Pagkalinawan, Homer M.

Salmo III, Severino G.

English

Ateneo de Manila University: Journals Online

State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports 77Simulation of Impacts of Sea Level Rise on Mangrove Survival in Central and Eastern VisayasHomer M. Pagkalinawan1 and Severino G. Salmo III21COBENA Business Analytics and Strategy, Inc. 2Department of Environmental Science, Ateneo de Manila UniversityI. IntroductionSea level rise (SLR) is the change in sea level due to climate change. It is affected by the expansion of oceans due to warming, and the transfer of water stored from land to water, e.g., glacier and ice sheets (Church et al. 2013). The SLR is perceived to be the most serious threat on the long-term survival of mangroves (McLeod & Salm 2006). David et al. (2015) classified 11 geographical clusters of Philippine waters based on climate hazard exposures such as SLR, extreme heating, extreme rainfall events, disturbed water budget, and increasing ocean temperature (Fig. 1). Among the hazards, only SLR was prevalent in all clusters.Although mangroves can adapt to rising sea level (Gilman et al. 2007), areas that will be affected will vary depending on several bio-physical and geomorphological factors. In this study, the effects of SLR on the distribution of mangrove extent in the Central and Eastern Visayas regions were determined using spatial modeling.II. Data and MethodologyMangrove cover in Central and Eastern VisayasThe Central and Eastern Visayas regions are composed of the islands and provinces of Negros Oriental, Cebu, Bohol, Siquijor, Biliran, Leyte, Southern Leyte, Samar, Northern Samar, and Eastern Samar (Fig. 2). These regions are home to an estimated 31,500 ha of mangroves (as of 2015; Phil-LiDAR 2 CoastMap 2016). Historically, the total mangrove extent coverage is at 47,100 ha in 2000 and 41,100 ha in 2010 (Table 1; Long & Giri 2011; Long et al. 2014). Among these provinces, Bohol has the largest mangrove cover with 8,200 ha, followed by Northern Samar and Eastern Samar with 6,400 and 5,900 ha, respectively.State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports78Figure 1. Climate hazard clusters of Philippine waters (David et al. 2015).Figure 2. Mangrove extent per province, Central and Eastern Visayas outlined in red (Phil-LiDAR 2 CoastMap 2016).Table 1. Mangrove area estimates, in ha (Phil-LiDAR 2 CoastMap 2016). There is no available data from Siquijor probably because of the low resolution of satellite images or mangroves are thinly/sporadically distributed.Province 2000 2010 2015Biliran 100 100 -Bohol 10,600 8,700 8,200Cebu 2,900 2,400 2,200Eastern Samar 6,400 6,900 5,900Leyte 7,000 6,600 4,000Negros Oriental 2,300 1,700 1,700Northern Samar 5,200 3,800 6,400Samar 11,800 10,500 2,900Siquijor - - -Southern Leyte 800 400 200Cluster I - extreme heating events, sea level riseCluster II - extreme heating events, extreme rainfall events, disturbed water budget, sea level riseCluster III - disturbed water budget, sea level riseCluster IV - extreme heating events, sea level riseCluster V - sea level riseCluster VI - sea level riseCluster VII - sea level rise, increasing ocean temperatureCluster VIII - extreme heating events, increasing ocean temperature, sea level riseCluster IX - extreme heating events, sea level riseCluster X - extreme heating events, increasing ocean temperature, disturbed water budget, sea level riseCluster XI - sea level riseState of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports 79From 2010 to 2015, the largest decrease in mangrove cover was observed in Samar (69 %) and Leyte (36 %). This period includes the occurrence of Super Typhoon Yolanda (international name: Haiyan; 2013) affecting the area and could have been a possible cause of mangrove losses (Fig. 3). However, during the mangrove summit, it was evident that these mangrove area estimates were not consistent with provincial datasets and therefore need to be validated.Sea level rise measurementsIn a global scale, sea level is monitored through satellite altimetry—a height measuring technique using radar pulses. Satellites such as Topex/Poseidon, Jason-1, Jason-2, and Jason-3 have observed sea level since the 1990s. According to the Centre National d’études Spatiales (CNES 2017), the global rate of SLR is at 3.31 mm/year (Fig. 4). The data resolution is one degree or approximately 110 km.On the other hand, the regional rate for North Pacific, where the Philippines is located, is at 2.89 mm/year (Fig. 5). These rates have high inter- and intra-site spatial variability and is expected to accelerate in the succeeding years (Nerem et al. 2017).Similarly, in-situ measurements on surface elevation change were made through a USAID-PEER project in selected mangrove sites across the country (Table 2). However, due to differences in scale and resolution of satellite altimetry data, the differences in values between the two datasets are large.Figure 3. Historical mangrove extent coverage in Central and Eastern Visayas. Highlighted in red are decreased coverage in the provinces of Samar and Leyte (adapted and modified from Phil LiDAR 2 and Long & Giri 2011).Table 2. Difference in SLR between in-situ and satellite altimetry values (in mm) (USAID-PEER Project Data; unpublished).Site In-situ (Average) Altimetry DifferencePangasinan 32.6 3.8 28.8 Masinloc 1.1 3.7 (2.7)Subic 22.4 3.5 18.9 Palawan (7.8) 3.8 (11.6)Kalibo 18.1 3.5 14.6 Bantayan (4.6) (0.1) (4.5)Palompon 17.3 (0.3) 17.6 Manila Bay (LPPCHEA)(5.0) (4.2) 0.8 Puerto Galera (5.1) 2.4 (7.5)Calapan (4.2) (1.3) (2.9)Samar (55.6) 4.1 (59.7)Scenario simulationThe simulation determined how much mangroves will be affected or vulnerable to SLR at varying degrees, as 0.1 m, 0.2 m, 0.5 m, and 1 m rise. Spatial analysis was run using the latest mangrove coverage and SLR data. Elevation values for mangroves were assigned using the open sourced digital elevation model (DEM) from Shuttle Radar Topography Mission (SRTM) with a 90 m resolution.BiliranEastern SamarNorthern SamarSouthern LeyteBoholLeyteSamarCebuNegros OrientalSiquijor2000  2010 2015Year12,0009,0006,0003,0000Area (ha)State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports80Figure 4. Global sea level rise observation (EU Copernicus Marine Service/CNES/LEGOS/CLS 2018).A BFigure 5. Historical sea level observation in North Pacific (A) and mean sea level rise in the Philippines in mm (B; CNES 2017)State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports 81To supplement the assessment of vulnerability, normalized difference vegetation index (NDVI) data from 250-m resolution Moderate Resolution Imaging Spectroradiometer (MODIS) was added (Fig. 6). NDVI, as an indicator of vegetation health, indicates that lower values corresponds to higher vulnerability while higher values corresponds to lesser vulnerability.Figure 6. SRTM Digital Elevation Model (A) and MODIS Normalized Difference Vegetation Index (B).Table 3. Vulnerability matrix.NDVI/Elevation Change > 1m < 1 < 0.5 < 0.2 < 0.1> 0.75 Less vulnerableLess vulnerable No ChangeModerately vulnerableHighly vulnerable0.5 – 0.75 No Change No Change No Change Moderately vulnerableHighly vulnerable< 0.5 Moderately vulnerableModerately vulnerableModerately vulnerableModerately vulnerableHighly vulnerable< 0.25 Highly vulnerableHighly vulnerableHighly vulnerableHighly vulnerableHighly vulnerableThe degree of vulnerability was assessed using a matrix that combined NDVI and elevation change values (Table 3).Sample simulation using field observations was also performed using the average value in Palompon, Leyte—the only observation site within the study area A BState of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports82that has a positive measurement (i.e., sea level rise). Mean values in both Samar and Bantayan Island, Cebu were negative (i.e., sea level lowering).III. Results and DiscussionAt increasing sea level, larger mangrove areas in more provinces were affected (Table 4; Fig. 7). At 0.1-m rise, 3,980 ha of mangroves in 23 provinces (in the Philippines) will be affected and increased to 8,813 ha in 32 provinces at 1.0 m.  Among the leading provinces that will be affected are Bohol and Northern Samar in Central and Eastern Visayas, respectively.Focusing in Central and Eastern Visayas, the mangroves in Negros Oriental, Cebu, Leyte, Samar, and Southern Leyte are located in relatively higher elevation; thus, will be less vulnerable to SLR (Table 5).Using the values on annual rate on SLR, the years to take each scenario to happen were also estimated (Table 6). The slowest rate occurred in Leyte at 0.06 mm/year, while the fastest was at Negros Oriental at 4.14 mm/year. It will take around 242 years for Negros Occidental to be submerged in 1.0 m scenario as compared to 1,520 years in Leyte. In addition, Biliran was recorded to experience lowering of sea level, i.e., receding seawater.Table 4. Result of the simulation showing estimates of mangrove areas vulnerable to SLR.Sea Level Rise 0.1 m 0.2 m 0.5 m 1.0 mAffected mangroves (in ha)3,980 4,025 5,757 8,813Affected provinces23 23 28 32Top provinces and area (in ha)Northern Samar (923) Northern Samar (923) Zamboanga Sibugay (1,727) Zamboanga Sibugay (2,105)Sulu (778) Sulu (778) Northern Samar (1,129) Northern Samar (1,836) Basilan (587) Basilan (587) Sulu (859) Basilan (1,700)Zamboanga Sibugay (534) Zamboanga Sibugay (556) Basilan (587) Bohol (1,115)Tawi-tawi (510) Tawi-tawi (510) Tawi-tawi (529) Sulu (859) Bohol (435) Bohol (458) Bohol (505) Tawi-tawi (529)Iloilo (127) Iloilo (127)Figure 7. Location of most affected provinces.Northern SamarSulu BasilanZamboanga SibugayTawi-tawiBoholIloiloState of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports 83Table 5. Result of simulation showing mangrove areas (ha) vulnerable to SLR in Central and Eastern Visayas.Province 0.1 m 0.2 m 0.5 m 1.0 mNegros Oriental - - - -Cebu - - 24 29Siquijor 2 2 2 2Bohol 435 458 505 1,114Biliran 27 27 27 27Leyte - - - 8Southern Leyte - - - -Northern Samar 924 924 1,133 1,836Samar - - 20 60Eastern Samar 5 5 5 5Table 6. Result of sea level rise simulation in Central and Eastern Visayas.Province Annual rise (mm)Years to reach level0.1 m 0.2 m 0.5 m 1.0 mNegros Oriental 4.14 24 48 121 242Cebu 2.74 36 73 182 364Siquijor 3.97 25 50 126 252Bohol 3.22 31 62 155 310Biliran -0.51 (sea level getting lower)Leyte 0.06 152 304 760 1,520Southern Leyte 2.17 46 92 230 460Northern Samar 2.76 36 73 181 363Samar 1.78 56 112 280 560Eastern Samar 3.87 26 52 129 258State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports84Filling up the vulnerability matrix, 40,246 ha or 24.4 % of total mangrove areas in the Philippines can be considered as highly vulnerable (Tables 7 & 8). Northern Samar, Bohol, and Eastern Samar were included in the top ten provinces vulnerable to SLR (Table 9).In Central and Eastern Visayas, all provinces have at least 11 % of their mangrove areas considered as highly vulnerable (Table 10). Biliran has the largest percentage of vulnerable mangroves at 54.5 %, followed by Northern Samar and Southern Leyte at 47.7 % and 47.3 %, respectively.Table 7. Vulnerability matrix (Filled up with areas in ha).NDVI/Elevation Change > 1 m < 1 m < 0.5 m < 0.2 m < 0.1 m>0.75 25,437 154                -   22 2440.5 – 0.75 55,667 604 1,181 23 633< 0.5 40,271 1,263 146                -   1,839< 0.25 34,362 1,500 404                -   1,264Table 8. Summarized vulnerability matrixClassification Area (ha) %Highly Vulnerable 40,246 24.4Moderately Vulnerable 41,726 25.3No Change 57,451 34.8Less Vulnerable 25,591 15.5165,014No Data 51,093Table 9. Top 10 provinces with largest area of vulnerable mangroves.Rank Province Highly Vulnerable (ha)Total Area (ha) %1 Palawan 6,840 43,735 15.602 Sulu 4,883 20,860 23.403 Tawi-Tawi 4,163 12,219 34.104 Zamboanga Sibugay 3,059 12,442 24.605 Northern Samar 2,857 5,994 47.706 Basilan 2,307 8,271 27.907 Zamboanga del Sur 2,266 9,054 25.008 Quezon 1,994 10,541 18.909 Bohol 1,770 7,725 22.9010 Eastern Samar 1,428 6,043 23.60State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports 85Using in situ elevation measurement values in Palompon, Leyte (which is much higher than satellite altimetry values), it will take less time for mangrove areas to be affected by SLR (Table 11). If field observation was validated to be correct, then a more pro-active management intervention (through conservation and restoration programs) is needed to alleviate the effect of SLR.Table 10. Percentage of highly vulnerable mangroves in Central and Eastern Visayas.Province Highly vulnerable (ha)Total mangrove area (ha) %Biliran 111 204 54.5Northern Samar 2,857 5,994 47.7Southern Leyte 1,030 2,180 47.3Negros Oriental 398 1,336 29.8Eastern Samar 1,428 6,043 23.6Bohol 1,770 7,725 22.9Cebu 388 2,294 16.9Siquijor 7 61 12.7Leyte 474 3,905 12.2Samar 409 3,704 11.0Figures 8 and 9 show zoomed-in maps of selected areas in Central and Eastern Visayas. Mangrove areas near San Juanico Strait between the islands of Samar and Leyte are generally located in higher elevation; thus, less mangroves will be affected by a 1.0 m SLR. On the other hand, all mangroves in Bantayan Island, Cebu will be affected by 1.0 m SLR due to lower elevation; hence, more vulnerable to submergence.Table 11. Result of sea level rise simulation in Central and Eastern Visayas using field measurement in Palompon, Leyte.Annual rise (mm)Years to reach level0.1 m 0.2 m 0.5 m 1.0 m17.3 6 12 29 59State of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports86Figure 8. Affected mangrove area (highlighted in red) by 1.0 m SLR in the vicinity of San Juanico Strait between Samar and Leyte islands.Figure 9. Affected mangrove area (highlighted in red) by 1.0 m SLR in Bantayan Island, Cebu where in-situ elevation values were measured.Mangroves Affected by 1 mSea Level Rise in San JuanicoStrait AreaMangroves Affected by 1 mSea Level Rise in BantayanIsland, CebuState of the Mangrove Summit: Central and Eastern Visayas ProceedingsTechnical Reports 87IV. Summary and ConclusionAs of 2015, the Philippines has a remaining mangrove area of about 216,000 ha with Palawan having the largest share. SLR is expected to continue in the years to come. Regional rate for North Pacific is at 2.89 mm/year, however, it is expected to increase. A minimal increase of 0.1 m will affect about 3,980 ha of mangroves in 23 provinces, while a 1.0 m rise will affect a total of 8,813 ha of mangroves in 32 provinces.In Central and Eastern Visayas, the rates of SLR varied among provinces. It was slowest in Leyte (0.06 mm/year) and fastest in Negros Oriental (4.14 mm/year). Biliran, on the other hand, has a negative value. Supplementing NDVI (as indicator of vegetation health) to projected SLR provided an additional criterion in determining the vulnerability of mangroves. Three out of the ten provinces (Northern Samar, Bohol, and Eastern Samar) in the combined regions were included in the leading ten provinces with the largest area of highly vulnerable mangroves. Using field observations, the effect of SLR was expected to occur earlier as compared to the simulation based on satellite altimetry data. More in situ measurements are needed to calibrate the model.V. ReferencesCentre National d’études Spatiales (CNES). 2017. Mean sea level product and image interactive selection. Retrieved from https://www.aviso.altimetry.fr/en/data/products/ocean-indicators-products/meansea-level/products-images.html.Church JA, Clark PU, Cazenave A, Gregory JM, Jevrejeva S, Levermann A, Unnikrishnan AS. 2013. Sea level change. In: Climate change 2013: The physical science basis. Contribution of working group I to the fifth assessment report of the Intergovernmental Panel on Climate Change [Stocker TF, Qin D, Plattner GK, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.David LT, Borja-Del Rosario R, Peñaflor EL, Cordero-Bailey K, Villanoy CL, Hilario F, Licuanan WR. 2015. Developing a Philippine climate-ocean typology as input to national vulnerability assessments. Proceedings of the 36th Asian conference on remote sensing, Manila, Philippines. Gilman E, Ellison J, Coleman R. 2007. Assessment of mangrove response to projected relative sea level rise and recent historical reconstruction of shoreline position. Environmental Monitoring and Assessment 124 (105). doi.org/10.1007/s10661-006-9212-y.Long JB, Giri C. 2011. Mapping the Philippines’ mangrove forest using Landsat imagery. Sensors 11: 2972-2981. doi:10.3390/s110302972.Long JB, Napton D, Giri C, Graesser J. 2014. A Mapping and monitoring assessment of the Philippines’ mangrove forests from 1990 to 2010. Journal of Coastal Research 30(2):260–271. doi.org/10.2112/JCOASTRES-D-13-00057.1.McLeod E, Salm R. 2006. Managing mangroves for resilience to climate change. International Union for the Conservation of Nature and Natural Resources, Gland, Switzerland.Nerem RS, Beckley BD, Fasullo JT, Hamlington BD, Masters D, Mitchum G. 2017. Climate-change-driven accelerated sea-level rise detected in the altimeter area. Proceedings of the National Academy of Sciences of the United States of America 2018. doi/10.1073/pnas.1717312115.Phil-LiDAR 2 Aquatic resources extraction from LiDAR Surveys. 2016. 2015 Philippine mangroves (Shapefile).  UP Training Center from Applied Geodesy and Photogrammetry.

Simulation of Impacts of Sea Level Rise on Mangrove Survival in Central and Eastern Visayas

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Simulation of Impacts of Sea Level Rise on Mangrove Survival in Central and Eastern Visayas

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