1.3. Macajalar Bay and the Cagayan de Oro River catchment
1.3.6. The Study’s Overall Framework and an Overview of Each Chapter
This study focuses on the sedimentation process: the generation, downward transport, and concentration/deposition of terrigenous materials from sub-catchments down to the river channel; then at the river-coastal area where mangroves are found; and finally the underwater habitats of coral and seagrass communities (see Figure 1.4). The study sites include:
1) The Cagayan de Oro upper catchment where rainfall events were monitored.
2) The Cagayan de Oro River main channel where measurements of total river discharge and suspended sediment concentration (SSC) were conducted; and
3) The Cagayan de Oro River mouth and its vicinity where river-borne total suspended sediment concentration (TSS) data were collected, and where the distribution, composition and abundance of existing coral, seagrass and mangrove communities were assessed in relation to the presence and potential influence of sediments from the Cagayan de Oro River catchment.
Chapter 1 provides a general introduction of the thesis. It describes catchment and coastal connectivity as an environmental issue affecting human communities globally. In particular, it focuses on local issues in the Philippines, with the Cagayan de Oro River catchment area and its coastal marine environment within Macajalar Bay. The chapter introduces the ridge-river-reef approach as an appropriate framework for research and for applying measures to address coastal area sedimentation problems caused by land-based activities in the uplands. It also reviews the previous research on each major ridge-river-reef model component.
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Rainfall –runoff study using MLRA
Rainfall + catchment spatial heterogeneity – runoff study using SWAT model
Sub-catchments’ sediment yields River discharge & sedimentation level
Catchment discharge and suspended sediments
Simulated river discharge + tides + key bay forcing factors’ effect on river plume using Delft3D model
Coastal current circulation and river sediment distribution in the river mouth and its coastal marine environs
River plume outflow (discharge + sediments)
River-borne sediments dispersal in mangrove, coral, and seagrass sites
Survey of mangrove trees using Delft3D – modeling tool for water current flow Catchment erosion vulnerability assessment
Perceived threats and proposed management measures
Figure 1.4: A conceptual framework of the present study showing the flow and connectivity of the three main chapters through sedimentation dynamics. Each chapter contains the inputs, methods, expected results/outputs, analyses, outcomes and proposed management program. The overlapping sections connect the chapters.
Chapter 2 begins with the erosion process that occurs in the uplands. It focuses on the influence of the Cagayan de Oro River catchment’s spatial heterogeneity, management practices and local rainfall seasonality on soil loss and river discharge. The chapter first
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examines the rainfall variation in selected monitored sub-catchments and uses multiple linear regression analysis (MLRA) to correlate rainfall data with the corresponding river measurements taken during a ten-month sampling period. Following this, the chapter introduces SWAT as a modelling tool to examine the role of specific sub-catchment physical features (e.g., topography and soil), land management practices (e.g., land use and land cover) and rainfall factors affecting run-off and sediment flow. Statistical measurements such as the Nash and Sutcliffe efficiency (NSE) (Nash & Sutcliffe, 1970) and the PBIAS(%) models (Gupta et al.,1999) evaluate the model’s performance. Chapter 2 highlights the identified ‘erosion hotspots’ or priority sites in the catchment requiring urgent government and local community rehabilitation programs.
Chapter 3 describes the behaviour of upland-derived sediments driven by various forcing factors and conditions as they reach the river and coastal waters. The first phase of the study describes the TSS and salinity concentrations of both Macabalan seagrass and Bonbon coral sampling sites, based on measurements collected once a month for eight months. The study’s second phase employs the Delft3D model to simulate the extent of river sediment plume flow and the bay’s coastal current circulation patterns. It also identifies the key factors that influence the dispersal and fate of river sediments at the river mouth and offshore. Finally, it presents normal- and worst-case weather scenarios affecting river discharge and the sedimentation implications for seagrasses, corals and the adjacent human communities.
Chapter 4 examines the relationship between river-borne sediments and the condition of each of the three coastal habitats. Each habitat’s ecological profile is described in terms of its geographical distribution, composition and abundance. The chapter compares historical maps and satellite images of the river mouth showing mangrove cover to ascertain physical changes over time resulting from sedimentation. It also examines satellite plume images,
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simulated maps and observed TSS and salinity results to determine the extent of river plume encroachment on seagrass and coral communities. Ecological profiles of coastal habitats are assessed and then examined in relation to sedimentation using the related literature as a reference. Finally, the chapter presents future scenarios for both coral and seagrass communities, based on the threat of continuing sedimentation from the uplands.
Chapter 5 details the key management principles, as well as specific management and rehabilitation measures for the entire ridge-river-reef continuum, based on the study’s major findings. It identifies the key factors that influence ridge-river-reef sedimentation, while highlighting the connectivity of the erosion-to-transportation process with the concentration or deposition of terrigenous materials at the river mouth and offshore. The chapter borrows four key management principles (integration, sustainability, precautionary and adaptive) from Boesch (2006) as a basis for integrating the recommended management and rehabilitation measures. As on-going management plans and activities already exist, it is hoped that the actions recommended here will both reinforce and bridge certain gaps in the existing plans and strategies of local government and other groups.
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