The University of Notre Dame Australia ResearchOnline@ND Theses 2017 Sedimentation dynamics of the Cagayan de Oro river catchment and the implications for its coastal marine environments Mars P. Tan The University of Notre Dame Australia Follow this and additional works at: https://researchonline.nd.edu.au/theses Part of the Environmental Sciences Commons COMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING The am terial in this communication may be subject to copyright under the Act. Any further copying or communication of this material by you may be the subject of copyright protection under the Act. Do not remove this notice. 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 This dissertation/thesis is brought to you by ResearchOnline@ND. It has been accepted for inclusion in Theses by an authorized administrator of ResearchOnline@ND. For more information, please contact [email protected]. Chapter 3: Sediment plume behaviour and coastal current circulation patterns in the coastal marine environments of the Cagayan de Oro River catchment 3.1. Introduction 3.1.1. Catchment—Coastal Connectivity and the Key Factors Affecting its Sedimentation Dynamics Upland erosion and the subsequent sedimentation are not limited to the catchment areas and affect more than the freshwater ecosystem and habitats. A continuum from the ridges to the lowlands also naturally transports water, sediments and other eroded upland materials downstream through the catchment’s river systems and finally into the bay or ocean (McKergow et al., 2005; Saxton et al., 2012). Within the river channel, as flowing sediments approach the bay zone, various factors and conditions (e.g., sediment size and weight, river flow velocity channel morphology, coastal bathymetry) act on it and influence the plume’s structure and trajectory (Ashworth & Ferguson, 1986; Milliman & Syvitski, 1992). As the plume leaves the river mouth and navigates offshore, various bay forces (e.g.. waves, tides, wind, Coriolis force) continue to affect the plume’s (and therefore the sediment’s) movement and direction until the deposition phase (Geyer et al., 2000; Dong et al., 2004). 3.1.2. The Cagayan de Oro River Catchment and Macajalar Bay The present study identifies Macajalar Bay as the place of confluence for river discharge and other upland materials from the Cagayan de Oro River catchment via the Cagayan de Oro River (see Figure 3.1). Two other major river systems, the Iponan and the Tagoloan, drain into the same bay. The Cagayan de Oro catchment was chosen over these specifically, due to its increased vulnerability to degradation. This degradation results from a) the presence of large-scale land-based activities in the uplands (e.g., crop plantations, agricultural farm expansion) (Ecosystem Alliance, 2015); b) a rapid increase in urban population and infrastructural development (Philippine Statistics Authority, 2013); and c) the proliferation of manufacturing plants along Macajalar’s coastal areas (Quizon, 2005). 89 N Figure 3.1: The Cagayan de Oro River catchment and Macajalar Bay connected by the Cagayan de Oro River that transports sediments and other materials to coastal areas and offshore (base map from Google, 2015). A close connection between the Cagayan de Oro River catchment and Macajalar Bay, in particular, the river plume and the coastal marine environs, was suspected due to actual observations and previous studies of coastal habitats in the bay. Increased river sediment plumes were observed during strong rain events in the uplands. Siltation at the river mouth was also observed to worsen after heavy rains and typhoons. Further, previous ecological surveys (Quiaoit et al. 2010) have reported the threatened status of marine ecosystems and fishery resources in the bay (to be discussed in the next sub-section). In the present study, the river sediment source is located proximate to existing seagrass and coral habitats in the bay. 90 However, no scientific study has yet been undertaken to demonstrate a relationship between the present conditions of marine resources in the bay and river-borne sediments. Two main sets of information are needed in this study: a) the potential location(s) of sediment deposition in the bay, and b) the weather- and bay-forcing factors that determine the SSC level at the affected site. Demonstrating sedimentation-coastal habitat connectivity is challenging. In fact, the extent and direction of river plume trajectory could continually vary, depending on the influences of various factors and conditions, such as the river run-off, wind force and direction, tidal action, coastal bathymetry, re-circulating currents and boundary forces (Walker et al., 2005; Warrick et al., 2007; Hu et al., 2009). Due to the number of factors and conditions that could potentially influence the plume and the complexity of interactions over time, a modelling tool is needed to quantify the effect of each and all factors on sediment transport and persistence in the bay. The resulting interplay of these factors and conditions within the bay has had a direct bearing on the affected site and the marine resources within it. River sediment plume threatens coastal marine habitats such as seagrass and corals, as determined by the prevailing weather conditions and the site’s bay-forcing factors (Storlazzi et al., 2004; Carballo, 2006; Devlin & Schaffelke, 2009;). 3.1.3. The Study’s Objectives This present study aims to predict the extent of area encroached upon by river sediment plumes on the sampling sites of seagrass and corals, as influenced by key forcing factors during different events. The selection of potential influential factors was based on the datasets required by the Delft3D model and on Hurlburt et al.’s (2011) studies. Using the Delft3D model, this study had four specific research objectives: 1) To demonstrate the general coastal current circulation flow in the bay near the river mouth. 91 2) To determine the suspended sediment dispersal behaviour under three different river discharge conditions. 3) To identify the bay-forcing factors that determine river sediment dispersal pattern. 4) To identify the potential areas of highest sediment concentration and deposition within the coastal marine environs. The Delft3D, as a modelling tool, simulated actual coastal current circulation near the river mouth and predicted the fate of river-suspended sediment within the river-coastal continuum, as influenced by the interplay of various bay-forcing factors and conditions (Flemming, 1981; Schoellhamer, 1996). Both coral and seagrass habitats are important biological indicators of sediment presence, both suspended and settled (Rogers, 1990; Neil et al., 2002; Erftemeijer & Lewis, 2006). Sedimentation effects on these habitats could be seen in increased water turbidity reducing sunlight penetration to the bay floor, or by the physical burial and smothering of organisms. Knowledge of sediment plume movement patterns in the bay is vital for drafting appropriate intervention measures for effective integrated coastal and catchment management programs (Xue et al., 2004; Bunn etal., 2007;). 3.1.4. The Study’s Significance Continuous sedimentation in the bay has had adverse effects on its three coastal habitats and fish populations within the vicinity (Newcombe & MacDonald, 1991; Wilber & Clarke, 2001). Previous studies of Macajalar Bay have reported the declining conditions of ecological and fishery resources in certain parts of the bay. Atrigenio et al. (1998) and Quiaoit et al. (2008) have reported a decline of fishery and marine resources in Macajalar Bay. Live coral cover also declined from good at 59% (Atrigenio et al., 1998) to fair at 38% (Quiaoit et al., 2008) between these two studies. Seagrass cover has increased in some areas and declined in other parts of the bay (Atrigenio et al., 1998 and Quiaoit et al., 2008). 92 Suspected sedimentation draining from the Cagayan de Oro River has most likely influenced reef and other marine habitat deterioration in certain parts of the bay. What is not clear is the extent in distance and plume concentration by which sediments have affected the marine and fishery resources. Previous studies of the bay’s resources were limited to assessments of the bay’s ecological and fishery resources. The present study was confined to coastal habitat sites near the Cagayan de Oro River mouth. However, the study’s results and findings can assist in understanding the dispersal patterns of sediments in other parts of the bay, affording a larger picture of plume influence here. Overall, knowledge of river plume characteristics, the extent of coverage, and its relation to coastal marine habitat conditions are useful for effective coastal and bay management planning. It is hoped that the study will also increase awareness among government authorities and local communities (both uplands and lowlands) regarding the vulnerability of marine environments to catchment impacts and exacerbating factors. 3.1.5. The Study’s Scope and Limitations Given the limited time and resources, this study has focused on and limited its scope to the following research concepts and related methods: 1) The study focuses on the coastal surface current circulation net effect on the distribution of river-borne suspended sediments in the coastal waters, as determined by the bay- forcing factors. No analysis was presented of other water movements, such as deep currents and vertical mixing. 2) Sediment plumes from the Iponan River were not considered in the study. The next chapter focuses on the reefs near the Cagayan de Oro River mouth, and the Iponan River is located further west of the Cagayan River mouth. 93 3) Salinity and TSS sampling sites in the east and the west were contained within the imaginary river plume-covered areas, which were as close as possible to the locations of coral reefs and seagrass meadows. 4) Bay-forcing factors and other variables were measured within a 10-month period between November 2012 and June 2013.