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HomeMy WebLinkAbout20070812 Ver 1_US Geological Suvery Draft Review_20070624~, DRAFT Review of Selected Documents Related to Flooding at City of Salisbury Facilities on the Yadkin River upstream from High Rock Dam, North Carolina Jerad Bales U.S. Geological Survey June 24, 2007 prepared in cooperation with North Carolina Division of Water Resources • , DRAFT Introduction The City of Salisbury has operated awater-supply intake at the confluence of the Yadkin and South Yadkin Rivers since 1917 (fig. 1). The City maintains that the operation of High Rock Dam, completed in 1927 and currently operated by Alcoa Power Generating Inc. (APGI), has created a sediment delta in the headwaters of High Rock Reservoir, and that this delta has caused (1) increased flooding of the water-supply pump station, (2) sediment deposition around the pump intake, and (3) increased flooding at a wastewater treatment plant pump station located near the mouth of Grants Creek (fig. 1). Figure 1. Yadkin River, South Yadkin River, Grants Creek, and upstream end of High Rock Lake, North Carolina APGI is currently (2007) seeking to relicense four hydroelectric stations, including High Rock Dam, on the Yadkin -Pee Dee River. The license for the hydroelectric stations and the reservoirs that support them is granted by the Federal Energy Regulatory Commission (FERC). The North Carolina Department of Environment and Natural Resources (NCDENR) is participating in the relicensing process. NCDENR has signed a Relicensing Settlement Agreement with APGI, and must subsequently respond to an application by APGI for a section 401 water quality certification for the stations. The City of Salisbury has requested that APGI address the issue of flooding and sedimentation at the City's facilities on the Yadkin during the FERC relicensing process 2 0 1 2 4 Miles 9 +-~-+ I ~ ~ ~~ DRAFT settlement agreement. APGI maintains that their operation is too far downstream to have effects on the City's pump stations, that the Yadkin has a naturally high sediment load, and that the pump station is in the floodplain. Because of APGI's position, Salisbury retained consultants to develop hydraulic and sediment transport models for the reach in question and to study sediment transport as it affects their intake. The final report was submitted to FERC on February 26, 2007, as part of Salisbury's relicensing scoping comments. APGI also has funded studies of their own, including a review of the City's documents. Because of the complexity of the issues, NCDENR asked the U.S. Geological Survey (USGS) to review both APGI's and the City of Salisbury's documents related to flooding at the City's facilities. The independent review is needed in order to assist NCDENR in their review of APGI's application fora 401water-quality certification and subsequent renewal of the FERC license for High Rock Dam. Purpose and Scope The purpose of this report is to document a review of the hydraulic and sediment transport models developed by the City of Salisbury, APGI, and FERC, as well as related data and information. The objective of the review was to determine if the documents submitted by Salisbury clearly demonstrate that the presence of High Rock Dam has led to an increase in water levels at Salisbury facilities, or conversely if APGI documents demonstrate that High Rock Dam has not had an effect on water levels at Salisbury facilities. Documents which were included in the review are listed in the following section. The review included a site visit to the Yadkin and South Yadkin Rivers and downstream to High Rock Lake. No new data were collected as a part of the review, and the models developed by involved parties were not tested during the review. Some historical records were checked as part of the review. This report is structured as follows. First, the documents which were reviewed are listed. Next, hydraulic modeling and related documents are reviewed. Sediment transport modeling conducted by the City's consultant is then reviewed, along with comments provided by APGI's consultant. A brief review of other documents provided by the City and APGI is followed by a summary and recommendations. Materials Reviewed The documents reviewed during this study were as follows: Salisbury Documents: • Technical Report: High Rock Dam and High Rock Lake Sedimentation Flooding Effects as Estimated using HEC-RAS Modeling, January 2006; prepared by Hazen and Sawyer for Salisbury-Rowan Utilities. (Hereafter referred to as SAL-1) Letter from V. Randall Tinsley to Secretary Magalie Salas, Federal Regulatory Commission, February 23, 2006 (SAL-2) DRAFT • Numerical Sedimentation Investigation, Yadkin River, North Carolina, February 20, 2007; prepared by R.R. Copeland, Mobile Boundary Hydraulics for City of Salisbury (SAL-3). • High Rock Dam and Sediment Delta Flooding and Sedimentation Effects (1927 - 2058) on City of Salisbury Critical Infrastructure, February 2007; prepared by Martin Doyle for City of Salisbury (SAL-4) Yadkin / Alcoa Power Generating, Inc. Documents: • Review of January 1998 Flood of Yadkin river, February 1998; prepared by Stone & Webster for Yadkin, Inc. (hereafter referred to as APGI-1). • Yadkin Hydroelectric Project-FERC No. P-2197-073, Alcoa Power Generating Inc., Responses to Federal Energy Regulatory Commission's September 14, 2006 and November 22, 2006 Additional Information Requests; Apri16, 1998; prepared by Julian Polk, Yadkin, Inc. (APGI-2). • Letter from Mr. Gene Ellis, Licensing and Property Manager, APGI, to Secretary Magalie Salas, FERC, December 13, 2006; response to the Additional Information Request for Yadkin Project (APGI-3). • Affidavit of David Williams, Ph.D., P.E., March 26, 2007 (APGI-4). • Affidavit of Paul F. Shiers, P.E., March 27, 2007 (APGI-5). • Consolidated answer of Alcoa Power Generating Inc. to petitions to intervene and comments in response to Scoping Document 1; prepared by LeBoeuf, Lamb, Greene, and MacRae LLP, Counsel to Alcoa Power Generating Inc. undated (APGI-6). • Sediment Fate and Transport Report, Final Report, November 2005; prepared by Normandeau and Associates, Inc. and PB Power (APGI-7). FERC Documents: • Letter from Jerrold Gotzmer, Director, FERC to Mr. Julian Polk, Yadkin, Inc., dated March 11, 1998 (hereafter referred to as FERC-1). • Letters from Jerrold Gotzmer, Director, FERC to Mr. Ron Qualkenbush, dated May 6, 1998 and May 21, 1998 (FERC-2). • Hydraulic Modeling Report for Yadkin Project-North Carolina, Alcoa Power Generating, Inc. (APGI), June 2003; prepared by Federal Energy Regulatory Commission (FERC)., and transmittal letter from FERC to Mr. David Treme, Mr. Ron Qualkenbush, and Mr. Milton Crowther (FERC-3). 4 DRAFT One-Dimensional Hydraulic Modeling One-dimensional hydraulic models were developed by Stone & Webster for APGI (APGI-1), FERC (FERC-2), and Hazen and Sawyer for Salisbury -Rowan Utilities (SAL-1). Important aspects of these models are summarized in table 1. Stone & Webster: In January 1998, a campground approximately 20 miles (mi) upstream from High Rock Dam was flooded during high flows. The Stone & Webster model was created to evaluate "the effect that the operation of High Rock Dam may have had on this flooding" (APGI-1). The model extended from High Rock Dam to the confluence of the Yadkin and South Yadkin Rivers, 19.4 miles (mi) upstream from High Rock Dam (or RM 19.4). Cross sections were scaled from topographic maps and were assumed to trapezoidal in shape. The report does not specify how the bottom and/or top width of the cross sections were determined, nor how the side slopes were determined. Apparently more detailed cross-sectional data were available (APGI-7) from 1997 surveys, but these data were not used. The channel bed slope was based on the assumption of a liner slope between the dam and RM 18.1; the slope was assumed to be zero between RM 18.1 and RM 19.4. It does not appear that the model was calibrated. The report concluded that for existing conditions, the water level at RM 19.4 was independent of the lake level at High Rock Dam. The report also concluded that water levels at RM 19.4 were affected by a channel constriction at the railroad bridge near the NC 150 bridge and by a narrow bend in the river at about RM 18.4. The conclusion regarding the relation of water levels at High Rock Dam and water levels at RM 19.4 for current conditions is reasonably supported by the modeling for current conditions. The conclusions could be strengthened by conducting a sensitivity analysis of the effects of the assumed channel geometry and bed slope on simulated water levels at RM 19.4. There is no evidence given in the report for the conclusion that the channel constriction and narrow bend in the river controlled water levels at RM 19.4 during high flows. A sensitivity analysis of the effects of the expansion and contraction coefficients and simulations without the bridges in place are needed to give credibility to the conclusion regarding the effects of bridge constrictions. It is not clear how results from one-dimensional model were used to make conclusions regarding the effects of the bend. Doyle (SAL-4) gives a brief discussion of HEC-RAS modeling that was conducted to evaluate the effects of the bridges on water levels at the Salisbury facilities. This analysis indicates that the upstream effects of the bridges are minimal, even under high flows. There is essentially no detail given in that report regarding any details of the model, so it is impossible to assess these conclusions. The results of the Stone & Webster modeling are not directly relevant to the issue of the effects of the delta on flooding at RM 19.4. This is because no simulations for pre-dam or pre-delta conditions were performed for comparison with 1998 conditions. 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FERC: FERC obtained the HEC-RAS model used by Stone & Webster (APGI-1) to conduct additional one-dimensional hydraulic modeling investigations (FERC-3). The purpose of modeling was to "Determine the influence of the High Rock Reservoir elevation on the water surface elevation at the confluence of the Yadkin and South Yadkin Rivers," and in particular during a 2003 flood. The model was extended upstream to about RM 21.3, and additional cross sections were added to the model to provide greater resolution at locations where FERC concluded that Stone & Webster cross sections were too widely spaced. The model was used to simulate water-surface profiles for the March 2003 flood for a range of water levels at High Rock Dam. FERC used a digital elevation model to develop a topographic map that perhaps provided more detailed cross-sectional information than was in the Stone & Webster model, but the detailed data from the 1997 survey were not used in the model. The approach used by FERC was much the same as that used in the Stone & Webster study, other than the source of the topography data and the flood to which the model was applied. As a result, the conclusions from this study were essentially the same as for the Stone & Webster study-water level at RM 19.4 is essentially independent of water level at High Rock Dam for current conditions and flows between 40,000 cfs and 70,000 cfs. As with the Stone & Webster study, no simulations were done for pre-dam or for 1917 bathymetric conditions for comparison to current conditions. Hazen and Sawyer: Hazen and Sawyer (SAL-1) constructed a HEC-RAS model for High Rock Lake and a portion of the Yadkin River to "evaluate the effects of High Rock Dam and dam-induced sediment accumulation on the frequency and magnitude of flooding ... at the Yadkin -South Yadkin confluence." This statement presumes the conclusion that all of the sediment accumulation in the reservoir is "dam-induced." The modeling effort was unique in two ways: (1) 1997 topography was used to construct cross sections for current conditions model input and (2) pre-impoundment conditions were simulated. The 1997 topographic survey was conducted by APGI contractors when the lake level was at 12 ft below full pool. The survey extended out from the full pool elevation about 0.25 mi. A topographic map with 2 ft contour intervals was prepared (APGI-7). Hazen and Sawyer also simulated pre-impoundment conditions by using 1917 topographic data, obtained from maps provided by APGI, and estimating channel cross-sectional shape below the 1917 water surface. Simulations were made assuming the dam was in place and using 1917 topography. Results indicated that the presence of the dam alone, with no sediment accumulation (i.e., 1917 bathymetry), may have led to an increase in water levels at RM 19.4 for flows less than about 80,000 cfs. At a flow of about 20,000 cfs, the increase may be about 6 ft relative to pre-impoundment conditions. The increases relative to pre-impoundment conditions were greater when simulations were made using 1997 topography/bathymetry, with an increase of about 10 ft at a flow of about 20,000 cfs. Water levels for estimated 9 DRAFT 2038 conditions were linearly extrapolated by assuming that the average annual increase in water-surface elevation for each flow rate during 1927 - 1997 continued through 2038. Various aspects of the Hazen and Sawyer model are poorly documented, including the Manning's n, expansion and contraction coefficients, and treatment of bridges for historic conditions (i.e., did the 1917 simulations include the 2005 bridges, 1917 bridges, or no bridges). In addition, like the Stone & Webster and the FERC models, the Hazen and Sawyer model does not appear to have been calibrated or compared to observed conditions. Nevertheless, this model clearly demonstrates that the presence of the dam and the change in the bathymetry likely have had an effect on water levels at the Salisbury intakes relative to the pre-dam conditions. The exact magnitude of that change is less certain primarily because of uncertainties in current and historic bathymetry, because model performance has not been tested through comparison to measured conditions, and because all aspects of the model were not documented. Doyle (SAL-4) summarized the work of Hazen and Sawyer (SAL-1) in his report. The hydraulic modeling aspects of Doyle's report were subsequently reviewed by APGI's consultant, Parsons Brinkerhoff (PB) (APGI-5). The PB review identified four major areas of concern, which are listed below, along with some observations regarding the concerns. It should be noted, however, that none of these criticisms by PB relate directly to the issue of the effects of High Rock Lake on possible increased water levels at Salisbury's facilities, but rather to some peripheral issues related to the details of the analysis by Salisbury's consultants. The resolution of the topographic data used in Hazen and Sawyer's modeling, in particular the pre-dam conditions, does not justify Doyle's conclusions. In fact, Doyle does report water-surface elevations to the nearest 0.1 ft for future conditions. These numbers should be used with caution. Copeland (SAL-3) does address the issue of uncertainty in the sediment transport modeling, although uncertainty in topography and bathymetry are not addressed. Doyle calculated the "design flow " of 121, 000 cubic feet per second (cfs) at RM 15.2 rather than at the water-supply intake at RM 19.4. Doyle apparently did use records collected at RM 15.2 to represent conditions at RM 19.4. The increase in drainage area between the two sites is about 90 mil, or less than 3 percent of the total drainage area. The at-station 100-year recurrence interval flow at RM 15.2 is 166,000 cfs based on records collected 1896 - 1927 (Pope and others, 2001), which is less than Doyle's "design flow." These issues, however, do not directly relate to the question of the effect of High Rock Lake on water levels at Salisbury's intake, but rather to the magnitude of the effect at selected conditions (100-year flow; "design flow," etc.). As an aside, the methods used by Doyle (SAL-4) and Copeland (SAL-3) might underestimate the 100-year flood at the confluence of the Yadkin and South Yadkin Rivers. Pope and others (2001) provide methods for estimating flood flows at ungaged sites. Greater use of at-station flood-frequency information for Yadkin River at Salisbury and South Yadkin River at Cooleemee might also be helpful. ^ PB believes that Doyle mischaracterizes APGI's statements regarding causes for sediment accumulation in the reservoir. This criticism appears to be valid. APGI has 10 DRAFT argued that sediment accumulation is caused, in part, by natural bends in the river and that channel constrictions from bridges are contributing to flooding at Salisbury's facilities. As noted above, however, the evidence presented by APGI to substantiate these claims is weak, at least in the documents available for review. PB also indicates that Doyle does not present any evidence that the river was in geomorphic equilibrium prior to 1927. Indeed, Doyle does not present any quantitative evidence for this assertion, although Copeland (SAL-3) does appear to have examined the stage-discharge relation for Yadkin River at Yadkin College and found the rating to be stable. ^ There is an issue related to Doyle 's characterization of flooding at the Grants Creek wastewater pump station, and design of the pump station to account for possible flooding. This issue is unrelated to the effects of High Rock Lake on increased flooding at Salisbury's facilities. One-Dimensional Sediment Transport Modeling A one-dimensional unsteady sediment transport model was constructed "to evaluate the potential impact of continuing delta aggradation in the Yadkin River at the upstream end of High Rock Lake" (SAL-3). The study seems to presume that all of the sediment accumulation is directly attributable to the presence of High Rock Dam. The model was applied to evaluate four alternatives for reducing sedimentation at City of Salisbury facilities near RM 19.4. The HEC-6T model, which is a proprietary version of the U.S. Army Corps of Engineers HEC-6 model, was used for the simulations. Cross sections for the sediment transport model were obtained from the Hazen and Sawyer HEC-RAS model, from LIDAR data, and from bathymetric survey data. Additional cross sections were surveyed between RM 21.3 and 31.3, and these cross sections were assumed to be stable over the simulation period. The pre-impoundment river bottom elevations used in the HEC-6T model were, however, adjusted upward from those used in the Hazen and Sawyer HEC-RAS model. Fairly extensive sediment inflow records were available for the model boundary conditions. Sediment inflow rates for future conditions were assumed to be the same as those for historical conditions, but the sensitivity of model results to this assumption were evaluated. Assumptions regarding inflow hydrographs were required, and these assumptions seem reasonable, although a number of other approaches could have been used. The model was calibrated such that measured and simulated bed profiles for 1917 and 1997 were in general agreement. The model also was calibrated to water levels measured at RM 19.4 for selected events. The sensitivity of model results to changes in sediment inflows, the sediment transport equation used in the model, and to variability in bottom elevation was evaluated. The model does not simulate natural adjustments in channel width; it has been noted elsewhere (APGI-7, SAL-3) that channel widths in the region of concern have changed over the last 70 years. 11 DRAFT The model was applied to simulate the growth of the delta in the headwaters of High Rock Lake during 1928 - 97. Changes in water-surface elevations at Salisbury facilities for 3 flow conditions as a function of changes in bed elevation are presented for the period 1920 - 2057. Copeland's report provides a good documentation of the data and assumptions used to construct the model and provides a reasonable assessment of uncertainty in model results. The data, analyses, and simulations presented in the report add to current understanding of sediment transport and accumulation in the High Rock Lake and the Yadkin River. The report does not, however, address the issue of the relative effects of High Rock Dam on sediment accumulation, because the model was not applied to simulate conditions without the dam in place. The study also does not address the relative effects of the bridges on sediment accumulation in the upper reach of High Rock Lake. As a result, there is much improved understanding of sediment transport and accumulation, but there is no new information on the effects of High Rock Dam or bridges on sediment accumulation relative to what might have occurred naturally. Copeland's report was reviewed by Dr. David Williams (APGI-4). The review identified three major areas of concern regarding Copeland's sediment transport modeling. The base condition (or pre-impoundment condition) was not modeled, so the effect of High Rock Dam on sediment accumulation is not known. This is a valid criticism of Copeland's work, just as it is for the Stone & Webster (APGI-1) and FERC (FERC-3) hydraulic modeling. The actual increase in sediment deposition in the reach of the Yadkin River between High Rock Dam and RM 19.4 attributable to the presence of the dam cannot be determined from Copeland's results. The sediment transport model appears to have numerical instabilities when results are displayed for each computational time step. The computational time step used by Copeland was one day, except during high-flow events (SAL-3). Figures 1, 3 and 4 of Williams' report (APGI-4) shows oscillations of about 6 ft in the bed elevation at RM 19.4. Similar oscillations are not evident upstream of RM 19.4. The largest oscillations (Williams' figures 3 and 4) begin at about day 22,000 from the beginning of the simulations, which would seem to correspond to sometime in the mid-1980's (although this is an estimate based on material presented in the reports). Oscillations appear to have an approximately annual frequency. Copeland notes that his model simulated sand extraction during 1965 - 1984 and 1988 -end of simulation at locations near RM 19.4; sand was extracted by the model at the end of each water year. It seems likely that the oscillations in bed elevation presented by Williams are the result of the annual sand extraction in the model, particularly because the oscillations seem to be present at RM 19.4, where extraction occurred, but not at other nearby cross sections. Additional documentation is needed to verify this. ^ Williams also points out that model results seem unusual during the first two years of the simulation (Williams' figure 2), in that there are large changes in bed elevation during this period. Some numerical models require a `warm-up' period because 12 DRAFT initial conditions cannot be known throughout the model domain. Therefore, this `warm-up' period is used to transport the estimated initial conditions out of the model domain before boundary conditions begin to affect model simulations throughout the model domain. Simulation results are normally not considered useful during this `warm-up' period. Copeland does not address the issue of initial conditions, a `warm- up' period, or the bed elevation changes during the first two years of simulation. Williams' criticism of a flaw in the modeling approach could be valid, but certainly Copeland should have addressed the issue in his documentation. It does not seem reasonable, however, to invalidate the entire modeling activity because of "unusual" bed elevation changes during the first two years of a 70-year simulation, particularly when these patterns do not seem to be repeated later in the simulations. Other Studies Normendeau Associates, Inc. and PB Power prepared a report on sediment fate and transport in the Yadkin -Pee Dee River basin (APGI-7). The study provides a very comprehensive review of literature on sediment transport in the Yadkin River basin. Although the material in the report is quite interesting and well-documented, most of the information is not directly relevant to the question of the effects of High Rock Dam on water levels at RM 19.4. The study does present a comparison of the 1917 bathymetry with 1997 topography in the upper 12 ft of the reservoir, although a full bathymetric survey of current conditions has not been completed. Sedimentation patterns also were analyzed by comparing the 1917 and 1997 topographic maps. The report notes "the deepest portion of the river has narrowed" from the I-85 bridge upstream to the Yadkin - South Yadkin confluence. Doyle (SAL-4) presents an overview of reservoir sedimentation processes. A time series of aerial photographs of High Rock Lake also are presented, and Doyle discusses sedimentation patterns and the growth of the delta in the upstream end of the reservoir. The river bed profile also is reconstructed for pre-impoundment and 2000 conditions. This discussion is a very useful documentation of sedimentation in High Rock Lake. The discussion by itself, however, does not unequivocally demonstrate that High Rock Dam is responsible for all or part of the sedimentation. Doyle's arguments, the aerial photographs, and a general understanding of river morphology would lead one to conclude that sedimentation in the portion of the Yadkin River that is now the upper end of High Rock Lake has increased as a result of High Rock Dam. The amount of increase attributable to the presence of High Rock Dam and to the bridge abutments in the reach has not been quantified through either Doyle's presentation or Copeland's modeling. Doyle argues that the Yadkin River was in morphological equilibrium prior to 1917 and continues to be in equilibrium, meaning that the overall sediment deposition and erosion in the river is in general balance with the sediment supply. Doyle offers no quantitative evidence of this assertion. Cross sections measured at USGS gaging stations (Yadkin River at Salisbury at about RM 15.2, South Yadkin River at Cooleemee, and Yadkin River at Yadkin College) could be examined to document changes in the river geometry 13 DRAFT at measurement sections for both pre-dam and post-impoundment periods, although the channel width at these cross sections may be constrained by bridge abutments. Summary .and Recommendations The primary conclusions from this review are as follows: ^ The hydraulic models of Stone & Webster and FERC did not assess the effects of changes in bathymetry on changes in flood levels at the confluence of the Yadkin and South Yadkin Rivers. In other words, pre-impoundment conditions were not simulated, so the effect of High Rock Dam and post-impoundment sedimentation on water levels at RM 19.4 can not be evaluated from these studies. ^ The hydraulic modeling performed by Hazen and Sawyer seems to indicate that both the presence of the dam in the absence of any post-impoundment sedimentation and changes in bathymetry between pre-dam and 1997 conditions have resulted in increased water levels relative to pre-impoundment conditions at Salisbury facilities on the Yadkin River for a fairly wide range of flows. ^ The degree to which the dam and the changes in bathymetry have affected flood levels at the Salisbury facilities relative to pre-impoundment conditions is open to dispute because of uncertainty in topographic/bathymetric data and the absence of calibration and sensitivity testing of the hydraulic models. ^ None of the 3 hydraulic models appear to have been calibrated to or tested against measurements, and no sensitivity testing was reported. ^ Copeland's sediment transport model was calibrated to estimated bed elevations and to water levels at RM 19.4. The model is well documented (except for issues related to initial conditions and possible numerical instabilities) and provides a good understanding of the expected growth of the sediment delta in the upper end of High Rock Lake. ^ The actual increase in sediment deposition in the reach of the Yadkin River between High Rock Dam and RM 19.4 attributable to the presence of the dam or to bridges cannot be determined from Copeland's results because the base (no dam) condition was not simulated. Some recommendations for further analyses and studies to improve understanding of the relation of High Rock Lake to sedimentation in the lake and changes in the flood regime in the upper part of High Rock Lake include the following: Several elevation datums are used in the various documents, including the Yadkin datum, "USGS datum," NGVD 29 datum, and some datums which are unspecified. This leads to a good bit of confusion in interpreting results from reports. All parties should agree that elevations will be specified in the standard NAVD 88 datum. A detailed bathymetric survey of High Rock Lake and the Yadkin River from High Rock Dam to 20 mi upstream from the dam would provide the correct information needed for hydraulic (and sediment transport) modeling of the reach for current 14 ~' DRAFT conditions. Detailed topographic data of the region is available from the North Carolina Floodplain Mapping Program. The current bathymetry and topography could be combined to generate realistic channel cross-sections for hydraulic modeling of current conditions. Pre-dam conditions were already simulated by Hazen and Sawyer, although some clarification and documentation of their approach is needed. Copeland's model could be used to simulate sediment accumulation in the absence of High Rock Dam, with High Rock Dam in place but with no bridges, and in the absence of High Rock Dam but with bridges in place. Sediment accumulation attributable to the dam and to bridges could then be distinguished from what might have happened for a natural condition. Questions regarding initial conditions and possible numerical instabilities should be addressed prior to further simulations. ^ In the absence of realistic bathymetry for current conditions, a sensitivity analysis could be performed to determine the effects of changes in channel cross sections and slope on final results. A sensitivity analysis for pre-dam conditions, for which it is not possible to know channel cross sectional geometry, could be conducted. Likewise effects of bridges could be evaluated for current and pre-impoundment conditions. ^ Cross sections measured at USGS gaging stations (Yadkin River at Salisbury at about RM 15.2, South Yadkin River at Cooleemee, and Yadkin River at Yadkin College) could be examined to document changes in the river channel for both pre-dam and post-impoundment periods. ^ Sediment coring and age-dating methods could be investigated as a means to evaluate sediment throughout the dam for both the post-dam and pre-dam periods. It might be possible to estimate accumulation rates for both pre-dam and post-impoundment conditions. ^ Flood data from existing and discontinued gages could be examined to determine if there has been a change in the flood flow regime. Improved estimates of flood recurrence intervals could be obtained by using the methods of Pope and others (2001) and historical at-gage flood frequencies. References Pope, B.F., Tasker, G.D, and Robbins, J.C., 2001, Estimating the magnitude and frequency of floods in rural basins of North Carolina-Revised: U.S. Geological Survey Water-Resources Investigations Report O 1-4027, 44 p. 15