Author

Mars P. Tan, The University of Notre Dame Australia

Sedimentation dynamics of the Cagayan de Oro river catchment and the implications for its coastal marine environments

Date of Award

2017

Degree Name

Doctor of Philosophy Science

Schools and Centres

Arts & Sciences

First Supervisor

Dr Rene N. Rollon

Second Supervisor

Dr Tolentino B. Moya

Third Supervisor

Dr Dylan Korczynskyj

Abstract

Declines in coastal environment condition can often be attributed to land-based activities in the uplands. This may be the case in some parts of Macajalar Bay, where river plume is observed almost daily. This present study aims to demonstrate the erosion-sedimentation process along the Cagayan de Oro River Catchment to its river mouth, and its implication for the marine coastal habitats. Highlighted in the study are the various natural factors that have influenced the erosion-sedimentation process: its volume; direction; and effects on the coastal habitats.

In the uplands, to account for the influence of catchment spatial heterogeneity and local rainfall on run-off rates and sediment yield, the Soil Water Assessment Tool (SWAT) model was employed. The model predicted high (15 > 50 t/ha/yr) to very high sediment yields (>50 t/ha/yr) in a few sub-catchments and slight to moderate yields (0 > 15 t/ha/yr) in most sub-catchments. However, during heavy and prolonged rainfall events, a number of sub-catchments became highly prone to erosion, due to existing large cultivated lands and very steep slopes. On normal rainfall days, the model predicted continuous transport downstream of slight to moderate amounts of sediments which could have implications for coastal marine environments within the river mouth vicinity.

In the bay, the Delft3D model was employed to investigate the direction and location of total suspended sediment distribution. The model predicted coastal current circulation and sediment dispersal patterns in the months of April to May and December to be predominantly east and southeast. Based on the simulation results, most of the flowing suspended sediments were trapped at the river mouth (average discharge: 30-50 mg/L; extreme discharge: 1200-1600 mg/L), while some were dispersed east of the opening. The amount of dispersed sediments in inshore waters varied according to the river discharge conditions: low to average discharge (~113.49 m3/s) amounted to minimally higher-than-normal total suspended solid (TSS) concentrations in ambient water (10-30 mg/L), while extremely high discharge (~1245,33 m3/s) resulted also in high-TSS concentrations (200-500 mg/L). Given that most sediment particles were predicted to be concentrated at the river mouth (e.g., with shallow depth water and mudflat presence), sedimentation may have influenced mangrove establishment and growth. Likewise, there may have been an association between river-sediment plume and the present ecological conditions of both corals and seagrasses.

To determine any relation between river sedimentation and marine coastal habitats, the existing distribution, composition and abundance of each coastal marine habitat were scrutinised using satellite images, historical maps, previous related studies and Chapter 3 results on river plume extents and concentrations. Analysis results revealed that river sedimentation reinforced by human intervention has contributed to land changes at the river mouth, either through accretion (~35.21 ha) or through erosion (~5.10 ha). Formation of new land forms has in turn contributed to mangrove colonisation, albeit slow, either through natural growth (~4.5 ha) or through human plantation (~2.0 ha). With regard to corals and seagrasses, their natural locations and distributions in Macajalar Bay have most likely been influenced by salinity and sediment concentration levels. As to their composition and abundance, massive corals dominate sites furthest from the river mouth but no clear association between seagrass abundance and river-borne sediment encroachment. At best, the results imply that sedimentation in the catchment does have implications for the distribution of the three major coastal habitats within the river’s vicinity.

Based on the major findings of the study, specific rehabilitation and management measures were recommended to address erosion-sedimentation issues in the uplands, the coastal areas and the coastal marine habitats while taking into account existing government plans and projects. Four key management principles, namely, integration, sustainability, precautionary and adaptive (Boesch, 2006) were used as basis for the integration of the recommended management measures.

Limitations of the study in each chapter were recognised. In the catchment, the model simulated sediment data showed poor agreement with the observed data, and the validation results were weak. Thus, longer data collection period is recommended for future monitoring and modelling studies. In the sediment transport near the river mouth, there was disparity between model and measured suspended sediment concentration data. It is recommended for future studies that several collections of samples be done following different stages of river flow to approximate the value of model simulated data. As regards the coastal marine habitats, the study results can be strengthened by long-term information on the distribution, abundance and diversity of coral reefs and seagrass meadows within the river mouth vicinity.

Publication Details

Tan, M. P. (2017). Sedimentation dynamics of the Cagayan de Oro river catchment and the implications for its coastal marine environments (Doctor of Philosophy Science). University of Notre Dame Australia. https://researchonline.nd.edu.au/theses/165