10 th International Conference on Hydroinformatics HIC 2012, Hamburg, GERMANY RAINFALL-RUNOFF STUDY FOR SINGAPORE RIVER CATCHMENT CHI DUNG DOAN (1)(3), JIANDONG LIU (1), SHIE-YUI LIONG (1), ADRI VERWEY (2), DADIYORTO WENDI (1), ANH TUAN DAO (1) (1): Tropical Marine Science Institute, National University of Singapore, 12A Kent Ridge Road, Singapore, 119223 (2): Deltares,, Rotterdamseweg 185, Delft, The Netherlands (3): Singapore-Delft Water Alliance, Block E1 Level 08-25, No 1 Engineering Drive 2, Singapore, 117576 Several rainfall-runoff concepts are studied for Singapore River catchment. The urban rainfall-runoff model with Horton infiltration equation, the Green-Ampt infiltration method, the exponential loss method (also called HEC s nonlinear loss-rate function), as well as the very simple constant loss rate method are investigated for this highly urbanized catchment in Singapore. Also, some of the above mentioned approaches are further explored in a gridded domain. The study utilizes the software packages HEC-HMS with HEC-RAS (U.S. Army Corps of Engineers) and SOBEK (Deltares) to accomplish the task. The results and the suitability of applying each model for this typical highly urbanized catchment are discussed. Keywords: Singapore, rainfall-runoff modeling; urbanized catchment; HEC; SOBEK. INTRODUCTION Singapore River is approximately 15 kilometers long from its source at Kim Seng Bridge to its mouth at the Esplanade, where it flows into the Marina Channel and finally into the Singapore Strait [1]. Singapore has a tropical rainforest climate with no distinctive seasons and temperatures ranging from 22 C to 34 C. June and July in Singapore are the hottest months, while November and December are generally wetter in the monsoon season. Moreover, forest and nature reserves only occupy about 23 percent of Singapore s land area. Therefore, Singapore is a highly urbanized city. The Singapore River catchment area (Figure 1) is about 18 square kilometers. In 2010 and 2011, several floods, caused by heavy downpours, inundated some main roads in the downtown area. Especially on 16 June 2010, the excess water submerged some shopping malls and car park basements along the Orchard Road in the central business district. For a good understanding of what happened it is very important to describe an accurate relationship between rainfall and runoff as boundary conditions for hydrodynamic simulations. The main focus of the paper is to investigate the effects of excessive rainfall events by comparing various approaches in the modeling of the rainfall-runoff processes, in order to recommend the most suitable approach for a highly urbanized area like Singapore. For this
purpose, several rainfall-runoff models implemented in two software packages (SOBEK- Urban from Deltares and HEC-HMS with HEC-RAS from the US Army Corps of Engineers) were chosen for the simulation of Singapore River catchment. Figure 1. Marina catchment and Singapore River sub-catchment METHODOLOGY Each conceptual rainfall-runoff model consists of a transformation process from precipitation to runoff by various types of loss (infiltration and percolation) methods and surface runoff routing to river and channel systems. The rainfall-runoff models investigated are listed in Table 1. Table 1. Overview of rainfall-runoff models with their loss/infiltration and surface routing concepts Software package SOBEK-Urban HEC-HMS with HEC-RAS HEC-HMS alone Rainfallrunoff model SOBEK- Urban HEC-HMS HEC-HMS with gridded precipitation Loss (infiltration) method Horton infiltration Initial and constant Exponential loss Green-Ampt infiltration Initial and constant Routing method Linear reservoir Linear reservoir /kinematic wave ModClark In the study, the Singapore River catchment is divided into 24 sub-catchments for the simulation of the rainfall-runoff process. The time steps of simulation in HEC-HMS with HEC-RAS and SOBEK-Urban are 1 minute.
SOBEK-Urban SOBEK-Urban is an integrated 1D and 2D simulation model, developed by Deltares. SOBEK-Urban incorporates hydrology and 1D and 2D hydrodynamic modules [6]. In SOBEK-Urban, after subtraction of the initial loss, the infiltration is estimated by utilizing the Horton equation. Horton [2] presented a three-parameter empirical equation for the infiltration capacity expressed by (1) where: is the loss rate (mm/hour); is the initial infiltration capacity (mm/hour); is the final constant infiltration capacity (mm/hour) and is the factor representing the rate of decrease in the infiltration capacity. The Horton infiltration equation is quite popular because of its simplicity. However, some skill is required to estimate the parameters of the Horton infiltration equation from experimental data based on the land use and soil characteristics in each area. In SOBEK- Urban, each sub-catchment is mainly divided into (semi) pervious and impervious areas. The parameters of Horton equation have to be determined for up to four classes of such areas. After extracting losses and infiltration from total rainfall, a linear reservoir method is utilized for surface routing. HEC-HMS The U.S Army Corps of Engineers developed a suite of water resources simulation packages, containing a hydrological modeling system (HEC-HMS), river analysis system (HEC-RAS) and related tools. HEC-HMS is designed to simulate the precipitation-runoff processes of dendritic watershed systems [7]. HEC-RAS performs one-dimensional steady and unsteady flow river hydraulics calculations [8]. In the study, HEC-RAS simulates the hydrodynamics with the inflow derived from HEC-HMS. HEC-HMS provides functionality for lumped as well as gridded modelling. Three loss (infiltration) methods are presented in the lumped model: i) initial and constant loss method; ii) exponential loss method; and iii) Green-Ampt infiltration method. Similar with SOBEK, each sub-catchment can be divided into pervious and impervious areas for each lumped loss method. The kinematic wave is selected as the runoff routing method for all three loss methods. Initial and constant loss method The initial and constant loss method is a very simple method, which contains only two parameters, initial loss and constant infiltration rate. The initial loss specifies the amount of precipitation that will be initially lost by depression, tree canopies, etc. before infiltration and surface runoff begins. The constant rate determines the rate of infiltration that will
occur after the initial loss is satisfied. Both parameters are determined by insight followed by calibration trials. Exponential loss method (HEC s nonlinear loss-rate function) The Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers has developed an exponential loss method [4], describing the parts of precipitation not available to direct runoff. The exponential loss method can be expressed by where: is the loss rate (mm/hour); is the loss coefficient at the start of a storm; is the coefficient controlling the rate of decrease; is the accumulated loss during the storm (mm); is the rainfall intensity (mm/hour) and is the exponent ranging between 0.3 and 0.9, with a most commonly selected value of 0.7. The parameters are storm dependent, which can be determined from monitored rainfall and runoff relationships. The exponential loss method is not suitable for continuous simulation. Green-Ampt infiltration method Green and Ampt [3] proposed an infiltration method based on Darcy s law of soil water movement. The Green-Ampt infiltration equation is expressed by where: is the loss rate (mm/hour); is the effective hydraulic conductivity (mm/hour); is the wetting front soil suction head (mm); is the porosity; is the initial moisture content and is the cumulative infiltration. As compared to the empirical infiltration equations, the parameters, and of the Green-Ampt model can be computed from the soil properties. Gridded precipitation in HEC-HMS The gridded model is also explored and still on-going. The gridded precipitation is used as input of the rainfall-runoff model. In the gridded model, the aforementioned three loss methods can be applied for each individual grid cell. However, for surface routing in the gridded rainfall-runoff model only the ModClark [9] transformation is available. The ModClark transformation method is developed based on Clark s UH conceptual method, which accounts for a time-area relationship of the watershed, translated to a hydrograph and routed through a linear reservoir [9]. Different from Clark s UH method, the ModClark method considers the spatial variability of the stream runoff transformation and the spatial variability of the rainfall. The spatial distribution in ModClark is represented by a collection of grid cells covered by a watershed. Each cell, defined with uniform properties, is characterized by parameters to describe the flow length of the cell and the area it covers [10]. Each cell accounts for the specific rainfall amount it receives. Losses are subtracted to define the excess precipitation to be transformed by the ModClark method (2) (3)
to generate specific cell hydrographs. The entire set of derived cell hydrographs is then added together to represent the total runoff at the catchment outlet. This gridded model is still under exploration. In this study only lumped loss methods are evaluated and simulated by HEC-HMS alone, without integration with a hydrodynamic simulation based upon HEC-RAS. RESULTS AND DISCUSSIONS The performance of different rainfall-runoff models is examined for two periods with observed rainfall and discharges recorded at 10 minute intervals. It is noted that the data collected for these periods are from the time before the Marina Barrage was completed. Therefore, the hydrographs exhibit the tidal influence. The first rainfall period started on 17 Dec 2006 and ended on 20 Dec 2006. This event has been used for calibration. The second period selected is from 26 Dec 2006 to 28 Dec 2006 and has been used for validation ( Figure 2). a) Observed rainfall and discharge for calibration (17 Dec 2006 20 Dec 2006) b) Observed rainfall and discharge for validation (26 Dec 2006 28 Dec 2006) Figure 2. Hyetographs and corresponding hydrographs for calibration and validation Two performance indicators are used for the comparison. Correlation Coefficient (CC) (4) Nash Sutcliffe index (R2) (5) The results for calibration and validation obtained with different rainfall-runoff models are shown in Figure 4 and Figure 5, respectively.
Figure 4. Comparisons of hydrographs from different rainfall-runoff models for the calibration period Figure 5. Comparisons of hydrographs from different rainfall-runoff models for the validation period The correlation coefficients (CC) and Nash-Sutcliffe index (R2) for the calibration and validation events obtained for different rainfall-runoff models are listed in Table 2. Table 2. Correlation coefficients and R-squared values for calibration and validation events obtained for different rainfall runoff models
Calibration Validation CC R2 CC R2 SOBEK-Urban 0.76 0.56 0.82 0.61 HEC Constant 0.89 0.76 0.76 0.58 HEC Exponential 0.89 0.78 0.77 0.59 HEC Green- Ampt 0.89 0.78 0.77 0.59 HEC Gridded Precipitation 0.84 0.65 0.48 0.11 The performance indices show that 4 types of rainfall-runoff models ranging from simple to complicated perform equally well for the catchment setup. The decision of choosing the best rainfall-runoff model for Singapore River catchment is still inconclusive. Further research needs to be carried out with the following remarks. It is noted that the comparison is affected not only by the use of the different rainfallrunoff modeling concepts but also by the hydrodynamic routing. Although both SOBEK and HEC-RAS utilize dynamic wave routing along the river, different numerical schemes could result in slightly different outputs. It would be ideal if the comparison could be performed in the upstream sub-catchments only, where the flow is not affected by the tide or by backwater effects. In such case, the assessment can be carried out purely for the rainfall-runoff model without the need of coupling the hydrodynamic routing. However, until very recently Singapore s upstream sub-catchments rarely have flow gauging stations and that condition hinders us from doing so. It could be expected that the gridded rainfall model of HEC-HMS would give better or equally good result for the validation period. However, due to the fact that the model is not yet coupled with 1D hydrodynamic routing, the output could not reflect the effect of the tide, which is a major component in the validation period. CONCLUSIONS Four rainfall-runoff concepts have been assessed for an 18km 2 sub-catchment of Singapore River with rainfall and flow data monitored in Dec 2006, when Singapore River was not yet dammed by the Marina Barrage. These concepts range from a simple initial and constant loss model to the physically-based Green-Ampt method. The comparison, for this calculation period, temporarily reveals that 4 methods perform equally well. More data would be needed and further study would have to be performed to achieve conclusive results. The results of HEC-HMS with gridded precipitation are very encouraging and the incorporation of HEC-RAS in the simulations will be carried out in the near future.
ACKNOWLEDGMENTS The authors gratefully acknowledge the support & contributions of the Tropical Marine Science Institute and Singapore-Delft Water Alliance ( Multi-objective Multiple Reservoir Management research programme (R-264-001-005-272)). REFERENCES [1] Wikipedia, Singapore River, retrieved from http://en.wikipedia.org/wiki/singapore_river on 6th Jan 2012. [2] Horton, R. E., Analyses of Runoff-Plat Experiments with Varying Infiltration Capacity, Trans. Am. Geophys. Union, Vol. 20, (1939), pp. 693-711. [3] Green, W. H. and Ampt, G., Studies of Soil Physics, Part I: The Flow of Air and Water through Soils, J. Agric. Sci., Vol. 4, No. 1, (1911), pp. 1-24. [4] Feldman, A. D. and Goldman, D. M., Infiltration and Soil Moisture Redistribution in HEC-1, TP-95, U.S. Army Corps of Engineers, Hydrologic Engineering Center, Davis, CA, (1984). [5] Bupta, Ram S., Hydrology and Hydraulic Systems, Second Edition, Waveland Press, Illinois, U.S., (2001). [6] Delft Hydraulics part of Deltares, SOBEK Online Help, Delft Hydraulics part of Deltares, Netherlands, (2009). [7] U.S. Army Corps of Engineers, Hydrologic Modeling System HEC-HMS, Technical Reference Manual, CPD-74B, U.S Army Corps of Engineers, Hydrologic Engineering Center, Davis, CA, (2000). [8] U.S. Army Corps of Engineers, HEC-RAS, River Analysis System Hydraulic Reference Manual, CPD-69, U.S. Army Corps of Engineers, Hydrologic Engineering Center, Davis, CA, (2010). [9] Kull, D., Nicolini, T., Peters, J. and Feldman, A., A Pilot Application of Weather Radar Based Runoff Forecasting, Salt River Basin, MO, U.S. Army Corps of Engineers, Hydrologic Engineering Center, Davis, CA, (1996). [10] Scharffenberg, W. A., HEC-HMS User s Manual Version 2.1, U.S. Army Corps of Engineers, Hydrologic Engineering Center, Davis, CA, (2001).