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Floodplain wetlands. Associated with the drainage networkVulnerable to changes in river hydrologyAvailability
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1. River hydrology and sediment flux: implications for wetland geomorphology Suzanne Garden, Fred Ellery & Michael Grenfell
School of Environmental Science
University of KwaZulu-Natal
Howard College Campus
2. Floodplain wetlands Associated with the drainage network
Vulnerable to changes in river hydrology
Availability & frequency of surface water and sediment is linked to wetland origin, health and persistence
3. How does discharge impact wetland geomorphology?
4. the longitudinal profile
6. Floodplain wetlands Wetlands are generally located in zones of deposition
Has 2 geomorphic impacts:
Slope steepening & slope reduction
Results in wetlands being rather precariously located
Steepened gradients on lower end of wetland are vulnerable to changes in discharge & sediment regimes
7. Mfolozi Floodplain Wetlands
8. Aim & Methodology Aim:
Assess long term variability and trends in discharge data and associated impacts on wetland geomorphology
Calculated mean annual flow values
For each year on record, difference from the mean was calculated as a % deviation from the mean
9. Long term periodicity
10. Comparative frequency of wet & dry years 63% of years dryer than the LT mean
Wet years are wetter than dry years are dry: much larger extremes in flood years
11. long term discharge trends
12. mean annual rainfall at Hlobane
13. Mfolozi Floodplain wetland Generally experiences low sediment inputs, punctuated by extreme, short lived flood events that deposit large amounts of sediment
E.g. Cyclone Demoina 1984 – 4m
Conforms to idea of punctuated equilibrium
Wetland undergoes the greatest amount of change during extreme events
Dynamics may be static during dry years, but rapid sediment accumulation during flood events may alter the system
Cooper’s (1994) river dominated estuaries
Experience scour during flood events, opposite to floodplains
However:
Floodplains and estuaries both characterized by long static periods and extreme flood events
May be linked to climatic variability
14. Floodplain wetlands in climatically variable settings Erosion during dry years is prevented although the system may be sediment starved
Low discharges lack erosional power
Climatic variability lengthens the longevity of some systems
If discharges tended towards average and high flows, sediment deposition would be more rapid
Result in rapid oversteepening and eventually erosion of the system
15. Other wetland types Other hydrogeomorphic types reaction to climatic variability is likely to differ
Largely because of varied importance of surface and groundwater inputs
i.e. groundwater fed wetlands more likely to erode during wetter periods as surface flow increases
Some wetlands may be characterized by periods of erosion
Intermittent periods of progradation and degradation may be a function of climatic control in some instances
16. Other wetland types
Climatic variability may extend the longevity and persistence of some wetlands
Variable deposition (and possibly erosion) maintains gradient at a critical slope threshold
Long term trends of warming and increased rainfall for wetlands in northern KwaZulu-Natal
Will probably increase the rate of sediment deposition, resulting in more rapid oversteepening and eventually erosion in long term
17. sediment flux
How true is the relationship between discharge and sediment flux and can it be rationalized?
Management of the estuary mouth and St. Lucia
Understanding of the sediment flux associated with average flood events is critical
18. Methodology 5 Locations selected on straight reaches of the river in the floodplain
Suspended sediment was sampled with a water trap sampler, current velocity was measured with a SEBA current meter and bedload was sampled with a Helley Smith bedload sampler
21. Sampling strategy
22. bedload sediment transport
23. bedload transport vs. discharge
24. Velocity profile of Dx
25. bedload transport vs. velocity
26. Further analysis Calculate bed- and suspended load transport for each cross-section
Develop an equation linking velocity or discharge and sediment transport
Link to long term discharge records
Compare sediment transport between cross-sections – areas of erosion or deposition