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HomeMy WebLinkAboutAPA2426• ""-'··"<''-"!' [' r ,,. ENVIRONMENTAL IMPLICATIONS OF DESIGN CHANGES ENVIRONMENTAL EVALUATION MEMORANDUM NO. l PROJECT OPTIMIZATION AND JUSTIFICATION The text attached to this memorandum is an initial draft of the Environmental Appendix to the EngineeriQg Report ''Susitna Project -Initial Review and ~valuation, Part B: Project Optimization and Justification" presently being prepared. That report evaluates alternative development options for the Suaitna Project and the 'nvironmental Appendix considers the env1ron- mental implications of these engineering alternatives. The information contained in this appendix, which 1s designed to be a self-sufficient document, has been developed from Discussion Memoranda 1, 2 and 3 (February-March 1983); review of Exhibit E of the FERC license application, as filed on 28 February 1983 (particularly chapters 3 and 10); the Acres December, 198~ Development Selection Report and Appendices; the Decemberj 197~ Corps of Engineers Interim Feasibility Report and Appendices; the September, 197~ Corps of Engineers Draft Envir- onmental Impact Statement for the Susitna Project; other [}:{)&00~& 0 ~liD&®©© Susitna Joint Venture Document Number Please Return To DOCUMENT CONTROL • .. • project reports as identified; and further evaluations of the relative impacts of t~e identified engineering alternatives. In this appendix, the compar1sons of environmental impacts of alternative developments have been based on available information. The comparisons consider only those aspects which ll differ significantly between alternatives and therefore influence the selection of a development plan. Consideration of impacts which are similar in magnitude or which are relatively insignificant and which will not influence the selection process have not been included in this evaluation. In rev1ew1ng and commenting on the adequancy of the evaluation presented in this appendix, it should be noted that the state and federal agencies were provided an opportunity to rev1;w and comment on a draft of the Development Selection Report in 1981. The basic comment received from ADF&G was that "it would have been a helpful process for Acres to involve ADF&G, USFWS and others in such an analysis to discuss alternative positive/negative impact possibilities. .may have led to conclusions which were the same or potentially quite different from the Acres analysis of the situtation" (letter ~f_ • • .. • fP ! ·,' dated August 4, 1981). To the max1mum extent possible, our present analysis needs to be sufficiently comprehensive and CODV1UC1ng that the conclusions are readily acceptable by all interests. Comments and information for improving the evaluation of alternative impacts are requested. In order that this material be ready to accompany the engineering report when it 1s transmitted to the Alaska Power I Authority, it is requested that any comments, corrections or supplemental information concern1ng the material contained herein be provided to E. F. Dudley by f f'"_; l I 1 • .. • f·~ '. ~:3 . f t Part I APPENDIX ENVIRONMENTAL IMPLICATIONS OF ALTERNATIVE DEVELOPMENT CONCEPTS TABLE OF CONTENTS -INTRODUCTION Part II -ENGINEERING MODIFICAITONS CONSIDERED Part III -ENVIRONMENTAL IMPLICATIONS OF DEVELOPMENT CONCEPTS Watana Alternatives Area of A. B. c. Inundation Loss of grayling spawning Removal of Vegetation Removal of Moose Habitat and rear1ng hebitat D. Inundation of Jay Creek Mineral Lie/<. E. Impacts on Other Wildlife Borrow Material Shorter Construction Period Em e r g en c y F 1 ow s t o --Ts us en a C r e e k "elict Channel Flood Control --- Devil Canyon Development High Devil Canyon Alternatives Vee Canyon Development Part IV -ENVIRONMENTAL IMPLICATIONS OF OTHER DESIGN CHANGES Arch Dam vs. Fill Dam Discharge Facilities Part V -ENVIRONMENTAL IMPLICATIONS OF PROJECT OPERATIONAL MODES (fr-vec--f r~;VJ'\.ft:t~dr~ P~J n;f aA/Ovi~ 4"-~.r -t;vn ~) • • .. • r. j; I; r 1 \.• I' L • "' APPENDIX ENVIRONMENTAL IMPLICATIONS·OF ALTERNATIVE DEVELOPMENT CONCEPTS INTRODUCTION Due to rather drastic changes expected in load fore- casts for the Railbelt area, it is possible that sizeable changes may be required 1n the configuration of the Susitna Project. One aspect of the Susitna Project Optimization and Justification Study is to consider the environmental implications of possible alternative development concepts. Exhibit E of the FERC license application as filed, considers all aspects of project construction and operation in relation to probable impacts on the physical, biological and social resources of the reg1on affected by the Project as proposed. Exhibit E has been based on the initial construction of the Watana Development with normal maximum re~ervdir elevation of 2185, immediately followed by construction of the Devil Canyon dam and reservoir. -!- • r r r ! Changes 1n the s1ze or configuration of project features or sequence of construction may result 1n different project impacts and therefore necessitate revisions to the existing env~ron- mental discussion as contained ~n Exhibit E. There is also the possibility that (major) changes 1n the design of the Project, even if these changes improve project economics and/or reduce anticipated environmental impacts~ may be viewed as significant changes in project planning and therefore cause delays in the FERC license review process. This appendix presents a discussion of the relative impacts related to each of the design and operational alternatives considered in the Project Optimization and Justificaiton Report. Following selection of a specific project configuration and operational mode, the environmental implications of the selected project will be elaborated together with necessary modifications to the Exhibit E discussion of project impacts and planned mitigation measures. A/ originally proposedl this information would be presented as an Environmental Design Modifications for incorporation FERC License Application Revisions. -1-,...._ Report on all 7 ( r • ' into the June) Project report on .. • • r;: f ~ I i '' i" I ,. a u ENGINEERING MODIFICATIONS CONSIDERED The Project Optim~zation and Judtification Report exam1nes whether the proposed Susitna Project remains the most economic project to meet the future load growth of the Railbelt Region by evaluating the following questions: • What are the preferable timing, sequence and s1zes of the Watana and Devil Canyon developments? • Will the Watana and Devil Canyon developments provide ~ the least-cost electricity to consumers when compared with any other projects or combinations of projects? • Can reasonable financial arrangements be established for funding the construction of the proposed initial Watana development? The latter two questions have relatively little, if any, purely environmental implications as considered 1n Exhibit E. The first question~ however, introduces several alternative -J- • .. .. • " " -' ' • ·....,o,_ ___ ._ ............. __ W __ _ ril ,. :· development schemes with potentially significant environmental implications. Both design and operational variations have been considered as part of the Project Optimization Study. The following design options have been examined in relation to their . . eng~neer1ng, econom1c, and environmental desirabilty. e Should the max~mum normal water surface elevation of !i the Watana Development be maintained at elevation 2185 or would a lower elevation (e.g., 2100, 2000 or r 1900) be preferable? • Should the Devil Canyon Development be built pr~or to the Watana Development? e Should both developments be constructed as presently designed and scheduled with no appreciable design changes or time lag? • Are other Susitna hydro development alternatives (e.g., construction of Watana with a tunnel to the Devil Canyon .. • r ,,, ~-t r. Ji ~ ~ ~ J site 1n lieu of the dam, construction of High Devil Canyon with subsequent construction of a dam at the Vee site, etc.) viable alternatives to the proposed project? Ten alternative project operation patterns for the Watana and Devil Canyon developments have been identified and studied to provide answers to these questions. These alternatives are identified 1n Table 1 and the individual developments are characterized in Table 2. In addition, potential developments at the High Devil Canyon and Vee Canyon sites have been reevaluated alone and in combinations. Other potential development sites identified in studies of the hydroelectric po~ential of the Upper v previous Susitna River have not been included 1n the present reevaluation. These sites include: e Gold Creek, • Olson, f) Devil Creek, f) Susit.na III, -)- • .. .. Table 1 PROJECT DESIGN ALTERNATIVES A. Watana-Devil Canyon Alternatives B. Watana Normal Maximum Water Surface Elevation Single Development Operation 1 . 2 .. 3. 4. 5. 6. 2185 2100 2000 1900 Joint Develop~ent Operation 7 ~ 2185 8. 2100 9 . 2000 10. 1900 Devil Canyon Normal Maximum Water Surface Elevation (Drawdown) 1455 (50 ft) 1455 (100 ft) 1455 (50 ft) 1455 (50 ft) 1455 (50 ft) 1455 (50 ft) Other Site Possibilities/Combinations 11. High Devil Canyon 1750 12. High Devil CanJon site but only dev~1oped to 1455 (some height as at the Devil Canyon Development) 13. Vee Canyon 2330 14.-17. High Devil Canyon 1455 plus each of the four Watana alternatives 18. High Devil Canyon 1750 plus Vee (2330) 19. Watana 1900 plus Devil Canyon (1455) plus Vee (2330) C. Reregu1ating Dams Devil Canyon RlOOO High Devil Canyon R1130 • .. .. I i , I I II I • r L J~ I' t , ff u f", ! j ~ ' . . : -: ;q :."' ( :~:, ···:·, ·<,· ... · . . . · .. 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FEE i~~--11 ---·-1r . . 1 ··-----·. l H-r -----1 -~--f-1·-H , -r~r+ ~- co !I l II I 1: i l I I l l I I I : I I ' I l I i I l l ' IH 'I " " I !i . : :I l ! I l I i I I -!I L ____ I -' . ----" Jill -J..l I II I I I I ~ I i i l . -" "" ------,_ ____ "' ----.1::------··--=--·----------" - 6~1 ,.AOC IN US A, --·-.......... -------·---·--·------------·-------·~------------···-------~-.:-------~-~--;;-:-:-·-....,-~~..,~--------~~---· ~~-:--.. --~--··--!1 ·-~ . t li ~ L ·r·. ~·~~ If' 'o '' . . ; 1 • • s Maclaren, • Denali, • Butte Creek, and • Tyone. These sites were all eliminated from prev1ous studies for one or more of the following reasons and there does not appear to be reason to alter this decision: e Potential blockage of salmon migrations up to Portage Creek (Gold Creek, Olson); Excessive environmental impacts on big game and water- fowl (~yone, Butte Cree~, Denali, Maclaren); or ~ Better alternative sites (Devil Creek, Butte Creek, Susitna III). In all, 19 development concepts have been evaluated 1n terms of their ability to economically meet the electrical needs of the Railbelt Region an environmentally acceptable man~ G::ree multi-development 1n schemes are shown schematically 1n Exhibit I. These are: .I / f -;- • .. • fi 1 I l L L !J' b. • i li ll ~ ' ~ ' IL ~ • L • the Devil Canyon -Watana 2185 scheme as described 1n • the license application, High .Devil Canyon(-' Vee Canyon, and • Devil Canyon -Watana 1900 -Vee Canyon. For each development concept, alternative operational modes have been considered. For any operational mode resulting in fluctuating project discharge flows, reregulating structures will be incorporated into project plans to virtually eliminate flow fluctuations. Two potential reregulating structures, at the Devil Canyon and High Devil Canyon sites, are shown in Table 2. In terms of anticipated environmental implications of the alternatives, the development concepts will differeutially impact the reg1on upstream of Portage Creek. and the operational modes will affect the river downstream from Portage Creek through differential flow release patterns. -8- • ,. • • ENVIRONMENTAL IMPLICATIONS OF DEVELOPMENT CONCEPTS The dam and reservo1r characteristics of the eight single- development alt~rnatives, plus two reregulating structures, are presented in Table 2. The remaining development concepts are combinations of these individual developments. Individual reservoir areas range from 38,000 acres for the Watana 2185 development to 6,300 acres for the High Devil Canyon 1455 development. Four of the eight single-development alternatives are alternative heights for a development at the Watana site and are defined in terms of the maximum normal water surface elevation. Similarly, two alternative heights for a development at the High Devil Canyon site are included. Exhibit E of the license application considers all aspects of project construction and operation in relation to probable impacts on fish, vegetation, wildlife and other physical, biological and socio-economic resources of the project area. That discussion is based on the Watana 2185 alternative combined with subsequent construction of the Devil Canyon dam and . reservo1r. The following sections of this memorandum consider the differences in impact if a lower mhx1mum ~ormal water surface elevation is selected at the Watana site or if other development concepts are selected. -I- ... • ... ,_..,,; .., ' • ,' • • ~ • • I 't l . ' ' . I t I 1 L J I 1 r t L L. • 10 Watana Alternatives The majority of the anticipated impacts on terrestrial and aquatic resources resulting Nm the construction and operation of the two dam project (aB described in the license application) are related to the first phase of development, the Watana 2185 dam and reservo1r. The relative i~pacts of the Watana alter- natives are therefore compared to those for the base case Watana 2185 development as discussed in Exhibit E. Lowering the maximbm normal water surface elevation at the Watana site from 2185 feet to 2100, 2000 or 1900 feet would result 1n the following changes in project characteristics: a) less area inundated~ b) less ~orrow material needed, c) 1 to 3 years shorter construction period, d) elimination of the potential for emergency releases to Tsusena Creek, -rtJ- • l . r t I i i e) more modest remedial measures to seal the relict channel, and f) less inherent capacity for flood control and less seasonal flow regulation. These changes, 1n turn, will modify the impacts that are described in many sections of Exhibit E. Alterations of project height may also lead to alterations in installed capacity (but not the turbine discharge capacity) and project operation schedules which in turn will result in alterations in downstream flows. These downstrealli alterations are considered in a subse- quent portion of this Appendix. Area of Inundation. Table 2 shows that lowering the reservo1r from 2185 to 2100, 2000 or 1900 reduces the length of the reservoir by 5, 10 and 15 miles respectively, reduces the area by 26, 48 and 62 percent respectively, -•'···-~ l 32' 58 and 7 s'\percent \,.... ""---~-··--/ and reduces the active storage capacity by respectively. -f/- ( • • I I • .. L • Exhibit E identifies five maJor impact ~ssues directly related to the amount of area inundated by the Watana development and therefore to the planned normal maximum water surface elevation. These impact issues relate to: A. Loss of grayling spawn1ng and rearing habitat B. Removal of vegetation C. Removal of moose habitat D. Inundation of Jay Creek ..Mineral ;(i'ck E. Impacts on other wildlife. A. Loss of grayling spawn1ng and rear~ng habitat. The Watana 2185 reservo~r will flood 54 miles of Susitna River mainstream habitat and 28 miles of tributary habitat, including 10.0 miles~/ along Watana Creek, as well as portions of other tributaries. The primary long-term impact results from the loss ~/10.0 miles according to Table E.2.25 of license application (Exhibit E). Data derived from U.S.G.S. maps. Recent project maps show inundation to extend 54,650 feet of the creek or 10.4 miles. -1'2.- .. • " .. of clear water tributary habitat that curr~ntly supports a substantial population of grayling. Future aquatic habitats within the reservoir area are not expected to support a significant grayling population (page E-3-121).~/ Identified measures to . . . m~n~m1ze impoundment impacts would he to .. substantially lower the surface elevation of the reservoir or to maintain surface level during the embryo \1 incubation period (page E-3-171). maintain constant reservoir period~ay and June), but It will not be feasible to elevations during the incubation the alternative Watana developments would substantially lower the surface elevation of the reservoir and thereby inundate correspondingly fewer stream miles of tributary habitat than the 28 miles inundated by the elevation 2185 development (Table 3). Deadman, Watana, Kosina and Jay Creeks would still be impacted by a reservoir at elevation 1900, but to a considerably smaller extent. Goose Creek has an elevation of approximately 2060 feet at its confluence with the Susitna River and would not be adversely affected by the two lower alternatives. Oshetna River would be inundated only by the Watana 2185 development. b I 1 h . Un ess ot erw~se Exhibit E of the 1983. identified, page references are to license application as filed, February 28, -)}- • .. .. .. Table 3 PRINCIPAL TRIBUTARY STREAMsa/ INUNDATED BY WATANA RESERVOIR Stream Location R.M. Elevation Deadman Creek 186.7 1,513 Watana Creek 194.1 1,552 Kosina Creek 206.9 1,670 Jay Creek 208.6 1,700 Goose Creek 231.2 2,060 Oshetna River 233.5 2,110 Length (miles) Inundated by Reservoir At ---- 1900 2000 2100 218.5 (!), 7 J.~ /. 7 ~.3 6. 1 7. 7 9.2 10.4 2.2 3.2 3.9 4.6 1 ..,, • I 2.3 3.0 3.6 0.3 1 . 1 1. 9 ~/In addition, the lower portions of 39 smaller, unnamed tributaries will be inundated, for 0.1 to 3.9 miles, by all four alternatives with an additional 4J12 and 13 tributaries inundated by the elevation 2000, 2100 and 2185 alternatives respectively. -/'1- • • • • f t B. Removal of Vegetation. Construction of the dam, spillway and impoundment areas for the Watana 2185 development will result in removal of about 36,600 acres of vegetation (page E-3-225). A reduction in the total reservoir area associated with the smaller projects will mean a corresponding reduction 1n total area requiring removal of vegetation. The Watana 2100, 2000 and 1900 alternatives would result in preservation of 9,700, 18~000 and 24,000 acres of natural vegetation, respectively. c. Removal of Moose Habitat. Removal of vegetation for the Watana 2185 development will reduce the carrying capacity of the spr1ng and winter range by the equivalent of 266 moose. In years of average snowfall, the impoundment zones are most important as a source of early spring foods and as calving areas. These zones also contain several large areas of r1ver valley bottomland with mixed spruce deciduous woodlands that provide critical moose habitat in years of heavy snowfall. Reduction of reservoir area, particularly in the length of mainstream and tributary stream inundated~ will reduce the magnitude of this impact. A reduction in the extent of inundation along Watana may be particularly significant. -15"- .. 1 • I · •, • "' ' ' . . . . . .. ·.. ' . ' . .... ' ·. • .. r; ! D. Inundation of Jay Creek Mineral Lick. Partial inundation of the Jay Creek mineral lick and blockage of access to the lick may reduce the carrying capacity of the area for Dall sheep. With the reservoir at elevation 2185, up to 42 percent of the surface area of the mineral lick will be inundated by the Watana impoundment (page E-3-512). The lick extends from elevation 2000 to 2450, so lower elevations of the reservoir will inundate less of the lick area or may totally avoid it. E • Impacts on Other Wildlife. Reservoir clearance and general J ground disturbance associated with the Watana development will have adverse impacts on many other spec1es of wildlife (pages E-3-512 to 517 and Tables E.3.149 to 158). Lower reservo1r elevations with less needed clearing and general ground disturbance will reduce construction and inundation impacts on all wildlife spec1es 1n the area. Borrow Material. "Removal of floodplain gravel can cause eros1on, siltation, increased turbidity, increased ice buildup -fh~ .. • • • ,_, ~ -, ... _., .... ~.-... _.,,L ' . caused by ground water overflow, fish entrapment, and alteration of fish habitat" (page E-3-155). Borrow material requirements for the dam are shown 1n Table 4. A project at elevation 2100 reduces the volume of the dam by 26 percent, at elevation 2000 by 53 percent and a development at 1900 by percent as compared with the base case ~reject with normal . max1mum reservoir elevation at 2185. Borrow areas A and D are located 1n upland areas away from the reservo1r. Borrow area E 1s a large alluvial fan deposit at the confluence of Tusuena Creek and ranges 1n elevation :rom a low of 1410 feet near the r1ver to 1700 feet against the valley walls. Although the mined area will be rehabilitated to provide productive feeding and overwintering fish habitat following construction, some increased turbidity will doubtless occur from the mining activities. Reducing the volume needed from this site will tend to reduca the extent and duration of turbidity and sedimentation in the . r1ver downstream during construction. Also, r~ducing the volume needed from this area will reduce impacts on the existing riparian habitat for moose and other spec1es. -t?- ,. .. • r~ .. fr n ll I r Projec~ Elevation :t! lOO 2000 1900 Borrow Area Source Table 4 DAM FILL VOLUMES (million cu. yds.) Total Volume Riprap Volume %Reduction 61.8 1. 5 45.8 26 1 . 1 29.1 53 0.8 A Shorter Construction Period. Core Gravel & Filters Volume %Reduction 8.3 52 5.8 38.9 25 3.9 24.4 47 n E Many project impacts discussed ~n Exhibit E are essentially time dependent in that the shorter the construction period, the less the cumulative impact. Of particular concern is increased hunting and fishing pressure and the general disturbance that will occur throughout the construction period. Reducing the dam elevation and therefore the construction period will thereby reduce the overall impacts. Some of the types of impacts which would be minimized in this way include: --;8- . .. .f1) J in • ff1 n .. lj I' fl <j n .. • Erosion Potential for Oil and Hazardous Material Spills Blasting River Diversions Reservoir Filling Water Quality Changes Maintenance of Access and Temporary Camps Aircraft Disturbance Emergency Flows to Tsusena Creek. The present proposal includes an emergency spillway to pass flood flows 1n excess of 150,000 cfs (recurrance interval of less than once 1n 10~000 years). The emergency spillway will consist of a long straight chute excavated in rock and leading in the direction of Tsusena Creek. An erodible fuse plug at the upstream end will remain in place until overtopped. Flows of up to 140,000 cfs 1n excess of the combined ma1n spillway and outlet facility capacities may be released to Tsusena Creek, thus preventing overtopping of the main dam -;r- • under Probable Maximum Flood (PMF) ·conitions. It ~s estimated n that flows down the emergency spillway to Tsusena Creek would continue for a period of 20 days under PMF conditions. n At lower reservo1r elevations, construction of this emergency channel would be more expens~ve than expans~on of the main spillway facilities and therefore emergency spills would no longer be diverted to Tsusena Creek. Although such flows have an extremely low frequency of occurence, their removal from Tsusena Creek would remove a potential source of project impact. Relict Channel. An ancient channel, now filled, exists 1n the north bank of the reservoir approximately 2,600 feet upstream of the dam. This channel runs from the Susitna River gorge to Tsusena Creek and represents a potential source of leakage from the Watana . reservo~r. The controlling bedrock surface of the channel is at elevation 1740 and contains up to 454 feet of glacial deposits. To preserve the integrity of the r1m of the Watana 2185 -/-.P- • • • • I i .... ' I • I • r u I r reservo1r and to control losses due to potential seepage, a number of remedial measures have been proposed. These measures will have a net result of disturbance to the vegetation and wildlife resources of that zone. For lower reservo1r elevations (2100 to 1900), needed remedial measures will be greatly reduced resulting 1n considerably less ground disturbance. Flood Control. The Watana 2185 project as described 1n the FERC license application is designed so that the powerhouse and outlet facilities, plus reservo1r storage, will have sufficient capacity to pass the once in fifty year summer flood without operating the main spillway. During the flood, the . reservo1r will be allowed to surcharge to elevation 2193. By containing the fifty year flood without needing to use the spillway structure~ problems related to nitrogen supersaturation and resultant fish kills will be minimized. If a lower elevation for the Watana project is considered (2100 to 1900), project facilities should be designed so that equal protection from nitrogen supersaturation is provided, such • • .. .. r I as requiring up to 1 in 50 year floods to be passed without operating the main spillway. Sufficient flood routing studies will be necessary to assure that the project can adequately meet these criteria. Devil Canyon Development (el. 1455, R.M. 152) This development, as presently being considered, 1s virtually identical to that described in the license applica- tion. Incorporation of this development into the recommended project plan will not add differential impacts 1n relation to those described 1n the license application. High Devil Canyon Alternatives (R.M. 156) Two alternative heights are under consideration for a development at the High Devil Canyon site. One o£ these would be at the same elevation as the Devil Canyon Development (1455 feet) and thus would be comparable to it except that the reser- voir would be four miles shorter with correspondingly less area and volume. Only Cheechako Creek and an unnamed steep (1175 ft/mile) creek would remain unaffected as compared with the Devil Canyon Development. <"'}<'1--,....r • .. I I I I J. The higher of the two developments under consideration for the High Devil Canyon site would have a normal maximum water surface elevation a~ 1750 feet. This development would be located wholly within the reach of river inundated by the combined Devil Canyon -Watana developments ~s discussed in the license application. It would inundate the Watana site by 290 feet making future development there infeasible. Comparisons of the High Devil Canyon 1750 development with the Devil Canyon - Watana develoRment, singly or combined, are shown in Table 5. Table 5 COMPARISON OF DEVELOPMENTS Development Characteristic HDC DC/W W2185 DC Total Reservoir Area (Acres) 24,000 45,500 38,000 7,500 River Length Inundated (miles) Active Reservoir Volume (106AF) 56 1. 8 86 54 4.1 3.75 Environmenal impacts of the single High Devil Canyon 1750 development would be less than those for the combined Devil 32 0.35 Canyon -Watana development and equal to or less than those for • • .. • r J ; I -{;J, e,. 'W ~ ~ 2 1 8 5 de v e 1 o. pm en t a 1 one . Th e Hi g h D e vi 1 C any on 1 7 50 3. 3 ,.,; I~ development would inundate ~ of Tsusena Creek, including the falls (an aesthetic resource of the area) but would inundate o.~ rf.h.e,y-"1'rrre.~'" -l,.;6,7a.rr~ less of Watana Creek~an any of the Watana alternatives (Table 6). As compared with the Watana 2185 development, the High Devil Canyon 1750 reservoir would spare a considerable amount of deciduous forest (birch and aspen) that exists along the south- facing slopes of the Susitna Canyon and along some of the tributaries. This is the only area of any extent that contains this type of habitat, and its associated avifauna, within the Upper Susitna Basin. Table 6 INUNDATION OF TRIBUTARIES BY ALTERNATIVE DEVELOPMENTS (miles) Tributary R.M. Devil Creek 161.4 Fog Creek 176.7 Tsusena Creek 181.9 Deadman Creek 186.7 Ivatana Creek 194.1 Kosina Creek 206.9 Jay Creek 208.6 _=Y fh;h P~i / ~.ryv }?_/ P ~t I Cc-zy (NV f:.J ;;.// ~~ sf HDC17.50 ?,,").... :r y ':1"i' J. 0 'II --";(- J,j l"' p,S *' 3. 7 0.9 0.5 Development Jl DC(HDC)l455 Watana2185 1.4 ~ N/A 1.0 .;y N I,\ 0.2 -;y N/A N/A 2.3 it' N/A 10.4 N/A 4.6 N/A 3.6 • • • li' E) J 1 l i 1-i '' ! ' l.; Vee Canyon Development (el 2330, R~M. 227) ~ The Vee Canyon 2330 development would inundate 3~ miles of the Susitna River channel including 22 miles that would not be inundated by any other development~ It would also inundate approximately 14 miles of Tyone River and approximately 2.2 miles of Oshetna River that would not be affected by any other development. Possibly one of the most significant aspects of the Vee site is that much of the reservoir area has not been included 1n the area tha~ has been studied to document baseline conditions and likely impacts of the Susitna project. In general, the baseline studies did not extend upstream beyond river mile 242 on the mainstream of the Susitna River, halfway up the length of the Vee Canyon Reservoir. Thus, detailed infor ~~tion is not available to make more specific comparisons of alternative project impacts. The Vee Canyon development would inundate a "~ large area river bottomlands upstream of the Oshetna River, 1\ particularly in the upper end of the reservo1r area. This area is utilized by three sub-populations of moose that range 1n the northeast section of the basin and would otherwise be little • .. r· p ' i' n f u L.~ f: affected by the Susitna Project. This additional inundated area is also used by the Nelchina caribou herd, particularly moving to th{rty' calving grounds near Kosina Creek. This additional inundation would result in a greater potential for division of the Nelchina caribou herd's range, as well as inundating part of their range. The Vee Canyon reservo1r area 1s considered to be more J.n important to some key furbearers, particularly red fox, than comparable lands affected by other developments downstream. On the other hand, impacts on birds and black bears would be less than that for comparable projects downstream. The Vee rese~vo1r would also flood the mouth of the Tyone River with a fluctuating and turbid pool and would, in all likelihood, severely decrease the present resident fish population of this, the main clearwater tributary of the Upper Susitna River. The Vee development would also create access to more wilderness azea than would the downstream developments. The area at and around the mouth of the Tyone River has a long history of occupation and use by man, and is a valued area by native people. Previous reviews of potential impacts of the .. ,. • • • .. Vee Canyon development indicate there 1s a high potential for discovery of archaeological sites in the area to be affected. The overall compar1son presented in Exhibit E of the License application between impacts of the Devil Canyon -Watana Combination as opposed to the High Devil Canyon -Vee Develop- ment (Table E.l0.19) shows that the former (Devil Canyon- Watana 2185) is preferable in all environmental characteristics except for potential impacts on birds and black bears. The High Devil Canyon ~ Vee combination would flood more floodplain habitat such as balsam poplar forests and more lakes and wetlands, while the Watana -Devil Canyon scheme would inundate r' more birch and aspen forests. In general, the incremental i j .• impacts of the Vee development outweigh any lessening of impacts r. r from the High Devil Canyon development as compared with the Devil Canyon plus Watana 2185 development. The potential impacts of the Vee development are particularly sensitive to relatively small chanses 1n reservo1r elevation. I£ the elevation were raised by 25 to 50 feet (to approximately elevation 2370, the reservo1r would extend upstream on the Tyone River to include Susitna Lake and Lake -/..7- • • I ~ I • I Louise and the surrounding area. On the other hand, adverse impacts of the development could be substantially reduced by lowering the proposed level of the reservoir to approximately elevation 2240. At that elevation, the reservoir would no longer reach the mouth of the Tyone River and its associated floodplain and wetlands. The reservoir would be reduced 1n length by approximately 14 miles along the Susitna River. Such a reduction would bring the upper limit of the reservoir downstream into the canyon area and away from some of the valuable moose and caribou habitat located further upstream. re&tl/c.t ivY\ i.. re-s-e-r-vt?i r e-lt:Ne-t.l~ ~M ~u; lL}llfrt ~ Such ~reservoir to the area that has been covered by the baseline studies. ENVIRONMENTAL IMPLICATIONS OF OTHER DESIGN CHANGES Du~ing the development and costing of project alternatives, possible design changes for specific project features have been considered as well as the alternative development concepts and operational modes. The elimination of the emergency spillway to Tsusena Greek has already been considered in relation to lowering the Watana dam. Two other general changes in project design have also been considered that may influence project bnp acts. These are the possible substitution of concrete arch -';l._g- • "' "' r f r I n I n dams for the fill dams at all sites and elevations except for the Watana 2185 development and the elimination of the cone valves in favor of low level intakes leading to modified flip bucket discharge facilities that will disperse the flow and prevent nitrogen supersaturation. Environmental implications of these chan~es are considered below. ARCH DAM VS. FILL DAM The possibility of constructing concrete arch dams rather than fill dams 1s one option being considered. This appears to be a viable alternative at all locations except the Watana 2185 development where the lateral flair of the upper slopes would make this type of construction inappropriate. The basic environmental difference between a fill dam and concrete arch dam at a g1ven location is in their construction. In general, the arch dam will requ1re one to two years less constructon time> and will require consideraly less borrow material than a comparable fill dam. These changes are similar to the ch~es previously discussed when considering lowering the normal surface elevation at the Watana development. In ,, • .. general, reducing borrow material ~nd construction time requirements will both tend to reduce construction impacts at the site. Site specific factors will be incorporated into the environmental evaluation upon completion of preliminary design for the development alternatives. DISCHARGE FACILITIES The Alaska State Standard for man-caused gas saturations /10 cannot exceed ft%. The incorporation of "cone-valves" into project design 1s repeatedly mentioned 1n the license application as the method of mitigation for gas supersaturation. Chapter 2 of Exhibit E (page E-2-187 and 188) states: "The avoidance of gas supersaturation will be achieved by the inclusion of fixed-cone valves as the "normal" outlet facilities. "By us1ng the . reservo1r storage capacity coupled with the minimum summer powerhouse flow and the fixed-cone valve discharge, all r-)rJ- • • ... .. • f flow releases with a recurrence interval of up to 1:50 years will be discharged with minimum potential for nitrogen supersatura- tion. As previously described ~n Section • I 4.1.3, s~x 78-inch (2m) diameter valves with a design capacity of 4000 cfs each, will be located approximately 125 feet (38 m) above normal tailwater levels. These valves will discharge the flow as highly diffused jets to achieve significant energy dissipation without a stilling basin or plunge pool." "Little literature and no precedent data were available regarding the performance of fixed-cone valves in reducing or preventing supersaturated discharges. As such, a theoretical assessment of their anticipated performance was conducted based upon available studies of the aeration efficiency of similar Rowell-Bunger valves (fixed-cone) and the physical and geometric characteris- n " u I tics of diffused jets discharging freely into the atmosphere." "The results of the assessment indicated that no serious supersaturation of nitrogen is likely to occur with flow releas~ ........., through the valves. Estimated gas con- centrations that would occur as a result of flow release are 101 percent at Watana and 102 percent at Devil Canyon. For releases of greater frequency at less discharge, the concentrations are expected to be slightly lower." "To support these conclusions, a field test of similar valves was undertaken at the Lake Comanche Dam on the Mokelumne River . 1n California (Ecological Analysts, 1982). The results of the tests indicate that the valves prevented supersaturation and, to a limited extent, may have reduced existing nitrogen concentrations. Flows of 4000 cfs .. " .. • • • n ,j with a dissolved nitrogen concentration of 101 percent at the intake structure were passed through four Howell-hunger valvef. n J.J Gas concentrations in the discharge were 97 percent. At 330 feet and 660 feet (100 and H (" u 200 m) downstream, concentrations were 95 and 97 percent respectively." Alternative design configurations are being considered whereby the cone valves, which were planned to discharge into the air approximately 105 feet above the river, may be replaced by a modified flip bucket specifically designed to disperse the flow and prevent penetration of the discharge plume more than 25 feet below the tailwater surface. Theoretically, this con- figuration will keep dissolved gas concentrations at or below those provided by the cone valves. The design of these dis- charge facilities will continue to limit the use of the main spillway to flow releases with a recurrence interval of no greater than 1 ~n 50 years. Definitive tests of the alternative discharge facilities and their ability t~ prevent gas super- saturation will be performed as part of the spillway model tests. -3J- .. n· u n lJ ~ L n lJ f'l ' l ! ' u ~ l t. . ~J n \ i u Fl /I l l w n k 1 =-.) !l ' I .w ·-l i ~ i l ' \,) ~ ~ ~ " '1 \J ~ ~ ~ ~ ""' 'U ~ " s: ~ ' ....,..._ ~ '!\. 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