GEOMORPHOLOGY AND INUNDATION BEHAVIOR OF THIRTY CRITICAL WATERSHEDS IN MINDANAO

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1 GEOMORPHOLOGY AND INUNDATION BEHAVIOR OF THIRTY CRITICAL WATERSHEDS IN MINDANAO George R. Puno (*1), Rose Angelica L. Amper (1) (1) Central Mindanao University, Musuan, Dologon, Maramag, Bukidnon, 8710, Philippines KEYWORDS: flood plain, GIS, hydrologic behavior, morphometric parameters ABSTRACT: The geomorphology of the watershed characterizes the formation of the river flood plains as it plays key roles on the dynamic hydrologic behavior in relation to flooding. To determine the extent of geomorphologic impact on the percent coverage of inundated area within the floodplain, this study analyzed 30 rivers across Mindanao with majority identified as critical base on the impaired biotic and biotic status due to socioeconomic roles and susceptibility, and potential threats to flooding by the Mindanao Rivers Reconstruction Task Force (MMRTF). Using ASTER Global DEM (ASTGTM) 30m Digital Elevation Model (DEM), watershed boundaries and river networks were determined using ArcGIS of Esri. Inundated areas of each watershed were likewise delineated based on the Mines and Geosciences Bureau (MGB) Maps. Regression analysis was conducted between inundated areas and the various geomorphologic values such as basin size, form factor, elongation ratio, circularity ratio, length of overland flow, basin relief, slope, number of stream, stream order, stream frequency, and stream density among others. Results revealed that inundated area is explained by the geomorphologic attributes of the watersheds. This implies that geomorphologic features of watershed are good indicators of flood prone zone within the landscape. Awareness on the geomorphologic characteristics of the watershed is helpful to the disaster risk reduction management council both at the local and national level particularly in prioritizing areas which need preventive mitigation measures and relief goods distribution for cost effective and efficient management operations. 1. INTRODUCTION The watersheds morphometry referring to its topographic characteristics generally controls the hydrologic activity. These characteristics as an expression of land topography by way of area, slope, shape, elevation among others affect streamflow patterns on catchments through concentration time influence and bears vital roles to the understanding of the hydrological behavior in the drainage basins as whole (Ajibade et al., 2010; Nayar and Natarajan, 2013). In studying morphometric parameters, delineation with the use of GIS, a system aimed in understanding morphometry of the river channel and its drainage network, has become a common geomorphologic task (AltÕn et al., 2010; Koshak and Dawod, 20). Immense utility is found through this technique towards river basin evaluation, watershed prioritization for flood, and water conservation and natural resource management as seen in the extensive use of remote sensing and GIS tools in studies on flood hazard and risk mapping as well as in flood monitoring all throughout the globe (AltÕn et al., 2010; Adel et al., n.d.). With the use of same technology, 30 critical rivers in Mindanao identified based on its impaired socioeconomic roles and susceptibility to flooding by Mindanao Rivers Reconstruction Task Force (MMRTF) were investigated with a view to understand the processes operating within them. The study aimed to evaluate the extent of inundated floodplain of the thirty watersheds in Mindanao in response to the geomorphologic characteristic of the watersheds. 2. METHODOLOGY The core file utilized for map delineation is ASTER Global DEM (ASTGTM) 30m Digital Elevation Model (DEM). Watershed boundary is generated through Basin Hydrology Tool while flood plains were determined using SAR 10m DEM considering elevation below 120masl and 18 degrees slope. Delineation of the inundated areas was based on the available hazard maps of Mines and Geosciences Bureau (MGB), worked in ArcGIS environment of Esri. Regression analysis was performed between inundated areas and the various geomorphologic values such as basin size, form factor, elongation ratio, circularity ratio, length of overland flow, basin relief, slope, number of stream, stream order, stream frequency, stream density among others to determine the linear relationship between the different variables. Through this, a causal inference was established between percent coverage of inundation and geomorphologic attributes of the watershed.

2 3. RESULTS & DISCUSSIONS 3.1 Description of Study Sites Table 1 shows the details of the 30 rivers with their geographical location and the number of inundated barangays. Rivers under the study are found across the six regions of Mindanao. Table 1. Geographical locations of the 30 critical rivers in Mindanao. Rivers Location Geographical Location Dipolog Sindangan Salug Iponan Mandulog Kingking Manurigao Padada Kraan Big Lun Big Sapu Glan Little Lun Luan Makar Malabulen Malapatan Matinao Pange Saboay Sigil Cabadbaran Surigao Tandag Tago Zamboanga del Norte Zamboanga del Norte Zamboanga Del Norte Cagayan de Oro City, Northern Mindanao. Lanao del Sur Compostela Valley Davao Oriental Davao del Sur Sultan Kudarat Saranggani General Santos City General Santos City General Santos City Agusan Del Norte Surigao Del Norte Surigao Del Sur Agusan Del Sur N lat. and to E long N lat. and E long N lat. and E long to N lat. and to E long to N lat. and to E long to N lat. and to E long to N lat. and to E long N lat. and E long. 6 7'27.57"N lat. and '47.29"E long. 6 1'36.09"N lat. and '46.63"E long. 5 55'21.28"N lat. and '28.57"E long. 5 49'30.57"N lat. and '35.49"E long. 6 2'40.24"N lat. and '32.77"E long. 6 5'52.77"N lat. and '51.71"E long. 6 5'22.56"N lat. and 125 9'3.67"E long. 6 6'48.65"N lat. and 125 '26.08"E long. 5 58'4.08"N lat. and '9.51"E long. 6 6'20.88"N lat. and 125 9'50.07"E long. 6 2'13.81"N lat. and '57.06"E long. 6 6'40.82"N lat. and 125 8'53.83"E long. 5 57'32.56"N lat. and 125 6'1.02"E long N lat. and E long N lat. and to E long N lat. and to E long N lat. and to E long. No. of Barangays

3 Daywan Alegria Puyo Carac-an Surigao del Norte Surigao del Norte Agusan del Norte and Surigao del Norte Surigao del Sur ARMM Matling Lanao del Sur N lat. and E long N lat. and E long to N lat. and to E long to N lat. and to E long to N lat. and to E long Based on the Corona s classification of climate, rivers belonged under the three different types among the four climate classification which is based on the monthly rainfall received. 9 located in Zamboanga peninsula is consist of 2 climate types. The western part including Salug watershed is under type III climate, characterized by short dry season usually from February to April. The eastern portion on the other hand which includes watersheds of Dipolog and Sindangan is under type IV climate, where rainfall is almost evenly distributed all throughout the year. 10 and its watersheds commonly experience type III climate with short dry season from February to April. The watersheds under and 12 all experiences type IV climate characterized by almost evenly distributed rainfall during the whole year, short dry season and pleasant climate all year round. These areas include provinces of Davao and Compostella Valley, and South Cotabato, Sarangani and Sultan Kudarat located at the southeastern and central part of Mindanao, respectively. The watersheds in 13 located at the northeastern portion of the Island experience type 2 climate characterized by no dry period all throughout the year with pronounced wet season from November to February. Moreover, Matling, the sole watershed under this study in Autonomous of Muslim Mindanao (ARMM) experiences type 3 climate characterized by short dry season, usually from February to April. 3.2 Watershed Geometry Figure 1. Map showing the locations of the watersheds studied in Mindanao. The largest watershed is Tago of Surigao del Norte while the smallest is Alegria of the same region. Amount of precipitation is influenced by the size of the watershed. The watershed of Tago in this study is hence considered to

4 have the greatest amount of rainfall. Moreover, larger watershed size and longer total channel length cause a slower travel of rainfall discharge to reach main channel (20). Form factor values imply that most of the watersheds are elongated in shape indicating lower runoff rates due to uneven distribution of rainfall. Values of circularity and elongation ratio confirming elongated shapes of most of the watersheds further implies manageable watersheds due to slow disposal of water compared to the circular one which allows quick runoff (Kanth and Hassan, 2012). Narrow and smaller size watersheds such as Tandag, Salug, Manurigao, Kingking, Dipolog, Glan, Alegria, Saboay, Pange, Malapatan, Makar, Luan, Little Lun and Big Sapu have higher tendency of immediate peak runoff during shorter storm events. However, it has been established that the complex topography of each areas plays major factor affecting the geometry of each watershed (Koshak and Dawod, 20). 3.3 Stream Features The properties of the channels networks of basins are considered as the major characteristics of the watershed morphometry (Koshak and Dawod, 20). All watersheds have low stream frequency values which may imply low discharge runoff and high infiltration capacity (Rekha et al., 20). Moreover, low stream densities which likely to occur in regions of highly resistant of highly permeable subsoil material under dense vegetative cover and low relief indicating high water holding capacity. Relatively long overland flow of surface water is expected in areas with low drainage density values. Bifurcation ratio ranges between 3 and 5 is geologically characterized by homogeneous materials without structural disturbances to the drainage of the watershed (RaoLiaqat, et al., 20). Among the 30 rivers, only Mandulog, Matinao and Padada are with ratios falling within the ranges suggesting a watershed of reduced structural disturbances. Except Saboay which is alone in having very high bifurcation value indicating geological control, the rest of the watersheds are with lower values implying production of sharp peak discharge (Agarwal, 1998 as cited by Nayar and Natarajan, 2013). 3.4 Relief Features Relief features of the watersheds are mainly described by the elevation, slope and the ratio of relief. Relief varies across the different watersheds. Watersheds of Davao Oriental, South Cotabato, Sultan Kudarat, Saranggani and Agusan del Norte areas are with higher elevation. Relief ratio values reveal that all of the watersheds are with higher relief and steeper slopes implying higher runoff rates. 3.5 Statistical Analysis The main statistical tool used in this study is regression analysis to the value of a dependent variable is affected by values of independent variables (Mason, 2012). Individual correlation analysis was conducted between the different variables referring to the geometric features of the watershed affecting the flood extent coverage. Regression values indicating relationship between the attributes and the inundation extent in the floodplain and percentage of variation caused by the said variable are shown in Table 2. Table 2. Regression analysis relationship between the different independent variables to inundated area coverage. Variables Coefficient of Correlation (R) Interpretation Coefficient of Determination (R 2 ) % variation caused by the variable Area (sqm) Weak negative relationship % Form Factor Ratio Very weak positive relationship % Elongation Ratio Very weak positive relationship % Circularity Ratio Very weak positive relationship % Texture Ratio (T) Very weak positive relationship % Basin Relief (m) Very weak negative relationship % Relief Ratio Weak positive relationship % Mean Elevation (m) Weak negative relationship % Mean Slope (%) Very weak negative relationship % Ruggedness Number (R n ) Very weak negative relationship % No. of Streams Weak negative relationship % Mean Slope of River (%) Weak positive relationship %

5 Stream Frequency Very weak negative relationship % Drainage Density (m) Very weak negative relationship % Bifurcation Ratio Weak positive relationship % No. of 1 st Order Stream Weak negative relationship % Looking at the individual impacts of each of the 16 morphometric attributes subjected to correlation analysis reveals a majority of very weak relationship and several other weak relationships towards the inundation coverage. There are nine (9) variables having negative relationship towards extent of flooded area namely area of watershed, basin relief, mean elevation, mean slope, ruggedness number, number of streams, stream frequency and drainage density. These variables pose inverse effect causing a decrease of flood extent coverage as their values increase. A bigger watershed area, the same with higher basin relief, elevation, slope and so on implies a decrease of inundation extent in each watersheds floodplain. On the contrary, the rest of the geomorphologic attributes such as form factor ratio, elongation ratio, circularity ratio, texture ratio, relief ratio among others show a positive relationship towards extent of flood area. The increase of their values implies an increase of inundation coverage. A study of Koshak and Dawod (20) correlating the main morphometric parameters of the six sub-basins under study revealed that basin area is the most effective element while total relief has the non-significant impact to the total length of orders. Coefficient of determination (R 2 ) shows the percentage of variation caused by the variables. Mean elevation of the watershed variable has the highest (10.45%) R 2 implied as the greatest contributing factor on the coverage of the flooded area in the floodplain. This is followed by the relief ratio responsible for the percentage variation of 8.99% on the flood extent. Multiple regression analysis was performed to see the changed caused by the interaction of several independent variables to the predicted values (Table 3). R value reveals very strong correlation of the predictor variables which are the geomorphologic characteristics to the inundation percentage. R 2 explains that 83.88% of Table 3. Multiple Regression Analysis the variation and causal factors in the percentage of Regression Statistics inundation are caused by these independent variables. The value of adjusted R 2 of 59.47% further implies a Multiple R high percentage of indication that the incidence and R Square occurrence of the flooding covering that percentage area is accounted to the set of geomorphological Adjusted R Square attributes. Adjusted R 2 account both the sample size Standard Error and the number of predictors set. Observations CONCLUSIONS Through the utilization of the GIS technology, geomorphologic attributes were computed and flood extents in each watershed are determined. Results of analysis in this study has demonstrated that extent of inundation in the watershed can be predicted by its geomorphologic attributes. Awareness on the geomorphologic characteristics of the watershed is helpful to the disaster risk reduction management council both at the local and national level particularly in prioritizing areas which need preventive mitigation measures and relief goods distribution for cost effective and efficient management operations. ACKNOWLEDGMENTS The author is grateful to Central Mindanao University administration and to the faculty and staff of the College of Forestry and Environmental Science, CMU. REFERENCES Adel, O., Schröder, D., El Rayes, A., Geriesh, M. n.d. Flood Hazard Assessment in Wadi Dahab, Egypt Based on Basin Morphometry Using GIS Techniques.Geospatial GI_Forum '. Herbert Wichmann Verlag, VDE VERLAG GMBH, Berlin/Offenbach by Car, A., Griesebner, G. & Strobl, J. (Eds.) (20):. ISBN

6 Ajibade, L.T., Ifabiyi, I.P., Iroye, K.A., Ogunteru, S Morphometric Analysis of Ogunpa and Ogbere Drainage Basins, Ibadan, Nigeria. Ethiopian Journal of Environmental Studies and Management. Vol.3, No.1. AltÕn, T.B., AltÕn, B.N Drainage morphometry and its influence on landforms in volcanic terrain, Central Anatolia, Turkey. Elsevier Ltd. Selection and/or peer-review under responsibility of The 2nd International Geography Symposium- Mediterranean Environment. Geena, G.B., Bullukraya, P.N. 20. Morphometric Analysis of Korattalaiyar River Basin, Tamil Nadu, India: A GIS Approach. International Journal of Geomatics and Geosciences, Vol. 2(2): Hilario, F.D., Cinco, T.A., de Guzman, R.G., Ares. E.D. 20. Climate change in the Philippines.. DOST- PAGASA Climatology and Agrometeorology Division (CAD) Agham Road. Kanth T.A., Hassan Z.U Morphometric Analysis and Prioritization of Watersheds for Soil and Water Resource Management in Wular Catchment using Geo-spatial Tools. International Journal of Geology, Earth and Environmental Sciences. Available at Koshak.N., Dawod.G. 20. A GIS morphometric analysis of hydrological catchments within Makkah Metropolitan area, Saudi Arabia. International Journal of Geomatics and Geosciences. Volume 2, No 2, 20 Mason, J.O Performing Regression Analysis Using Microsoft Excel International Journal of Arts and Commerce. Vol. 1 No. 5. Nayar, V., Natarajan, K Quantitative Morphometric analysis of Kosasthalaiyar sub basin (Chennai basin) using remote sensing (SRTM) data and GIS techniques. International Journal of Geomatics and Geosciences. Volume 4, No. 1. Puno, G., Villar, R., Bruno, A.G., Casas, J. 20. GIS-based Mapping and Morphometric Analysis of Flood Prone Sites in the Three Watersheds of Bukidnon. Journal of Environmental Science and Engineering A 3 (20) RaoLiaqat, A.K., Rehman, A.Z., Alia, Y. 20. Morphometric Analysis of Drainage Basin Using Remote Sensing and GIS Techniques: A Case Study of Etmadpur Tehsil, Agra District, U.P. International Journal of Research in Chemistry and Environment. Vol. 1 (2):36-45