The URL can be used to link to this page

Home My WebLink AboutAPA4143 This document is copyrighted material. Permission for online posting was granted to Alaska Resources Library and Information Services (ARLIS) by the copyright holder. Permission to post was received via e-mail by Celia Rozen, Collection Development Coordinator on December 16, 2013, from Kenneth D. Reid, Executive Vice President, American Water Resources Association, through Christopher Estes, Chalk Board Enterprises, LLC. Five chapters of this symposium are directly relevant to the Susitna-Watana Hydroelectric Project, as they are about the Susitna Hydroelectric Project or about the Susitna River. This PDF file contains the following chapter: Forecasting the effects of river ice due to the proposed Susitna Hydroelectric Project by Ned W. Paschke and H.W. Coleman .................................................................. pages 557-563 Assigned number: APA 4143 American Water Resources Association PROCEEDINGS of the Symposium: Cold Regions Hydrology UNIVERSITY OF ALASKA-FAIRBANKS, FAIRBANKS, ALASKA Edited by DOUGLASL.KANE Water Research Center Institute of Northern Engineering University of Alaska-Fairbanks Fairbanks, Alaska Co-Sponsored by UNIVERSITY OF ALASKA-FAIRBANKS FAIRBANKS, ALASKA AMERICAN SOCIETY OF CIVIL ENGINEERS fECHNICAL COUNCIL ON COLD REGIONS ENGINEERING NATIONAL SCIENCE FOUNDATION STATE OF ALASKA, ALASKA POWER AUTHORITY STATE OF ALASKA, DEPARTMENT OF NATURAL RESOURCES U.S. ARMY, COLD REGIONS RESEARCH AND ENGINEERING LABORATORY Host Section ALASKA SECTION OF THE AMERICAN WATER RESOURCES ASSOCIATION The American Water Resources Association wishes to express appreciation to the U.S. Army, Cold Regions Research and Engineering Laboratory, the Alaska Department of Natural Resources, and the Alaska Power Authority for their co-sponsorship of the publication of the proceedings. American Water Resources Association 5410 Grosvenor Lane, Suite 220 Bethesda, Maryland 20814 JULY COLD REGIONS HYDROLOGY SYMPOSIUM AMERICAN WATER RESOURCES ASSOCIATION 1986 FORECASTING mE EFFECTS 00 RIVER ICE IXJE TO THE PROF03ED SUSITNA HYIH>EUX:I'RIC PinJEX:l' Ned w. Paschke and H.W. Coleman* ABSTRACT: River ice processes affect the physical and hydraulic properties of many of the world's rivers. Although winter flaws are characteristically low, the additional friction and ice displacement within an ice-covered river can greatly increase the water surface elevation. The Susitna River, located in south- central Alaska, is generally subject to river ice processes for 6 or 7 months of each year. Environmental studies in connection with the proposed Susitna Hydroelectric Project (Alaska Power Authority, 1985) included documentation of natural (pre-project) river ice condi- tions and forecasting the effects of the project on river ice. In this regard, a numerical river ice model was calibrated and applied to an 85~ile reach of the Susitna River downstream of the proposed project. This paper presents a summary of the river ice modeling process, ob- served trends in natural ice conditions and the expected effects of the proposed project. (KEY TE~S: cold regions: river ice: winter hydro operation, river ice model- ling.) IN'I'R>IXJCI'IOO Proposed Susitna Hydroelectric Project The proposed Susitna Hydr~lectric Project includes the constt~ction of two large dams on the Susitna River (Figure 1). Watana Dam, an earthfill structure with an ultimate planned height of 885 feet, would be located 184 river miles upstream from the river mouth at Cook Inlet (i.e., "RM 184"). I:evil Canyon Dam, a 645-foot high concrete arch structure, \<Tould be located at RM 152, i.e., 32 miles downstream of Watana Dam. The project is planned for construc- tion in 3 stages as follows: Stage I -Watana Dam would be constructed to an initial height of 700 feet. Stage II -Devil Canyon Dam (full height) would be constructed. Stage III -Watana Dam would be raised to its ul- timate height of 885 feet. * Respectively, Hydraulic Engineer, Harza Engineering Company, 150 South Wacker Drive, Chicago, Illinois 60606 (Presently, Assistant Director of Engineering and Planning, Madison Metropolitan Sewerage District, 1610 Moorland Road, Madison, ~tisconsin 53713); and Head, Hydraulic Analysis and Design Section, Harza Engineering Company, 150 South Wacker Drive, Chicago, Illinois 60606. 557 Stage I is planned to begin operation in the year 1999. Stages II and III would be added in accordance with energy demand. Figure 1. Susitna River Location Map Environmental Background Environmental concerns regarding ice processes on the Susitna River include potential effects on the salmon popu- lation. A number of slough and side channel areas along the river provide habitat for spawning and juvenile over- wintering. These areas are generally isolated from the rnainstem by a natural berm at the upstream entrance to the slough or side channel. During the winter, these areas are often warmer than the mainstem due to upwelling of rela- tively warm (e.g. 3°C) groundwater. Ice- induced stage increases periodically overtop some of the berms under natural conditions, flooding the slough with 0°C rnainstem water and possibly harming the developing salmon. With the proposed 558 project in operation, changes from the natural flows and stream temperatures will affect the river ice conditions and the frequency and severity of the slough overtopping events. The river ice rrodel therefore focuses on the timing and magnitude of ice-induced river stage variations at the slough and side channel locations. METHOOOI.DGY Study Reach River ice modeling was limited to the "middle reach" of the Sus i tna River, i.e., the 85-mile reach from the Watana damsite to the "three-river confluence" forned by the Susitna, Chulitna and Talkeetna Rivers (Figure 1). Downstream of this confluence, the substantial incoming tributaries are expected to lessen the relative effects of the future project. Typical natural river flew rates in the vicinity of the damsites range from 30,000 cfs in June to less than 2,000 cfs in March. With the pro- ject operating reservoir releases would generally be 8,000 to 13,000 cfs year- round. Susitna River Ice Observations Ice observations on the Susitna River have been documented for the past five winters (R&M Consultants 1981, 1982, 1983, 1984, 1985}. Natural ice processes on the Susitna typically begin in early October with the generation of frazil ice, i.e., small ice crystals probably formed in supercooled surface water exposed to subfreezing air temperatures (Ashton, 1978}. Frazil ice is first observed in the middle and upper reaches of the Susitna which are subject to the coldest air temperatures. Reaches of low solar radiation and high water turbulence appear to be highest in frazil ice pro- duction. As the frazil is carried downstream, it coalesces into pans or rafts of "slush" ice which are often 2 to 5 feet in diameter and which may cover as much as 80% of the river surface. Border ice is observed to form as some of the slush ice pans come to rest and freeze together in law velocity zones along the river banks. Typically late in October, an accumu- lation of slush ice becomes jammed and freezes together near the river mouth, forming a stationary ice bridge across the river. Formation of the ice bridge appears to be triggered by a high con- centration of slush ice pans, law air temperatures and a high tide event in Cook Inlet which substantially reduces river velocities for several ndles up- stream. Following formation of the ice bridge, slush ice pans accumulate against its upstream edge and thereby advance the ice cover in an upstream direction. Same slush is observed to be swept beneath the ice front and is apparently deposited downstream on the underside of the cover, thereby thickening the ice cover. Pe- riodic mechanical oampression or "shoving" of the advancing ice cover (Pariset et. al., 1966) is observed, whereby as much as 2000 feet of the slush ice cover consolidates and thickens. The advancing ice cover often reaches the three-river confluence (RM 98) in November and the vicinity of Gold Creek (RM 137) in late I:ecember or January, rut varies with weather and flow conditions. Observed ice front progression rates in the middle reach typically range from 0 to 2 miles per day. Ice front progres- sion generally becomes undefined upstream of Gold Creek, where intermittent bridg- ing of border ice precedes the arrival of the ice front. River stage increases of 2 to 6 feet in the middle reach are ccmnon during progression of the ice front, and over- topping of some slough and side channel areas has been observed. Slush ice cover thicknesses are observed to vary substan- tially along the river and particularly across the channel width. Often little or no ice is observed in a central, high ~locity core area whereas accumulations as great as 12 feet thick reaching the channel bottom have been observed closer to the river banks. Following progres- sion, the upper surface of the slush ice cover begins to freeze into solid ice. The solid ice portion of the ice cover is observed to reach typical thicknesses of 559 2 to 4 feet by February or March. Spring breakup of the ice cover typi- cally occurs in early May and is largely due to the natural flow increases which lift and fracture the cover. Sporadic ice jams caused by blocks of the frac- tured ice cover are observed to cause greater stage increases and more frequent slough overtoppings than those of the initial ice cover progression. River Ice Model The numerical river ice model was based on the work of Calkins (1984) on the ottauquechee River and was modified and calibrated by Harza-Ebasco ( 1984) for application to the Susitna River. The model provides. a daily summary of hydrau- lic and ice conditions throughout the study reach during the period from November 1 to April 30. A detailed description of the model and its govern- ing equations has been presented by Calkins (1984). The general features of the model are briefly summarized as follows: 1. Hydraulic profiles are computed daily based on the Bernoulli and Manning equations (standard step method). Computations include the effects of the ice cover and border ice where appropriate. 2. Frazil ice production within reaches of 0°C open water is com- puted by a heat transfer coeffi- cient approach (Ashton, 1978). Cumulative frazil flaw rates are tabulated as the ice travels downstream. 3. Border ice growth is oamputed as a function of air temperature and water velocity and is calibrated to Susitna observations. 4. Hydraulic conditions at the ice front determine if the slush ice pans are swept beneath the ice cover or accumulated at its upstream edge, thereby advancing the ice front (Figure 2). repo- sition of slush ice beneath the cover is conputed based on the ice supply and water velocity under the ice cover. Thicknesses of the advancing ice cover are computed in accordance with OAM Pariset, et. al. ( 1966). Ice front progression rates are based on river geometry and the supply of ice reaching the front. 5. Solid ice growth within the slush ice cover is conputed (Figure 2). 6. Melting of the ice cover and re- treat of the ice front are c~ puted when warm water (i.e. above 0°C) reaches the ice front. Water temperature decay beneath the ice cover is also computed. Mechanical breakup of the ice cover is not simulated by the m:xiel. WATER r;MP > I'J°C I WATER TEMP= 0°~ OPEN WATER FLOW PLAN PROFILE Figure 2. River Ice Schematic SOLID ICE GROWTH Surveyed river cross-sections at 102 locations between Watana damsite (RM 184) and the "three-river confluence" (RM 98) were used in the m:xiel. Manning's "n" values ranging from .022 to .065 were selected to calibrate the open-channel portion of the model to stage-discharge measurements. Daily air temperatures and wind speeds recorded at 3 locations along the study reach were used for the various ice processes in the model. For simula- tions of natural (pre-project) condi- tions, daily flow rates and frazil ice discharges were input at the upstream boundary based on observations at Gold 560 Creek (RM 137). For with-project condi- tions, flow rates and water temperatures upstream of the ice front were provided by corresponding reservoir and stream temperature simulations. Starting dates for the simulated with-project ice front progression at the three-river confluence were based on tabulation of the total ice volumes supplied to the lower Susitna River and the time required to advance the lower Susitna ice front from Cook Inlet to the three-river confluence. The river ice model is primarily in- tended to simulate the timing and mag- nitude of river stage variations as- sociated with ice. Simulated natural ice conditions shaw reasonably good agreement with field observations (Figure 3). Limitations of the model relate primarily to its one-dimensional nature. Veloc- ities and ice cover thicknesses computed by the model are mean or characteristic values intended to represent an entire cross-section. Actual velocities and ice thicknesses are likely to be quite non- uniform within the cross-section. z 0 150 140 ~-130 8o!! ~~ 120 Z.!: oa: ~-110 w !,! w 14 "' ~ 12 ~ 10 ~-; 8 -I ~~ 6 i 4 x 2 ~ 0 NOV DEC JAN FEB REFERENCE LINE J 3000 cfs OPEN WATER SURFACE PROFILE I I I .I I 1\ .....J..--; 100 _... I 110 . 120 RIVER MILE "' MAR APR J I I 130 140 1r~-~---w ----++----+----, MF---+--------i--f4l--+----¥N------If--+----+--+-t+-ll,lllll ~ ~1oo~----~110~~u_~1ro~~~~~130~~~~140~~ RIVER MILE LEGEND -SIMULATED • OBSERVED Figure 3. Sample River Ice Calibration Winter 1982-1983 RESULTS Sample results of river ice simula- tions for natural conditions and the three stages of the project are shown in Figures 4 and 5. These simulations are based on weather conditions of 1981-82 (an average winter in terms of mean air temperatures) and shaw typical trends. with Stage I in q;>eration, ice front progression at the "three-river confluence" during an average winter is expected to be delayed until rrdd- December, about 3 weeks later than that of natural conditions (Figure 4). With the operation of the project Stages II and III, respectively, the ice cover progression is expected to be further delayed until late December or early January (Figure 5). Spring meltout in the middle reach of the Susitna River with Stage I operating is expected to be ccrrpleted by late April (Figure 4), about 2 weeks earlier than the natural breakup. With q;>eration of -130 ~ ~ i 120 ~ > ! 110 100 14 12 10 100 REFERENCE LINE· _r:__)ooo ef1 OPEN WAi-ER SURFACE PROFILE I ....., .... I . ~· \. I !/ . ./ -""'., ·-· " , I 110 120 l\1.~~ \ II' -, --I 130 140 Stages II and III, the meltout would be further advanced (Figure 5) , occurring in late to early March, respectively. The delayed ice front progression and the earlier-than-natural ice meltout with the project operating is due primarily to warmer-than-natural water temperatures released from the project reservoirs during the winter months. The maxLmum upstream extent of the ice cover during an average winter is expected to be in the vicinity of RM 139 with Stage I operating. This ice cover extent would be reduced to near Rt1 133 with Stage II q;>erating and further reduced to the vicinity of RM 114 with Stage III operating (Figure 5). Little or no ice is expected upstream of these locations with the project operating, whereas under natural conditions these reaches become covered by border ice growth. The total thickness of the river ice cover with Stage I operating is expected to be generally sirrdlar to that of natu- ral conditions (Figure 4). Ice cover 150 140 ~ ;:: 130 "-go! .J:E !Z.t 120 Oa: ff-110 100 2 0 REFERENCE LINE / 3000cft OPEN WATER SURFACE PROFILE I I I --lo;. ~ ... ------- t 100 110 ~~ -~ ...... 120 RIVER MILE ~-.--...\ r\ "' /'-~' --~ ~. 130 140 ·~I~---·---lM:l)U.Jfi,JIIIII li1~-ijtt11}lfl-l.llll IOO 1)0 I20 I30 ,10 RIVER MILE fUVER MILE LEGEND' -NATURAL CONDITIONS ·-·-STAGE I OPERATING • NATURAL SLOUGH BERM ELEVATION Figure 4. Simulated River Ice Conditions Stage I vs. Natural 1981-1982 Weather Conditions 561 LEGEND ---COLD WINTER 1971-72 --AVERAGE WINTER 1981-82 ·-•••• WARM WINTER 1976-77 • NATURAL SLOUGH BERM ELEVATION NOTE' ALL SIMULATIONS ASSUME WATANA DAM IS ON-LINE Figure 5. Simulated River Ice Conditions Stages I, II and Ill 1981-1982 Weather Conditions thicknesses are expected to be progres- sively reduced with the addition of Stages II and III (Figure 5). The reduced extent and thicknesses of the ice cover with the project operation again primarily reflect the warmer-than-natural reservoir release temperatures. Maximum river stages within the ice- covered reaches during operation of Stages I, II and III are expected to be generally several feet higher than those of natural conditions. This is due primarily to the greater-than-natural flow rates with the project in operation. The frequency and magnitude of the slough overtopping events upstream of the ice front with the project operating are therefore expected to be less than or equal to those of natural conditions. The simulation results discussed above are based on the average winter weather conditions of 1981-82. Simula- tions were also made for a cold winter (1971-72) and a warm winter (1976-77). Although these sinulations (Figure 6) were based on a different construction and operational schedule than Figures 4 and 5, they serve to indicate the sensi- tivity of the simulated river ice processes to weather conditions. With the project in operation during a cold winter, for example, the ice front would be expected to begin several weeks earlier and would extend several rrdles further upstream than for an average winter. Maximum ice cover thicknesses and river stages during a cold winter would also be about 2 feet greater than those during an average winter. During a warm winter, conversely, ice cover thicknesses and river stages are likely to be about 2 feet less than for the average winter. StHtARY A numerical river ice model was applied to the Susitna River to forecast the effects of the proposed Susitna Hydroelectric Project. The model simula- tions predicted delayed ice cover formation, reduced ice cover extent and earlier and rrdlder spring meltout rela- tive to natural conditions. Greater than natural river stages were predicted for 562 ,. 12 10 100 ~ ,.... REFERENCE LINEo 3000cfs OPEN WATER SURFACE PROFILE 1 __.. ~------'1- I 110 _, .. .. .......... / ., . 120 RIVER MILE . ' ' ""~ ... "\ lo"i . ·~ , ~\! ' 130 140 1'1 I b-kiHflfH,J 1111 0 I I 100 110 120 130 1CO RIVER MILE LEGEND: STAGE I OPERATING STAGE II OPERATING STAGE Ill OPERATING • NATURAL SLOUGH BERM ELEVATION Figure 6. Simulated River Ice Results For Various Weather Conditions sane reaches, and mitigation rreasures will be proposed therein. Weather condi- tions and project stage were shown to substantially affect the expected river ice conditions. REFERENCES Alaska Power Authority, 1985, Amendment to the Application for Major License, The Susitna Hydroelectric Project (Draft), prepared by Harza-Ebas~ Susitna Joint Venture. Ashton, George D., 1978, "River Ice", Annual Reviews in Fluid Mechanics, Vol. 10, pp 369-392. Calkins, D.J., 1984, "Nurrerical Simulation of Freezeup on the ottauquechee River", Workshop on Hydraulics of River Ice, June 20-21, 1984, Frederickton, New Brunswick, w 247-277. Gerard, R., 1984, Notes fran short course "Ice Engineering for Rivers and Lakes", University of Wisconsin. Harza-Ebasco Susitna Joint Venture, 1984, "Instream Ice Sinulation Study" for Alaska Power Authority. Harza-Ebasco Susitna Joint Venture, 1985, "Instream Ice Sirrulations; Supplemen- tary Studies for Middle Susitna River", for Alaska Power Authority. Pari set, E. , Rene Hausser and Andre Gagnon, 1966, "Fonnation of Ice Covers and Ice Jams in Rivers", ASCE Journal of Hydraulics Division, Vol. 92, HY6. R&M Consultants, Inc., 1981, "Ice Obser- vations, 1980-81", for Acres American for Alaska Power Authority. R&M Consultants, Inc., 1982, "Winter 1981-82, Ice Observations Report", for Acres American for Alaska Power Authority. R&M Consultants, Inc., 1983, "Susitna River Ice Study, 1982-83", for Harza- Ebasco for Alaska Power Authority. R&M Consultants, Inc., 1985, "Susitna River Ice Study, 1983-84", for Harza- Ebasco for Alaska Power Authority. R&M Consultants, Inc., 1985, "Susitna River Ice Study, 1984 Freeze-Up", Draft Report for Harza-Ebasco for Alaska Power Authority. 563