Importance of Land use management on the Flood Management in the Chi River Basin, Thailand Kittiwet Kuntiyawichai Bart Schultz Stefan Uhlenbrook F.X. Suryadi Ann van Griensven 4 th International Symposium on Flood Defence 6 – 8 May 2008, Toronto, Canada

Problem definition Objectives of the study Model setup Introduction Contents Conclusions & Recommendations Discussions SWAT & 1D/2D SOBEK simulations

Introduction 49,477 Area = 49,477 km 2 Chi River, Main system: Chi River, 765 km river length 6.9 millionPopulation = 6.9 million people 20 Sub-basins: 20 60% Land use: 60% (30,000 km 2 ) = agricultural land 1,200 Precipitation: 1,200 mm/year 1,290 Potential evaporation: 1,290 mm/year Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Chi River Basin Model simulations

Introduction  Selection as a test area  based on the availability of input data   results will be applied & provide benefit to the whole Chi River Basin at later stage Study area: Yang River Basin Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Introduction 4,145 Area = 4,145 km 2 83% Land use: 83% (3,440 km 2 ) = Agricultural land (Paddy field) 1978 Major flood events: 1978 entire Chi River Basin in the Yang River Basin 5 Sub-basins: 5 Yang River Basin Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Problem definition Large flood in 2001  315 Large flood in 2001  Precipitation = 315 mm/3 days Stagnant & urban flooding ‘How will the ongoing changes in land use, water management & flood protection influence floods & flooding, and what would be the implications for flood management?’ Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Objectives of the study ‘To identify both structural & non-structural measures in order to reduce flood impacts to settlements in the Yang River Basin’ Under different scenarios Under different scenarios:   Storage improvement   Land use management changes Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Model setup Hydrologic model (SWAT)  Hydrologic model (SWAT)  calculate a tributary inflow at all the selected points on the river system. Hydraulic model (1D/2D SOBEK)  Hydraulic model (1D/2D SOBEK)  simulate the flow of water through a river channel network & create flood inundation extents for the specified return periods. Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Model setup SWAT1D/2D SOBEK, Relationship between SWAT & 1D/2D SOBEK, & the physical aspects Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Calibration results  1/6/ /10/2001  Calibration period: 1/6/ /10/2001 SWAT simulations Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations   Calibration accuracy:

SWAT simulations Table:areal rainfallpeak discharges Table: Calculated areal rainfall & peak discharges for various return periods (ignoring flooding). SWAT  SWAT  calculate a tributary inflow on the Yang River for different return periods.  SWAT Observed rainfall for each station (4 stations)  converted into areal rainfall (Thiessen method) & used as input to SWAT. Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

1D SOBEK calibration results  1/6/ /10/2001 Calibration period  1/6/ /10/2001  Manning’s roughness Calibration parameter  Manning’s roughness 1D/2D SOBEK simulations Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

1D/2D SOBEK simulations Natural flood storage 100-year10-year Target: arbitrarily set to limit a 100-year incident event to a 10-year target peak flow. Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

1D/2D SOBEK simulations The modelled flood extents Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

1D/2D SOBEK simulations Possible natural flood storage simulated 100-year flood level Basis: considering the simulated 100-year flood level in relation to the topographic condition at the locations where the floodwater needed to be released to reduce flood risk downstream Starting risk location Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

1D/2D SOBEK simulations Possible natural flood storage Peak discharge reduced with the magnitude close to the 10-year event. Comparison between the simulated 100-year discharge with/without flood retention for the same section of the downstream river channel. Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Discussions  The methodology used to investigate the flood management measures  Numerical modelling of inundation processes at the floodplains. SWAT  SWAT  calculate the tributary inflow to the Yang River.  SWAT Calibration results  SWAT is able to simulate the discharge reasonably. However, further calibration efforts for longer period & model validation are required to obtain more accurate/reliable results. 1D/2D SOBEK  1D/2D SOBEK  predict flooding behaviour within a river system for different flood magnitudes. 1D/2D SOBEK  1D/2D SOBEK simulation outputs  it seems viable to provide enough flood storage to limit 100-year event to 10-year target peak flow by using the natural floodplain. 3 flood storages+flood diversion channel  3 flood storages + flood diversion channel were identified by considering the calculated 100-year flood level in relation to the topographic condition upstream of the risk location  The simulated 100- year peak discharge was reduced only about 3.5% with the magnitude close to the 10-year target peak flow. Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Conclusions SWAT & 1D/2D SOBEK Coupling of SWAT & 1D/2D SOBEK is being developed to enable a better modelling of the actual physical behaviour & processes. Storage improvement land use & management Storage improvement options have been formulated as proposed flood management scheme. A step towards land use & management scenarios. 1D/2D SOBEK 1D/2D SOBEK is found capable of simulating flood events with/without flood mitigation measures & delivering reliable results. Moreover:   It can provide the maximum inundation level, which is predominant for determining the damage at downstream risk location.   A sustainable method of flood risk management can also be established at later stage, which will be a useful component additionally to conventional flood defences in the Chi River Basin. Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

Recommendations Future study will consider:  Areas that could not be used for storage  Built-up area, i.e. urban, towns, villages (to minimize damage). Buffer zone to protect the built-up area from flooding. Full operation of flood detention basins, with many levels of floodwater sub-diversions, based on different floodwater levels. Examine the economic impacts for land within the assumed flood extent for flood storage by:   Categorize the land use types with respect to elevation   Define the cost for each land use type   Estimate an inundated area (incl. damage) per land use type Introduction Objectives Problem definition Model setup Discussions Recommendations Conclusions Model simulations

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