Technical note River modelling to infer flood management framework Darrien Yau Seng Mah1*, Ching Poon Hii2, Frederik Josep Putuhena2 and Sai Hin Lai3 1River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Penang, Malaysia 2Department of Civil Engineering (Water & Environment), Faculty of Engineering, Universiti Malaysia Sarawak, km-17 Kuching Samarahan Expressway, 94300 Kota Samarahan, Sarawak, Malaysia 3 Department of Civil Engineering, Faculty of Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia Abstract River hydraulic models have successfully identified the weaknesses and areas for improvement with respect to flooding in the Sarawak River system, and can also be used to support decisions on flood management measures. Often, the big ques- tion is ‘how’. This paper demonstrates a theoretical flood management framework inferred from Sarawak River modelling outputs. The river model simulates the movement of flood waters through river reaches. Information on flood levels and overtopping of embankments is used to guide a flood early warning system. The above-mentioned elements were combined in a logical framework that showed logic sequences and impact indicators for improved flood relief activities in the city of Kuching, Malaysia. Keywords: Flood, framework, InfoWorks RS, logframe, river modelling Background South China N The Sarawak River system has been the sub- Sea ject of numerous studies, due to the location of Kuching, the capital of Sarawak State, Malaysia, within the basin. The state government has invested huge amounts of resources and time Salak River in researching the river. Two very important Sarawak River outputs of those funded projects have been the East Malaysia States Kuching topographical and hydrological data essential to City Sabah river modelling (Dyhouse et al., 2003). Without Sarawak Kanan River these data, investigations aimed at understand- Sarawak ing the hydrological behaviour of the Sarawak River would be futile. Earth surface data repre- sented the elevation, profile curvature, flow path length, upslope area and conveyance capacity of the river. Hydrological data represented the Sarawak Kiri meteorological and rainfall-runoff mechanism River (Yu et al., 1999). However, as it is difficult to Kuching City Aerial View imagine the effects of these factors, a river computer model is useful in enabling visualisa- tion of the system (Liang and Molkenthin, 2001; Figure 1 Sinnakaudan et al., 2003). The modelling outputs support deci- Sarawak River Basin sion making (Hämäläinen et al., 2004; Oxley et al., 2004) and can be used, as in this study, to formulate a flood management tributaries, namely, Sarawak Kanan River and Sarawak Kiri framework. River. The 2 tributaries meet near Batu Kitang, some 34 km The Sarawak River (see Fig. 1) rises in the Bungoh Ranges upstream of Kuching. Kuching city is very flat and low-lying. to the south of Kuching city, at the border with Indonesia, Parts of the city are susceptible to flooding from fluvial and meanders across a wide coastal floodplain and through the tidal events. The average annual rainfall in the catchment is capital city before discharging into the South China Sea. The about 3 800 mm (Memon and Murtedza, 1999). river basin has an area of approximately 1 430 km2 and a On the eastern edge of the city, the Sarawak River divides, stream length of about 120 km, which consists of 2 principal and prior to 1998 2 exits to the sea to the sea existed, on the Sarawak River and Santubong River. In order to protect the * To whom all correspondence should be addressed. city, the Sarawak State Government blocked the 2 rivers and +604-5995874; fax: +604-5941036; allowed only 1 outlet to the sea, via the Kuching Barrage, e-mail: [email protected] aimed at controlling the river and tidal flows (KTA Consulting Received 14 April 2010; accepted in revised form 13 December 2010. Engineers, 1994). The river flow was modified from the natural Available on website http://www.wrc.org.za ISSN 0378-4738 (Print) = Water SA Vol. 37 No. 1 January 2011 ISSN 1816-7950 (On-line) = Water SA Vol. 37 No. 1 January 2011 121 Kuching City Aerial View 122 Stage-Time Boundary was imposed at the outlet the of bypass. modelled as an extended river channel to Salak River. Atidal bypass channel, anew barrage and tide The levels. bypass was pendent data the for second event, and including the flood onefor event, can be utilised to predict discharge, using inde begun (MWH Soft, 2008). 80% confidence of before any the intended investigationswere model had been to able mimic the real river behaviours at withable, acorrelation coefficient 0.80. over of The calibrated during the February 2003 flood eventis accept 2003) (DID, 2, theFig. matching the of recorded and modelled hydrographs As demonstratedal. (2005-2006) and Mah et al. in (2010). description the of modelling steps can be found in Putuhena et and modelled hydrographs Sarawak of River.Adetailed andnel floodplain friction as free parameters, against recorded simulating the existing conditions was calibrated, using chan as radial gates. With these hydrological data, abase river model as atidal Stage-Time Boundary. Barrage gates were modelled Boundaries. The downstream end, at the estuary, was treated et al.,2007). The 2upstream ends were treated as Flow-Time Kpg Git, which been have calibrated from time to time (Mah curves are available 2upstream for stations, Buan Bidi and thereHowever, is no direct measurement flow data. of Rating River, which record hourly rainfall and water data. level ling Sarawak River systems. reports Jenny of et al. (2007) and Salim on model et al. (2009) modelling,1-dimensional as previously (1-D) indicated the by data was the best available at the time. These were adequate for earth-surface observation datasets, the available topographical Consulting Engineers, In the 2003). absence more of advanced the Sarawak River Mitigation Options Study (Jurutera Jasa survey exercise was carried out in 2000 in conjunction with intervals 1km of in urban areas and 5km in rural areas. The survey exercise that produced river cross-sectional profiles at 1:50000 topographical map rural of areas and ariver corridor from 3sources: topographical a1:25000 map urban of areas, a The basin-wide digital terrain model (DTM) was derived topographical data (Benito et al.,2004; Sinnakaudan, 2009). simulation. Such amodel relies profoundly on the accuracy of tight coupling functionality GIS of and full dynamic flow lised modelling for the Sarawak RS involves River.InfoWorks A MWH River Soft model, Simulation InfoWorks (RS) was uti modellingRiver rivers. time the Sarawak River will be separated into and lower upper facilities are expected to be in full at which operation 2015, by Sarawak River wouldbe cut off asecond by barrage. These to Salak River (Jurutera Jasa Consulting Engineers, 2006). construction an of 8km floodbypass channel from Paroh Kpg the flooding problems, the stategovernment announced the 2003, January 2004 and January further To 2009. mitigate stream regime flow altered,floods major placetook February in significant till the river confluence at BatuKitang. tidal influx into the Sarawak River system.Tidal effects were was operated in such as away to allow acertain degree of ing barrage of gates flushing, for de-siltation and navigation, city (Sharp and the Lim, opening 2000). However, and clos constructed along aland isthmus just downstream Kuching of tidal regime to aregulatedby meansof a system flow, of gates The base model, carefully calibrated against discharge There are 24 hydrological stations along the Sarawak After the completion the of barrage, and with the down - - - - - - - Tebas) (at Muara Lock (c) Ship (a) Batu KawaBridge (b) Satok Bridge The river model computed the floodwater level floodand to produce an inundation analysis along the river corridor. engineering solutions. channel Both models 4). (Fig. enabled the exploration many of Barrage was included. Model 2incorporated the floodbypass existing mitigation conditions where the Kuching 3), (Fig. the upcoming floodbypass channel.Model 1 was derivedfor but has 2mitigation structures, the i.e. Kuching Barrage and did aany not have documented flood management framework, run through the developed model. The Sarawak River basin 100-year flood event 2004).(DID, The floodevent flows were tide table was accessible from the marine department. gaugesNo were available at Salak River at present. a However, Stage (m MSL) Stage (m MSL) Stage (m MSL) -3 -2 -1 -2 -1 -3 -2 -1 0 1 2 3 4 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 Model calibration of February 2003 floodevent at locations For purposesFor demonstration, of Model 2was utilised The January 2004 flood event wasgenerally quoted as a (a) Batu Kawa(a) Bridge, Satok (b) Bridge Ship and (c) Lock 0000 0000 0000 0100 0100 0100 ISSN 1816-7950 0200 0200 0200 0300 0300 0300 0400 0400 0400 0500 0500 0500 ISSN 0378-4738 (Print) = Water SA Vol. 37No.1January2011 Vol. SA Water ISSN 0378-4738(Print) = 0600 0600 0600 0700 0700 0700 0800 0800 0800 3-Feb-03 3-Feb-03 0900 3-Feb-03 0900 0900 1000 1000 1000 1100 1100 1100 1200 1200 1200 1300 1300 1300 1400 1400 1400 1500 1500 1500 1600 1600 1600 1700 1700 1700 1800 1800 1800 1900 1900 1900 2000 2000 2000 2100 2100 2100 2200 2200 2200 2300 2300 2300 0000 0000 0000 KawaBatu at Flood Bridge of Feb Calibration 0100 0100 0100 0200 Calibration of Feb Flood at U/S Ship Lock 0200 0200 Calibration of Feb Flood at Satok Bridge (On-line) 0300 0300 0300 0400 0400 0400 0500 0500 0500 0600 0600 0600 0700 0700 0700 (at Muara(at Tebas) 0800 0800 0800 4-Feb-03 4-Feb-03 0900 4-Feb-03 0900 0900 1000 1000 1000 Available onwebsitehttp://www.wrc.org.za Available 1100 1100 1100 1200 1200 1200 1300 1300 1300 1400 1400 1400 Figure 2 1500 1500 1500 1600 1600 1600 1700 1700 1700 = Water SA Vol.