Basin Hydrology and Wetland Dynamics of Agusan Marsh

BASIN HYDROLOGY ANDWETLAND DYNAMICS OF

AGUSAN MARSH

Guillermo Q. Tabios IIIDepartment of Civil Engineering andNational Hydraulic Research Center

University of Philippines, Diliman, Quezon City

Agusan Marsh Scientific ConferenceBalanghai Hotel, Butuan City

May 21-23, 2007

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

On Sustainable Development of Water Resources

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.

…. On Sustainable Development of Water Resources

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.

On Ecosystem-based Management

Ecosystem – interaction between all-living and non-living elements

Ecosystem-based approach – managing natural resources based on interdependence of soil, water, forest, terrain and human settlements (and their activities).

Also, ecosystem-based management recognizes that people as part of living elements are dependent on a healthy ecosystem for food, water and shelter.

Main elements of the Agusan River Basin environment

• water• forest• soil• terrain• human settlements (and their activities)

Water, forest and soil are the most critical elements in the Agusan River basin (CTI-HALCROW-WOODFIELDS 2006)

… Water as critical element of the Agusan River Basin environment

Water 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.

… Forest as critical element of the Agusan River Basin environment

The massive logging of forests in Agusan in the early 1950’s 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.

… Soil as critical element of the Agusan River Basin environment

The 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.

Alternative objectives in managing Agusan marsh to satisfy the environmental and ecological goals

1. 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.

…. Alternative objectives in managing Agusan marsh to satisfy the environmental and ecological goals

2. 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.

…. Alternative objectives in managing Agusan marsh to satisfy the environmental and ecological goals

3. 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.

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

Agusan River Basin

(from CTI-HALCROW-WOODFIELDS 2007)

THREE DIMENSIONAL VIEW

OF THE PROJECT SITE Vertical Exaggeration: 7 Sun Azimuth: Northeast: 32.69o

Sun Altitude: Low: 30.00o

Agusan Marsh

Agusan River Basin

(from CTI- HALCROW- WOODFIELDS 2007)

Agusan River BasinRiver Network

(from CTI-HALCROW- WOODFIELDS 2007)

Agusan River Basin17 Sub-Basins


R iv e r In d e x

N u m b e rS u b -Ba si n s

C a tc h m e n t A re a

P e rc e n t o f th e B a sin

Are a1 B ug abo s 170. 5 5 1 .4 32 O jot 924. 2 0 7 .7 43 L ib ang 246. 5 6 2 .0 74 M a as a m 418. 2 0 3 .5 05 K as ilan 376. 8 9 3 .1 66 A dg aon 984. 8 1 8 .2 57 K ay ona n 729. 4 7 6 .1 18 H ao an 746. 3 4 6 .2 59 L ogo m -B ao bo 291. 2 0 2 .4 410 M a nat 1 , 758. 2 0 14. 7311 S im ulao 978. 3 8 8 .2 012 L agc oga ngan 159. 7 5 1 .3 413 G ibon g 926. 3 8 7 .7 614 W aw a-A ndan on 764. 1 4 6 .4 015 T agu ibo 75. 7 2 0 .6 3

Tot a l 9 , 550. 7 8 80. 01B a sin Are a 1 1, 936. 5 5


Agusan River BasinSlope Map


Agusan River BasinSoil Texture


0 1 2 3 4 5 6 7 8 9 10 11 12Months

0

100

200

300

400

500

600

700

Mon

thly

Rai

nfal

l (m

m)

Average Rainfall

Maximum

Minimum

La Suerte, Prosperidad, Agusan del Sur

Annual RainfallIsohyetal Pattern

2250

4000

3750

2750

3000

3750

3500

3250

4000

2500


Flood Inundation Map Generated from 30-yr

Rainfall


Agusan Marsh Flood Hydrograph Generated from 30-yr Storm Rainfall

Total Discharge EnteringTributaries that Flow intoAgusan Marsh

Flow into Agusan MarshAgusan Marsh Outflow


Existing and Proposed Water Resources Development in Agusan River Basin

• CARDBP-PMO (2003)• CTI-HALCROW-WOODFIELDS (2007)• HASSALL (2003)

CARDBP-PMO (2003) Proposed dam constructions including characteristics of main river channels and major tributaries in Agusan River Basin.

List of Proposed Flood Management ProjectsARB Master Plan, 2005-2006

No. Proje ct Sub -Bas inAm o unt

(P000 at 2006 Pr ices )

FIRR EIRRBenefit:Co st Ratio @12% pe r ann um

1Implementation of the Butuan City Drainage Sys tem Master Plan

Minor 948,500no

revenue17.0% 1.3

2

Monkayo Manat 2,801 Mon tevista Manat 2,801 Nabun turan Manat 2,801 Talacogon Kasilan 2,801 San Francis co Gibong 2,801 Bayugan Waw a-Andanan 2,801 R T Romualdez Minor 2,801 Esperanza Ojot 2,801 Las Niev es Minor 2,801 Magallanes Minor 2,801To tal 28,008

3

Feasibility Study f or a Multipurpose Outlet Sys tem to Increase Surcharge Capac ity of the A gusan Marsh (Flood, Water Supply and Irr igation )

A gusan Marsh 22,418

4Update and Implementa tion of the Upper A gusan Flood Con trol and Drainage Sys tem Project

Manat 1 ,974,111no

revenue15.3% 1.5

5Establishment of Flood Warning and Forecasting Sy stem Project

basinw ide 11,550

To tal Flo od M anage m ent Program 2,984,587

study phas estudy phas estudy phas e

sy stem could be installed w ithin s ix months

study phas e

study phas e

study phas e

Preparation of Drainage System Master Plan (one package pro ject; cost w as div ided equally fo r allocation purposes only ):

study phas estudy phas estudy phas estudy phas estudy phas e

CTI-HALCROW-WOODFIELDS (2007)Proposed Flood Management Projects, ARB Master Plan



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

Water situation of Agusan River Basin and Agusan Marsh in particular (CARDBP-PMO, 2003)

Impacts of planned reservoirs in the Agusan River Basin and Agusan Marsh in particular

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.

To maintain its ecological function, a major hydrologic criterion is to maintain its historical flood inundation frequency.

Percentages of time the area (highlighted) is flood inundated

RIVER STRAIGHTENING PROBLEMConsider a meandering river.

flow 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.

η

x (arc length coordinate)

A

B

RIVER STRAIGHTENING (contd.)Now suppose the bends between points A and B are cut off due to river straightening.

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.

AB

flow

η

x (arc length coordinate)

A

ambient slope

steepened slope

ambient slope

RIVER STRAIGHTENING (contd.)

( ) tqqx

1xq

t xxtxttttt Δ−

Δ+η=η→

∂∂

−=∂η∂

Δ+Δ+

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.

η

x

A

B

U

Dqt

The sediment transport rate is higher over the steepened reach.

RIVER STRAIGHTENING (contd.)

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.

Initial long profile after straightening.

η

x

A

B

U

D

Evolving long profile.

Final equilibrium long profile: ∂qt /∂x = 0 thus ∂η/∂t = 0

Stream types and classification

Maintenance of equilibrum river meander geometry – based on bankfull flow, which is below flood overflow levels.

From Agusan National High School Einstein Class 1999 (web)

Saltwater intrusion further inland in Agusan River at Butuan City estuary if freshwater inflow is reduced

Adverse impact to ecosystem health of mangrove forest due to reduced groundwater recharge and flux

Future Work or Discussions• Planners, engineers, ecologists and scientists should

learn from each other’s 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

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.

Documents

Basin Hydrology and Wetland Dynamics of Agusan Marsh