Flood-level Forecasting Using Intensity-Duration-Frequency Curves

Transcription

1 Australan Journal of Basc and Appled Scences, 3(4): , 2009 ISSN Flood-level Forecastng Usng Intensty-Duraton-Frequency Curves 1 2 A.R.Ghavaseh, A. Norouz 1 Assstant professor, Power and Water Unversty of Technology, Tehran- Iran. 2 M.S. Rver Engneerng, Power and Water Unversty of Technology, Tehran- Iran. Abstract: One of the effectve flood damage mtgaton methods s usng Flood Warnng System. The system s bascally conssted of three elements: Montorng, Forecastng and Decson makng. Ths paper presents a useful method whch can be appled n flood forecastng by means of Intensty- Duraton-Frequency ranfall curves. Dependng on senstvty and mportance of the propertes n a vulnerable zone of flood plan and the level of protecton needed, a desgn IDF curve may be consdered for the zone. Then, the precptaton for the next tme nterval (say 30 mnutes) can be extracted from the curve. Afterwards, ranfall-runoff and flood routng models may be used to determne the flood level at floodplan. The procedure was appled to the flood warnng system of Madarsou basn n north-east of Iran. Madarsou basn s one of the most vulnerable areas to floodng n Iran and ts flood warnng system has been nstalled n The precptaton hyetographs of 26 flood events recorded from 1983 to 2004 were acqured. Settng the results of the ranfall-runoff and flood-routng models as references, the effcency of the procedure was verfed by applyng on-tme forecastng. Checkng out several IDF curves, confrmed that for the Madarsou basn, the 10 year curve s a proper desgn curve whch deduces the most accurate results. Key words: flood warnng system, flood level forecastng, ntensty-duraton-frequency curves, forecastng model. INTRODUCTION Gorganroud basn s one of the man Caspan basns n north of Iran. Flood damage news n the basn s not chockng now. Nearly each year, a serous flood takes place n ths basn. There were 433 losses of lves just n Most of the dead were not natve, they were toursts. Madarsou s one of the Gorganroud subbasns. However, land use changes n the basn s the man cause of flash floods, but t seems that launchng a flood warnng system could decrease sgnfcantly dead losses n the basn. One of the flood dsaster characterstcs s the lag-tme between the precptaton event and the flood peak occurrence n a basn : Lead tme s the nterval tme between flood warnng and flood event n a specfc flood zone elevaton (Norouz, A., et al 2007). The am of a flood warnng system s to maxmze the lead tme. Accurate forecastng of the ncdent moment and the ntensty of flood peak perod s vtal to execute flood damage mtgaton programs on tme. These programs, ncludng people and propertes evacuaton, makng temporary flood barrers and movng all obstacles out of access roads, must be desgned or prepared before a flood event. Usually, three components of a flood warnng system are: montorng, forecastng and decson-makng (Krzysztofowcz, R., 1993). Montorng needs some clmatologc and rver flow measurng nstruments to record and transfer the weather, ranfall and water level nformaton to a control center. Forecastng refers to a stochastc predcton ranfall model (e.g. Artfcal Neural Network, ARMA, ARIMA), a ranfall-runoff and flood routng model (e.g. HEC-HMS). So the relablty of flood warnng not only depends on a consstent and on tme measurng and communcaton factors but also requres an adequate long ranfall and rver flow tme seres to guaranty proper model calbraton and satsfactory results (Chow, V.T., et al. 1988). Decson-makers usually want to ncrease the relablty and enlarge the lead tme of the system by usng advanced montorng tools, mprovng forecastng methods and performng warnngs at lower flood levels n rver. Consderng the trade-off between relablty and lead tme, ncreasng one wll decrease the other. Readers nterested n ths topc may refer to Krzysztofowcz papers ((Krzysztofowcz, R., 1993), (Krzysztofowcz, R., et al, 1994)). However, based on montorng nstruments and technques, applyng a Correspondng Author: A.R.Ghavaseh, Assstant professor, Power and Water Unversty of Technology, Tehran- Iran. E-mal: ghavaseh@pwut.ac.r 4384

2 Aust. J. Basc & Appl. Sc., 3(4): , 2009 relable flood forecastng method not only ncreases the lead tme but also t wll decrease the uncertantes of the system whch leads to false warnngs or mssed floods (Krzysztofowcz, R., et al, 1994). Ths paper proposes a method based on usng ranfall IDF curves to apply n forecastng procedure. It may be appled n flood warnng systems, especally where there s no relable and adequate observed data and so regonal hydrologcal methods should be used. Furthermore, the effect of the proposed approach wll be assessed on potental lead tme of flood warnng system of Madarsou basn, n north east of Iran. Methodology: Recorded ranfalls for 22 years are avalable n the basn. Ranfall-runoff and flood routng modellng s appled to selected events to deduce flow dscharges and flood levels n dfferent zones of the basn. The smulated nformaton s used as observed data. On the other hand, by applyng a real-tme forecastng for each event, water level n the zone may be estmated whch s used as forecasted data Desgn Curve Determnaton: The approach profts ntensty-duraton-frequency curves for flood forecastng and estmaton of tme-stage curves for a control cross secton. Prncpally, the ntensty-duraton-frequency curves can be obtaned for varous return perods based on the recorded ranfall events. Accordng to the accepted flood rsk for the downstream vulnerable zone, one of these curves should be selected as Desgn Curve. A hgh return perod curve (e.g. 50 years) leads to premature warnng and so wth hgh uncertanty. It wll augment the false warnngs. In contrary, a low return perod curve (e.g. 2 years) may cause n retard warnngs. It wll ncrease the mssed floods. So dfferent return perod curves should be checked for the recorded ranfalls and floods to select an optmum return perod curve. Another varable n the IDF curves s the ranfall duraton whch should be determned before forecastng. As the ranfall events are proposed to be ndependent events, by settng a forecast tme nterval, at each moment, the same precptaton duraton may be appled for all ranfall forecastng n an event. The forecast tme nterval can be set from 10 mnutes to one hour. However, for ts determnaton, the clmatologcal and physcal characterstcs of the basn should be consdered. For a small mountanous basn wth severe ranfalls a short forecast tme nterval should be consdered, but t can be longer for a vast basn where flash floods are rarely seen. Recorded precptaton events are helpful to determne the IDF curve return perod and the forecast th duraton (.e. desgn curve wth forecast tme nterval): for the nterval, the total precptaton at the end of tme nterval, hf, s the sum of real tme observed precptaton at the begnnng of the tme nterval, ho, and the forecasted precptaton n the t mnutes tme nterval, ht : hf = ho + ht (1) ht s extracted from the consdered desgn curve wth t mnutes ranfall duraton. The forecasted precptaton, hf, can be compared wth the total recorded precptaton at the end of tme nterval, ho : +1 ho +1 = ho + Äho (2) where Äho s the precptaton recorded for the tme nterval. If the forecasted precptatons for varous tme ntervals n dfferent observed ranfall events were not satsfactory, the desgn IDF curve should be changed. It means that an alternatve curve wth dfferent return perod should be consdered. For the Madarsou basn, the 2 year to 50 year return perod curves wth dfferent tme ntervals were checked and fnally the 10 year return perod curve wth 30 mnutes tme nterval was selected as the desgn curve and the tme nterval for forecastng Forecastng Procedure: When precptaton begns n a basn, the water level s montored at the control secton contnuously. If the water level exceeds a predefned level (.e. trggerng threshold), the montorng component actvates the forecastng component. It estmates at frst, the precptaton of the next tme nterval (e.g. 30 mnutes) and then, models the output hydrographs and the water levels n the flood vulnerable areas as well (Krzysztofowcz, R., 1993). In the decson makng component, based on the flood level and the vulnerablty of the zone, 4385

3 Aust. J. Basc & Appl. Sc., 3(4): , 2009 warnng to the concerned sectors wll be actvated (Fg. 1). The warnng may be dvded nto three levels: low, medum and hgh. An acton plan, accordng to each warnng level, wll be proceeded. The acton plans must be prepared before dsaster events, based on potental local and regonal facltes. It should be noted that f the concentraton tme of a basn s long enough, just montorng the ranfalls n sub-basns may be suffcent to estmate the flood level n the vulnerable downstream zones. So the precptaton estmaton for the next tme nterval may be not needed n all cases. Fg. 1: Flood Warnng system components To estmate the precptaton (ranfall depth), the desgn IDF curve and the forecastng tme nterval must be defned. The poston, sgnfcance and vulnerablty of the downstream zone as well as the physologcal and clmatologcal features of the basn should be consdered to determne a proper IDF curve and the best forecastng tme nterval. 3. Madarsou Basn Case Study: 3.1 Flood Warnng System of Madarsou Basn: Madarsou Rver basn, whch s located n the north east of Iran, s one of the dsaster-affected areas n ths regon. The number of vctms durng 2001 and 2002 summer floods were 260 and 60, respectvely. Furthermore, thousands of lvestock were lost and a lot of nfrastructures, such as brdges and roads, were washed out or destroyed. 4386

4 Aust. J. Basc & Appl. Sc., 3(4): , 2009 The Madarsou rver basn s located n the Golestan provnce. Its source s located n the Alborz Mountans. It flows from the east to the west through the northern part of the country, and jons the Gorgan rver that fnally ends to the Caspan Sea (Fg.2). It receves the most mportant trbutary, Cheshmekhan rver, near Dasht vllage. The Madarsou rver has a catchment area of 2,360 square km and a length of about 100 km. The populaton n ths basn s about 60,000 people and the average annual ranfall s close to 1,000 mm. Fg. 2: Locaton of Madarsou basn and vulnerable areas (Japan Internatonal Cooperaton Agency, 2006). The destructve flood events n 2001 and 2002, whch caused mportant lfe and property loses, were the man ncentve of nstallng the rver flood forecastng and warnng system n In ths paper, as the case study of presented methodology, three vulnerable areas (.e. Dasht vllage, brdge of Golestan road and Tangrah vllage), and one water level control cross secton at the brdge of Dasht were selected (Fg. 2). Accordng to topographcal characterstcs and land use condtons, the warnng thresholds are assumed to be at 3m, 3.5m and 4m for each recorded flood event smulaton. The 10 year IDF curve wth 30 mnutes forecastng tme nterval s selected for the ranfall depth estmaton Modellng and Forecastng: Lackng precptaton staton n the basn, recorded ranfalls by two neghborng statons were used. By applyng the so-precptaton maps of 2001and 2002 summer floods, Fg. 3, the data recorded n Laleh-bagh and Agh-ghala statons was allocated to Cheshmehkhan and Madarsou reaches, respectvely. As a result, the ntensty-duraton-frequency curves of Madarsou and Cheshmehkhan are plotted n the fg. 4. Amongst the 260 recorded ranfalls n Laleh-bagh and Agh-ghala statons, 26 events were selected. These events were both smultaneous and the synthetc smulaton results showed that these events cause flood n the Madarsou and Cheshmehkhan reaches. The 10 year return perod IDF curves, selected as desgn curves, yeld 10.23mm and 3.97mm ranfalls n 30 mnutes for Agh-ghala and Laleh-bagh statons, respectvely. One trggerng threshold was selected to trgger the forecastng component (.e. S1 = 2.5m). When the water level reaches to the trggerng threshold at the control secton, the forecastng component wll be actvated and the flood hydrograph and the flood level wll be predcted for the vulnerable zones. To realze whether the water level reaches to the trggerng threshold, the ranfall-runoff smulaton was appled n real-tme modelng for every 26 ranfall events. The ranfall was transformed to runoff and then t was routed n the rver reaches to control pont usng HEC-HMS model. If the smulated flood level exceeds the trggerng threshold, the forecastng component s actvated and otherwse the ranfall-runoff smulaton process s repeated for the next precptaton tme nterval. Clarke model s consdered for ranfall-runoff smulaton and Muskngham-Cunge method for flood routng (Moussa, R. et al., 1996). The downstream hydrographs were transformed to tme-stage dagram at control 3/2 secton by usng rectangular wer formula Q = 1.7BH, where B s the wdth of brdge and H s the flow depth. The relatonshp was calbrated by the 2005 flood at control secton. Table (1) presents the flood peak estmatons 4387

5 Aust. J. Basc & Appl. Sc., 3(4): , 2009 a) summer flood n 2000 b) summer flood n 2001 Fg. 3: Iso-precptaton map (Japan Internatonal Cooperaton Agency, 2006) for the consdered flood events. Forecastng procedure s stopped when the flood level arrves at warnng threshold. That's why the maxmum forecasted flood levels are less than the maxmum observed flood levels. When the forecastng results show floodng stuatons, the forecastng procedure s stopped and t s decded to warnng. Fg, (5) shows the procedure flow dagram. 4388

6 Aust. J. Basc & Appl. Sc., 3(4): , 2009 Table 1: The Results of hydrologcal smulaton of Madarsou basn for 26 flood events NO. Maxmum Observed Maxmum Forecasted Maxmume Observed Maxmume Forecasted Flowrate Flowrate Level Level 3 (m /s) 3 (m /s) m m Fg. 4: Intensty-Duraton-Frequency curves 4389

7 Aust. J. Basc & Appl. Sc., 3(4): , 2009 Fg. 5: Procedure of observed (H) and forecasted (S) flood level estmaton based on precptaton depth n Madarsou basn 3.3. PLT Calculaton: The tme perod between a flood warnng and the flood occurrence n a zone s called Potental Lead Tme (Krzysztofowcz, R., 1993). Decson makers prefer to ncrease the PLT as much as possble to decrease flood damages. But, there s a tradeoff between the potental lead tme and the relablty of flood warnngs. It means that, n general, by ncreasng the PLT, the number of false warnngs ncreases too. However, by usng the IDF curves, forecastng tme procedure decreases and the PLT wll be ncreased wthout any real effect on warnng relablty. Besdes the physologcal characterstcs of basn, PLT depends on the tme of data collecton and the trggerng threshold. The use of IDF curves decreases the data collecton tme by excludng the meteorologcal forecast/modelng. In Madarsou basn, the real tme needed for the meteorologcal forecastng s not avalable. So, comparng the PLT wth/wthout IDF curves s not possble. However, t may be estmated based on the trggerng threshold, S1, and the flood celerty. All of the PLT calculatons were carred out n control pont. Control pont s very close to Dasht Vllage and so the calculated PLT can be extended to the vllage. The flood travelng tme from control pont to Brdge and Tangrah s 2 and 4 hours, respectvely. 4390

8 Aust. J. Basc & Appl. Sc., 3(4): , 2009 Consderng three warnng threshold for each vulnerable zone, PLT s computed and presented n the table (2). By ncreasng the zone elevaton, PLT ncreases as well. Table 2: PLT (hr) for three senstve areas wth constant trggerng threshold Locaton Zone Elevaton m 3.5m 4m Dasht Vllage Brdge of Glestan Road Tangrah Vllage Conclusons: Proper usng of Flood Warnng System can reduce sgnfcantly the flood damages of a vulnerable zone. Decson makers may proft the Potental Lead Tme deduced by on-tme warnngs n a relable FWS to evacuate people and propertes, and to prepare the zones n flood rsk (accordng to the Flood Acton Plan). A desgn IDF curve may be used n forecastng component of FWS to ncrease the PLT. The desgn curve should be selected based on senstvty of the zone and densty of people and propertes. In ths paper, a procedure for flood level forecastng, based on the precptaton, s developed. To trgger a flood warnng, ths forecasted level must be compared wth observed flood levels. In Madarsou basn, the 10 year return perod IDF curve deduces very close but underestmated forecasted to observed flood peaks. Accordng to the land use of regon, the curve s sutable. But n the future, f the densty of propertes changes, the desgn curve should be changed too. ACKNOWLEDGMENTS The study was fnancally supported by Water Resources Management organzaton of Mnstry of Energy n Iran. Useful comments on the text were provded by Dr. S.Almohammad.(PWUT). REFERENCES Chow, V.T., et al Appled Hydrology, McGraw-Hll, pp: 572. Japan Internatonal Cooperaton Agency, The Study on Flood and Debrs Flow n the Caspan Coastal Area Focusng on the Flood-Ht Regon n the Golestan Provnce, Fnal Report. Krzysztofowcz, R., A Theory of flood warnng systems, Water resources research, 29(12): Krzysztofowcz, R., et al., Relablty of flood warnng systems, Journal of Water Resources Plannng and management, 120(60): Mays, L.W., Hydraulc Desgn Handbook, McGraw-Hll, pp: Moussa, R., C. Bocqullon, Crtera for the choce of flood-routng methods n natural channels, Journal of Hydrology, 186(1-4): Norouz, A., A.R. Ghavaseh and J. Attar, Uncertanty evaluaton of Flood Warnng System: nd Relablty and Trade-off, Electronc Proceedng of 32 Congress of IAHR, Vence, Italy. Poulard, C. and A.R. Ghavaseh, et al., Dynamc Slow Down: From Integrated Management to Flood Mtgaton, Internatonal Conference "Toward natural Flood Reducton Strateges", Warsaw, Poland. 4391