River hydrology and sediment flux: implications for wetland geomorphology

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