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LakeForestMappingANALYSIS OF SHOALING AND POOL VOLUMESCenter for Applied Coastal Engineering & ScienceUNIVERSITY OF SOUTH ALABAMA 150 JAGUAR DRIVE, MOBILE, AL 36688i

Lake Forest MappingAnalysis of Shoaling and Pool VolumesFinal ReportBret M. WebbUniversity of South AlabamaDepartment of Civil, Coastal, & Environmental Engineering150 Jaguar Drive, Shelby Hall 3142Mobile, Alabama 36688-0002 USAAugust 15, 2016Prepared for:Lake Forest Property Owners Association1 Golf Terrace DriveDaphne, Alabama 36526City of Daphne1705 Main StreetPO Box 400Daphne, Alabama 36526Mobile Bay National Estuary Program118 North Royal Street, Suite 601Mobile, Alabama 36602ii

ACES CenterLake Forest MappingExecutive SummaryThis report presents the data and results of topographic and bathymetricmapping of Lake Forest in the city of Daphne, Alabama. The work wasperformed by the University of South Alabama’s Center for Applied CoastalEngineering and Science under the direction of Dr. Bret Webb.Data collection was performed over the period May 24, 2016 – June 21, 2016and consisted of topographic and bathymetric elevation measurements madewith highly accurate Global Positioning System (GPS) and acoustic sonarequipment. Over 12,000 discrete locations were sampled for position andelevation within the study area boundary. That boundary was generally the lakeshoreline, truncated by the Bayview Drive bridge to the east on D’Olive Creek,just upstream of the Tiawasee Creek delta to the south, and the dam to thewest. Elevation data were screened for instrumentation errors. These data wereused to generate two-dimensional Digital Elevation Models (DEMs)of the 2016lake bed surface. A 1958 topographic survey was georeferenced and itselevations digitized using Geographic Information System software. Theseelevations were used to generate a DEM of the 1958 lake bed surface forcomparison to present day. This DEM served as the “pre-construction” conditionsthat existed before completion of the Lake Forest dam in 1973.Comparison of the 1958 and 2016 DEMs reveals that nearly 313,296 yd3 ofsediment has accumulated, or shoaled, over 78% of the study area, most likelysince construction of the dam. This accumulation of sediment has reduced thelake’s open water surface area from 61.6 acres in 1974 to 43.3 acres in 2016. Thecorresponding shoaling rates in volume and weight are 7,285 yd3/yr and 16,272tons/yr, respectively, over the 43-yr period between 1973 and 2016.The accumulation of sediment within the impoundment has reduced the floodattenuation capacity of the reservoir. Within the present study area boundary,the available pool volume has decreased from 356 acre-feet in 1958 to less than197 acre-ft in 2016 (relative to the normal pool elevation). In other words, thelake has lost nearly 45% of its flood volume storage. This value may be as high as61% considering the original open water extent of the Tiawasee Creek arm ofthe lake at the time of dam construction.Prior studies provide some limited confirmation of these findings. While it isdifficult to directly compare the various study results, there is an order ofmagnitude agreement in the shoaling volumes and rates. The existing literaturestates that shoaling rates were very high in the late 1970s and early 1980s, buthave since reduced for various reasons. Therefore, the values presented in thisstudy should be considered averages over the period in question.iii

ACES CenterLake Forest MappingTable of ContentsExecutive Summary . iiiTable of Contents. ivList of Tables . vList of Figures . viIntroduction . 9Goals & Objectives . 9Study Area . 10Background . 11Methods & Mapping . 12Land-Based Mapping . 12Water-Based Mapping . 13Georeferencing . 14Mapping Procedures . 16Error Analysis . 16Results . 17Discrete Elevation Data . 17Two-Dimensional Elevation Maps . 19Two-Dimensional Depth Map - 2016 . 21Survey Elevation Profiles. 21Discussion . 21Changes in Lake Bed Elevation . 22Sediment Volume and Weight . 25Estimated Pool Volumes . 26Works Cited . 27Appendices . 28Appendix A . 28Appendix B . 35iv

ACES CenterLake Forest MappingList of TablesTable B- 1. Summary of grain size parameters for Lake Forest sediment coresamples. . 37v

ACES CenterLake Forest MappingList of FiguresFigure 1. Map of the Lake Forest study area showing aerial imagery andelevation contours. The inset panel shows the location (arrow) relative toBaldwin County. . 11Figure 2. Aerial photography of the study area from 1938 - 2009. . 11Figure 3. Topographic RTK GNSS mapping in D'Olive Creek on May 24, 2016. . 13Figure 4. Modified ocean kayak. . 13Figure 5. The RTK base station location used for the bathymetric mapping. . 14Figure 6. Pre-construction 1958 topographic survey of the Lake Forest area. . 15Figure 7. Overview of the georeferenced 1958 survey shown with 2008 aerialimagery. . 15Figure 8. Map showing the distribution, and values, of elevations digitized fromthe 1958 survey. 16Figure 9. All topographic and bathymetric survey data collected in the LakeForest mapping study. . 18Figure 10. Detailed overview of survey data collected near the Tiawasee Creekdelta. . 18Figure 11. Detailed overview of survey data collected near the delta of D'OliveCreek. . 19Figure 12. Triangulation of the digitized 1958 topographic survey data. . 20Figure 13. Triangulation of the 2016 discrete elevation data within the study area. 20Figure 14. Two-dimensional map of 2016 lake depths (colors, numbers). 21Figure 15. Elevation differences between 1958 and 2016. Positive values (browns)represent shoaling or shallowing of the lake. The negative values (greens)indicate areas where the lake has deepened since 1958. . 23Figure 16. Areas of the lake indicating shoaling greater than (orange) or lessthan (black) 5 ft since 1973. . 23Figure 17. Areas of the lake indicating shoaling greater than (orange) or lessthan (black) 10 ft since 1973. . 24Figure 18. Estimation of shoaling areas by accretional depth over the period1958 to 2016. . 24Figure 19. Estimated sediment volume yield by height above the preconstruction land surface. . 25vi

ACES CenterLake Forest MappingFigure 20. Estimated sediment volume yield by elevation within the studyboundary. . 26Figure A- 1. Location and identification of elevation profiles within the studyarea. Profiles are labeled by easting coordinate. . 28Figure A- 2. Elevation profiles at transect E1844750. View is of an observer lookingto the west. . 29Figure A- 3. Elevation profiles at transect E1844500. View is of an observer lookingto the west. . 29Figure A- 4. Elevation profiles at transect E1844250. View is of an observer lookingto the west. . 29Figure A- 5. Elevation profiles at transect E1844000. View is of an observer lookingto the west. . 30Figure A- 6. Elevation profiles at transect E1843750. View is of an observer lookingto the west. . 30Figure A- 7. Elevation profiles at transect E1843500. View is of an observer lookingto the west. . 30Figure A- 8. Elevation profiles at transect E1843250. View is of an observer lookingto the west. . 31Figure A- 9. Elevation profiles at transect E1843000. View is of an observer lookingto the west. . 31Figure A- 10. Elevation profiles at transect E1842750. View is of an observerlooking to the west. . 31Figure A- 11. Elevation profiles at transect E1842500. View is of an observerlooking to the west. . 32Figure A- 12. Elevation profiles at transect E1842250. View is of an observerlooking to the west. . 32Figure A- 13. Elevation profiles at transect E1842000. View is of an observerlooking to the west. . 32Figure A- 14. Elevation profiles at transect E1841750. View is of an observerlooking to the west. . 33Figure A- 15. Elevation profiles at transect E1841500. View is of an observerlooking to the west. . 33Figure A- 16. Elevation profiles at transect E1841250. View is of an observerlooking to the west. . 33vii

ACES CenterLake Forest MappingFigure A- 17. Elevation profiles at transect E1841000. View is of an observerlooking to the west. . 34Figure A- 18. Elevation profiles at transect E1840750. View is of an observerlooking to the west. . 34Figure A- 19. Elevation profiles at transect E1840500. View is of an observerlooking to the west. . 34Figure B- 1. Sediment sample location (white circles) map. 35Figure B- 2. Wet sediment core samples obtained from Lake Forest. . 36Figure B- 3. Oven and air dried sediment core samples (numbering is LF00x). . 36Figure B- 4. Grain size distributions for all Lake Forest sediment core samples. . 38Figure B- 5. Grain size histograms for all Lake Forest sediment core samples. . 38Figure B- 6. Grain size distribution (left) and histogram (right) for Lake Forestsample LF001. . 39Figure B- 7. Grain size distribution (left) and histogram (right) for Lake Forestsample LF002. . 39Figure B- 8. Grain size distribution (left) and histogram (right) for Lake Forestsample LF003. . 39Figure B- 9. Grain size distribution (left) and histogram (right) for Lake Forestsample LF004. . 40Figure B- 10. Grain size distribution (left) and histogram (right) for Lake Forestsample LF005. . 40Figure B- 11. Grain size distribution (left) and histogram (right) for Lake Forestsample LF006. . 40Figure B- 12. Grain size distribution (left) and histogram (right) for Lake Forestsample LF007. . 41Figure B- 13. Grain size distribution (left) and histogram (right) for Lake Forestsample LF008. . 41viii

ACES CenterLake Forest MappingIntroductionThis report outlines the facts and findings of a limited field investigation in theLake Forest lake/reservoir (hereafter referred to simply as Lake Forest) located inDaphne, Alabama. This study was of limited scope, schedule, and budget witha very narrow focus: mapping lake bed elevations and documenting areas andvolumes of shoaling. The text that follows in this introduction will summarize thestudy goals and objectives, describe the study area, and present some limitedbackground information that provides context for this specific study. The othermajor report sections will describe the data collection and analysis procedures(Methods & Mapping), the study findings (Results), and provide a cursoryanalysis of the results (Discussion).This work was performed for the Lake Forest Property Owners Association andthe City of Daphne, under contract with the Mobile Bay National EstuaryProgram, by the University of South Alabama’s (USA) Center for Applied CoastalEngineering and Science (ACES) under the direction of Dr. Bret Webb, Ph.D.,P.E., D.CE, Associate Professor of Civil, Coastal, and Environmental Engineering,and Director of ACES. Additional field assistance was provided by USA studentsPatrick Hautau and Brittany McMillan.Goals & ObjectivesThe goal of this mapping exercise was to document existing lake bed elevationsfor the purpose of estimating: 1) the remaining normal pool volume available forflood routing; and 2) the amount of sediment within the lake. The more specificobjectives of the mapping study were:1.   To survey existing elevations, above and below water, within the lakestudy area;2.   To correct and document the survey data, as appropriate, for use byothers in future projects and/or decision-making;3.   To prepare and deliver topographic-bathymetric (topo-bathy) crosssections of the lake;4.   To prepare and deliver two-dimensional maps of the topo-bathy surveydata; and5.   To document survey-specific information and metadata in the form of asummary report.In pursuit of this mapping study, USA-ACES was provided with a digital image filecontaining a 1958 topographic survey of the Lake Forest area prepared by Irby& Rester Engineering & Surveying Co., Inc. (dated April 14, 1958). The 1958 surveydata were used to extend the mapping study beyond the original fiveobjectives. These additional tasks included:6.   To georeferenced and digitize the 1958 topographic survey to establishpre-construction (of the dam) lake bed elevations within the study area;9

ACES CenterLake Forest Mapping7.   To assess the change in lake bed elevations between 1958 and 2016;8.   To identify the locations, areas, and volumes of shoaling between 1958and 2016;9.   To determine sediment volume yields as a function of height above the1958 pre-construction elevations; and10.  To evaluate the change in pool volume between 1958 and 2016.These additional tasks were not included in the original scope and fee proposalas it was unclear, at the time, whether use of the 1958 survey data would evenbe possible owing to the lack of a coordinate reference system.In the performance of this work and discussions with the City of Daphne itbecame apparent that some information about the basic characteristics ofsediment within the shoaling areas would aid in future work. Some limitedsampling of surficial sediments will be performed within the study area usingshallow, benthic push cores (2 ft in length, 3 in diameter sample). These data willbe provided as Appendix B when available.Study AreaThe Lake Forest lake is located within the Lake Forest Subdivision in the city ofDaphne, Alabama. A map of the study area is provided in Figure 1. The generallocation of the lake is in the northwest portion of the subdivision development,which is immediately east and south of US Highway 98 and US Highway 90,respectively. The lake is a man-made feature that resulted from the constructionof the existing gravity dam in 1973 (MBNEP, 2010).The study area for this project was mostly limited to the existing open watersections of Lake Forest due to the nature of our mapping methods (as describedin a later section). This open water area is generally restricted to areas below the20-ft elevation contour on the map shown in Figure 1. The current operating poollevel is at approximately 19 ft NAVD881. In 1974 the approximate surface areaof open water in Lake Forest was 61.6 acres. The extent of open water surfacearea in the lake during this 2016 survey was measured at 43.3 acres, orapproximately one-third less than the 1974 value immediately after constructionof the dam. A series of aerial photographs in Figure 2 shows the progression ofchanges in the area from 1938 to 2009.Some effort was made to map elevations above the pool level, specifically atthe mouths of D’Olive Creek and Tiawasee Creek. However, extensive datacollection in these areas was not possible due to the excessive overhead treecover and difficulty accessing upstream sections of those streams. In addition tothe two major streams listed above, four lesser unnamed tributaries deliver waterand sediment to the lake from within the D’Olive watershed (Cook and Moss,2008; Cook, 2010).1North American Vertical Datum of 198810

ACES CenterLake Forest MappingFigure 1. Map of the Lake Forest study area showing aerial imagery and elevation contours. The inset panelshows the location (arrow) relative to Baldwin County.BackgroundThis study is not the first attempt at mapping lake elevations and evaluating theextent of sediment shoaling. Prior studies have been performed by Isphording(1981), Douglass (1994), Cook and Moss(2008), Cook (2010), and MBNEP (2010).These documents have been brieflyreviewed in the performance of thisstudy. The elevation/depth mappingperformed by Isphording (1981) andDouglass (1994) were of a limited nature.The information contained in Cook andMoss (2008) and MBNEP (2010) providespecific information and estimatesrelated to potential sediment transportloads and rates, water quality andchemistry, and biological activity in theFigure 2. Aerial photography of the study areastreams and tributaries within the D’Olivefrom 1938 - 2009.watershed. The brief report by Cook11

ACES CenterLake Forest Mapping(2010) presents an analysis of trapping efficiency and capacity that was helpfulin the preparation of this report.The author of this report is unaware of any comprehensive survey of lake bedelevations since construction of the dam in 1973 and the pre-construction surveydated 1958. While some discussion of our findings relative to those contained inthe reports previously cited will be provided, the intent of this study was not todraw specific comparisons to previous works, figures, and estimates. In fact,doing so would be impractical given the varied nature of these studies.Methods & MappingThe mapping for this project was performed over the period May 24, 2016 toJune 21, 2016. A combination of land-based and water-based mapping wasused within the study area. Each of these is described more fully in the sectionsthat follow. A brief overview of the materials and methods used to generate thefinal maps is also provided below.Land-Based MappingTopographic and very shallow water (i.e., waist-deep water and shallower)elevations were collected by wading survey (see Figure 3). Elevations weresampled at discrete locations ( 1500 points) using two Magellan (Ashtech)ProMark 500 Global Navigation Satellite System (GNSS) receivers andMobileMapper CX field terminals. Each of the GNSS receivers were equippedwith cellular connections to the Alabama Department of Transportation (ALDOT)Continuously Operating Reference Station (CORS) network, thereby providingReal Time Kinematic (RTK) corrections to the position data. Typical horizontaland vertical root mean square errors were commonly 0.1 ft or less (i.e., HRMS 0.1 ft; VRMS 0.1 ft). Points were sampled and recorded in the Alabama StatePlane Coordinate System (in feet) using a vertical datum of NAVD88 (ref. Geoid12a).The topographic mapping was conducted over a two-day period in late May2016. Over 1000 points were surveyed along D’Olive Creek between theBayview Drive bridge and the terminus of the creek delta in Lake Forest on May24, 2016. An additional 500 points were surveyed at the terminus of theTiawasee Creek delta on May 25, 2016.12

ACES CenterLake Forest MappingFigure 3. Topographic RTK GNSS mapping in D'Olive Creek on May 24, 2016.Water-Based MappingThe original methodology proposed for bathymetric mapping of the lake, usingthe South Alabama Jag Ski system, was attempted on May 25, 2016 butabandoned due to operational safety concerns. The Jag Ski was launched andused on May 25 but sustained substantial damage during the process. Thedecision was made to retrieve the watercraft and develop an alternative plan.Lack of adequately safe access to the lake, inaddition to the abundance of freshwater vegetation,makes use of a personal watercraft impractical.Based on the shallow nature of the lake system,presence of vegetation, and lack of access options,the decision was made to proceed with a kayaksurvey. All of the required instrumentation wastransferred from the Jag Ski to a 12-foot-long oceankayak, which was modified to perform this study (seeFigure 4).The bathymetric mapping was conducted on June21, 2016 using the modified ocean kayak system.Depth sampling was performed with a SonTek M9RiverSurveyor Acoustic Doppler Profiler (ADP) and RTKFigure 4. Modified oceankayak.GNSS position referencing system. An RTK base stationwas established on top of the dam as shown in Figure 5. Samples were obtainedand recorded to instrument memory at a rate of 1 sample per second. Theresulting sample spacing was typically less than 4 ft. Over 10,400 samples wereobtained over a four-hour period.The depth samples were recorded in a geographic coordinate referencesystem (i.e., WGS84) and measured relative, in the vertical dimension, to the lakesurface. The lake surface elevation, relative to NAVD88, was obtained before13

ACES CenterLake Forest Mappingand after the data collection: the water surface elevation remained constant at 18.9 ft NAVD88. Depth samples were later corrected for changes in the speedof sound based on water temperature and conductivity throughout the studyarea. These corrections were applied in the SonTek RiverSurveyor Live softwareusing conductivity-temperature measurements made with a YSI CastAwayprofiler (orange device shown in Figure 4). Finally, all depth sample coordinateswere transformed to the Alabama State Plane system for consistency with thetopographic elevation data.Figure 5. The RTK base station location used for the bathymetric mapping.GeoreferencingThe original 1958 topographic survey (Figure 6) was georeferenced and itselevations digitized using Geographic Information System (GIS) software. The1958 survey contained no horizontal coordinate reference system nor any statedvertical datum to which elevations were referenced. There were not enoughcommon, readily identifiable features on the 1958 topographic survey by whichto directly georeference the data. However, I was able to scale and fit the 1958survey image to aerial imagery of Lake Forest and, combined with theunderstanding that the approximate 20 ft elevation contour represents thecurrent shoreline position, identify 10 common points within Google Earth for thepurpose of referencing (see Figure 7). Once completed, over 800 elevationswere manually digitized from the 1958 survey (see Figure 8). As the survey wasconducted in 1958, it was assumed that the elevations were measured andrecorded relative to NGVD292.2National Geodetic Vertical Datum of 192914

ACES CenterLake Forest MappingFigure 6. Pre-construction 1958 topographic survey of the Lake Forest area.Figure 7. Overview of the georeferenced 1958 survey shown with 2008 aerial imagery.15

ACES CenterLake Forest MappingFigure 8. Map showing the distribution, and values, of elevations digitized from the 1958 survey.Mapping ProceduresAfter reviewing all survey data for errors, a common horizontal reference system(Alabama State Plane, West FIPS 0102) and vertical datum (NAVD88) wereselected and the data transformed/corrected as needed. These data weresubsequently triangulated within the study area boundary to create twodimensional digital elevation raster images. Coordinate transformations andtriangulation were performed within GIS software.Error AnalysisErrors are ubiquitous in this type of work. It is possible, however, to identifycommon sources of error and minimize them wherever practical.Instrumentation errors are the most obvious sources for the topographic andbathymetric mapping performed in this study. These can be, and have been,eliminated by reviewing data samples and eliminating points with unacceptablylarge errors. For example, large HRMS and/or VRMS values (i.e., 0.25 ft) in RTKGNSS data, large pitch/roll values (i.e., 5o) in the ADP data, and/or largebeam standard deviations in the ADP depth samples were identified andremoved.Other relevant sources of errors addressed in this analysis are those associatedwith the unknown vertical datum of the 1958 topographic survey. A logical16

ACES CenterLake Forest Mappingassumption is that survey elevations were reported in the NGVD29 verticaldatum. The local difference between NGVD29 and NAVD88 is only 0.15 ft.Cumulatively, that difference would constitute an error of less than 6% in thesubsequent calculations of volumes in this report. The 1958 elevations weretransformed to NAVD88 in order to eliminate that possible source of error.

Lake Forest lake/reservoir (hereafter referred to simply as Lake Forest) located in Daphne, Alabama. This study was of limited scope, schedule, and budget with a very narrow focus: mapping lake bed elevations and documenting areas and volumes of shoaling. The text that follows in this introduction will summarize the