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HomeMy WebLinkAboutSuWa116Alaska Resources Library & Information Services Susitna-Watana Hydroelectric Project Document ARLIS Uniform Cover Page Title: Hydrology and power studies to date, 15 April 2011 SuWa 116 Author(s) – Personal: John Haapala Author(s) – Corporate: MWH Americas Inc. AEA-identified category, if specified: Briefing and Technical Documents AEA-identified series, if specified: Series (ARLIS-assigned report number): Susitna-Watana Hydroelectric Project document number 116 Existing numbers on document: AEA11-022 TM-03-0002-041311 Published by: [Anchorage, Alaska : Alaska Energy Authority, 2011] Date published: April 15, 2011 Published for: Prepared for Alaska Energy Authority Date or date range of report: Volume and/or Part numbers: Final or Draft status, as indicated: Document type: Technical memorandum Pagination: 7 p. Related work(s): Pages added/changed by ARLIS: Notes: All reports in the Susitna-Watana Hydroelectric Project Document series include an ARLIS- produced cover page and an ARLIS-assigned number for uniformity and citability. All reports are posted online at http://www.arlis.org/resources/susitna-watana/ TECHNICAL MEMORANDUM HYDROLOGY AND POWER STUDIES TO DATE 15 APRIL 2011 ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 PREPARED FOR: ALASKA ENERGY AUTHORITY PREPARED BY: JOHN HAAPALA MWH AMERICAS INC. ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041511 MWH Americas Inc. Page 1 of 7 4/15/2011 TECHNICAL MEMORANDUM ON HYDROLOGY AND POWER STUDIES TO DATE The purpose of this technical memorandum is to summarize progress on hydrology and reservoir operation and power studies at the current early stage of studies. Results presented herein are preliminary. HYDROLOGY Hydrologic data is being gathered for two general purposes, first for presentation in the Pre- Application Document (PAD), and second for developing a long-term record of reservoir inflows and flood frequency data at the Watana dam site. Table 1 summarizes the available USGS flow data in the Susitna watershed. The two most useful USGS gaging stations for developing flow at the Watana dam site are the downstream gage at Gold Creek (6,160 square miles drainage area) and the upstream gage at Cantwell (4,140 square miles drainage area). For comparison, the drainage area at the Watana dam site is 5,180 square miles. Table 1: USGS Streamflow Gages in the Susitna Watershed Periods of concurrent flow data can be used for fill-in and extension of data sets, or for developing data at ungaged sites. Figure 1 shows the chronological availability of USGS flow data in the Susitna watershed. For example, the 17 years of Cantwell data is concurrent with the Gold Creek data, which provides a useful flow record both upstream and downstream from the Watana dam site. Figure 1 also shows an active period of flow gaging in the early 1980s. USGS Gage Number Gage Name Drainage Area (sq.mi) Latitude Longitude Gage Datum (feet) Available Period of Record 15290000 Little Susitna River near Palmer 62 61°42'37" 149°13'47" 916.6 63 years: 1948 - 2011 15291000 Susitna River near Denali 950 63°06'14" 147°30'57" 2440 27 years: 1957 - 1976; 1978 - 1986 15291500 Susitna River near Cantwell 4,140 62°41'55"147o32'42"1900 17 years: 1961 - 1972; 1980 - 1986 15292000 Susitna River at Gold Creek 6,160 62o46'04" 149 o41'28"676.5 57 years: 1949 - 1996; 2001 - 2011 15292400 Chulitna River 2,570 62°33'31" 150°14'02" 520 19 years: 1958 - 1972; 1980 - 1986 15292700 Talkeetna River near Talkeetna 1,996 62°20'49" 150°01'01" 400 39 years: 1964 - 1972; 1980 - 2011 15292780 Susitna River at Sunshine 11,100 62o10'42" 150 o10'30"270 5 years: 1981 - 1986 15292800 Montana Creak near Montana 164 62°06'19" 150°03'27" 250 4 years: 2005 - 2006; 2008 - 2011 15294005 Willow Creek Near Willow 166 61°46'51" 149°53'04" 350 25 years: 1978 - 1993; 2001 - 2011 15294010 Deception Creak near Willow 48 61°44'52" 149°56'14" 250 7 years: 1978 - 1985 15294100 Deshka River near Willow 591 61°46'05" 150°20'13" 80 21 years: 1978 - 1986; 1988 - 2001 15294300 Skwentna River near Skwentna 2,250 61°52'23" 151°22'01" 200 23 years: 1959 - 1982 15294345 Yentna River near Susitna Station 6,180 61°41'55" 150°39'02" 80 6 years: 1980 - 1986 15294350 Susitna River at Susitna Station 19,400 61°32'41" 150°30'45" 40 19 years: 1974 - 1993 ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041311 Page 2 of 7 4/15/2011 Figure 1: Susitna Watershed USGS Flow Data – Chronological Availability Susitna River flow is highly seasonal as shown on Figure 2, which is based on daily flows at the USGS gage at Gold Creek. Figure 2 exhibits the characteristic shape of streamflow for many Alaskan rivers, but the flow characteristics will change at least somewhat at different locations on a river and among different rivers. Numerous tables and plots are currently in preparation for the PAD to display the flow characteristics of the Susitna watershed. By subtracting the monthly recorded Cantwell gage flows from the Gold Creek gage flows yields a 17 year data set of flow between gages that includes the Watana dam site. Analysis of this flow between gages data set has shown that it has monthly flow characteristics that are somewhat different from the Gold Creek gage, primarily due to the timing of snowmelt at different elevations and a slightly different runoff per unit area. A long-term record of reservoir inflows at the ungaged Watana dam site must be developed for use in the reservoir operation and power studies. It is expected that the Gold Creek flows will be adjusted to the Watana dam site using monthly factors derived based on a comparison of the concurrent flow at Gold Creek and the runoff between gages. The result will be a 55 year period (excluding the recent USGS provisional flow data) of daily inflows to the proposed Watana reservoir. We understand that two or three years of flow data was gathered at or near the Watana dam site by R&M in the early 1980s. Efforts are being made to retrieve this flow data, which will then be evaluated for accuracy and utility in improving the estimate of long-term flows at Watana. USGS Station Number Station Name 194819491950195119521953195419551956195719581959196019611962196319641965196619671968196919701971197219731974197519761977197819791980198119821983198419851986198719881989199019911992199319941995199619971998199920002001200220032004200520062007200820092010201115290000 Little Susitna River near Palmer 15291000 Susitna River near Denali 15291500 Susitna River near Cantwell 15292000 Susitna River at Gold Creek 15292400 Chulitna River 15292700 Talkeetna River 15292780 Susitna River at Sunshine 15292800 Montana Creak near Montana 15294005 Willow Creek Near Willow 15294010 Deception Creak near Willow 15294100 Deshka River near Willow 15294300 Skwentna River 15294345 Yentna River near Susitna Station 15294350 Susitna River at Susitna Station Note: Data are on a calendar year basis. Legend Complete years of record Partial years of record ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041311 Page 3 of 7 4/15/2011 Figure 2: Flow Frequency – Susitna River at Gold Creek HYDROLOGIC CHANGE Global warming scenarios have projected greater temperature increases at higher latitudes. Over the past 50 years, Alaska has warmed at more than twice the rate of the rest of the United States’ average. Its annual average temperature has increased 3.4oF, while winters have warmed even more, by 6.3oF (Karl, et al, 2009). As a result, climate change impacts could be expected to be much more pronounced in Alaska than in other regions of the United States. Among other effects, higher temperatures should contribute to earlier spring snowmelt, a higher percentage of precipitation falling as rain instead of snow, and glacier retreat. The effect of increasing average annual temperatures on annual average streamflow is not easily predicted. Major factors other than temperature to be considered would include climate change effects on precipitation, evaporation, transpiration, snow ablation (direct change in phase from solid to vapor), and the rate of net loss to glaciers. Increased flows from glacial melt can be more than balanced by reduced runoff due to increased evaporation and transpiration. Projections of future average precipitation at a location are generally considered to be much less certain than projections of future average temperatures. 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecDaily Flow (cfs)5% Exceedance 10% Exceedance 25% Exceedance 50% Exceedance 90% Exeedance Based on Historic Recorded Daily Flows in the Susitna River at Gold Creek - USGS Gage 15292000 ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041311 Page 4 of 7 4/15/2011 For the purpose of assessing the effects of climate change on long-term reservoir inflows to the Watana Hydroelectric Project, it is fortunate that a long-term USGS record of streamflow exists at Gold Creek. Average annual flows at Gold Creek for the complete calendar year period of record are plotted on Figure 3. A best fit linear trendline is also plotted as the straight red line through the recorded data. The trendline shows an essentially constant average annual flow rate over the period of record, with only a very slight, almost negligible, upward trend. Figure 3: Average Annual Recorded Flows – Susitna River at Gold Creek Analysis of monthly flow data at Gold Creek presents an entirely different picture from the annual data. As shown on Figure 4 for February data and Figure 5 for April data, the linear trendlines show a pronounced increase in average monthly flows over time. Statistical tests of significance indicate with very high reliability that the observed trends in streamflow are not random. The April trends are most significant and undoubtedly result from an earlier initiation of the spring snowmelt as well as more precipitation falling as rain instead of snow. If annual average flows remain constant while winter and early spring flows are increasing, flows in other months must be decreasing. A statistically significant decrease in flows has been observed in June, as shown on Figure 6. Although the percentage decrease in flows is less than the percentage increase in flow in other months, June is the month with the highest average flow (Figure 2) so that a smaller percentage change can have a greater effect. 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 1950 1960 1970 1980 1990 2000 2010Flow (cfs)Year ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041311 Page 5 of 7 4/15/2011 Figure 4: February Monthly Recorded Flows – Susitna River at Gold Creek Figure 5: April Monthly Recorded Flows – Susitna River at Gold Creek 0 500 1,000 1,500 2,000 2,500 1950 1960 1970 1980 1990 2000 2010Flow (cfs)Year 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 1950 1960 1970 1980 1990 2000 2010Flow (cfs)Year ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041311 Page 6 of 7 4/15/2011 Figure 6: June Monthly Recorded Flows – Susitna River at Gold Creek The observed seasonal flow trends on the Susitna River are significant because:  Over the past 60 years, the unregulated flow trends have been similar to the seasonal flow changes that would result from reservoir operations, although they have been much smaller in magnitude.  The observed flow trends may establish a different unregulated flow baseline for comparison with the projected future flow regime with the Watana Hydroelectric Project.  The general trend to increased winter flows, the earlier initiation of spring snowmelt, and the reduction in peak snowmelt runoff are all favorable for increased firm and average total hydroelectric energy production. Reservoir operation and power studies have traditionally used historic flow records as the basic hydrologic input data. Our plan is to use Watana reservoir inflows developed directly from USGS records as the basic hydrologic input data set for the reservoir operation and power studies. However, the foregoing information has indicated that there is reason to consider alternative hydrologic input data sets that account for potential future hydrologic change. The most authoritative global projections of climate change result from an ensemble of 23 Atmosphere-Ocean General Circulation Models (AOGCMs) (IPCC 2007). One method to 0 10,000 20,000 30,000 40,000 50,000 60,000 1950 1960 1970 1980 1990 2000 2010Flow (cfs)Year ALASKA RAILBELT LARGE HYDRO ENGINEERING SERVICES AEA11-022 TM-03-0002-041311 Page 7 of 7 4/15/2011 develop hydrologic data sets that account for climate change would be to begin with results from the 23 AOGCMs, downscale the temperature, precipitation and evaporation results for the Susitna watershed, and then develop a rainfall-runoff model to produce the Watana reservoir inflows. The AOGCMs yield the highest confidence for temperature results, with much lower confidence in precipitation results. After introducing substantial uncertainty in future evapotranspiration, this method would produce a wide variation in projected future runoff at Watana. Instead, we feel that the historic USGS flow data can be used to project potential future Watana inflows with much greater reliability. Developing potential future Watana hydrologic inflows could also involve stochastic hydrology methods beginning with alternative projections of monthly flow data statistics. RESERVOIR OPERATION AND POWER STUDIES Development of a reservoir operation and power study model is just being initiated at the present time. The initial input data set to the model will rely on basic input data used in previous studies for the Watana site including reservoir elevation-capacity data, the tailwater rating curve, and environmental release requirements. Generation results from the initial model run will be compared to previous results and any significant differences will be resolved to the extent possible. Input parameters will be revised as necessary, for example to determine the effects of increasing the plant capacity. The general operating objective will be to maximize firm energy during the low flow period from November 1 through April 30 and to maximize total energy during the remainder of the year. REFERENCES IPCC, 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 996 pp. Karl, Thomas R., Jerry M. Melillo, and Thomas C. Peterson, (eds.), 2009. Global Climate Change Impacts in the United States, Cambridge University Press.