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James Cook University (2004)

Groundwater flow in arid tropical tidal wetlands and estuaries

Susilo, Adi

Titre : Groundwater flow in arid tropical tidal wetlands and estuaries

Auteur : Susilo, Adi

Université de soutenance : James Cook University

Grade : Doctor of Philosophy (PhD) 2004

Résumé
The flow of groundwater in mangrove forests is very important to the mangrove trees and organisms that live within the soil. Groundwater flow is responsible for preventing salinity of the water around the mangrove roots from reaching lethal levels. Further the flow of nutrients between the swamp and creeks is important to the nutrient budget of the swamps. Groundwater flow in mangrove swamps is complicated by the presence of abundant animal burrows. These have the effect of greatly increasing the hydraulic conductivity of the soil. However, using conventional methods, it is difficult to determine the bulk hydraulic conductivity of the soil when the effect of the burrows is taken into account. In this work, a simple method is described for determining the hydraulic conductivity of mangrove sediment, including the effect of macropores such as crab burrows. The method uses the existing animal burrows as piezometers. Experiments to measure the hydraulic conductivity of the sediment were carried out in a variety of mangrove forests. It was found that hydraulic conductivity varied from around 1 m/day to 10 m/day, which is at least 10 times greater than would be expected if there were no burrows. In order to check the validity of the method, conventional piezometers were used to determine the free water table level in an area of mangroves fringing a creek. From these measurements, hydraulic conductivity was determined independently and found to be consistent with the new methodology.
Tidal groundwater in a mangrove swamp can return to the mangrove creek by one of two mechanisms : (a) it can either flow through the swamp soil due to the water table difference between the creek and the groundwater in the swamp ; or (b) it can flow via tidal flushing of animal burrows. The second section of this thesis compares the magnitude of these two mechanisms for different regions of a mangrove swamp. Direct groundwater flow rates resulting from water stored in the sediment as a consequence of infiltration, especially during and after tidal inundation, were calculated for every square meter in the surface of a mangrove forest from piezometer data. Flow rates of water due to burrow flushing were determined based on published surveys, by estimating the burrow volume and the percentage of the burrow water that is flushed at each tidal inundation. Although direct groundwater flux was found to decrease further away from the creek compared to close to the creek, it was also found to have a similar range as burrow flushing flow. Specifically, direct groundwater flow ranged from 0.004 to 0.04 m³/(m² day), whilst burrow flushing flux ranged from 0.01 to 0.04 m³/(m² day). Considering the errors involved in the experiments and calculations, these ranges can be considered as being the same and none of the two processes can be considered as negligible compared to the other.
The final section of the thesis is the development of a simple analytical model that was used to calculate the groundwater flow to the creek. Data from piezometers were used in the model calculation. It was found that the model results agreed well with, experimentally determined calculations of groundwater fluxes. Fluxes ranged from 0.007 to 0.026 m³/m² day with the lowest fluxes being recorded at the end of the neap tide period. This model is easily adaptable to most mangrove geometries.

Mots Clés : groundwater flow, mangrove wetlands, tidal wetlands, dry tropics, mangrove wetlands hydrology

Présentation et version intégrale

Page publiée le 12 décembre 2011, mise à jour le 10 juillet 2017