DIRECT RUNOFF HYDROGRAPH FOR UNGAUGED BASINS USING A CELL BASED MODEL P. B. Hunukumbura & S. B. Weerakoon Department of Civil Engineering, University of Peradeniya, Sri Lanka PUB Session 3rd AOGS Meeting July 2006

Overview Introduction Objectives of the study Model development Case study Results Conclusions

PUB - Relevance to Sri Lanka  Sri Lanka has 103 river basins with sizes varying from km 2. The basin is divided into several grids.  Water resources development potentials exist mostly in ungauged river basins : irrigation, water supply, small hydropower,..  Most basins/ sub basins are ungauged as there are only 35 stream flow measuring stations.

To develop a model to predict direct runoff hydrograph in an ungauged basin using basin’s rainfall, topography, land use and stream network data. Objective of the study

Model development The basin is divided into grids  Canal network is superimposed  Time required to flow water generated by 1 mm/hr rainfall from each grid cell to the basin outlet (travel time) is calculated S-curve for the basin is derived by cumulative distribution of the travel times Unit hydrograph of the basin is obtained from the S-curve

Model development…..  Rectangular grid cell is considered as a plane  Surface roughness for overland flow depends only on land use type  Single land use type for a given grid (uniform surface roughness)  Overland flow in the grid cell/ canal is a steady, uniform flow

Flowchart

Computerized model – (CellBasin Model) A computerized model with a Graphical User Interface (GUI) was developed using Visual Basic programming language Input file path File select dialog box

Case study The model was applied to the Upper Kotmale basin which is the upper most basin of the Mahaweli River, Sri Lanka. Model predictions were compared with the observed stream flow data at the basin outlet, Thalawakelle.

Upper Kotmale basin Basin area = 304 km2 Elevation= 1200m to 2500m Annual rainfall= 2200 mm to 2600 mm

GIS data used for the model 90 m SRTM - DEM 90 m Slope grid 90 m Flow accumulation grid 90 m Land use grid 90 m Flow direction grid

One hour unit hydrograph of the basin Results

Comparison of the Unit Hydrograph with the Unit Hydrograph derived form the observed data Comparison of the results

Observed and predicted direct runoff hydrograph Event 7/13/ :00 Comparison of the CellBasin Results

Observed and predicted direct runoff hydrograph Event 5/16/ :00 Comparison of the CellBasin Results

Observed and predicted direct runoff hydrograph Event 4/21/ :00 Comparison of the CellBasin Results

Conclusions The unit hydrograph obtained from the model was compared with the unit hydrograph derived from the observed data. Both unit hydrographs show similar in shape. The CellBasin model predictions for the Upper Kotmale basin is compared with the predictions obtained from the Snyder’s synthetic unit hydrograph of the basin developed using regionalized parameters. The CellBasin model predictions showed good agreement with the observed data than the Snyder’s unit hydrograph predictions.

Overland flow Overland flow can be modeled using Manning's equation by assuming the flow is steady, uniform and fully turbulent (Chow, 1988, Nurunnisa and Yilmaz, 2002). In this model Manning’s equation is used to calculate the flow velocity through each grid cell assuming steady, uniform, fully turbulent flow. The overland flow through a grid cell can be assumed as a flow through a wide canal and hence the value of the hydraulic radius ( R) is taken as the flow depth (Y) in calculating the flow velocity through a grid cell. As the final target is to get the unit hydrograph for the basin, the flow depth (Y) is calculated due to constant rainfall rate of 1mm/hr. Considering the continuity and Manning’s equation, Y (m) can be written as Flow Velocity through a grid cell is ;

Canal flow Measured cross section details and the condition of the canals are used to estimate a reasonable value for the hydraulic radius and roughness coefficient of the particular section of the canal. In this regard, starting point and ending point of the canal are defined by the cell value of the FAccGrid. For the canal section, If [Upstream FAccGrid value] < [(cell value of FAccGrid]< [Downstream FAccGrid value], then; Where, MnReach and HRadius represent Manning’s roughness coefficient and hydraulic radius of the canal respectively

Derivation of the observed unit hydrograph

Assuming the Upper Kotmale basin is ungauged, the Snyder’s unit hydrograph for the basin is obtained using the regional parameters the Snyder’s unit hydrograph are given bellow. Basin Area = 304 km 2 Length of the main stream = 31.5 km Distance from the centroid of the basin to the basin outlet = 13.3 km C t = 1.13 and C p = 0.48 Comparison of the results….. Comparison of the Unit Hydrograph with Snyder’s unit hydrograph model

Snyder’s coefficients for Sri Lanka

Snyder’s unit hydrograph for the Upper Kotmale basin Comparison of the CellBasin Results

Land use type Manning’s roughness coefficient Water body0.01 Build up land0.011 Tea0.17 Forest0.8 Grass land0.24 Other crop land0.17 Source: USDA, 1986, Nurunnisa and Yilmaz, 2002

Flow accumulation value Hydraulic radius Manning’s roughness coefficient Effective flow depth According to Table > Source: USDA, 1986, Connecticut, 2001

Source: USDA, 1986, Connecticut, 2001

Infiltration capacity of hydrological soil groups (SCS, 1986)