Basin Hydrology and Wetland Dynamics of Agusan Marsh

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<p>BASIN HYDROLOGY AND WETLAND DYNAMICS OF AGUSAN MARSHGuillermo Q. Tabios III</p> <p>Department of Civil Engineering and National Hydraulic Research Center University of Philippines, Diliman, Quezon CityAgusan Marsh Scientific Conference Balanghai Hotel, Butuan City May 21-23, 2007</p> <p>Outline of Presentation Considerations in managing Agusan Marsh Hydrology of Agusan River basin Wetland dynamics of Agusan marsh Implications and impact of water resource development on the hydrology, wetland dynamics including ecology of Agusan Marsh</p> <p>On Sustainable Development Resources</p> <p>of Water</p> <p>The sustainable utilization and development of watershed, river and coastal resources must balance economic development and environmental protection. Economic development is to satisfy basic needs, to alleviate poverty, to enhance economic and social equity, and to improve the quality of human life. Environmental protection is to ensure ecological integrity of the environment to support biological life in general and to maintain genetic diversity, resilience and biological productivity in particular.</p> <p>. On Sustainable Development Resources</p> <p>of Water</p> <p>Thus, a sound and enhanced environment is a necessary condition to ensure economic efficiency and sustainability. An environmentalist or ecologist treat sustainability as preservation of the natural function and status of the ecological system; whereas an economist emphasizes on the maintenance and enhancement of human living standards as indices of sustainability.</p> <p>On Ecosystem-based ManagementEcosystem interaction between all-living and non-livingelements</p> <p>based on interdependence of soil, water, forest, terrain and human settlements (and their activities).</p> <p>Ecosystem-based approach managing natural resources</p> <p>Also, ecosystem-based management recognizes that people as part of living elements are dependent on a healthy ecosystem for food, water and shelter.</p> <p>Main elements of the Agusan River Basin environment water forest soil terrain human settlements (and their activities)</p> <p>Water, forest and soil are the most critical elements in the Agusan River basin (CTI-HALCROW-WOODFIELDS 2006)</p> <p> Water as critical element of the Agusan River Basin environmentWater quantity concerns are due to competing uses of water such as: domestic water supply versus irrigation; alleviation of flooding in populated areas versus floodwater needs of the marsh; and, upland water needs versus coastal urban and ecological water needs. Water quality concerns are mainly due to contamination of surface and groundwater resources from: mining; agricultural wastes especially chemical and pesticides; and, urban liquid and solid wastes.</p> <p> Forest as critical element of the Agusan River Basin environmentThe massive logging of forests in Agusan in the early 1950s and 1960 has reduced forest cover. Due to lack of forest cover, soil erosion and eventual sedimentation in low lying rivers and floodplains as well as occasional landslides had increased. Also, through private and government efforts, the endemic forest have been replaced with exotic trees, and some of the original forest lands were converted to banana plantations.</p> <p> Soil as critical element of the Agusan River Basin environmentThe lack of forest cover, uncontrolled mine tailings, steep slope farming and kaigin farming, has significantly increased soil erosion. The eroded soil increased suspended sediment loads (thus turbidity) as well as siltation of water bodies (thus shallowing of rivers and floodplains). High turbidity and heavy siltation adversely affect the productivity and diversity of aquatic flora and fauna.</p> <p>Alternative objectives in managing Agusan marsh to satisfy the environmental and ecological goals1. Maintain or restore the natural condition based on historical past. In this case, the hydrologic and hydraulic conditions that is conducive to fish and aquatic life such as frequency and extent of flooding of the wetland, recession rates in the drying and flooding cycles, and desirable ranges of velocity and flow depths will be maintained or restored. This approach is attractive when directed towards diverse biological communities.</p> <p>. Alternative objectives in managing Agusan marsh to satisfy the environmental and ecological goals2. Achieve a certain specific ecological criteria for selected species. If the ecologic criteria of certain species can be determined, then the river or marsh conditions specific to their needs can be maintained. These conditions include river flow properties needed at different life stages, acceptable levels of sediment in the flow, and magnitude and frequency of flooding.</p> <p>. Alternative objectives in managing Agusan marsh to satisfy the environmental and ecological goals3. Maintain stable river basin system. This may not always satisfy various ecological criteria. However, it minimizes sudden or abrupt perturbations in the river system causing adverse impacts on the biological communities. A stable river means river in dynamic equilibrium, not static.</p> <p>Hydrology of Agusan River Basin in general and Agusan Marsh in particular Basin topography Watershed delineation and river network Slope and Soil Properties Rainfall patterns Streamflow regimes</p> <p>Agusan River Basin</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Agusan River Basin OF THE PROJECT SITEVertical Exaggeration: 7 Sun Azimuth: Northeast: 32.69o Sun Altitude: Low: 30.00o</p> <p>THREE DIMENSIONAL VIEW</p> <p>Marsh</p> <p>Agusan</p> <p>(from CTIHALCROWWOODFIELDS 2007)</p> <p>Agusan River Basin River Network</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Agusan River Basin 17 Sub-Basins</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>R iv e r In d e x N um be r 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15</p> <p>S u b -B a si n s B ug abo s O jot L ib a n g M a as a m K a s il a n A dg aon K ay ona n H ao an L o g o m -B a o b o M a nat S im ulao L agc oga ngan G ibon g W a w a -A n d a n o n T a g u ib o To t a l B a sin A r e a</p> <p>C a tc h m e n t A re a 170.5 5 924.2 0 246.5 6 418.2 0 376.8 9 984.8 1 729.4 7 746.3 4 291.2 0 1,758.2 0 978.3 8 159.7 5 926.3 8 764.1 4 75.7 2 9,550.7 8 1 1,936.5 5</p> <p>P e rc e n t o f th e B a sin Are a 1 .4 3 7 .7 4 2 .0 7 3 .5 0 3 .1 6 8 .2 5 6 .1 1 6 .2 5 2 .4 4 14.73 8 .2 0 1 .3 4 7 .7 6 6 .4 0 0 .6 3 80.01</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Agusan River Basin Slope Map</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Agusan River Basin Soil Texture</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>La Suerte, Prosperidad, Agusan del Sur700Average Rainfall Maximum</p> <p>600</p> <p>Minimum</p> <p>Monthly Rainfall (mm)</p> <p>500</p> <p>400</p> <p>300</p> <p>200</p> <p>100</p> <p>0 0 1 2 3 4 5 6 7 Months 8 9 10 11 12</p> <p>2250 4000</p> <p>2500 4000 3750 3500 3000</p> <p>3250 3750 2750</p> <p>Annual Rainfall Isohyetal Pattern</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Flood Inundation Map Generated from 30-yr Rainfall</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Agusan Marsh Flood Hydrograph Generated from 30-yr Storm RainfallTotal Discharge Entering Tributaries that Flow into Agusan Marsh Flow into Agusan Marsh Agusan Marsh Outflow</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Existing and Proposed Water Resources Development in Agusan River Basin CARDBP-PMO (2003) CTI-HALCROW-WOODFIELDS (2007) HASSALL (2003)</p> <p>CARDBP-PMO (2003) Proposed dam constructions including characteristics of main river channels and major tributaries in Agusan River Basin.</p> <p>List of Proposed Flood Management Projects CTI-HALCROW-WOODFIELDS (2007) ARB Master Plan, 2005-2006 Proposed Flood Management Projects, ARB Master PlanAm o unt No. P r oje ct Sub -Bas in (P0 00 at 2006 Pr ice s ) 1 Implementation of the B utuan City Drainage Minor Sys tem Master Pla n 948,500 no revenu e 17.0% FIRR EIRR Be ne fit:Co s t Ratio @ 12 % pe r ann um 1.3</p> <p>2</p> <p>Preparation of Drainage System Ma s ter Plan (one p ac kage pro ject; cost w as divided equally fo r allocation purposes only ): Mon ka yo Mon tevista Nabun turan Tala cogon San Francis co Bayug an R T Romualdez Esperanza Las Niev es Mag allan es To tal Feasibility S tudy f or a Multipurpose Outlet Sys tem to Increase Surcharge Capac ity of the A gusan Ma rsh (Floo d, Water Supply and Irrigation ) Upd ate and Implementa tion of the Upp er A gusan Flood Con trol and Drainag e Sys tem Project Establishme nt of Flood Wa rning and Forecasting Sy stem Project To tal Flo od M anage m e nt Pr ogr am Manat Manat Manat Ka silan Gibong Waw a-A nda nan Minor Ojot Minor Minor 2,801 2,801 2,801 2,801 2,801 2,801 2,801 2,801 2,801 2,801 28,008 study phas e study phas e study phas e study phas e study phas e study phas e study phas e study phas e study phas e study phas e</p> <p>3</p> <p>A gusan Marsh</p> <p>22,418</p> <p>study phas e</p> <p>4</p> <p>Manat</p> <p>1 ,974,111</p> <p>no revenu e</p> <p>15.3%</p> <p>1.5</p> <p>5</p> <p>basinw ide</p> <p>11,550 2 ,984,587</p> <p>sy stem co uld be ins talle d w ithin six months</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>(from CTI-HALCROWWOODFIELDS 2007)</p> <p>Implications and Impacts of Existing and Proposed Water Resources Development in Agusan River Basin Planned upstream reservoirs will: decrease surface water inflow to Agusan Marsh; modify flooding frequency and spatial extent of Agusan Marsh; lower water table due to decrease watershed recharge; decrease freshwater inflow towards Agusan River in Butuan City; and, decrease groundwater flux to coastal zone causing saltwater intrusion and hypersalinity condition in the mangrove forest in the Agusan River estuary Planned diking and river straightening destroys dynamic equilibrium resulting in other flooding and sedimentation problems</p> <p>Water situation of Agusan River Basin and Agusan Marsh in particular (CARDBP-PMO, 2003)</p> <p>Impacts of planned reservoirs in the Agusan River Basin and Agusan Marsh in particular</p> <p>Candaba Swamp as Detention Pond for Ecological and Flood Control Purposes (an example)The Candaba swamp serves as a habitat for local and migratory birds thereby an ecotourism site. It also acts a natural flood detention basin during the wet season.Percentages of time the area (highlighted) is flood inundated</p> <p>To maintain its ecological function, a major hydrologic criterion is to maintain its historical flood inundation frequency.</p> <p>RIVER STRAIGHTENING PROBLEMConsider a meandering river.</p> <p>flow</p> <p>A</p> <p>In 1D approaches, the relevant streamwise coordinate is measured as an arc length down the channel centerline. The long profile of the river is as illustrated below. The fluctuations are induced by the meanders themselves.</p> <p> B</p> <p>x (arc length coordinate)</p> <p>RIVER STRAIGHTENING (contd.)Now suppose the bends between points A and B are cut off due to river straightening.B A flow</p> <p>ambient slope A steepened slope </p> <p>The elevation drop between A and B is the same as before straightening (at least initially), but the distance between A and B is much shorter. As a result bed slope is elevated between A and B compared to the reaches upstream and downstream.ambient slope</p> <p>x (arc length coordinate)</p> <p>RIVER STRAIGHTENING (contd.)U A</p> <p>q = t t x</p> <p> t + t = t +</p> <p>1 qt x qt x</p> <p>(</p> <p>x + x</p> <p>)t</p> <p> B D qt x</p> <p>The sediment transport rate is higher over the steepened reach.</p> <p>The reach UA must degrade (bed elevation must decrease in time), as there is more sediment output than input. The reach between B and D must aggrade (bed elevation must increase in time), as there is more sediment input than output.</p> <p>RIVER STRAIGHTENING (contd.)U</p> <p>Initial long profile after straightening.A</p> <p>Evolving long profile. B D</p> <p>Final equilibrium long profile: qt/x = 0 thus /t = 0</p> <p>x</p> <p>The upstream part of the reach must degrade and the downstream part must aggrade until a new equilibrium profile is established. The above 1D picture is a simplified version of reality: too much aggradation downstream will cause the river to avulse (jump channel). Nevertheless, it captures the essentials of the problem.</p> <p>Stream types and classification</p> <p>Maintenance of equilibrum river meander geometry based on bankfull flow, which is below flood overflow levels.</p> <p>Saltwater intrusion further inland in Agusan River at Butuan City estuary if freshwater inflow is reduced</p> <p>From Agusan National High School Einstein Class 1999 (web)</p> <p>Adverse impact to ecosystem health of mangrove forest due to reduced groundwater recharge and flux</p> <p>Future Work or Discussions Planners, engineers, ecologists and scientists should learn from each others perspectives and knowledge to properly manage our environment in general and water resources systems in particular In the planning and management of Agusan River Basin, objective measures should be established to ensure and assess that ecological goals and targets The natural course and dynamics of nature must be recognized and respected since tampering nature can can be costly Modeling and simulation studies is a useful tool in the planning and management Agusan River Basin to properly assess the impacts and implications of any water resource developments on the wetland dynamics and ecological integrity of Agusan Marsh in particular</p> <p>The author gratefully acknowledges Dr. Jurgenne Primavera for invitation and travel funds to this conference and Dr. Rey Medina of Woodfields Consultants for allowing the use various figures in this presentation.</p>

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