MODELING OF RIVER ICE BREAKUP DATE AND THICKNESS IN THE LENA RIVER

Transcription

1 Ice n the Envronment: Proceedngs of the 16th IAHR Internatonal Symposum on Ice Dunedn, New Zealand, 2nd 6th December 2002 Internatonal Assocaton of Hydraulc Engneerng and Research MODELING OF RIVER ICE BREAKUP DATE AND THICKNESS IN THE LENA RIVER X. Ma 1, T. Yasunar 1 and Y. Fukushma 2 ABSTRACT In order to determne the cause of floodng, ths study analyzed meteorologcal data and modeled ce breakup dates. The data were for 16 years: 9 years (from 1986 to 1994) from the GAME-Sbera Commttee and 7 years (after 1994) from NOAA/NCDC. The GAME-Sbera Commttee has rver ce data for the Lena Rver basn for October to May n ten-day sets from 1986 to Ths study used a smple accumulated degree-day method to estmate rver ce breakup dates n cold regons. We prevously appled the method to the Lena Rver basn for and showed that the breakup dates of 43 rver sectons n the basn could be modeled. Here, four sectons (Yakutsk, Olekmnsk, Vtm and Krensk) of the Lena Rver were selected and a long-term smulaton of breakup dates and rver ce thckness was carred out. The results showed that 1) the rver breakup dates and changes n ce thckness could be modeled; 2) over the prevous 16 years, the dfference between the breakup date upstream (Krensk) and that mdstream (Yakutsk) ranged from several to 69 days. INTRODUCTION Rver freezng s a common phenomenon n northern regons. The process of rver-ce formaton not only affects the rver dscharge regme, but also nfluences the envronmental and ecologcal systems n the basn or regon. The Lena Rver, one of the four largest rvers flowng nto the Arctc Ocean, freezes over completely from early December untl late Aprl. Each sprng, snowmelt and ce jams cause floodng n the Lena Rver basn; n the last 30 years, catastrophc floods have occurred n 1998, 1999, and 2001 (Global Actve Archve of Large Flood Events, artsc/geog/floods/). The 2001 flood was the worst n over 100 years. Although there s no conclusve evdence of a relatonshp between catastrophc floods and global clmate change, the clmate n the area has changed. Records of rver ce processes n the basn would help to montor clmate change; however, only three years of ce data for the basn (1986 to 1988) are avalable, from the GAME-Sbera Project. To estmate the varaton n rver ce processes assocated wth clmate change, ths study conducted a numercal analyss of rver ce break-up dates along the Lena Rver usng meteorol- 1 Fronter Research System for Global Change, Showamach, Kanazawa-ku, Yokohama , Japan 2 Research Insttute for Humanty and Nature, Owake-cho, Ktashrakawa, Kyoto , Japan

2 Fgure 1: Map of the Lena Rver basn showng the sx meteorologcal statons ogcal data. STUDY AREA The Lena Rver (Fg. 1) s one of the three largest rvers n Sbera; t s 4,400 km long, and flows from southern mountanous regons at 52 N lattude to the lower northern plans at 73 N. The mean annual temperatures range from about 11 C at Zhgansk to 3 C at Krensk. Table 1 shows the annual precptaton and temperature at four meteorologcal stes n the basn for the perod Accordng to data measured n 1986, 1987, and 1988, freezng starts near the outlet n late October and proceeds to the mddle and upstream n early and late November, respectvely. Thawng begns from the upstream reaches n late Aprl, and then extends nto the mddle and downstream regons. Downstream, the rver does not thaw untl late June (Ma and Fukushma, 2002).

3 Table 1: Annual precptaton and annual mean ar temperature of the four stes n the Lena Rver basn (1986~1994). Ste Lattude (degree) Longtude (degree) Elevaton (m) Precptaton (mm) Ar temperature ( C) Zhgansk Yakutsk Aldan Krensk METHOD Stefan s formula was used to predct ce thckness: 0.5 t 0.5 Ê2K ˆ È Z = Á Í ( Tm -Ts)dt Ë ρλ Ú (1) Î 0 where Z s the ce thckness, ρ s ce densty, λ s the latent heat of fuson, K s the thermal conductvty of ce, and T s and T m are the temperatures at the top and bottom of the ce layer, respectvely. Startng the ce decay process when the ar temperature turns postve, the tme at whch the ce depth reaches zero can be calculated usng: È QZ w t 1- -ρλ ( Zt -Z0) Kρλ ( Tm -Ts) Í K( Tm -Ts) t = - ln Í (2) 2 Qw ( Qw) Í QZ w 0 1- Í Î K( Tm -Ts) where t s the tme step, Z t and Z 0 are the fnal and ntal values of ce thckness durng the tme step perod, and Q w s the heat flux from water to ce. In general, the ar surface temperature of the ce s set to equal the ar temperature. Ths study gnored any snow layer. APPLICATION AND RESULTS Ths smple model was appled to the Lena Rver. Four sectons along the rver were selected: Krensk, Vtm, Olekmnsk, and Yakutsk (see Fg. 1). They are located upstream and mdstream n the rver. Meteorologcal data for 16 years were avalable; the frst nne years (1986 to 1994) were from the GAME-Sbera Commttee and the remanng data (after 1994) were from NOAA/NCDC. Both sets contan daly routne data, ncludng ar temperature, precptaton, etc. Rver ce thckness data for 1986 to 1988 are also avalable from the GAME-Sbera Commttee. Table 2: Comparson of rver breakup date between observaton and calculaton from 1987 to Secton Observaton Calculaton Observaton Calculaton Yakutsk 15 May 22 May 15 May 2 May Olekmnsk 5 May 5 May 5 May 1 May Vtm 5 May 5 May 5 May 28 Aprl Krensk 5 May 5 May 25 Aprl 28 Aprl

4 Table 3: Comparson of maxmum rver ce thckness between observaton and calculaton from 1987 to (unt: cm). Secton Observaton Calculaton Observaton Calculaton Yakutsk Olekmnsk Vtm Krensk The model was calbrated for the perod Ma et al. (2001) reported the rver breakup dates for 43 sectons of the Lena Rver basn for (Fg. 2). Table 2 shows modeled and actual rver breakup dates for four sectons of the Lena Rver n the perod Most of the dfferences are smaller than 10 days, ndcatng the success of the model. The calculated and observed ce thcknesses for the Lena Rver are compared n Table 3. Ths comparson shows that the model tends to overestmate the ce thckness because snow cover on the surface of the rver ce s not consdered. Nevertheless, the calculated results do show the observed trend to deepenng ce along the rver. Therefore, estmates of ce thckness can be used to predct changes n rver ce. Fgure 2: Comparson of observed and calculated breakup date at the 43 rver sectons n the Lena Rver basn n 1987 usng the smple model Fgure 3 shows a long-term smulaton of the breakup dates for the four sectons from 1987 to In the frst three years, the dfference between the breakup date at Yakutsk

5 and that at Krensk was less than 2 weeks. Snce 1990, however, the dfference has ranged from several days to over two months. A maxmum value of 69 days was seen n the sprng of 1998, a year n whch a dsastrous flood occurred n the downstream area. Fgure 3: Tme seres of the estmated rver ce breakup date along the Lena Rver from 1987 to 2002 CONCLUSION A smple model was used to smulate breakup dates and ce thckness on the Lena Rver to determne the mechansm of flood formaton. The performance of the model was checked for the calbraton perod The rver breakup dates and changes n ce thckness could be modeled. Long-term varaton n the rver ce breakup dates of the Lena Rver was estmated usng GAME-Sbera and NOAA/NCDC data. The result shows that the dfference between the breakup date at Krensk and that at Yakutsk ranged from several days to several months. A maxmum dfference of 69 days occurred n the sprng of REFERENCE Ma, X. and Fukushma, Y. A numercal model of the rver freezng process and ts applcaton to the Lena Rver. Hydrologcal Processes 16 (n press). Ma, X., Fukushma, Y. and Ohata, T. Hydrologcal modelng of rver ce processes n cold regons. In Sol-Vegetaton-Atmosphere Transfer Schemes and Large-Scale Hydrologcal Models (Proceedngs of a symposum held durng the Sxth IAHS Scentfc Assembly at Maastrcht, The Netherlands, July 2001), A.J. Dolman, A.J. Hall, M.L. Kavvas and T. Ok, eds., IAHS Publ. 270 (2001)