HYDRAULIC MODELING ASSESSMENT OF THE TURNER FALLS IMPOUNDMENT TURNERS FALLS HYDROELECTRIC PROJECT (NO. 1889) AND NORTHFIELD MOUNTAIN PUMPED STORAGE

Transcription

1 HYDRAULIC MODELING ASSESSMENT OF THE TURNER FALLS IMPOUNDMENT TURNERS FALLS HYDROELECTRIC PROJECT (NO. 1889) AND NORTHFIELD MOUNTAIN PUMPED STORAGE PROJECT (NO. 2485) FEBRUARY 2013

2 TABLE OF CONTENTS 1.0 BACKGROUND - TURNERS FALLS PROJECT BOUNDARY BATHYMETRIC AND HYDRAULIC MODELING BACKGROUND Bathymetry and Digital Terrain Model Hydraulic Model Model Updates HYDROLOGY HYDRAULIC MODELING ASSESSMENT TO ASSESS TF PROJECT IMPACTS Hydraulic Modeling at 120,000 cfs At what flow does the hydraulic control of the TF Impoundment shift to the Gorge? How far upstream is the influence of the TF Project under low flow conditions? If the TF Dam was removed, how would its absence impact water levels in the TF Impoundment under flood flows? CONCLUSIONS REFERENCES...9 APPENDIX A. INUNDATION MAPS February 2013 i

3 LIST OF TABLES Table 1: FEMA Reported Flood Flows (Source: FEMA FIS, Revised February 1982)...4 Table 2: Estimated Flood Flows at Turners Falls Dam (Log-Pearson Type III)...5 Table 3: Change in Channel Top Width at Three Transects near the Gorge (under 120,000 cfs and starting downstream boundary of ft msl)...6 February 2013 ii

4 LIST OF FIGURES Figure 1: Key Features along Turners Falls Impoundment Figure 2: Transects through Turners Falls Impoundment (Source: HEC-RAS) Figure 3: 50-Year Flood Flow (120,000 cfs) TF Impoundment Water Surface Profile with Starting Downstream Water Surface Elevations at the TF Dam of 176 feet msl, feet msl and 185 feet msl Figure 4: 50-Year Flood Flow (120,000 cfs) TF Impoundment to just above Gorge- Water Surface Profile with Starting Downstream Water Surface Elevations at the TF Dam of 176 feet msl, feet msl and 185 feet msl Figure 5: Transect No at 120,000 cfs, Top Width= 304 feet Figure 6: Transect No at 120,000 cfs, Top Width= 393 feet Figure 7: Transect No at 120,000 cfs, Top Width= 670 feet Figure 8: Turners Falls Impoundment from TF Dam to Vernon Tailrace Water Surface Profiles for Various Flows Figure 9: Turners Falls Impoundment from TF Dam to just above Gorge- Water Surface Profiles for Various Flows Figure 10 Inundation Map of the Turners Falls Impoundment at 15,928 cfs, 30,000 cfs, 120,000 cfs and the existing Project Boundary Figure 11: Turners Falls Impoundment- Water Surface Profile under Flow of 1,433 cfs with starting downstream WSEs at Turners Falls Dam of 176 feet msl and 185 feet msl Figure 12: Turners Falls Impoundment- Water Surface Profile under Flow of 1,433 cfs with starting downstream WSEs at Turner Falls Dam of 176 feet msl and 185 feet msl- Blown up of Figure 11 from Station to (at Vernon Tailrace) Figure 13: Turners Falls Impoundment from TF Dam to Vernon Tailrace Water Surface Profiles at 15,938 cfs (Plant Capacity) under Dam-in and Dam-out Conditions Figure 14: Turners Falls Impoundment from TF Dam to Vernon Tailrace Water Surface Profiles at 30,000 cfs under Dam-in and Dam-out Conditions February 2013 iii

5 LIST OF ABBREVIATIONS DEM DGPS DTM FEMA FIS GIS Gorge HEC-RAS MassDOT msl NF NF Project TF TF Project USACOE USGS WSE WSP Digital Elevation Model Differential Global Positioning System Digital Terrain Model Federal Emergency Management Agency Flood Insurance Study Geographic Information System French King Gorge Hydraulic Engineering Center River Analysis System Massachusetts Department of Transportation mean sea level Northfield Northfield Mountain Pumped Storage Project Turners Falls Turners Falls Hydroelectric Project United States Army Corps of Engineers United States Geological Survey water surface elevation water surface profile February 2013 iv

6 1.0 BACKGROUND - TURNERS FALLS PROJECT BOUNDARY With approval for construction of the Northfield Mountain Project (NF Project) in 1968 and the use of the Turners Falls (TF) Impoundment as the lower impoundment for pumped storage operations, the TF Impoundment was enlarged to provide an additional 12,600 acre-feet of useable storage by raising the normal water surface elevation (WSE) approximately 5.4 feet to elevation 185 feet mean sea level (msl) at the TF Dam. The TF Project Boundary was also enlarged to coincide with the NF Project Boundary, except for certain lands that are utilized solely for the NF Project. The Project Boundary from the TF Dam to the Narrows was based on the inundation area at the 50-year flood, estimated at the time as 126,000 cfs. From the Narrows to the base of Vernon Dam the Project Boundary was based on the inundation area at the 50-year flood plus three (3) feet vertically due to possible inaccuracies in historical data used in estimating the backwater elevations. The upstream geographic extent of the Project Boundary was determined to extend approximately 20 miles upstream, due to the possible backwatering of the TF Impoundment to the base of Vernon Dam. Figure 1 is a plan map showing key features along the TF Impoundment including two major tributaries, Ashuelot and Millers Rivers, the Northfield Mountain Project tailrace, and the location of the French King Gorge (Gorge). Within the TF Impoundment the river has several natural constrictions; the most pronounced is the Gorge located approximately 0.6 miles upstream from the Millers River confluence. Other less pronounced restrictions occur at the Narrows, about 1.1 miles above the TF Dam, and in the Horse Race extending about 2.6 to 3.1 miles above TF Dam. The purpose of this document is to summarize more recent methods of determining the width of the Project Boundary above the Gorge and the upstream geographic extent of the TF Impoundment. The findings contained herein demonstrate that the TF Impoundment does not backwater to the base of the Vernon Dam and that the upstream influence of the TF Project is located approximately 9,000 feet downstream of Vernon Dam, or just below Stebbins Island. The findings also show that hydraulic control of the river shifts from the TF Dam to the Gorge at a flow of approximately 30,000 cfs. Accordingly, FL intends to propose a geographic scope for its relicensing studies limited to the zone of impact of the TF Project. In addition FL will propose modifying both the width and upstream geographic extent of the Project Boundary as part of its relicensing proposal. 2.0 BATHYMETRIC AND HYDRAULIC MODELING BACKGROUND As discussed above, since the original hydraulic assessment, more precise data collection methods and hydraulic modeling tools are available. Specific improvements include: Greater geographic coverage and higher resolution bathymetric mapping of the TF Impoundment; More detailed upland topographic mapping of the riverbanks and upland areas; Hydraulic models are available to predict the WSE at numerous transects along the TF Impoundment for a given flow, and; Hydraulic modeling results can be readily uploaded to a Geographic Information System (GIS) such that inundation areas can be readily shown on a plan map of the TF Impoundment. FirstLight has conducted numerous studies in the TF Impoundment. One such study was conducted by Field Geology Services for use in studying fluvial processes/shoreline erosion in the TF Impoundment. To support the Field study, in 2006, a bathymetric map and hydraulic model of the TF Impoundment was developed by Woodlot Alternatives Inc. (Woodlot) as summarized in the report entitled: Connecticut River Hydraulic Analysis, Vernon Dam to Turners Falls Dam (Woodlot, July 2007). Summarized below are sections of that report describing the bathymetric data collection and hydraulic modeling. February

7 2.1 Bathymetry and Digital Terrain Model To develop a hydraulic model of the TF Impoundment, transects perpendicular to the impoundment and upland river banks were needed. Per the Woodlot report, bathymetric data of the TF Impoundment was collected between July 30 and August 5, The geographic study area covered approximately 20 miles and stream widths ranging from 200 to 3,000 feet. No bathymetric data were collected, however, in the area between the TF Dam and its boat barrier, a distance of approximately 1,500 feet. Similarly, from the Vernon Dam downstream approximately 460 feet, no bathymetric data were collected. It is assumed that data were not collected in these locations for safety purposes. Per the Woodlot report, bathymetric data were obtained through a series of stream-wise transects along the study reach. Shoreline transects were obtained as close as possible to each bank and along the shorelines of instream islands. As with the river shoreline, transects were also obtained as close as possible to the perimeter of islands within the study reach. In addition, extra survey transect lines were completed where there appeared to be large variability in the bathymetry that could result in variable currents (Woodlot, 2007). The report states that bathymetric data were collected using a survey boat equipped with recording fathometers, hand sounding probes, a Differential Global Positioning System (DGPS), an onboard computer and hydrographic surveying software. The onboard hydrographic surveying computer software collected and logged real time, depth, boat speed, bearing and positioning data at one second intervals. Upon completion of the field survey, a sounding (XYZ) data table was developed containing time, location (Lat/Lon- Northing/Easting), sounding depths (ft), and river bottom elevations (corrected by bench marks /tide gage elevations). This information was subsequently used to develop a bathymetric map of the TF Impoundment. Woodlot then combined publicly available topographic data and the collected bathymetric data to develop a digital terrain model (DTM). The DTM was compiled from three elevation data sources, USGS 10 meter digital elevation model (DEM), 1:5000 Massachusetts DTM (MA DTM), and the bathymetric field data. The three elevation data sources were converted to the same vertical datum. 2.2 Hydraulic Model In 2006, Woodlot Alternatives developed a one dimensional hydraulic model of the TF Impoundment using the United States Army Corps of Engineers (USACOE) Hydraulic Engineering Center River Analysis System (HEC-RAS). HEC-RAS is a steady state model, meaning that the flow cannot change over the simulation period. The HEC-RAS model was developed using the HEC-GeoRAS Geographic Information System (GIS) extension for the purpose of defining reasonable lateral extents. From the integrated bathymetric and upland topographic data, transects were cut (perpendicular to the flow path) through the river and upland area as shown in Figure 2 for use in the hydraulic model. It is at these transects where the WSEs are predicted for a given flow and a water surface profile (WSP) along the length of the TF Impoundment can be developed. The hydraulic model includes approximately 200 transects over the 20 mile long reach or approximately 1 transect per 500 feet. 2.3 Model Updates For the present analysis, the HEC-RAS hydraulic model developed by Woodlot was used in determining the upstream influence of the TF Project. A few updates were made to the model as described below. Based on reviewing the model results for the 100-year flood flow, it was determined that the WSE extended beyond the river bank limits at six transects meaning the upland topography did not extend high enough to contain the flow. When this occurs, the HEC-RAS model defaults to extending a vertical line at the river bank February

8 to contain the flow. For those transects in which this occurred (6 out of 200 transects), the model was modified to accurately reflect the upland topography such that all flood flows were contained within the transect limits. Additional transects upstream and downstream of five bridges (described below) were imported into the model to better assess the constriction and expansion of flow around these structures. The original transects in Woodlot s hydraulic model used three Manning n 1 values across each transect divided into the left overbank, channel, and right overbank. When flows are high, the left and right overbanks will become inundated and these areas typically have a higher roughness or Manning n value. In the original model, a Manning n value of 0.05 was used in the left and right overbanks regardless of land composition over the length of impoundment. Similarly, a Manning n value of was used to reflect channel roughness along the length of the impoundment. Because the right and left overbank areas vary ranging from forests, heavy vegetation and pastures-- the overbank Manning n values were adjusted to reflect on-the-ground conditions. Data obtained from the National Land Cover Database of 2006 developed by the United States Geological Survey (USGS) was used to establish the roughness. Manning n values were adjusted to match the land cover data in the left and right overbank areas. No changes to the channel Manning n value was made. In addition, ineffective flow areas were also included in the revised model to determine where it made sense that flow would expand into the overbanks based on constrictions and expansions of flow due to the topography or man-made structures. Five bridges cross the Turners Falls Impoundment, which were not included in the original model. Bridges can impact the WSE in the Turners Falls Impoundment due to piers occupying space, and depending on the height of water, the low chord of the bridge could cause pressure flow. The following five bridges were added to the hydraulic model in upstream to downstream order: the abandoned Boston and Maine Railroad Bridge (only 5 of the original 6 piers remain upright and the bridge deck no longer remains), the Schell Memorial Bridge (two piers), the Central Vermont Railroad Bridge (three piers), the Route 10 Bridge (two piers), and the French King Bridge (no piers). Since actual survey data was unavailable for the bridges, other sources were utilized. The pier widths at the abandoned Boston and Maine Railroad Bridge in Vernon, VT, were determined using Google Earth measuring tools. Only five of the original six piers were included in the model since one has fallen. The Schell Memorial Bridge and the Central Vermont Railroad Bridge were both modeled using existing Flood Insurance Study (FIS) microfiche data from 1983 to determine the widths and elevations of the piers. For these bridges, the high and low chord of the bridge decks were determined based on the FIS reports and contours of the area developed in GIS. Pier data and deck elevation for the Route 10 Bridge were obtained from record drawings provided by the Massachusetts Department of Transportation (MassDOT). The French King Bridge road deck elevations and abutment stations were determined using DEM data and GIS contours. The low chord of all five bridges traversing the TF Impoundment did not influence the hydraulics in the TF Impoundment. Even under the 500-year flood, the model shows that the WSE in the TF Impoundment never became high enough to reach the low chord of the bridges. 1 Manning n is a unitless measure of roughness. For example, the Manning n values for sand would be less than for cobble. Similarly, a Manning n value is even higher if flow is traveling through trees or overbank vegetated areas to reflect the greater resistance to flow. February

9 3.0 HYDROLOGY As noted above, the 50-year flood was used to set the current Project Boundary in The Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) for Montague, MA was reviewed to identify flood flows. The most recent FIS for Montague, MA was published as a Revised FIS in February Per the 1982 FEMA study, flood flows on the Connecticut River were determined as follows (page 9 of FIS): The Connecticut River discharges for the 10-, 50-, 100- and 500-year floods were obtained from rating curves developed in a study by the U.S. Army Corps of Engineers. The analysis was based on existing historical records which were modified to account for 16 flood control dams throughout the Connecticut River Basin. In the study, peak discharge frequencies for natural conditions were determined using a log-pearson analysis. These natural frequency curves were combined with hydrographs based on routing analysis of the flood control structures throughout the drainage area to produce the final stage-discharge rating curves for the entire Connecticut River. The River gage used in the Montague study was as follows: Gage Period of Record Connecticut River at Montague City, MA (No ) Based on the above, the FIS reported flood flows at the Turners Falls Dam as shown in Table 1. Table 1: FEMA Reported Flood Flows (Source: FEMA FIS, Revised February 1982) Location Drainage Area 10-yr 50-yr 100-yr 500-yr Turners Falls Dam 7,163 mi 2 99,000 cfs 138,000 cfs 1 157,000 cfs 1 207,200 cfs 1 1 The FIS also reported the following The water surface elevations from these discharges were lower than the 30-year elevations with a discharge of 126,000 cfs. Because three decades have passed since FEMA s FIS was revised, a more up-to-date Log-Pearson Type III flood frequency analysis 3 was conducted to estimate flood flows at the TF Dam. To compute the flood flows, the USGS Gage at Montague City, MA was selected, which has been operational since This gage is located a short distance below FirstLight s Cabot Station. The drainage area at the gage is 7,860 mi 2, whereas the drainage area at Turners Falls Dam is 7,163 mi 2. The difference in drainage area is primarily due to the Deerfield River, which discharges into the Connecticut River just upstream of the Montague City gage. Two periods of record were selected to estimate flood flows. The first period extends from (58 instantaneous peak flows) to represent flood conditions prior to the construction of the several USACOE flood control facilities. The second period extends from 1962 to 2011 (50 instantaneous peak flows) to represent flood conditions after the USACOE flood control facilities were constructed in the Connecticut River Basin 4. Using the annual instantaneous peak flows at the Montague City USGS gage, the flood frequency analysis was 2 The FIS was published in February The FIS did not report the end of the period of record analysis, other than listing it as 1904 to present. It was assumed that the period of record ended in A weighted skew was used in the Log-Pearson Type III flood frequency analysis. 4 The seven USACOE flood control reservoirs located upstream of the Montague USGS gage and the date placed into operation include: Ball Mountain (1961), North Hartland (1961), North Springfield (1961), Otter Brook Lake (1958), Surry Mountain (1941), Townshend Lake (1961) and Union Village (1950). February

10 conducted to estimate the 10-, 50-, 100-, and 500-yr floods for the two periods of record. The estimated flood flows at the Montague City USGS gage were subsequently prorated, by a ratio of drainage areas, to estimate flood flows at the TF Dam (7,163 mi 2 /7,860 mi 2, ratio = 0.91) as shown in Table 2. Table 2: Estimated Flood Flows at Turners Falls Dam (Log-Pearson Type III) Location Drainage Area Period of Record 10-yr 50-yr 100-yr 500-yr Montague USGS Gage At Turners Falls Dam Montague USGS Gage At Turners Falls Dam 7,860 mi , 58 peak flows 142,200 cfs 191,000 cfs 212,800 cfs 266,800 cfs 7,163 mi 2 129,590 cfs 174,063 cfs 193,930 cfs 243,141 cfs 7,860 mi , 50 peak flows 110,600 cfs 131,300 cfs 139,600 cfs 157,900 cfs 7,163 mi 2 100,792 cfs 119,657 cfs 127,221 cfs 143,898 cfs As Table 2 shows, not surprisingly flood flows are higher prior to the construction of the USACOE flood control facilities. In comparing the post-usacoe flood control dam period of record ( ) with the original FEMA study (Table 1), the post-usacoe flood flows are lower for the 50-, 100- and 500-year floods. For purposes of this document, the 50-year flood flow reflecting post-usacoe flood control dams (119,657 cfs, rounded to 120,000 cfs) was used in the hydraulic model. As a side note, the instantaneous peak flow recorded at the Montague City USGS Gage on August 28, 2011 (Hurricane Irene) was 127,000 cfs. When prorating this flow, based on a ratio of drainage areas, the flow at Turners Falls Dam was approximately 115,570 cfs, just below the 50-year flood (based on the post USACOE dams). 4.0 HYDRAULIC MODELING ASSESSMENT TO ASSESS TF PROJECT IMPACTS 4.1 Hydraulic Modeling at 120,000 cfs Using the updated hydraulic model, the 50-year flood (120,000 cfs) was simulated under three different downstream boundary conditions. The HEC-RAS model requires a starting downstream boundary condition which is set at the lowermost transect in the model; in this case the lowermost transect is near the TF Dam buoy line. Three hydraulic models were developed for downstream boundary conditions at the TF Dam of 176 feet mean sea level (msl), feet msl and 185 feet msl. The current FERC license permits the TF Impoundment to fluctuate between elevations 176 feet msl and 185 feet msl. Elevation feet msl reflects the median WSE at the TF Dam based on the period The purpose for having three different starting WSEs at February

11 TF Dam under the same 50-year flood was to determine if the inundation area along the TF impoundment varied based on these three different starting WSEs. Logic would suggest that if the WSE at the TF Dam was at elevation 176 feet msl during a flood event it would result in less upstream flooding as compared to having the WSE at feet msl or 185 feet msl. Figure 3 shows the WSP along the TF Impoundment under a flow of 120,000 cfs and three different TF Dam starting WSEs (green line: 185 feet msl, red line: feet msl and blue line: 176 feet msl). Figure 4 is a blown-up version of Figure 3 showing the WSP from the TF Dam to just above the Gorge. The results show that between the TF Dam and the Gorge there is a measurable difference in the WSE between the three different starting WSEs (in other words there is a difference in the inundation area). However, upstream of the Gorge there is no measureable difference in the three WSPs. In sum, under a flow of 120,000 cfs, whether the WSE at the TF Dam is 176 feet msl or 185 feet msl it has little-to-no bearing on the WSEs (and hence inundation areas) above the Gorge. Thus, if the WSE at the TF Dam was lowered to 176 feet msl during a flood, this action has no influence on the WSP above the Gorge. The logical question is why is the WSE above the Gorge the same under three different starting WSEs at the TF Dam. Hydraulic controls in rivers are a function of the river s width and/or changes in the channel bed slope (positive and negative). In this case, the Gorge serves as a hydraulic control because of two factors - the river width narrows and the channel bed slope becomes negative. Of these two factors, the major contributor is the extreme narrowing of the river s width at the Gorge. Figure 4 shows the WSP and channel bed 5 from the TF Dam to just above the Gorge. Also on Figure 4 are three transects labeled in downstream to upstream order as Transect Nos , and Each of these three transects are separated by 500 feet. As Figure 4 shows, at the Gorge the channel bed slope is negative or inclines, which partially contributes to restricting the flow. Figure 5 (Transect No ), Figure 6 (Transect No ), and Figure 7 (Transect No ) show the WSE starting from the lowermost transect (Transect No ). Note that all three plots are on the same x- and y- scales to allow for comparison and the channel top width is based on a starting downstream boundary condition of feet msl. As shown in Table 3, moving in an upstream to downstream direction, the river width narrows by roughly 55% over a distance of 1,000 feet. Because of this abrupt narrowing of the river, the Gorge acts as a choke point, which results in water piling up behind the Gorge. In short, at flows of 120,000 cfs, the Gorge serves as a natural hydraulic control. Table 3: Change in Channel Top Width at Three Transects near the Gorge (under 120,000 cfs and starting downstream boundary of ft msl) Transect No. Channel Top 120,000 cfs Percent Reduction in Top Width Relative to Transect No feet feet 41% 5 The channel bed shown represents the thalweg, or lowest point along the transect. February

12 feet 55% 4.2 At what flow does the hydraulic control of the TF Impoundment shift to the Gorge? The same hydraulic model was used to determine at what flow the Gorge starts to control WSEs above the Gorge. In this case, a series of hydraulic model runs were made with flows ranging from 10,000-80,000 cfs (including the total hydraulic capacity of Station No. 1 and Cabot of 15,938 cfs 6 ). All modeling runs used the same starting downstream WSE of 185 feet msl (the maximum elevation for impoundment fluctuations per the FERC license). The WSP was plotted for each flow to determine at what flow the Gorge starts to control upstream WSEs. Figure 8 shows the WSP from the TF Dam to the Vernon Dam tailrace for a select number of flows. Figure 9 is a blown-up version of Figure 8 showing the WSP from the TF Dam to just above the Gorge. As Figure 8 and 9 show, at 15,938 cfs (blue line), the TF Dam controls the WSE throughout the TF Impoundment. However, at approximately 30,000 cfs (red line) the hydraulic control starts to shift from the TF Dam to the Gorge as the WSP above the Gorge steepens. In sum, the hydraulic modeling indicates that at approximately 30,000 cfs, the Gorge starts to controls WSEs above the Gorge. The next step was to develop inundation maps along the TF Impoundment for the following: 30,000 cfs- the approximate flow at which the Gorge controls upstream WSEs (yellow); 15,938 cfs- the total TF hydroelectric capacity (green); 120,000 cfs- 50-year flood (blue),and; the current Project Boundary (red). Figure 10 shows the inundation area for the three flows and the existing Project Boundary. Appendix A includes blown-up maps of the entire TF Impoundment. As expected, the area of inundation increases as the flow increases from 15,938 cfs to 30,000 cfs and then to 120,000 cfs but all inundation areas are less than the current Project Boundary. Because the Gorge, as opposed to operation of the TF Project, controls the WSP above the Gorge at flows of 30,000 cfs and above, FL intends to propose reducing the current Project Boundary above the Gorge accordingly as shown in Appendix A as part of its relicensing proposal. 4.3 How far upstream is the influence of the TF Project under low flow conditions? A sensitivity analysis was also conducted to determine the upstream influence of the TF Project under low flow conditions. To conduct this analysis, two hydraulic model runs were made under a flow of 1,433 cfs, the current minimum flow for the TF Project. To place this flow into perspective, 1,433 cfs is equaled or exceeded approximately 97% of the time (or conversely 3% of the time, the flow is less than 1,433 cfs) at TF Dam. One model run had a starting downstream boundary at the TF Dam of 176 feet msl and the second model run had a starting downstream boundary at the TF Dam of 185 feet msl. Shown in Figure 11 is the WSP for both model runs. The WSP is shown in blue for starting WSE of 176 feet msl and red for starting WSE of 185 feet msl. In addition, Figure 12 is a blown-up section of the TF Impoundment near the Vernon Dam, showing the WSPs merging well below Vernon Dam. These findings indicate that the even under low flows, the TF Project backwater does not extend to the base of Vernon Dam. In fact, the transect where the two WSP profiles converge is near Station 96000, which is located approximately 9,000 feet below Vernon Dam or just below Stebbins Island. Based on these findings, the influence of the TF Project under low flow conditions terminates approximately 9,000 feet below Vernon Dam. 6 Station No. 1= 2,210 cfs, Cabot Station= 13,728 cfs, Combined= 15,938 cfs. February

13 4.4 If the TF Dam was removed, how would its absence impact water levels in the TF Impoundment under flood flows? Another sensitivity analysis was conducted to determine if the TF Dam was not present, whether the inundation area above the Gorge would be less under a flow of 30,000 cfs. In this case, the hydraulic model simulated flows of 15,938 cfs (TF Plant Capacity) and 30,000 cfs with the TF Dam removed. The following assumptions were made: At the lowermost transect in the model (at the boat barrier), the starting downstream boundary condition was set at critical depth. No transects were modified in the model between the TF Dam and Gorge (or any other transect). In short, it was assumed that the channel bed would remain stable and that no sediment accumulation or erosion would occur if the dam were removed. Shown in Figure 13 is the WSP at a flow of 15,938 cfs under the following conditions: TF Dam Removed, Starting WSE at lowermost transect= critical depth= feet msl (green) TF Dam in Place, Starting WSE at lowermost transect= 176 feet msl (blue) TF Dam in Place, Starting WSE at lowermost transect= 185 feet msl (red) As Figure 13 shows, the WSEs eventually merge below the Vernon Tailrace. Shown in Figure 14 is the WSP at a flow of 30,000 cfs (hydraulic control shifts to Gorge) under the following conditions: TF Dam Removed, Starting WSE at lowermost transect= critical depth= feet msl (green) TF Dam in Place, Starting WSE at lowermost transect= 176 feet msl (blue) TF Dam in Place, Starting WSE at lowermost transect= 185 feet msl (red) Again, as Figure 14 shows, the WSEs eventually merge below the Vernon Tailrace. Based on this sensitivity analysis and above assumptions, the TF Project backwater does not extend far enough upstream to impact the Vernon tailrace elevation. 5.0 CONCLUSIONS The hydraulic analysis contained herein showed the following: At a flow of 120,000 cfs the WSP above the Gorge is the same regardless of whether the water elevation at TF Dam is 176 feet msl (the minimum drawdown elevation) or 185 feet msl (the maximum elevation). Hydraulic control of the river shifts from the TF Dam to the Gorge at a flow of approximately 30,000 cfs. Under low flow conditions of 1,433 cfs, and water elevation at TF Dam at 185 feet msl, the backwater of the TF Dam does not extend to the base of Vernon Dam. The TF Dam backwater terminates approximately 9,000 feet downstream of Vernon Dam or just below Stebbins Island and indicates that the TF Dam has no influence on the Vernon tailrace elevation. February

14 If the TF Dam was removed, under a flow of 15,937 cfs (TF Hydraulic Capacity) or of 30,000 cfs, the WSP profile is the same as with the TF Dam in place i.e., well below Vernon Dam. This finding also indicates that the TF Dam has no influence on the Vernon tailrace elevation. Based on the above analysis and findings, the impact of the TF Project does not extend to the base of Vernon Dam as contemplated in The TF Dam backwater terminates approximately 9,000 feet below Vernon Dam, and, as such, the Project Boundary should be reduced accordingly. Similarly, the width of the Project Boundary above the Gorge should also be contracted to encompass only those lands inundated at 30,000 cfs. Finally, the upstream geographic extent of FERC relicensing studies in the TF impoundment should also be limited to those areas in which the Project has a direct impact. 6.0 REFERENCES Field Geology Services, Fluvial Geomorphology Study of the Turners Falls Pool on the Connecticut River Between Turners Falls, MA and Vernon, VT (August 2007). Woodlot Alternatives, Connecticut River Hydraulic Analysis, Vernon Dam to Turners Falls Dam (July 2007). February

15 Ashuelot River Vernon Dam MASSACHUSETTS VERMONT NEW HAMPSHIRE MASSACHUSETTS Northfield Mountain Upper Reservoir Turners Falls Dam French King Gorge Millers River FIRSTLIGHT POWER RESOURCES CONNECTICUT RIVER INUNDATION MAPPING Miles 1 inch = 1.25 mile ³ Figure 1: Key Features along Turners Falls Impoundment Copyright 2011 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\figure_1.mxd

16 Vernon Dam ma_vt Turners Falls Dam Figure 2: Transects through Turners Falls Impoundment (Source: HEC-RAS) Notes: green lines are transects, red dots define the overbank areas ct _ ri ve r February

17 220 Turners Falls Dam FirstLight Plan: 50yr_BoundaryCondition 2/15/2013 CT_River MA_VT Vernon Tailrace WS 50 yr 185 ft 200 WS 50 yr ft WS 50 yr 176 ft 180 Gorge Ground Elevation (ft) Main Channel Distance (ft) Figure 3: 50-Year Flood Flow (120,000 cfs) TF Impoundment Water Surface Profile with Starting Downstream Water Surface Elevations at the TF Dam of 176 feet msl, feet msl and 185 feet msl February

18 FirstLight Plan: 50yr_BoundaryCondition 2/15/ CT_River MA_VT Turners Falls Dam Gorge WS 50 yr 185 ft 200 WS 50 yr ft WS 50 yr 176 ft Elevation (ft) Negative Slope Ground Main Channel Distance (ft) Transect No Transect No Transect No Figure 4: 50-Year Flood Flow (120,000 cfs) TF Impoundment to just above Gorge- Water Surface Profile with Starting Downstream Water Surface Elevations at the TF Dam of 176 feet msl, feet msl and 185 feet msl February

19 FirstLight Plan: 50yr_BoundaryCondition 2/15/ WS 50 yr ft Ground Bank Sta Elevation (ft) 200 Top Width = 304 feet 150 Transect No Station (ft) Figure 5: Transect No at 120,000 cfs, Top Width= 304 feet February

20 FirstLight Plan: 50yr_BoundaryCondition 2/15/ WS 50 yr ft 250 Ground Bank Sta Elevation (ft) 200 Top Width = 393 feet 150 Transect No Station (ft) Figure 6: Transect No at 120,000 cfs, Top Width= 393 feet February

21 FirstLight Plan: 50yr_BoundaryCondition 2/15/ WS 50 yr ft Ground Bank Sta Elevation (ft) 200 Top Width = 670 feet 150 Transect No Station (ft) Figure 7: Transect No at 120,000 cfs, Top Width= 670 feet February

22 220 FirstLight Plan: GorgeControl 2/15/2013 CT_River MA_VT Vernon Tailrace 200 Turners Falls Dam WS cfs WS cfs WS cfs Elevation (ft) WS cfs Ground 140 Gorge Main Channel Distance (ft) Figure 8: Turners Falls Impoundment from TF Dam to Vernon Tailrace Water Surface Profiles for Various Flows February

23 220 FirstLight Plan: GorgeControl 2/15/2013 CT_River MA_VT 200 Turners Falls Dam WS cfs WS cfs WS cfs Elevation (ft) WS cfs Ground 120 Gorge Main Channel Distance (ft) Figure 9: Turners Falls Impoundment from TF Dam to just above Gorge- Water Surface Profiles for Various Flows February

24 Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity FIRSTLIGHT POWER RESOURCES CONNECTICUT RIVER INUNDATION MAPPING Miles 1 inch = 1 mile ³ Figure 10: Inundation Map of Turners Falls Impoundment at 15, 928 cfs, 30,000 cfs, 120,000 cfs, and the Existing Project Boundary Copyright 2011 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\figure_10.mxd

25 200 Turners Falls Dam FirstLight Plan: MinFlow_BC 2/15/2013 CT_River MA_VT Vernon Tailrace WS 1433 cfs 185 ft 180 WS 1433 cfs 176 ft Ground Elevation (ft) Gorge Main Channel Distance (ft) Figure 11: Turners Falls Impoundment- Water Surface Profile under Flow of 1,433 cfs with starting downstream WSEs at Turners Falls Dam of 176 feet msl and 185 feet msl February

26 200 FirstLight Plan: MinFlow_BC 2/15/2013 CT_River MA_VT Vernon Tailrace ~9,000 feet WS 1433 cfs 185 ft WS 1433 cfs 176 ft Ground 180 Elevation (ft) 160 Water Surface Profiles merge at Station Main Channel Distance (ft) Figure 12: Turners Falls Impoundment- Water Surface Profile under Flow of 1,433 cfs with starting downstream WSEs at Turner Falls Dam of 176 feet msl and 185 feet msl- Blown up of Figure 11 from Station to (at Vernon Tailrace) - February

27 FirstLight Plan: NoDam_ /18/2013 CT_River MA_VT WS cfs-dam185 WS cfs-dam176 WS cfs -NoDam Ground Elevation (ft) Blow up Main Channel Distance (ft) Figure 13: Turners Falls Impoundment from TF Dam to Vernon Tailrace Water Surface Profiles at 15,938 cfs (Plant Capacity) under Dam-in and Dam-out Conditions February

28 FirstLight Plan: NoDam_30k 2/18/2013 CT_River MA_VT WS cfs-dam185 WS cfs-dam176 WS cfs-nodam Ground Elevation (ft) Main Channel Distance (ft) Figure 14: Turners Falls Impoundment from TF Dam to Vernon Tailrace Water Surface Profiles at 30,000 cfs under Dam-in and Dam-out Conditions February

29 APPENDIX A. INUNDATION MAPS February

30 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 1 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

31 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 2 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

32 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 3 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

33 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 4 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

34 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 5 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

35 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 6 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

36 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 7 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

37 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 8 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

38 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 9 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

39 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 10 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd

40 Service Layer Credits: Image courtesy of USGS 2013 Microsoft Corporation 2010 NAVTEQ AND Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, ipc, NRCAN, Esri Japan, ³ FIRSTLIGHT POWER RESOURCES TURNERS FALLS IMPOUNDMENT HYDRAULIC ASSESSMENT ,000 2,000 Feet Project Boundary SCENARIO 30,000 cfs Flow 50 Year Flood Plant Capacity Appendix A Map 11 Index Map 1 inch = 1,000 feet Copyright 2012 FirstLight Power Resources All rights reserved. Path: W:\gis\maps\Study_Plans\tf_hydraulic_assessment\tf_hydraulic_assessment_app_a.mxd