentation

SLIDE 19

Conclusion

3 common flood scenarios at tributary junctions in the region of high

spatial and temporal rainfall variability each with different sedimentation processes at the tributary junction,

‘Nogoa only’ floods ingress well up into the tributary with expansive

deposition zone

Nogoa-Medway floods disconnect Medway sediments and prevent

Nogoa sediments from ingressing into the tributary,

Role of constriction still being investigated with its influence probably

taking effect further upstream due to progradation,

Jan 2008 flood was the highest on record at Rutland, however waters

did not stay on the floodplain as long as previous big floods.

SLIDE 20

Acknowledgements

Marine and Tropical Research Facility

(Project 3.7.2) and

Australian Research Council (DP0449886)