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Published on January 4, 2008

Author: Olivia


River hydrology and sediment flux: implications for wetland geomorphology:  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 Floodplain wetlands:  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 Currently, the character and variability of the link between discharge and wetland geomorphology has not been adequately explored. How does discharge impact wetland geomorphology?:  How does discharge impact wetland geomorphology? Discharge = independent variable Sediment capacity & competence Slope Velocity Channel morphology Discharge can be used as a proxy for sediment loads Erosion, deposition or transport? the longitudinal profile:  the longitudinal profile Ideal profile Base level Sea Local Base Level LBL elevation distance A river is in equilibrium if its channel form and gradient are balanced such that there is no net erosion or deposition. Slide5:  Zone of deposition Slope reduced Slope steepened distance elevation Floodplain wetlands:  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 By investigating discharge regimes & using them as a proxy for sediment inputs, we can conceptualize wetland dynamics & evolution. Then calculate real discharge/sediment flux relationship. Mfolozi Floodplain Wetlands:  Mfolozi Floodplain Wetlands Aim & Methodology:  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 Negative values = discharge below the long term mean Positive values = discharge above the long term mean Long term periodicity:  Long term periodicity 9 – 12 year wet/dry cycles Comparative frequency of wet & dry years:  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 No incidences of discharge 100% lower than the LT mean 9 incidences of discharge 100% greater then the LT mean Overall, dryer than average years occur more frequently, but they are less extreme than flood years. long term discharge trends:  long term discharge trends mean annual rainfall at Hlobane:  mean annual rainfall at Hlobane Average increase of 0.29% / year Mfolozi Floodplain wetland:  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 Floodplain wetlands in climatically variable settings:  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 Other wetland types:  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 Other wetland types:  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 sediment flux:  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 What is the average sediment flux? Where does the sediment go? How much sediment is there? Methodology:  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 Slide19:  A B C D E Slide20:  D E C B A Sampling strategy:  Sampling strategy Plan view Side view bedload sediment transport:  bedload sediment transport Total transport on section D was 1.2 kg / s, This is the equivalent of 72 kg /min or 4.3 metric tonnes / hour bedload transport vs. discharge:  bedload transport vs. discharge At low stage, river was characterized by deep areas of slack water where little sediment was transported, and areas of shallow water (i.e. 40 cm) where velocities were frequently high. In these conditions, velocity adjacent to the bed is more important than the discharge for the entire water column. Velocity profile of Dx:  Velocity profile of Dx bedload transport vs. velocity:  bedload transport vs. velocity Statistically significant correlation at 95% confidence level Further analysis:  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 Assess the impact of varying discharge and bedload transport regime on water quality and floodplain and mouth geomorphology

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