Geological Circular 7 5-2

Shoreline Changes on Brazos Island and South Padre Island (Mansfield Channel to Mouth of the Rio Grande)— An Analysis of Historical Changes of the Texas Gulf Shoreline

OV: Robert A. Morton AND Mary J. Pieper

BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS AT AUSTIN AUSTIN,TEXAS 78712 W. L. FISHER, DIRECTOR 1975 Geological - Circular 7 5 2

Shoreline Changes on Brazos Island and South Padre Island (Mansfield Channel to Mouth of the Rio Grande)— An Analysis of Historical Changes of The Texas Gulf Shoreline

BY Robert A. Morton AND Mary J. Pieper

BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS AT AUSTIN AUSTIN,TEXAS 78712 W. L. FISHER, DIRECTOR

1975 Contents

Abstract 1 Brazos Island 17 Introduction 1 1854-1937 17 Purpose and scope 2 1937-1960 17 General statement onshoreline changes 2 1960-1970 17 Acknowledgments 2 1970-1974 17 Historical Shoreline Monitoring— General Methods South Padre Island 17 and Procedures Used by the Bureau of Economic Geology 4 1867-1880 to 1937 17 Definition 4 1937 to 1960 21 Sources of data 4 1960 to 1969-1970 21 Procedure 4 1969-1970 to 1974 21 Factors affecting accuracy of data 4 Net historic change (1854-1974) 22 Original data 4 Changes in position of vegetation line 24 Topographic surveys 4 1937 24 Aerial photographs 5 1955 24 Interpretationofphotographs 5 1937-1960 24 Cartographic procedure 6 1960-1970 25 Topographic charts 6 1970-1974 26 Aerial photographs 6 Factors affecting shoreline and vegetation line Measurements and calculated rates 6 changes 27 Justification of method and limitations 6 Climate 27 Sources and nature of supplemental Storm frequency and intensity 27 information 7 Destructive forces and storm Monitoringof vegetation line 8 damage 28 Previous work 10 Changes inbeachprofile during and Present beach characteristics 11 after storms 28 Texture and composition 11 Local and eustatic sea-level conditions 29 Beach profiles 11 Sediment budget 30 Human Alterations of Natural Conditions 13 Human activities 31 Brazos SantiagoPass 13 Evaluation of factors 31 Mansfield Channel 13 Predictions of future changes 32 Changes in shoreline position 14 References 33 Late Quaternary time 14 Appendix A. Determination ofshoreline changes 36 Historic time 15 Appendix B. Tropical cyclones affecting the Texas Brazos Santiago Pass 15 Coast, 1854-1974 38 Mouth of the Rio Grande 15 Appendix C. List of materials and sources 39

List of Illustrations

Figure- 8. Total discharge of Rio Grande recorded at Roma 1. Index map of the Texas Gulf shoreline 3 Station (1923-1954) and San Benito Station 2. Generalized diagram of beach profile 8 (1955-1970) 18 3. Generalized diagram showing location of shore- 9 Total suspended sediment of Rio Grande recordedat RomaStation(1929-1954) 19 line measurements, beach profiles, and 10. Generalized diagram of active processes in hurricane landfall 9 vicinity of BrazosIslandand southPadreIsland 20 4. Relative changes in position of shoreline and 11. Net shoreline changes along Brazos Island and vegetation line at selected locations, Brazos south Padre Island, based on variable time Island and southPadre Island 10 periodsfrom 1854-1880 to 1974 23 5. Beach profiles, Brazos Island and south Padre 12. Relative sea-level changes based on tide gage Island,recorded August 21and 22, 1974 12 measurements forPort Isabel, Texas 30 6. Proposed sea-level changes during the last 20,000 years 14 Table- 7. Changes in location of mouth of Rio Grande, 1. Short-term shoreline changes between 1854 and 1853-1-974 16 1937 nearBrazos Santiago Pass 15 Shoreline ChangesonBrazosIslandandSouthPadreIsland (Mansfield ChanneltoMouth of theRio Grande)— An Analysisof HistoricalChangesof the TexasGulf Shoreline

Robert A.Morton and Mary J. Pieper

Abstract

Historical monitoring along Brazos and south line changes were short-term erosional and accre- Padre Islands records the nature and magnitude of tionary cycles. changes in position of the shoreline and vegetation line and provides insight into the factors affecting Because of limitations imposed by the tech- those changes. nique used, rates of change are subordinate to trends or direction of change. Furthermore, values Documentation of changesis accomplished by determined for long-term net changes should be the compilation of shoreline and vegetation line used in context. The values for rates of net change position from topographic maps, aerial photo- are adequate for describing long-term trends;how- graphs, and coastal charts of various vintages. ever, rates of short-term changes maybe of greater Comparison of shoreline position based on topo- magnitude than rates of long-term changes, partic- graphic charts (dated 1854, 1867, 1879-1880, ularly in areas where both accretion and erosion 1917, 1934) and aerial photographs (taken in have occurred. 1937, 1960, 1970, 1974) indicates short-term changes of accretion and erosion along the beach Major and minor factors affecting shoreline between the mouth of the Rio Grande and changes include: (1) climate, (2) storm frequency Mansfield Channel. produces loss in and intensity, (3) local and eustatic sea-level Erosion a net conditions, (4) sediment budget, and (5) human land, whereas accretion produces a net gaininland. activities. The major factors affecting shoreline Comparison of the vegetation line based on the changes along the Texas Coast, including Brazos aforementioned aerial photographs indicates short- and south Padre Islands,are relative sea-level rise, term cycles of erosion related to storms (primarily compactional subsidence, and a deficit in sediment hurricanes) and recovery during intervening years supply. Changes in position of the vegetation line of low storm incidence. areprimarily related to storms.

Long-term trend or direction of shoreline Studies indicate that changes in shoreline and changes averaged over the 120-year time periodof vegetation line on Brazos and south Padre Islands this study indicates net erosion for south Padre are largely the result of natural processes, perhaps Island;maximum net erosion for this segment was expedited by man's activities. The apparent effect 1,400 feet or approximately 13.1 feet per year. of Falcon Dam upon the discharge of water and Minimum net erosion, in proximity to the north suspended sediment of the Rio Grande is marked, jetty at Brazos Santiago Pass, was 75 feet or less and the entrapment of sediment by the south than 1foot per year.The shoreline at the southern jetties at Brazos Santiago Pass and Mansfield tip of south Padre Island has undergone accretion Channel is obvious. Structures that retard or since construction of the jettiesin1935. eliminate sediment transport add to the sediment deficit already present in the littoral drift system. The long-term shoreline trend of Brazos A basic comprehension of these physical processes Island has been one of accretion; however, this is and their effects is requisite to avoid or minimize attributed to moderate and extreme accretion physical andeconomic losses associated with devel- between 1854 and 1937. After this period, shore- opment and use of the coast.

Introduction

The Texas Coastal Zone is experiencing expanse of beach along deltaic headlands, penin- geological, hydrological, biological, and land use sulas, and barrier islands is presently undergoing changes as a result of natural processes and man's considerable development. Competition for space activities. What was once a relatively undeveloped exists among such activities as recreation,construc- 2

tion, and occupation of seasonal and permanent tion. In advance of the final report and maps, a residential housing, industrial and commercial series of preliminary interim reports will be pub- development, and mineral and resource lished. This report covering Brazos Island and production. south Padre Island from Mansfield Channel to the mouth of the Rio Grande (fig. 1) is the second in Studies indicate that shoreline and vegetation that series. line changes on Brazosand southPadre Islands and along other segments of the Texas Gulf Coast are GeneralStatement on Shoreline Changes largely the result of natural processes. A basic comprehension of these physical processes and Shorelines are in a state of erosion or accre- their effects is requisite to avoid or minimize tion, or are stabilized either naturally or artifi- physical and economic losses associated with devel- cially. Erosion produces a net loss in land, accre- opment and use of the coast. tion produces a net gain in land, and equilibrium conditions produce no net change. Shoreline The usefulness of historical monitoring is changes are the response of the beach to a based on the documentation of past changes in hierarchy of natural cyclic phenomena including position of shoreline and vegetation line and the (from lower order to higher order) tides, storms, prediction of future changes. Reliable prediction of sediment supply, and relative sea-level changes. future changes can only be made from determina- Time periods for these cycles range from daily to tion of long-term historical trends. Topographic several thousand years. Most beach segments maps dating from 1854 provide a necessary exten- undergo both erosion and accretion for lower order sion to the time base, an advantage not available events, no matter what their long-term trends may through the use of aerial photographs which were be. Furthermore,long-term trends can be unidirec- not generally available before 1930. tional or cyclic; that is, shoreline changes may persist inone direction,either accretion or erosion, Purpose and Scope or the shoreline may undergo periods of both erosion and accretion. Thus,the tidal plane bound- In 1971, the Bureau of Economic Geology ary defined by the intersection of beach and mean initiated a program inhistorical monitoring for the high water is not in a fixed position (Johnson, purpose of determining quantitative long-term 1971). Shoreline erosion assumes importance along shoreline changes. The recent acceleration inGulf- the Texas Coast because of active loss of land, as front development provides additional incentive well as the potential damage or destruction of for adequate evaluation of shoreline characteristics piers,dwellings,highways,and other structures. and the documentation of where change is occur- ring by erosion and by accretion, or where the Acknowledgments shoreline is stable or inequilibrium. Assistance in preparation of this report was The first effort in this program was an provided by M. Amdurer. Drafting was under the investigation of Matagorda Peninsula and the supervision of J. W.Macon. Cartographic work was adjacent Matagorda Bay area, a cooperative study performed by D. F. Scranton. Critical review was by the Bureau of Economic Geology and the Texas providedby W. L.Fisher and L.F.Brown, Jr. General Land Office. In this study, basic tech- niques of historical monitoring were developed; The report was typed by Elizabeth T.Moore results of the Matagorda Bay project are now and edited by Kelley Kennedy. Composing was nearing publication (McGowen and Brewton, under the directionof Fannie M. Sellingsloh. 1975). Cooperation of personnel with the U. S,, In 1973, the Texas Legislature appropriated Army Corps of Engineers, Galveston District; funds for the Bureau of Economic Geology to Texas Highway Department; and General Land conduct historical monitoring of the entire 367 Office aided in the acquisition of materials and miles of Texas Gulf shoreline during the information. Meteorological data was supplied by 1973-1975 biennium. Results of the project will be the National Climatic Center and the National published ultimately in the form of maps sum- Hurricane Center. Detailed information concerning marizing the changes in shoreline position. Work changes in position of the mouth of the Rio versions of base maps will be on open file at the Grande was provided by B. L. Everitt, Inter- Bureau of Economic Geology until final publica- national Boundary and Water Commission. 3

Figure 1. Index map of theTexas Gulf shoreline. Historical Shoreline Monitoring—

General Methods andProcedures Used by the Bureau of Economic Geology

Definition Factors Affecting Accuracyof Data

Historical Shoreline Monitoring is the docu- Documentation of long-term changes from mentation of direction and magnitude of shoreline available records, referred to in this report as change through specific time periodsusing accurate historical monitoring,involvesrepetitive sequential vintage charts,maps,and aerial photographs. mapping of shoreline position using coastal charts (topographic surveys) and aerial photographs. This Sources of Data is in contrast to short-term monitoring which employs beach profile measurements and/or the Basic data used to determine changes in mapping of shoreline position on recent aerial shoreline position are near-vertical aerial photo- photographs only. There are advantages and disad- graphs and mosaics and topographic charts. vantages inherent inboth techniques. Accurate topographic charts dating from 1850, available through the Department of Commerce, Long-term historical monitoringreveals trends National Oceanic and Atmospheric Administration which provide the basis for projection of future (NOAA), were mapped by the U. S. Coast Survey changes, but the incorporation of coastal charts using plane table procedures. Reproductions of dating from the 1850's introduces some uncer- originals are used to establish shoreline position tainty as to the precision of the data. In contrast, (mean high water) prior to the early 19305. Aerial short-term monitoring can be extremely precise. photography supplemented and later replaced However, the inability to recognize and differ- regional topographic surveys in the early 19305; entiate long-term trends from short-term changes is therefore, subsequent shoreline positions are a decided disadvantage. Short-term monitoring also mapped on individual stereographic photographs requires a network of stationary, permanent and aerial photographic mosaics representing a markers which are periodically reoccupiedbecause diversity of scales and vintages.These photographs they serve as a common point from which future show shoreline position based on the sediment- beach profiles are made. Such a network of water interface at the time the photographs were permanent markers and measurements has not taken. been established along the Texas Coast and even if a network was established,it would take consider- Procedure able time (20 to 30 years) before sufficient data were available for determination of long-term Thekeyto comparisonof various data needed trends. to monitor shoreline variations is agreement in scale and adjustment of the data to the projection Because the purpose of shoreline monitoring of the selected map base; U. S. Geological Survey is to document past changes in shoreline position 7.5-minute quadrangle topographic maps and to provide basis for the projection of future = (1:24,000 or 1inch 2,000 feet)are used for this changes, the method of long-term historical moni- purpose. Topographic charts and aerial photo- toring is preferred. graphs are either enlargedorreduced to the precise scale of the topographic maps. Shorelines shown OriginalData on topographic charts and sediment-water interface mapped directly on sequential aerial photographs Topographic surveys.— Some inherent error are transferred from the topographic charts and probably exists in the original topographic surveys aerial photographs onto the common base map conducted by the U. S. Coast Survey [U. S. Coast mechanically with a reducing pantograph or opti- and Geodetic Survey, now called National Ocean cally with a Saltzman projector. Lines transferred Survey]. Shalowitz (1964, p. 81) states "...the to the common base map are compared directly degree of accuracy of the early surveys depends on and measurements are made to quantify any many factors, among which are the purpose of the changesinposition with time. survey, the scale and date of the survey, the 5 standards for survey work then in use, the relative However, only the central portion of the photo- importance of the area surveyed, and the ability graphs are used for mapping purposes, and and care which the individual surveyor brought to distances between fixed points are adjusted to the his task." Although it is neither possible nor 7.5-minute topographic base. practical to comment on all of these factors,much less attempt to quantify the error they represent, Meteorological conditions prior to and at the in general the accuracy of a particular survey is time of photography also have a bearing on the related to its date;recent surveys are more accurate accuracy of the documented shoreline changes. For than older surveys. Error can also be introduced by example, deviations from normal astronomical physical changes in material on which the original tides caused by barometric pressure, wind velocity data appear. Distortions, such as scale changes and direction, and attendant wave activity may from expansion and contraction of the base introduce errors, the significance of which depends material, caused by reproduction and changes in on the magnitude of the measured change. Most atmospheric conditions, can be corrected by photographic flights are executed during calm cartographic techniques. Location of mean high weather conditions, thus eliminating most of the water is also subject to error. Shalowitz (1964, effect of abnormal meteorological conditions. p.175) states "...location of the high-water line on the early surveys is within a maximum error of InterpretationofPhotographs 10 meters and may possibly be much more accurate than this." Another factor that may contribute to error in determining rates of shoreline change is the Aerialphotographs.— Error introduced by use ability of the scientist to interpret correctly what of aerial photographs is related to variation in scale he sees on the photographs. The most qualified and resolution,and to optical aberrations. aerial photograph mappers are those who have made the most observations on the ground. Some Use of aerial photographs of various scales older aerial photographs may be of poor quality, introduces variations in resolution with concomi- especially along the shorelines. On a few photo- tant variations inmapping precision.The sediment- graphs, both the beach and swash zone are bright water interface can be mapped with greater preci- white (albedo effect) and cannot be precisely sion on larger scale photographs, whereas the same differentiated; the shoreline is projected through boundary can be delineated with less precision on these areas, and therefore, some error may be smaller scale photographs. Statedanother way,the introduced. In general, these difficulties are line delineating the sediment-water interface repre- resolved through an understanding of coastal sents less horizontal distance on larger scale photo- processesand a thoroughknowledge of factors that graphs than a line of equal width delineating the may affect the appearance of shorelines on same boundary on smaller scale photographs. photographs. Aerial photographs of a scale less than that of the topographic base map used for compilation create Use of mean high-water line on topographic an added problem of imprecision because the charts and the sediment-water interface on aerial mapped line increases in width when a photograph photographs to define the same boundary is is enlargedoptically to match the scale of the base inconsistent because normally the sediment-water map. In contrast, the mapped line decreases in interface falls somewhere between high and low width when a photograph is reduced optically to tide. Horizontal displacement of the shoreline match the scale of the base map. Furthermore, mapped using the sediment-water interface is shorelines mechanically adjusted by pantograph almost always seaward of the meanhigh-water line. methods to match the scale of the base map do not This displacement is dependent on the tide cycle, change in width. Fortunately, photographs with a slope of the beach, and wind direction when the scale equal to or larger than the topographic map photograph was taken. The combination of factors base can generally be utilized. on the Gulf shoreline which yield the greatest horizontal displacement of the sediment-water Optical aberration causes the margins of interface from mean high water are low tide photographs to be somewhat distorted and shore- conditions,low beach profile,and strong northerly lines mapped on photographic margins may be a winds. Field measurements indicate that along the source of error in determining shoreline position. Texas Gulf Coast, maximum horizontal displace- 6

ment of a photographed shoreline from mean are calculated because: (1) time intervals between high-water level is approximately 125 feet under photographic coverage are not equal;(2) erosion or these same conditions. Because the displacement of accretion is assumed constant over the entire time the photographed shoreline is almost always period;and (3) multiple rates (n2^n,where nrepre- seaward of mean high water, shoreline changes sents the number of mapped shorelines) can be determined from comparison of mean high-water obtained at any givenpoint using various combina- line and sediment-water interface will slightly tions of lines. underestimate rates of erosion or slightly over- estimate rates of accretion. The beach area is dynamic and changes of varying magnitude occur continuously. Each pho- Cartographic Procedure tograph represents a sample in the continuum of shoreline changes andit follows that measurements Topographic charts— -The topographic charts of shoreline changestaken over short time intervals are replete with a1-minute-interval grid;transfer of would more closelyapproximate the continuum of the shoreline position from topographic charts to changes because the procedure would approach the base map is accomplished by construction of a continuous monitoring. Thus, the problems listed 1-minute-interval grid on the 7.5-minute topo- above are interrelated, and solutions require the graphic base map and projection of the chart onto averaging of rates of change for discrete intervals. the base map. Routine adjustments are madeacross Numerical ranges and graphic displays are used to the map with the aid of the 1-minute-interval present the calculatedrates of shoreline change. latitude and longitude cells. This is necessary Where possible, dates when individual photo- because: (1) chart scale is larger than base map graphs actually scale; (2) (expansion and contraction) were taken are used to determine distortions rates, in the medium (paper or cloth) of the original the time interval needed to calculate rather than the general date printed on the mosaic. survey and reproduced chart, previously discussed, adjustment; and (3) paucity of culture Particular attention is also paid to the month, as require well along the shore provideslimited horizontal control. as year of photography; this eliminates an apparent age difference of one year between photographs taken inDecember and January of the photographs.— Accuracy of aerial pho- Aerial followingyear. tograph mosaics is similar to topographic charts in that quality is related to vintage; more recent Justification ofMethod and Limitations mosaics are more accurate. Photograph negative quality, optical resolution,and techniques of com- The methods used in long-term historical piling controlled mosaics have improved withtime; monitoring carry a degree of imprecision, and thus, more adjustments are necessary when work- trends and rates of shoreline changes determined ing with older photographs. from these techniques have limitations. Rates of change are to some degree subordinate inaccuracy Cartographic proceduresmayintroduce minor to trends or direction of change;however, there is errors associated with the transfer of shoreline no doubt about the significance of the trends of position from aerial photographs and topographic shoreline change documented over more than 100 charts to the base map. Cartographic procedures do years. An important factor in evaluating shoreline not increase the accuracy of mapping; however, changes is the total length of time representedby they tend to correct the photogrammetric errors observational data. Observations over a short inherent in the original materials such as distor- period of time mayproduce erroneous conclusions tions and opticalaberrations. about the long-term change incoastal morphology. For example, it is well established that landward MeasurementsandCalculatedRates retreat of the shoreline during a storm is accom- panied by sediment removal; the sediment is Actual measurements of linear distances on eroded, transported, and temporarily stored off- maps can be made to one-hundredth of an inch shore. Shortly after storm passage, the normal which corresponds to 20 feet on maps with a scale beach processes again become operative and some of 1inch = 2,000 feet (1:24,000). This is more of the sediment is returned to the beach. If the precise than the significance of the data warrants. shoreline is monitored during this recovery period, However, problems do arise when rates of change data would indicate beach accretion; however, if 7 the beach does not accrete to its prestorm position, long as users realize and understand the limitations then net effect of the storm is beach erosion. of the method of historical monitoring, results of Therefore, long-term trends are superior to short- sequential shoreline mapping are significant and term observations. Establishment of long-term useful in coastal zone planning and development. trends based on changes in shoreline position necessitates the use of older and less precise Sources and Nature of SupplementalInformation topographic surveys. The applicability of topo- graphic surveys for these purposes is discussed by Sources of aerial photographs, topographic Shalowitz (1964, p. 79) who stated: charts, and topographic base maps used for this report are identified in appendix C. Additional "There is probably little doubt but that information was derived from miscellaneous the earliest records of changes in our coastline reports published by the U. S. Army Corps of that are on a large enough scale and in Engineers and on-the-ground measurements and sufficient detail to justify their use for quanti- tative study are those made by the Coast observations including beach profiles,preparedas a Survey. These surveys were executed by com- part of this investigation. Laws relating to the petent and careful engineers and were practi- improvement of rivers and harbors are synthesized cally all based on a geodetic network which minimized the possibility of large errors being in House Documents 379 and 182 (U. S. Army introduced. They therefore represent the best Corps Engineers,1940,1968a). evidence available of the condition of bur or years ago,and the coastline a hundred more Relative intensity, estimated from courts have repeatedly recognized their com- wave petency inthisrespect ...." photographs, and the general appearance of the beach dictate whether or not tide and weather Because of the importance of documenting bureau records should be checked for abnormal changes over a long time interval, topographic conditions at the time of photography.Most flights charts and aerial photographs have been used to are executed during calm weather conditions,thus study beach erosion in other areas. For example, eliminating most of this effect. On the other hand, Morgan and Larimore (1957), Harris and Jones large-scale changes are recorded immediately after (1964), El-Ashry and Wanless (1968), Bryant and the passage of a tropical storm or hurricane. For McCann (1973), and Stapor (1973) have success- this reason, photography dates have been com- fully used techniques similar to those employed to herein. Previous articles describing determinations pared with weather bureau records determine of beach changes from aerial photographs were the nature and extentof tropical cyclones prior to reviewed by Stafford (1971) and Stafford and the overflight. If recent storm effects were obvious others (1973). on the photographs,an attempt was made to relate those effects to aparticular event. Simply stated, the method of using topo- graphic charts and aerial photographs, though not Considerable data were compiled from absolutely precise, represents the best method weather bureau records and the U. S. Department available for investigating long-term trends in of Commerce (1930-1974) for manyof the dates shoreline changes. of aerial photography, These data, which include wind velocity and direction and times of predicted Limitations of the method require that tidal stage, were used to estimate qualitatively the emphasis be placed first on trend of shoreline effect of meteorological conditions on position of changes with rates of change being secondary. the sediment-water interface (fig. 2). Although rates of change from map measurements can be calculated to a precision well beyond the limits of accuracy of the procedure, they are most Northward from station 13 (fig. 3), ground important as relative values; that is, do the data control is lacking; therefore, transfer of data to the indicate that erosion is occurring at a few feet per base map is entirely dependent on the location of year or at significantly higher rates. Because the stable dunes and the shape of the back-island sequential shoreline positions are seldom exactly area. As a result, the degree of accuracy declines parallel, in some instances it is best to provide a slightly and the chance for error may increase in range of values such as 10 to 15 feet per year. As some areas. 8

Figure 2.Generalizeddiagramof beach profile.

Monitoring of VegetationLine photography may show a well-defined boundary for the same area, or vice versa. In general, these Changes inposition of the vegetationline (fig. difficulties are resolved through an understanding 4) are determined from aerial photographs in the of coastal processes and a thorough knowledge of same manner as changes in shoreline position with factors that affect appearance of the vegetation the exception that the line of continuous vegeta- line on photographs. For example, the vegetation tion is mapped rather than the sediment-water line tends to be ill defined following storms interface. Problems associated with interpretation because sand may be depositedover the vegetation of vegetation line on aerial photographs are similar or the vegetation may be completely removed by to those encountered with shoreline interpretation wave action. The problem of photographic scale because they involve scale and resolution of and optical resolution in determination of the photography as well as coastal processes.Inplaces, position of the vegetation line is opposite that the vegetation "line" is actually a zone or transi- associated with determination of the shoreline (see tion, the precise position of which is subject to page 5). Mapping the vegetation line is more interpretation; in other places the boundary is difficult on larger scale photographs than on sharp and distinct, requiring little interpretation. smaller scale photographs, particularly in areas The problems of mapping vegetation line are not where the vegetation line is indistinct, because just restricted to a geographic area but also involve larger scale photographs provide greater resolution changes with time. Observations indicate that the and much more detail. Fortunately, vegetationline vegetation line along a particular section of beach is not affected by processes such as tide cycle at may be indistinct for a given date, but subsequent the time the photography was taken. 9

Figure 3. Generalizeddiagram showinglocation of shorelinemeasurements, beach profiles, and hurricanelandfall. 10

Figure 4. Relative changes in position of shoreline and vegetation line at selectedlocations, BrazosIslandand south PadreIsland.

Previous Work

Brazos Santiago Pass has been the subject of southern end of south Padre Island, a 7.5-mile numerous studies by the U. S. Army Corps of segment of shoreline one mile north of Brazos Engineers, originating as early as 1850 and con- Santiago Pass was divided into one area of critical tinuing to the present.The early studiesmonitored erosion bracketed by two smaller areas of non- changesin width of the natural channel,as well as criticalerosion. depth of water within the channel and Laguna Madre. As a result of these studies, jetty construc- In a recent study, Seeligand Sorensen (1973) tion was proposed as the only means of main- presented tabular data documenting mean low- taining a navigable channel. Emory (1857), in an water shoreline changes along the Texas Coast; early survey for the Boundary Commission, also values calculated for the rates of shoreline change commented on the channels and bars of Brazos along Brazos Island and south Padre Island were Santiago Pass and the Rio Grande. included in their report. Their technique involved the use of only two dates (early and recent); the Erosional and accretionary shoreline changes change at any point was averaged over the time resulting from construction of the jetties at Mansfield Channel were discussed by Hansen period between the two dates. Cycles of accretion (1960) and the U. S. Army Corps of Engineers and erosion were not recognized and few inter- (1958b). Beach profiles surveyed by the U. S. mediate values were reported; thus, in certain Army Corps of Engineers (1968-1972) document instances, the data are misleading because of short-term shoreline changes in proximity to the technique. Furthermore, data retrieval is difficult Mansfield Channel jetties, Brazos Santiago jetties, because points are identified by the Texas coordi- and the Cameron-Willacy county line. nate system.Rates of accretion rangingfrom 2 to 8 feet per year are given for Brazos Island with A regional inventory of Texas shores was erosion of 5 and 18 feet per year in the vicinity of conducted by the U. S, Army Corps of Engineers the mouth of the Rio Grande. Rates of erosion (1971c). No quantitative data were given;however, determined by Seelig and Sorensen (1973, p. 13) the study delineated areas of critical and non- for south Padre Island range from 0 to 15feet per critical erosion along the Texas Coast. On the year. Present Beach Characteristics

Texture and Composition tions in beach profiles occasionally maybe attrib- uted to vehicular traffic and beach maintenance The beaches of Brazos Island and southPadre such as raking and scraping. Island are comprised of fine sand (Lohse, 1952; Hayes, 1965; Garner, 1967) which is well sorted Beach profiles presented in figure 5 were and contains abundant volcanic rock fragments, constructed using the method described by Emery volcanic quartz, and sanadine. These sediments (1961). The profiles, considered typical of certain were derived presumably from the volcanic areas of segments of Brazos and south Padre Islands, West Texas andMexico and transportedby the Rio represent beach conditions on August 21, 1974. Grande (Bullard,1942;Hayes,1964). The presence High tide mark was identified by sand wetness and of volcanic material gives the sand a reddish color position of debris line. Beach profiles in the in contrast to the grayish color typical of sand vicinity of Mansfield Channel, Brazos Santiago transported by other Texas rivers. Heavy mineral Pass, and justnorth of the Cameron-Willacy county analysis indicates a suite of minerals distinctive to line have also been surveyed by the Galveston the Rio Grande source composed of 60 percent District, U. S. Army Corps of Engineers basaltic hornblende and pyroxene, 10 percent (1968-1972). Comparison of beach profiles and green hornblende, and 30 percent of the more beach scour patterns by Herbich (1970) suggests durable minerals including zircon, garnet, stauro- that beach conditions (breaker bar spacing and lite,tourmaline,and rutile (Bullard, 1942). size) may be similar over a relatively longperiodof time except during and immediately following Beach Profiles storm conditions. Therefore, unless beach profiles are referenced to a permanent, stationary control Brazos and south Padre Islands are charac- point on the ground, comparison of profiles at terized by a diversity of beach conditions (fig. 5) different times may be very similar, but " the owing to the intermittent vegetated dunes and absolute position of the beach can be quite washover areas. Ingeneral, the beaches are narrow different. Thus,a beach profile may appear similar (about 150 feet wide) and steep in areas of (except after storms) for a longperiod of time but vegetated dunes. Beach width increases (200 to the entire profile may shift seaward (accretion) or 350 feet) and slope decreases in washover areas. landward (erosion) during the same period. Extant dunes are generally 15 feet in height and discontinuous because they are breached by Beach profile is controlled primarily by wave numerous active storm channels. Daily changes in action. Other factors determining beach charac- beach appearance reflect changing conditions such teristics are type and amount of beach sediment as wind direction and velocity, wave height, tidal available and the geomorphology of the adjacent stage, and the like. Accordingly,beach profiles are land (Wiegel, 1964). In general, beach slope is subject to change depending on beach and surf inversely related to grain size of beach material conditions that existed when measurements were (Bascom, 1951). Thus, beaches composed of fine recorded. In general, the most seaward extent of a sand are generally flat. Beach width along the beach profile is subjected to the greatest changes Texas Coast is primarily dependenton quantity of because in this area breakpoint bars are created, sand available. Beaches undergoingerosion due to a destroyed, and driven ashore. Under natural condi- deficit in sediment supply are narrower than tions, the landward portion of a beach profile is beaches where there is an adequate supply or affected only by spring and storm tides of more surplus of beach sand. For example, the beach on intense events such as tropical cyclones. With south Padre Island is not as wide as central Padre increased use of the beach, however, minor altera- Island where there is an adequate supply of sand. 12

Figure 5. Beach profiles, BrazosIsland and south PadreIsland, recorded August 21and22, 1974. HumanAlteration of NaturalConditions

Brazos Santiago Pass prevent bank erosion and protect the inner end of the jetties. Historically, Brazos Santiago Pass has been more important than other areas along southPadre Upon completion of the project, the channel and Brazos Islands because it serves as a natural from the Gulf had been dredged to 23 feet (U. S. pass from the Gulf to Laguna Madre and the Army Corps Engineers,1936). Channel dimensions mainland of the lower Texas Coast. were increased by dredging in 1939 and by 1940 the channel from the Gulf throughBrazos Santiago Prior to jetty construction, channel depth at Pass was 32 feet deep (U. S. Army Corps Engi- Brazos Santiago Pass varied from 6.5 to 11 feet neers, 1939, 1940b). Channel dredging continued (U.S. Army Corps Engineers, 1963), and channel until the channel through the pass was 35 feet deep position shifted continually. The width of the pass (U. S. Army Corps Engineers, 1941). At the same also varied from 700 to 1,700. feet between 1882 time the channel to Brownsville as well as the and1910 (U. S. Army Corps Engineers,1913). turning basins were deepenedand widened.

The first federalimprovement was initiatedin No further improvements were reported 1878 with the removal of a wreck from the during the 19405, but in 1950 new plans for channel. In 1881, the River and Harbor Act improvement called for a channel 38feet deepand authorized construction of a south jetty consisting 300 feet wide from the Gulf through Brazos of brush mattresses weighted down by clay bricks Santiago Pass; the proposed depth in all other (U. S. Army Corps Engineers, 1881). Work on the channels and basins was 36 feet (U. S. Army Corps jetty was suspended in 1884; the remaining jetty Engineers, 1950). Project maintenance and was destroyed by a major storm in 1887, which dredgingcontinued until completion in1960 (U. S. crossed the coast in Mexico (U. S. Army Corps Army Corps Engineers,1960). Major rehabilitation Engineers,1888, 1895). of, the north and south jetties in 1966 and maintenance dredging have been the onlyactivities Between 1887 and 1927, all jetty work was reported for this project since a stone embankment abandoned and an attempt was made to maintain was constructed in 1961 on Padre Island between the channel across the bar and within the Laguna the north jetty and the remains of the 1935 rock Madre at a desirable depth by dredging (U. S. groin (U. S. Army Corps Engineers, 1963). A Army Corps Engineers, 1900, 1905, 1913, 1914, 1,000-foot extension of the north jetty authorized 1916, 1919, 1928). This operation was largely by the River and Harbor Act of 1960 had notbeen unsuccessful because shoaling wasrapid. finished by 1972 (U. S. Army Corps Engineers, 1961,1965, and 1972). During 1927 and 1928, work commenced on two stone dikes extending1,400 feet from Brazos Mansfield Channel Island and 1,899 feet from Padre Island (U. S. Army Corps Engineers, 1928). The dikes, con- Southern Padre Island is traversed by one structed mainly as a protective device for the artificial pass, Mansfield Channel, located approxi- dredging barge, were ineffective in maintaining mately 38 miles north of Brazos Santiago Pass. The natural depth of the channel,and they deteriorated channel, 10 feet deep and 100 feet wide, was rapidly after construction. dredged through Padre Island by the Willacy County Navigation District in 1957. After the The River and Harbor Act of 1930 authorized channel was dredged, two jetties were constructed the construction of the present-day jetties. Not of concrete tetrapods. The north jetty extended until the present jetties were completed in 1,600 feet into the Gulf, and the south jetty February 1935, was any success achieved inmain- extended 900 feet (Hansen, 1960). Subsequent to taining a dependable navigation channel. The completion, extensive deterioration of both jetties jetties are 1,200 feet apart; length of the north occurred because of subsidence and erosion at the jetty is 5,370 feet, whereas the south jettyis 5,092 shore ends. With the effectiveness of both jetties feet long (U. S Army Corps Engineers, 1963). In destroyed, shoaling at the mouth of the channel 1935, small rock groins were constructed to occurred by 1958 making the channel useless for 14 navigation (Hansen, 1960). Hansen also reported improvement of Mansfield Channel as a Federal that the shoreline north of the channel entrance project. The project included dredging of the had undergone extensive erosional and accre- channel and construction of a north jetty and a tionary cycles since completion of the channeland south jetty extending 2,300 feet and 2,270 feet, jetties. respectively. Work under contract for construction of rubble stone jetties was completedMay 8, 1962 In September 1959, Congress authorized (U. S. Army CorpsEngineers,1962a).

Changesin ShorelinePosition

Late Quaternary Time as from fluvial-deltaic and marine processes. Lohse (1958) discussed the Recent history of the Rio Significant changes along the south Texas Grande delta and concluded that the distributary Coast during the past few thousand years have system during Early Recent time transported resulted from sea-level fluctuations (fig. 6) as well sufficient material to allow progradation of the

Figure 6. Proposed sea-levelchanges during the last 20,000 years; sketch defines use of ModernandHolocene used in text. 15 shoreline seaward of its present position. This is Table 1. Short-term shoreline changes between 1854 substantiated by outcrops of mud and poorly and 1937 near Brazos SantiagoPass. consolidated sandstone in the swash zone of south Padre Island. Lateral shifting of sites of active Point Point Point deltation, subsidence, and transgression of aban- doned subdeltas have been subsequently respon- Time Dist Rate Dist Rate Dist Rate sible for the present configuration of the coastline. ft ft/yr ft ft/yr ft ft/yr 854-1867 +450 +34.6 +675 +51.9 years, During the past few hundred conditions 867-1917 -1000 -20.0 -1200 -24.0 -525 -10.5 that promoted seaward accretion have been altered 917-1934 -125 -7.4 +150 +8.8 +325 +19.1 both naturally and, more recently, to some extent 934-1937 -75 -25.0 -75 -25.0 +50 +16.7 by man. Consequently, sediment supply to the Texas Coast has diminished and erosion is common. jetty, the shoreline has accreted 1,700 feet or 47 feet per year. The greatest period of accretion was Historic Time between 1934 and1937. Supplementary shorelines mapped on aerial photographs taken in 1955 and Shoreline changes and tabulated rates of 1968 confirm the accretionary trend established by change between 1854 and 1974 at 25 arbitrary comparison of the 1938, 1960, and 1970 shore- points spaced 10,000 feet apart along the map of lines. The shoreline has continued to accrete near Brazos Island and south Padre Island (fig. 3) are the north jetty (point 21) as indicated by the 1974 presented in appendix A. Excluding points in photographs. proximity to passes and the Rio Grande, Brazos Island has experienced one early period of accre- Until 1955, the accretionary rate adjacent to tion (1854-1937) and more recently a trend the south jetty was extreme (1934-1937) to major dominated by erosion (1937-1974). In contrast, (1937-1955). Accretion slowed to 15 feet per year south Paclre Island has undergone varying rates of between 1955 and 1960; total accretion between erosion since 1854 with the exception of stations 1937 and 1960 was 300 feet. Since 1960, the 20 and 21, which have been either in equilibrium shoreline has accreted slightly. or in accretion since construction of the Brazos Santiago jettiesin 1935. Mouth of theRio Grande

The following classification of rates of change Perhaps the most dramatic shoreline changes for the of describing have taken place on Brazos Island in the vicinity of is introduced convenience the Grande. range: Rio Repeated changes in the position changes that fall within aparticular of the mouth of the river have directly affected the shoreline along this segment of the coast (fig.7). Rate (ft/yr) Designation In1854, the mouth of the river was located in 0-5 minor approximately the same position as today (1975). 5-15 moderate Between 1854 and 1937, the mouth migrated to a 15-25 major point approximately 4,000 feet north of the 1854 >25 extreme location. Northward migration continued between 1937 and 1958; in 1958, the mouth reached its Brazos Santiago Pass most northern position since 1854 (8...L. Everitt, personal communication, 1974). Between 1958 Prior to jetty construction,Brazos Island and and 1960, the mouth migrated approximately south Padre Island experienced two periods of l?000 feet to the south. During high water accretion (1854-1867 and 1917-1934) and one produced by Hurricane Carla (1961), the river cut period of erosion (1867-1917) in the vicinity of a new course, shifting the position of the mouth Brazos Santiago Pass (table 1). Since completion of approximately 4,000 feet to the south, not far the jetties, the shoreline has accreted 1,050 feet or from its 1897 position (B. L. Everitt, personal 29 feet per year at the north jetty on southPadre communication, 1974). Beginning in 1962, the Island, the greatest rate being between 1937 and mouth again started a slow migration to the north. 1955. On Brazos Island,in proximity to the south During Hurricane Beulah (1967), the river changed 16

Figure 7. Changes inlocation of mouth of Rio Grande, 1853-1974. 17 its course resulting in a shift of the mouth again to 1960-1970.—Station 22, near the south jetty the south. The mouth of the river is currently at Brazos Santiago Pass, experienced moderate located approximately 750 feet north of the 1967 accretion (75 feet) between 1960 and 1970. The location. shoreline at station 23 remained relatively un- changed, whereas the two southernmost stations BrazosIsland (24 and 25) experienced erosion of 75 and 375 feet, respectively. The erosion may be attributed in The shoreline on Brazos Island has ex- part to a shift in the river course to a position perienced both accretion and erosion. The shore- line accreted between 1854 and 1937;after 1937, approximately 5,000 feet south of the 1960 the shoreline was characterized by a combination location. Also, straightening of the shoreline may of erosional and accretionary cycles at the indi- have been a factor. vidual stations. 1970-1974— Measurements for this time 1854-1937— During this time interval, all period are available only at stations 23 through 25. stations on Brazos Island experienced accretion. The shoreline between the south jetty and station Accretion ranged from 525 feet at point 22 to 22is not covered by the 1974photography. 1,425 feet at the mouth of the Rio Grande. Between 1970 and 1974, the shoreline at This time period is broken into four shorter station 23 accreted 25 feet, while the stations at increments (1854-1867, 1867-1917, 1917-1934, the southern end of Brazos Island (24 and 25) and 1934-1937); however, measurements are not experienced erosion of 100 and 125 feet, available for all stations (table 1). These shorter respectively. increments are also dominated by accretion. The greatest accretion (675 feet at point 22) occurred SouthPadreIsland between 1854 and 1867; however, between 1867 and 1917, erosion of 525 feet occurred at station The shoreline of south Padre Island has been 22. retreating at least since the late 1800's. Thebarrier island is traversed by numerous washover channels The 1937 photomosaics illustrate the fore- (fig. 10), which are opened periodically by storm dune erosion, which took place during the 1933 surges. Generally, washover channels heal quickly hurricane. Unfortunately, shoreline erosion related after a storm by normal littoral processes. The to the storm cannot be determined, but storm filled channels characteristically exhibit planar channels that breached the foredune remnants surfaces that are void of large dunes and lower than were still present in 1937. Boca Chica Pass was adjacent areas. opened by the storm surge but was again closed in 1937. 1867-1880 to 1937-The earliest shoreline position on south Padre Island is based on a 1937-1960.— During this time interval,station composite of U. S. Coast and Geodetic Survey 22 experiencedaccretion (300 feet),station 24 was charts dated 1867, 1879, and 1880. This early in equilibrium, and stations 23and 25 experienced composite shoreline was compared to the 1937 erosion. Shoreline erosion of 400feet at station 25 shoreline position mapped on aerial photomosaics. can be explained inpart by the northern migration Two intermediate maps are also available for of the mouth of the Rio Grande (fig. 7) and comparison in the vicinity of Brazos Santiago Pass: reorientation of a prominentbulge in the shoreline a 1917 U. S. Coast and Geodetic Survey chartand just north of the earlier position of the river a 1934 U. S. Geological Survey 15-minute mouth. Beginning with this time interval,there was topographic quadrangle. a marked change in the previous accretionary trend of Brazos Island. Probably this change, in part, Between 1867-1880 and 1937, all 21stations reflects the construction of Falcon Dam, which on south Padre Island experienced erosion. Four was completed in 1953 resulting ina sharpdecline stations experiencedmajor rates of erosion (15 to in volume of water and sediment carried down- 25 feet per year), 11 stations recorded rates stream from the structure by the Rio Grande (figs. between 5 and 15 feet per year, and 6 stations 8 and 9). recordedminor erosion (0 to 5 feet per year). 18

Figure 8. Total discharge of Rio Grande recorded at Roma Station (1923-1954) and San Benito Station(1955-1965). Figure 9. Total suspended sediment of Rio Grande recorded at Roma Station(1929-1954). 20

Figure 10. Generalizeddiagram of active processes in vicinity of BrazosIslandandsouthPadreIsland. 21

Greatest shoreline erosion,recorded atpoints to the base map within the limits of error 17 to 21, ranged from 900 to 1,250 feet. Erosion established for this study. During this 10-year for the remainingbeach segmentranged from 25 to period, the area was affected by two major 675 feet and averaged 375 feet. A total of 14 hurricanes (Carla, 1961, and Beulah, 1967). tropical cyclones, 6 of which were major Although Celia, in 1970, wasan extreme storm, its hurricanes, affected the lower part of the coast radius of damage was relatively small and did not during this period. Apparently the 1933 storm extendto southPadre Island. caused more damage to the island than any of the other storms between 1867 and 1937. During the Storm surge from Carla,ranging from 4.4 feet 1933 hurricane, a 12- to 15-foot storm surge was at Port Isabel to 5.5 feet on south Padre Island recorded at Brownsville (Sugg and others, 1971). (U. S. Army Corps Engineers, 1962b),increased in Price (1956a) reported that as a result of the height northward along the Texas Coast. Carla storm, 40 channels were opened and nearly all of surge opened Mansfield Channel and 40 storm the dunes were eroded from the southern part of channels onPadre Island (Hayes, 1967). Erosional Padre Island; however, no quantitative data on effects, however, wererestricted mainly to the area shoreline erosion are available. The extreme effects north of Mansfield Channel. A series of beach of the storm are exhibited on the 1937 photo- profiles in proximity to Mansfield Channel taken mosaics. The area recovered slowly because a by the U. S. Army Corps of Engineersafter Carla drought following the storm retarded growth of indicate shoreline erosion ranging from 60 ft to vegetation, thus slowing down the process of dune 160 ft (U. S. Army Corps Engineers,1962b). stabilization. Beulah (1967) crossed the Texas Coast just 1937 to i96o— Between 1937 and 1960, east of Brownsville. Maximum storm tides of 12 erosion continued along most of south Padre feet (measured high water mark at Port Isabel) and Island, the exception being between stations 20 18 feet (estimated at latitude 26° 4' N.) were and 21 where the shoreline was directly affected reported by Sugg and Pelissier (1968). Twenty- by the north jetty at Brazos Santiago Pass. These seven storm washover channels varying in width two stations experiencedaccretion of 200and 550 from several hundred feet to approximately 5,000 feet, respectively. The shoreline accreted 50 feet at feet breached the island between Andy Bowie point 13 and remained unchanged at point 14. County Park and Mansfield Channel (observations Erosion at the remaining17 points ranged from 25 made from 1967 Texas Highway Department to 450 feet; average erosion was 230feet. Areas of low-altitude aerial photos). Immediately south of most rapid shoreline erosion included stations 8to Andy Bowie CountyPark, the highway (Park Road 12and stations 1to 4. 100) was breached in several places; however, the washovers did not cut through the island. South of No major hurricanes made landfall in thearea this area, no washover channels are evident;erosion during this period; however, the area may have was restricted to the immediate beach and fore- been affected four times (July 1945, August 1947, dune area. June 1954, June 1958) by 4- to 5-foot surges associated with hurricanes crossingthe coast to the Comparison of June 1967 aerial photographs north or south of the area. (U. S. Army Corps Engineers) with September 1967 aerial photographs (Texas Highway Depart- 1960 to 1969-1970— Shoreline erosion on ment) reveals dune erosion of 50 to 350 feet that south Padre Island continued between 1960 and resulted in wave-cut faces on the seaward margins 1970. One station (14) recorded erosion of 325 of the foredunes. Areas of greatest dune erosion feet, and five points recorded erosion between 150 occurred along lateral margins of active washover and 225 feet. Shoreline erosion along the re- channels openedby the storm surge. maining segments was between 25 and 150 feet except at point 7 which remained unchanged. 1969-1970 to 1974.—The dominant erosional Again, points 20 and 21at the southernmost part trend on south Padre Island continued between of the island near the north jetty showed accre- 1970 and 1974. As before,points 20 and 21, near tionary values. Measurements are not available the north jetty, experienced accretion while the north of point 7 because lack of ground control rest of south Padre Island experienced erosion at made it impossible to transfer the 1970 shoreline highly variable rates; most of the island experi- 22 enced moderate erosion,however, the shoreline at 25, the 1867 shoreline from points 16 to 20, and points 9, 17, and 19 remained relatively un- the 1879-80 shoreline from points 1 to 15 as a changed. Erosion at the other points ranged from baseline; net shoreline change is equal to the 25 to 150 feet and averagedabout 70 feet. difference between this baseline and the 1974 shoreline (appendix A). Shoreline changes north of the Cameron- Willacy county line were not determined because The shoreline along Brazos Island has under- of the inability to transfer shoreline data to the gone net accretion since 1854; however,this trend base map with the desired degreeof accuracy. The is influenced predominantly by the moderate to lack of ground control along this portion of the extreme accretion which occurred between 1854 island in conjunction with migrating dunes makes and 1937. With the exception of point 22, where exact cartographic adjustment nearly impossible. Consequently, quantitative figures are not avail- accretion has continued due to entrapment of able, but generalized observations suggest that the sediment by the south jetty, the accretionary trend erosional trends typical of the shoreline from reversed after 1937, and Brazos Island entered a points 8 to 19 continue northward between points trend dominated by erosion at rates varying from 1and 7. 10 feet to 37.5 feet per year. Extreme erosion is restricted to south Brazos Island (point 25), which The shoreline adjacent to the south jetty at is greatly influenced by the location of the mouth Mansfield Channel accreted150 feet between 1967 of the Rio Grande (fig. 7). and 1974 or at a rate of 21.4 feet per year. This accretion was predictable because the jetty entraps South Padre Island has experienceda history sediment transported northward by the prevailing of erosion with the exception of the extreme longshore current. The shoreline adjacent to the southern tip of the island, which has undergone north jetty, where longshore sediment supply was accretion since construction of the jettiesin1935. deficient, eroded 100 feet or at a rate of 14.3 feet At many points,rates of erosion increased between per year. 1960 and 1969 with parts of the island ex- periencing extreme erosion. Comparison of net No tropical storms crossed this segment of the changes on south Padre Island is difficult because coast between 1970-1974. The erosional effects of of the composite nature of the late 1800's shore- Beulah are still visible; however, coppice mounds line 1970 1974 encroach upon the and the lack of and data on are again beginning to storm shoreline positions between points 1and 7. Never- channels. theless, the trend is clearly established because Shoreline changes documented by historical erosion prevailed at all points removed from the monitoring techniques, such as repetitive direct influence of the north jetty at Brazos sequential mapping using aerial photographs, have Santiago Pass. The most complete record for these been confirmed by field measurements. For points (8 through 20) indicates net erosion ranging example, in 1956 the Gilbert Kerlin #1 well from 725 to 1,400 feet and averaging about 990 (Cameron County) was drilled by Magnolia feet. Overall net erosion was moderate, ranging Petroleum Company about 5 miles south of the from 8 to13 feet per year. Cameron-Willacy county line. The well location plat, on file at the Texas Railroad Commission, These data indicate long-term net change,but shows that the well was drilled 330 feet landward the values should be used with caution because of from the mean tide line; the mean tide line is their misleading implications. The values for rates presently near the abandoned well. Beach erosion of net change are adequate for describinglong-term at point 11 of 300 feet between 1960 and1974is trends; however, rates of short-term changes may in close agreement with the measurement obtained be of greater magnitude than rates of long-term from the survey records of the oil well. changes, particularly in areas where both accretion and erosion have occurred. Furthermore,if for any Net Historic Change (1854-1974) reason the equilibrium between sediment supply and littoral processes is upset, then even long-term Net shoreline change (fig. 11) was calculated trends of the past may not reflect long-term trends using the 1854 shoreline between points 21 and of the future. 23

Figure 11. Net shoreline changes along BrazosIslandand southPadreIsland,based on variable time periods from 1854-1880 to 1974. Changes inPositionof VegetationLine

The first descriptive survey of Padre Island A general rather than quantitative description was recorded in 1828 by Domingo Fuente of changes in position of the vegetation line on (Meixner, 1948), who described the area of south Brazos Island and south Padre Island is necessary Padre Island between Brazos Santiago Pass to just because, in most instances, with the exception of a north of the present Cameron-Willacy county line portion of south Padre Island between the north as salty bays and dunes withvery little pasture land jetty and Park Highway 100, the vegetationline is except in the vicinity of the county line. North of characteristically discontinuous and confined to this line, the density of vegetation increased. He areas of stabilized dunes. noted the presence of small fresh-water lakes, willows, small oaks, and abundant grass on north 1937— The effects of the September 1933 Padre Island. hurricane on vegetation of south Padre Island and Brazos Island were still visible on the 1937 Apparently vegetation was moreprolific from photomosaics. The eroded dunes were separated by the early 1900's to about 1930.Pat Dunn, a cattle broad, smooth, washover channels; only isolated rancher,reportedcontinuous pasture land onPadre remnants of vegetated foredunes remained. Except Island extending110 miles longand 2V6 to 4 miles for the remnants, the southern half of Padre Island wide (Meixner, 1948). As many as 1,600 head of was virtually denuded of vegetation. cattle were on the island at one time; however, Vegetation Brazos Island also re- Dunn reported that after the 1933 storm it would on was stricted to erosional of the foredunes and have been possible tosupport 25head of cattle remnants not scattered coppice mounds seaward of theforedune on the southernmost 40 miles of Padre Island remnants. (Meixner,1948). The effect of extensivegrazing on the island's vegetationis onlyspeculative;however, 1955.— Despite the effects of extended it must be considered as a factor in the general droughts (1937-39, 1950-52, 1954-56), the lineof decline in abundance of vegetation (U. S. Dept. vegetation on Brazos and south Padre Islands Interior,National Park Service, 1959). advanced gulfward between 1937 and1955. Aerial photographs taken in 1955 show the effects of Price (1956a) reported that prior to 1933 drought on density and condition of the vegeta- there was a relatively continuous line of vegetated tion. During the drought,considerable unstabilized foredunes (15 to 25 feet high) on the islandbroken sand was permitted to migrate duenot only to the by irregularly spaced storm channels. The grass sparse vegetation but also to the poor condition of cover was destroyed and much of south Padre the vegetation. Major revegetation prior to the Island was reduced to relatively smooth sand plains 1954-1956 drought occurred on foredune at or below beach-ridge levels by the 13-foot surge remnants; minor revegetationoccurred in washover which accompanied the 1933 storm. By 1946, the areas which had been partially filled by coppice vegetation had begun to recover (Price, 1956a); mounds. Sparsely vegetated coppice mounds sea- progress was slow because droughts occurred in the ward of the foredunes were also abundant in 1955. area from 1933 to 1934 and from 1937 to 1939. — Droughts also affected the area in 1950-52 and 1937-1960. The vegetation line recovered 1954-56. The drought of 1954-56 was one of the considerably in the 23 years between 1937 and most severe experienced by the State (Lowry, 1960. Two major factors affecting this recovery 1959). During these droughts, the active dune were the low frequency of major storms striking fields were greatly expanded. the southern part of the Texas Coast during this period and the cessation of the 1954-56 drought. The effects of local droughts upon vegetation An increase in density was the most significant in this semiarid region are further exemplifiedby change in vegetation between 1955 and 1960 low-altitude aerial photographs taken in 1955 which can be attributed to increased rainfall. during the extended drought of the 19505. No Because of the lack of geographic control pointson major storms made landfall in this area in the late the 1937 mosaics, it is difficult to make detailed 1940's and 19505, thus the paucity of vegetation measurements comparing the 1937 and1960 vege- is related to drought conditions. tation line. 25

The vegetationon Brazos Island accreted both tember 25, 1967); however, erosion and denuda- seaward and parallel to the shoreline by 1960. tion of the vegetation from Beulah (fig. 4) was not Revegetation along the lateral margins of washover as severe as it was in 1933. Unfortunately, the channels occurred where dunes with sparse vegeta- September 25, 1967 photographs do not include tion partially infilled the washover channels; a Brazos Island. On south Padre Island, the vegeta- decrease in channel width of about 1,000 feet tion line experienced the same amount of erosion occurred in the areas immediately north of points as the foredunes because ;vegetation was restricted 24 and 25. Inaddition,coppice mounds developed to the stabilized foredune areas. Erosion of the across the washover channels and gulfward of the foredunes varied from 50 to 350 feet; the most continuous segments of vegetation. extensive dune erosion occurred along lateral margins of the washover channels. Between points A rather continuous vegetation line was 8-17 and 20-21, a vegetated zone up to 200 feet present on south Padre Island from point 21 to wide, which was buried by washover sand in1967, near point 20. Vegetation obscured two of the had recovered by 1970, and coppice mounds with three washover channels present along this segment sparse vegetationhad begun to form in front of the of the beach in 1937. One of the channels,present foredunes and within the lateral margins of the east of Park Road 100, had decreased in width by storm channels. about 200 feet. Although Celia (1970) was an intense storm, From point 20 to point 14, the vegetationin it was small in size and maximum effects were 1960 was confined to areas of stabilized dunes restricted to the vicinity of Corpus Christi. The which were cut by washover channels. These areas 1970 photographs were taken immediately after of stabilized dunes were more extensive than in Celia, and no erosional effects are visible on south 1937. Coppice mounds with sparse vegetation were Padre Island or Brazos Island. numerous within the washover channels and in front of the stabilizedforedunes. The vegetation line on Brazos Island experi- 1960, the vegetationnorth of point 14 enced net accretion between 1960 and 1970, In was within sparse and the area was dominated by actively except an area immediately north of the Rio Grande where the vegetation line receded 500 migrating dune fields. An exception to this oc- feet. In 1961, however, as a result of flooding curred one mile on either side of point 9 where associated with Carla, the Rio Grande changed vegetation was nearly continuous. This same area course by avulsion to a position about 4,500 feet was barren in1937 except for a few smallerosional south of its location in1960. remnants. 1960-1970— The difference between net The vegetation line along a segment one mile changes, and short-term changes is illustrated by on either side of station 23 was more continuous comparison of the position of vegetationlines from by 1970. Irregularities in the 1960 vegetation line 1960 to 1970. Air photos taken in June 1967 and were nearly eliminated by accretion up to 200 feet. September 1967 (appendix C) show the direct Two isolated vegetated areas located approxi- effect of Hurricane Beulah;however, the erosional mately 2,000 feet south of Brazos Santiago Pass effects of Beulah are overshadowed by the accre- accreted from 200 to 700 feet. In addition, tionary trend documented for the entire 1960-to- numerous coppice mounds developed between 1970 time period. these vegetatedareas and the south jetty. Between 1960 and 1970, two major hurri- In 1970, the vegetation line from point 21 to canes affected the southern part of the Texas point 20 was continuous. Although the vegetation Coast: Carla (1961) and Beulah (1967). Erosional line was approximately in the same position as in effects of Carla were restricted to active storm 1960, the vegetation was extended parallel to the channels on south Padre and Brazos Islands; shoreline to cover three washover channels that apparently the destructive effect of Carla on were present in 1960. vegetatedareas was only minor. A segment of the vegetation line extending Erosion of the dunes and vegetation line is 5,400 feet south from station 19 accreted from obvious on the post-Beulah photographs (Sep- 100 to 350 feet between 1960 and 1970, South 26

Padre Island was nearly barren of vegetation in 1970-1974.— During this period, the vegeta- 1960;perhaps the devegetation was associated with tion on Brazos and south Padre Islands continued predevelopment road construction. The same area to recover and advance. The area of greatest was covered by vegetationin 1970. accretion on Brazos Island was located just north of the Rio Grande. Storm channels were still For an area extending approximately 4,000 presentin 1974;however,between 1970 and1974, feet north of station 19, the vegetation line sparsely vegetated coppice mounds migrated into accreted from 0 to 200 feet seaward of the 1960 the channels. Vegetation existing on stabilized vegetation line. In addition, a storm channel located approximately 3,000 feet north of station dunes accreted 50 to 100 feet coincident with 19 was covered by vegetationin1970. development of a line of post-Beulah dunes sea- wardof the earlier foredunes (fig. 5). North from station18, the continuous vegeta- tion line ended, and vegetation was restricted to On south Padre Island, the vegetation areas of stable dunes. The dunes were cut by accreted 50 to 150 feet inareas of stabilized dunes. numerous washover channels. Coppice mounds Coppice mounds also formed seaward of the were present seaward of the foredune areas but not foredunes, but not to the extent that they did on within the channels. The shape and position of the Brazos Island.Most of the coppice mound develop- stabilized dunes changed little in the ten-yearspan ment was parallel to the beach and along the lateral between 1960 and1970. margins of storm channels. Factors AffectingShoreline and VegetationLine Changes

Geologic processes and, more specifically, sediment during the early Holocene. This,in turn, coastal processes are complex dynamic com- affected sediment budget by supplying additional ponents of large-scale systems. Coastal processes sediment to the littoral drift system. Droughts are are dependent on the intricate interaction of a a potential, though indirect, factor related to large number of variables such as wind velocity, minor shoreline changes via their adverse effect on rainfall, storm frequency and intensity, tidal range vegetation. Because dunes and beach sand are and characteristics, littoral currents, and the like. stabilized by vegetation, sparse vegetationresulting Therefore, it is difficult, if not impossible, to from droughts offers less resistance to wave attack. isolate and quantify all the specific factors causing Severe droughts have occurred periodically in shoreline changes. Changes in vegetation line are Texas; the chronological order of severe droughts more easily understood. However, in order to affecting Brazos and south Padre Islands is as evaluate the various factors and their inter- follows: 1891-1893, 1896-1899, 1916-1918, relationship, it is necessary to discuss not only 1937-1939,1950-1952,1954-1956 (Lowry,1959). major factors but also minor factors. Thebasis for future prediction comes from thisevaluation. Unfortunately, past changesin the position of vegetation line resulting from storms and droughts Climate generally cannot beindependently distinguished by sequential aerial photography. By monitoring Climatic changes during the 18,000 years hurricanes and droughts in relation to time of by since the Pleistocene have been documented available photography, however, one can correlate various methods. Ingeneral, temperature waslower the short-term effects of these factors, providing (Flint, 1957) and precipitation was greater the time lapse between photos is not too great. (Schumm, 1965) at the end of the Pleistocene than at the present; the warmer and drier conditions, which now prevail, control other factors such as Storm Frequency andIntensity vegetal cover, runoff, sediment concentration,and sediment yield. Schumm (1965) stated that The frequency of tropical cyclones is depen- "...an increase in temperature and a decrease in dent on cyclic fluctuations in temperature; precipitation will cause a decrease inannual runoff increased frequency of hurricanes occurs during and an increase in the sediment concentration. warm cycles (Dunn and Miller, 1964). Because of Sediment yield can either increase or decrease their high frequency of occurrence and associated depending on the temperature and precipitation devastating forces and catastrophicnature, tropical before the change." cyclones have received considerable attention in recent years. Accurate records of hurricanes Changes in stream and bay conditions,as well affecting the Texas Gulf Coastare incomplete prior as migration of certain plant and animal species in to 1887, when official data collection was initiated South Texas since the late 1800's, were attributed simultaneously with the establishment of the to a combination of overgrazing and more arid CorpusChristi weather station (Carr, 1967). climatic conditions (Price and Gunter, 1943). A more complete discussion of the general warming According to summaries based on records of trend is presented in Dunn and Miller (1964). the U. S. Weather Bureau (Price,1956a;Tannehill, Manley (1955) reported that postglacial air tem- 1956;Dunn and Miller,1964;Cry,1965),some 62 perature has increased 13° F in the Gulf region. tropical cyclones have either struck or affected the Furthermore, Dury (1965) estimated that many Texas Coast during this century (1900-1973). The rivers carried between 5 and 10 times greater average of 0.8-hurricane per year obtained from discharge than present-day rivers. His remarks these data is similar to the 0.67 per year average included reference to the Brazos and Mission reported by Hayes (1967) who concluded that Rivers of Texas. Observations based on geologic most of the Texas coastline experienced the maps prepared by the Bureau of Economic passage of at least one hurricane eye during this Geology (Fisher and others, 1972) confirm that century. He further concluded that everypoint on many rivers along the Texas Coastal Plain were the Texas Coast was greatly affected by approxi- larger and probably transportedgreater volumes of mately half of the storms classified as hurricanes. 28

Simpson and Lawrence (1971) conducted a Padre Island after the passage of Hurricane Carla. study of the probability of storms striking 50-mile Most tropical cyclones have potential for causing segments of the Texas Coast during any given year. some damage, but as suggested by McGowen and The 50-mile segment of the coast, which includes others (1970), certain types of hurricanes exhibit south Padre and Brazos Islands, has a 9-percent high wind velocities,others have high storm surge, probability of experiencing a tropical storm, an and still others are noted for their intense rainfall 8-percent probability of experiencing a hurricane, and aftermath flooding. and a 2-percent probability of experiencinga great hurricane. Hurricane surge is the most destructive ele- ment on the Texas Coast (Bodine, 1969). This is Comparisons of the different types of some of particularly true for south Padre Island because of the more recent hurricanes are available;the effects low elevations and lack of continuous foredunes of Hurricanes Carla (1961) and Cindy (1963) on that can dissipate most of the energy transmitted South Texas beaches were compared by Hayes by wave attack. Because of the role hurricane surge (1967). Hurricanes Carla, Beulah (1967),and Celia plays in flooding and destruction,the frequency of (1970) were compared by McGowen and others occurrence of high surge on the open coast has (1970); individual studies of Hurricanes Carla, been estimated by Bodine (1969). Included in his Beulah, Celia, and Fern were conducted by the report are calculations for Port Isabel which U. S. Army Corps of Engineers (1962b, 1968b, suggest that surge height of 8 feet can be expected 1971a, 1972). approximately four times every 100 years. Maxi- mum hurricane surge predicted was 12 feet. These Destructive forces and storm damage.—The estimates were based on the most complete records most intense storms on record to strike the south of hurricane surge elevations available for the Padre area were the storm of 1933 and Beulah; Texas Coast. Surge for specific storms was com- both storms were accompanied by surgesinexcess piled by Harris (1963). Wilson (1957) estimated of 12 feet (Price, 1956a; Suggand Pelissier,1968). deep-water hurricane wave height between 30 and Although both storms caused extensive erosion of 40 feet once every 50 years for the Brownsville foredunes and vegetation on Brazos Island and area. Maximum deep-water hurricane wave height south Padre Island, descriptions by Price (1956a) predicted for the same location was 45 feet with a indicate that more extensive erosion took place recurrence frequency of once every 100 years. during the 1933 storm. This can probably be Consequently, dissipated energy from breaking attributed to the angle and approach to the coast storm waves can be tremendous under certain when the hurricanes made landfall. The 1933 conditions. storm moved from the east approaching nearly normal to the shoreline, whereas Beulah moved in Changes in beach profile during and after from the southeast striking the coast obliquely; storms,— -Beach profiles adjust themselves to thus, the movement of the water during Beulah changing conditions in an attempt to maintain a was nearly parallel to the coast. Fresh-water profile of equilibrium; they experience their flooding also occurred as the result of heavy rains greatest short-term changes during and after associated with Beulah. Amounts varying from 10 storms. Storm surge and wave action commonly to 20 inches were recorded in South Texas. plane off preexisting topographic features and produce a featureless, uniformly seaward-sloping High velocity winds with attendant waves and beach. Eroded dunes and washover fans are com- currents of destructive force scour and transport mon products of the surge. The sand removed by large quantities of sand duringhurricane approach erosion is either (1) transportedand stored tempo- and landfall. The amount of damage suffered by rarily in an offshore bar, (2) transported in the the beach andadjoining areas dependson a number direction of littoral currents, and/or (3) washed of factors including angle of storm approach, across the barrier island through hurricane configuration of the shoreline, shape and slope of channels. Sediment transported offshore and Gulf bottom, wind velocity, forward speed of the stored in the nearshore zoneis eventuallyreturned storm, distance from the eye,stage of astronomical to the beach by bar migration under the influence tide, decrease in atmospheric pressure, and lon- of normal wave action. The processes involved in gevity of the storm. Hayes (1967) reportederosion beach recovery are discussed by Hayes (1967) and of 60 to 150 feet along the fore-island dunes on McGowen and others (1970). 29

Foredunes are the last line of defense against The second type of changeis characterizedby wave attack, and thus, afford considerable protec- stripping and complete removal of the vegetation tion against hurricane surge and washover. Dunes by erosion. This produces the featureless beach also serve as a reserve of sediment from which the previously described; oftentimes the wave-^cutcliffs beach can recover after a storm. Sand removed and eroded dunes mark the seaward extent of the from the dunes and beach, transported offshore vegetation line. Considerable time is required for and returned to the beach as previously described, the vegetation line to recover because of the slow provides the material from which coppice mounds processes involved and the removal of anynucleus and eventually the foredunes rebuild. Thus, dune around which stabilization and development of removal eliminates sediment reserve, as well as the dunes can occur. This processis wellillustratedby natural defense mechanism established for beach comparison of June 1967, September 1967, 1968, protection. 1969, 1970,and 1974 photographs between points 40 and 42and 30 and 32. Whether or not the beach returns to its prestorm position depends primarily on the Selective and incomplete removal of vegeta- amount of sand available. The beach readjusts to tion gives rise to the third type of change. normal prestorm conditions much more rapidly Frequently,long, discontinuous,linear dune ridges than does the vegetation line. Generally speaking, survive wave attack but are isolated from the the sequence of events is as follows: (1) return of post-storm vegetation line by bare sand. Recovery sand to beach and profile adjustment (accretion); under these circumstances is complicated and also (2) development of low sand mounds (coppice of long duration. The preserved dune ridge does provide a nucleus for dune development; at times, mounds) seaward of the foredunes or vegetation the bare sand is revegetatedand the vegetationline line; (3) merging of coppice mounds with fore- is returned to its prestorm position. This type of dunes; and (4) migration of vegetation line to erosion was not observed on Brazos and south prestorm position. The first stepis initiated within Padre Islands;however,ithas beendocumented on days after passage of the storm and adjustment is other segments of the Texas Coast. usually attained within several weeks or a few months. The remaining steps require months or Local and Eustatic Sea-Level Conditions possibly years and, in some instances, complete recovery is never attained. This sequence is Two factors of major importance relevant to idealized for obviously if there is a post-storm net land-sea relationships alongBrazos and southPadre deficit of sand, the beach will not recover to its Islands are (1) sea-level changes, and (2) compac- prestorm position; the same holds true for the tional subsidence, Shepard (1960b) discussed Holo- vegetation line. Occasionally the vegetation line cene rise in sea level along the Texas Coast based will recover completely, whereas the shoreline will on Cl4C14 data. Relative sea-level changes during not; these conditions essentially result inreduction historical time are deduced by monitoring mean in beach width. sea level as determined from tide observations and developing trends based on long-term measure- Apparently three basic types of shift in ments (Gutenberg, 1933, 1941; Manner, 1949, vegetation line are related to storms, and conse- 1.951, 1954; Hicks and Shofnos, 1965; Hicks, quently, the speed and degree of recovery is 1968, 1972). However, this method does not dependent on the type of damage incurred. The distinguish between sea-level rise and land-surface first and simplest change isattributed to deposition subsidence. More realistically, differentiation of of sand and ultimate burial of the vegetation. these processes or understanding their individual Although this causes an apparent landward shift in contributions,if both are operative,is an academic the vegetation line, recovery is quick (usually question;the problemis just asreal nomatter what within a year) as the vegetation grows through the the cause. A minor vertical rise in sealevel relative sand and is reestablished. An example of this can to adjacent land in low-lying coastal areas causes a be seen by comparison of aerial photographs taken considerable horizontal displacement of the shore- in June 1967, September 1967, 1968, 1969, 1970, line inalandward direction (Bruun, 1962). and 1974. The September 1967 photographs depict post-storm conditions following Hurricane Swanson and Thurlow (1973) attributed the Beulah. relative rise in sea level at Port Isabel to compac- 30 tional subsidence (fig. 12). Their conclusion was shelf sand by wave action. Sand losses are attrib- based on tide records between 1948 and 1971. uted to (1) transportation offshore into deep Interpreted rates of sea-level rise depend a great water, (2) accretion against natural littoral barriers deal on the specific time interval studied; thus, and man-made structures, (3) excavation of sand short-term records can be used to demonstrate for constructionpurposes, and (4) eolian processes. most any trends. On the other hand, long-term records provide a better indication of the overall The sources of sediment and processes trend and are useful for future prediction. Rates of referred to by Johnson have direct application to relative sea-level rise determined by previous the area of interest. Sources of sand responsible for workers range from 0.013 to 0.020 feet per year or the incipient stages of development and growth of 1.3 to 2.0 feet per century. It is readily apparent Brazos and south Padre Islands probably include that rises in sea level of this order of magnitude both sand derived from shelf sediment and the Rio may cause substantial changes in shoreline Grande. Van Andel and Poole (1960) and Shepard position. (1960a) suggested that sediments of the Texas Coast are largely of local origin. Shelf sandderived from the previously deposited sediment was apparently reworked and transported shoreward by wave action during the Holocene sea-level rise (fig. 6). The shelf off Brazosand south Padre Islands is underlain by fluvial-deltaic and interdeltaic sedi- ments comprised predominantly of mud with some interbedded sand. Therefore, reworked shelf sedi- ments in this area providedonly minor amounts of sand for beach maintenance.

Sediment supplied by major streams is trans- ported alongshore by littoral currents, Because of the orientation of the southern part of the Texas Coast, the prevailing south-southwest and east winds promote northward-flowing longshore cur- rents (fig. 10). The Rio Grande is the only major river in an updrift direction from Brazos and south Figure 12. Relative sea-level changes based on tide Padre Islands that supplies sediment directly into gage measurements for Port Isabel, Texas. the littoral zone. There are indications that sedi- ment discharge was greater during the early Holo- cene, but the time and maximum seaward extent of the Rio Grande delta duringits constructionare not known. Furthermore,it is not known precisely Sediment Budget when the destructive phase of the abandoned delta was initiated. Although there are indications that Sediment budget refers to the amount of sediment discharge was greater during the early sediment in the coastal system and the balance Holocene, most Texas streams were in the process among quantity of material introduced, tem- of filling their estuaries and were not contributing porarily stored, or removed from the system. significant quantities of sand to the littoral cur- Because beaches are nourished and maintained by rents. In addition to the natural decreases in sand-size sediment, the following discussion is sediment supply, both the construction of the limited to natural sources of sand for Brazos and jettiesat Brazos SantiagoPass (1935) and construc- southPadre Islands. tion of Falcon Dam (1955) have greatly decreased the availability of sediment from the Rio Grande Johnson (1959) discussed the major sources to south Padre Island. of sand supply and causes for sand loss along coasts. His list, modified for specific conditions Sand losses listed by Johnson (1959) do not along the Texas Coast, includes two sources of include sedimentremoved by depositionfrom tidal sand: major streams and onshore movement of deltas and hurricane washovers; these are two 31 important factors on the Texas Coast (fig. 10). was initiated in November 1933 and completed During storms, sand may be moved offshore in February 1935 (U. S. Army Corps Engineers, deeper water or into the lagoons through washover 1963). The present Mansfield jetties were com- channels;some sand is removed from the beach by pleted in 1962 (U. S. Army Corps Engineers, eolian processes. Sand removed by man-made 1962a).Projects suchas these serve to alter natural structures and for construction purposes is dis- processes. Their effects on shoreline changes are cussed in the following section on human activities. subject to debate, but it is an elementaryfact that impermeable structures interrupt littoral drift and Human Activities impoundment of sand occurs at the expenseof the beach downdrift of the structure. Thus, it appears Shoreline changes induced by man are dif- reasonable to expect that any sand trappedby the ficult to quantify because human activities south jettyis compensated for by removal of sand promote alterations and imbalances in sediment downdrift, thus increasing local erosion problems. budget. For example, construction of dams, erec- tion of seawalls, groins, and jetties, andremoval of Falcon Dam has had a profound effect on sediment for building purposes all contribute to both the discharge rate and suspended sediment changes in quantity and type of beach material load, which decreased significantly after construc- delivered to the Texas Coast. Even such minor tion of the dam in 1955 (figs. 8 and 9). Between activities as vehicular traffic andbeach scraping can 1854 and 1937, the accretionary rate of Brazos contribute to the overall changes, although they Island declined sharply and at two stations the are in no way controlling factors. Erection of trend reversed to an erosional cycle. The trend was impermeable structures and removal of sediment initiated prior to dam construction; however, the have an immediate, as well as a long-term effect, continued depletionof sediment contributes to the whereas a lag of several to many years may be continued erosion on Brazos Island. The erosional required to evaluate fully the effect of other rate increased on southPadre Island north of point changes such as river control and dam 16 between 1937 and 1960, the greatest rates construction. occurring between 1960 and 1969-70. This phenomenon may also reflect effects resulting Construction of the Brazos Santiago jetties from construction of the dam and jetties.

Evaluation of Factors

Shore erosion is not only a problem along Tropical cyclones are significant geologic United States coasts (El-Ashry, 1971) but also a agents and during these events, fine sand, which worldwide problem. Even though some local condi- characterizes most of the Texas beaches, is easily tions may aggravate the situation, major factors set into motion. Silvester (1959) suggested that affecting shoreline changes are eustatic conditions swell is a more important agent than storm waves (compactional subsidence on the Texas Coast)and in areas where longshore drift is interrupted and a deficit in sediment supply. Lohse (1958) con- sand is not replenished offshore. For the purposes cluded that shoreline retreat in the vicinity of the of this discussion,the individual effects of storms Rio Grande delta was causedprimarily by a deficit and swell is a moot question. Suffice it to say that in sediment budget. The deficit in sand supply is water in motion is the primary agent delivering related to climatic changes, human activities,and sand to or removing sand from the beach and the exhaustion of the shelf supply through super- offshore area. There is little doubt, however, that jacent deposition of finer material over the shelf storms are the primary factor related to changesin sand at a depthbelow wave scour. vegetation line. Predictions of Future Changes

The logical conclusion drawn from factual The shoreline could be stabilized at enormous information is that the position of shoreline and expense by a solid structure such as a seawall; vegetation line in this region will continue to however,any beach seaward of the structure would retreat landward as part of a long-term erosional eventually be removed unless maintained arti- trend. The combined influence of interrupted and ficially by sand nourishment (a costly and decreased sediment supply, relative sea-level rise, sometimes ineffective practice). The U. S. Army of and tropical cyclones is insurmountable except in Corps Engineers (1971b, p. 33) stated that "While seawalls may protect the upland, they do very local areas such as river mouths. There is no not hold or protect the beach which is the greatest that suggests a long-term reversal in evidence any asset of shorefront property." Moreover,construc- trends of the major causalfactors. Weather modifi- tion of a single structure can trigger a chain cation research includes seeding of hurricanes reaction that requires additional structures and (Braham and Neil, 1958; Simpson and others, maintenance (Inman and Brush,1973). 1963), but human control of intense storms is still in incipient stages of development. Furthermore, Maintenance of some beaches along the Outer elimination of tropical storms entirely could cause Banks of North Carolina has been the respon- a significant decrease in rainfall for the south- sibility of the National Park Service (Dolan and eastern United States (Simpson,1966). others, 1973). Recently the decision was made to cease maintenance because of mounting costs and Construction pits and borings on southPadre the futility of the task (New York Times,1973). Island (J. H. McGowen, personal communication, 1974;Rusnak,1960)indicate that sand is about 10 It seems evident that eventually nature will to 15 feet thick under mostof the island;the sand have its way. This should be given utmost consid- is underlain by mud. Thesand storedin the barrier eration when development plans are formulated. island, therefore, is insufficient to prevent future While beach-front property may demand the erosion without the addition of sediment to the highest prices, itmay also carry with it the greatest littoral drift system. risks. References

Bascom, W. N., 1951, The relationship between sand size Garner, L. E., 1967, Sand resources of Texas Gulf coast: and beach-face slope: Am. GeophysicalUnionTrans., Univ. Texas, Austin, Bur. Econ. Geology Rept. Inv. v. 32, no. 6, p. 866-874. 60, p. 85. Bodine, B. R., 1969, Hurricane surge frequency estimated Gould, H. R., and McFarlan, E., Jr., 1959, Geologic history for the Gulf Coast of Texas: U. S. Army Corps of of the chenier plain, southwestern Louisiana: Gulf Engineers, Coastal Eng. Research Center Tech. Mem. Coast Assoc. Geol. Socs. Trans., v. 9, p. 261-270. 26, 32p. Gutenberg, 8., 1933, Tilting due to glacial melting: Jour. Braham, R. R., Jr., and Neil, E. A., 1958, Modificationof Geology, v. 41, no. 5, p. 449-467. hurricanes through cloud seeding: Natl. Hurricane 1941, Changes in sea level, postglacial uplift, and Research Proj.Rept. 16, 12p. mobility of the earth's interior: Geol. Soc. America P., rise as a Bruun, 1962, Sea-level cause of shore erosion: Bull., v. 52, p. 721-772. Civil Engineers Proc, Jour. Waterways and Am. Soc. Hansen, E. A., 1960, Studies of a channel through Padre HarborsDiv., WWI, v. 88, p. 117-130. Island, Texas: Am. Civil Engineers Proc, Bryant, E. A., and McCann, S. 8., 1973, Long and short Soc. Jour. term changes in the barrierislands of Kouchibouguac Waterwaysand Harbors Div.,p. 63-82. Bay, southern Gulf of St. Lawrence:Can. Jour.Earth Harris, D. L., 1963, Characteristics of the hurricane storm Sci., v. 10, no. 10, p. 1582-1590. surge: U. S. Weather Tech. Paper48, 139 p. Bullard, F. M., 1942, Source of beach and river sands on Harris, W. D., and Jones, B. G., 1964, Repeatmapping for a Gulf Coast of Texas: Geol. Soc. AmericaBull., v. 53, record of shore erosion: Shore and Beach, v. 32, no. p. 1021-1044. 2, p. 31-34. Carr, J. T., 1967, Hurricanes affecting the Texas Gulf Hayes, M. 0., 1964, Grain size modesin Padre Island sands, Rept 49, Coast: Texas Water Development Board in Depositional Environments, South-Central Texas p. 58 Coast: Gulf Coast Assoc. Geol. Socs., Field Trip Cry, G. W., 1965, Tropical cyclones of the north Atlantic Guidebook, 30-31, 1964, p. Ocean: U. S. WeatherBur. Tech. Paper 55, 148 p. Oct. 121-128. Curray, J. R., 1960, Sediments and history of Holocene 1965, Sedimentation on a semiarid wave- transgression, continental shelf, northwest Gulf of dominated coast (South Texas) with emphasis on Mexico, in Shepard, F. P., Phleger, F. 8., and van hurricane effects: Univ. Texas, Austin,Ph. D. dissert., Andel, T. H., eds., Recent sediments,northwest Gulf 350 p. of Mexico: Tulsa, Okla., Am. Assoc. Petroleum 1967, Hurricanes as geological agents: Case studies Geologists, p. 221-266. of hurricanes Carla, 1961, and Cindy, 1963: Univ. Dolan, R., Godfrey, P. J., and Odum, W. E., 1973, Man's Texas, Austin, Bur. Econ. Geology Rept. Inv. 61, impact on the barrier islands of North Carolina: 54 p. Amer. Scientist, v. 61, no. 2, p. 152-162. Herbich, J. 8., 1970, Comparisonofmodeland beachscour Dunn, G. E., and Miller, B. 1., 1964, AtlanticHurricanes: patterns: Proc. 12th Conf. on Coastal Eng., LouisianaStateUniv. Press, 377 p. p. 1281-1300. Dury, G. H., 1965, Theoretical implications of underfit Hicks, S. D., 1968, Long-period variations in secular sea streams: U. S. Geol. Survey Prof. Paper 452-C, 43 p. level trends: Shore and Beach, v. 36, no. 1, p. 32-36. El-Ashry, M. T., 1971, Causes of recent increased erosion 1972, On the classification and trends of long along United States shorelines: Geol. Soc. America period sea level series: Shore and Beach, v. 40, no.1, Bull.,v. 82, p. 2033-2038. p. 20-23. ,and Wanless, H. R., 1968, Photo interpretationof ,and Shofnos, W., 1965, Yearly sea levelvariations shoreline changes between Capes Hatteras and Fear for the United States: Am Soc. Civil EngineersProc, (North Carolina):Marine Geol., v. 6, p. 347-379. Jour. Hydraulics Div., v. 91, paper 4468, no. HY 5, Emery, K. 0., 1961, A simple method of measuring beach p. 23-32. profiles: Limnology and Oceanography, v. 6, Inman, D. L., and Brush, B. M., 1973, The coastal p. 90-93. challenge: Science, v. 181, no. 4094, p. 20-32. Emory, W. H., 1857, Report of the United States and International Boundary and Water Commission, United Mexican boundary survey: House Ex.Doc. 135, 34th States and Mexico, 1931-1944, Flow of the Rio Cong., Istsess., serial 861. Grande and tributary contributions (Roma Station): Fisher, W. L., McGowen, J. H., Brown, L. F., Jr., and Water Bull.No. 1-14. Groat, C. G., 1972, EnvironmentalGeologic Atlasof 1945-1946, Flow of theRio Grande and tributary the Texas Coastal Zone— Galveston-Houston area: contributions (Roma Station): Water Bull. No.15-16. Univ. Texas,Austin, Bur. Econ. Geology, 91p. 1952-1970, Flow of the Rio Grande and related , Brown, L. F., Jr., McGowen, J. H., and Groat, data (Roma and San Benito Stations): Water Bull. C. G., 1973, Environmental Geologic Atlas of the No. 22-40. Texas Coastal Zone— Beaumont-Port Arthur area: Johnson, J. W., 1959, The supply and loss of sand to the Univ. Texas,Austin, Bur. Econ. Geology, 93 p. coast: Am. Soc. Civil Engineers Proc, Jour. Water- Frazier, D. E., 1974, Depositional-episodes:their relation- ways and HarborsDiv., WW3, v. 85, p 227-251. ship to the Quaternary stratigraphic frameworkin the 1971, The significance of seasonal beach changes northwestern portion of the Gulf basin:Univ. Texas, in tidal boundaries: Shore and Beach, v. 39, no. 1, Austin, Bur. Econ. Geology Geol. Circ. 74-1, 28 p. p. 26-31. 34

Lohse, E. A., 1952, Shallow marine sediments of the Rio Recent sediments, northwest Gulf of Mexico: Tulsa, Grande delta: Univ. Texas (Austin), Ph. D. dissert., Okla, Am. Assoc. Petroleum Geologists, p. 197-220. 113 p. 1960b, Rise of sea level along northwest Gulf of 1958, Mouth of Rio Grande: Gulf Coast Assoc. Mexico, in Shepard, F. P., Phleger, F. 8., and van GeoL Socs., Field Trip Guidebook, Oct. 30-Nov. 1, Andel, T. H., eds., Recentsediments, northwest Gulf 1958, p. 55-56. of Mexico: Tulsa, Okla., Am. Assoc. Petroleum Lowry, R. L., Jr., 1959, A study of droughts in Texas: Geologists, p. 338-344. Texas BoardWater Engineers Bull. 5914, 76 p. Silvester, R., 1959, Engineering aspects of coastal sediment Manley, G., 1955, A climatological survey of the retreatof movement: Am. Soc. Civil Engineers Proa, Jour. the Laurentide ice sheet: Am. Jour. Sci., v. 253, no. Waterways and Harbors Div., WW3, v. 85, p. 11-39. 5, p. 256-273. Simpson, J., 1966, Hurricane modification experiments: Manner, H. A., 1949, Sea level changes along the coastsof Hurricane symposium, Am. Soc. for Oceanography the United States in recent years: Am. Geophysical Pub. No. 1, p. 255-292. Union Trans., v. 30, no. 2, p. 201-204. Simpson, R. H., Ahrens, M.R., and Decker, R. D.,1963, A 1951, Changes in sea level determinedfrom tide cloud seeding experiment in Hurricane Esther,1961: observations:Proc. 2d Conf. on Coastal Engineering, Natl.HurricaneResearch Proj.Rept. 60, 30p. p. 62-67. , and Lawrence, M. 8., 1971, Atlantic hurricane 1954, Tides and sea levelin theGulf ofMexico,in frequencies along the U. S. coastline: Natl. Oceanic P. S. Galtsoff, coord., Gulf of Mexico, Its origin, and Atmos. Admin. Tech. MemoNWS SR-58, 14 p. waters, and marine life: U. S. Dept Interior, Fish and Stafford, D. 8., 1971, Anaerial photographic technique for WildlifeServ. Fish. Bull.,v. 55, p. 101-118. beach erosion surveys in North Carolina: U. S. Army McGowen, J. H., and Brewton, J. L., 1975, Historical Corps Engineers, Coastal Eng. Research Center Tech. changes and related coastal processes, Gulf and Memo. 36, 115 p. mainland shorelines, Matagorda Bay area, Texas: _, Bruno, R. 0., and Goldstein, H. M., 1973, An Univ. Texas, Austin, Bur. Econ. Geology Rept. Inv. annotated bibliography of aerial remote sensing in _, Groat, C. G., Brown,L. F., Jr., Fisher, W. L., and coastal engineering: U. S. Army Corps Engineers Scott, A. J., 1970, Effects ofHurricane Celia— a focus Coastal Eng. Research Center Misc. Paper No. 2-73, on environmental geologic problems of the Texas 122 p. Coastal Zone: Univ. Texas, Austin, Bur. Econ. Stapor, F., 1973, History and sand budgets of the barrier Geology Geol.Circ. 70-3, 35p. island system in the Panama City, Florida, region: Meixner,R. H., 1948, History ofPadre Island:Georgetown, MarineGeology, v. 14, p. 277-286. Texas,Southwestern University, Master'sthesis. Sugg, A. L., and Pelissier, J. M., 1968, The Atlantic Morgan, J. P., and Larimore, P. 8., 1957, Changes in the hurricane season of 1967: Monthly Weather Review, Louisiana shoreline: Gulf Coast Assoc. Geol. Socs. v. 96, no. 4, April, p. 242-247. Trans., v. 7, p. 303-310. _, Pardue, L. G., and Carrodus, R. L., 1971, Nelson, H. F., and Bray, E. E., 1970, Stratigraphy and Memorable hurricanes of the United States since history of the Holocene sedimentsin the Sabine-High 1873: Natl. Oceanic and Atmos. Admin. Tech. Island area, Gulf of Mexico, in Morgan, J. P., and Memo. NWS SR-56, 52p. Shaver, H., eds., sedimentation, modern R. Deltaic Swanson, R. L., and Thurlow, C. L, 1973, Recent subsi- and ancient: Soc. Econ. Paleontologists and Mineral- dence rates along the Texas and Louisiana coasts as ogists Spec. Pub. 15, p. 48-77. from Geophys. New York Times, Sept. 25, 1973, p. 13,col. 1. determined tide measurements: Jour. Price, W. A.,1956a, Hurricanes affecting the coast ofTexas Research, v. 78, no. 15,p. 2665-2671. from Galveston to the Rio Grande: Dept. of the Tannehill,I. R., 1956, Hurricanes, their nature and history: Army, Corps of Engineers, Beach Erosion Board Princeton, N. J., PrincetonUniv.Press, 308p. Tech. Memo.no.78, 17 p. TexasBoard of Water Engineers, 1961, Silt loadof Texas , and Gunter, G., 1943, Certain recent geological streams, a compilation report,June 1889-Sept. 1959 and biological changes in south Texas, withconsidera- (Roma and Brownsville Stations): Texas Bd. Water tion of probable causes: Texas Acad. Sci. Proc. and Engineers 8u11.6108, p. 221-228. Trans., v. 26, p. 138-156. Texas Water Development Board, 1970, Suspended- Rusnak, G., 1960, Sediments of Laguna Madre, Texas, in sediment load of Texasstreams, a compilationreport, Am. Petroleum Institute Project 51, Am. Assoc. Oct. 1963-Sept. 1965: Texas Water DeveL Board Petroleum Geologists, p. 152-196. Rept 106, p. 54-55. Schumm, S. A., 1965, Quaternary paleohydrology, in U. S. Army Corps of Engineers, 1850, Message from the Wright, H. E., Jr., and Frey, D. G., eds., The President of the U. S. communicating the report of Quaternary of the United States: Princeton, N. J., Lt. Webster of a survey of the Gulf Coast at the PrincetonUniv. Press, p. 783-794. mouth of the Rio Grande: S. Ex. Doc. 65, 31st Seelig, W. N., and Sorensen, R. M., 1973, Historic shoreline Cong., Ist sess., 12 p. changes in Texas: Texas A&M Univ. Sea Grant, 1881, Annual Report, Chief of Engineers, Im- TAMU-SG-73-206, COE-165, 19 p. provement of Rivers and Harbors: HouseEx. Doc. 1, Shalowitz, A. L., 1964, Shore and sea boundaries: U. S. 46th Cong., 3d sess.,pt 1, p. 176,pt. 2, p. 1272. Dept CommercePubl. 10-1, v. 2, 749 p. 1888, Annual Report, Chief of Engineers, Im- Shepard, F. P., 1960a, Gulf Coast barriers, in Shepard, provement of rivers and harbors: House Ex. Doc. 1, F.P., Phleger, F. 8., and van Andel, T. H., eds., 50th Cong., 2d sess., pt. 1, p. 176,pt. 2, p. 1320. 35

1895, Annual Report, Chief of Engineers, Im- ._ 1968a, Laws of the UnitedStatesrelating to rivers provement of certain rivers and harbors in Texas: and harbors: House Doc. 182, 90th Cong., Istsess. House Doc. 2, 54th Cong., Ist sess., pt. 1, p. 265,pt. 1968b, Report on Hurricane Beulah, 8-21 Sept. 3, p. 1819. 1967: GalvestonDist. Corps Engineers, 26 p. 1900, Annual Report, Chief of Engineers, Im- 1968-1972, Texas CoastalInletStudies:Galveston provement of harbor at Brazos Santiago, Texas: Dist. Corps Engineers. House Doc. 2, 56th Cong., 2d sess, pt. 1, p. 393, pt. 1971a, Report on Hurricane Celia, 30 July-5 4, p. 2340. August 1970: GalvestonDist. Corps Engineers, 13 p. 1905, Annual Report, Chief of Engineers, Im- 1971b, Shore protection guidelines: Washington, provement of harbor at Brazos Santiago, Texas: D. C, Dept. of the Army, Corps of Engineers, 59 p. House Doc. 2, 58th Cong., 3d sess., pt. 1, p. 380, pt. 1971c, National shoreline study, Texas coast 2, p. 2010. shores regional inventory report: Galveston Dist. 1913, Improvement of harborat Brazos Santiago: Corps Engineers, 26 p. House Doc. 200, 63dCong., Ist sess., 25 p. 1972, Report on Hurricane Fern, 7-13 Sept. 1971: 1913-1914, Index to reports ofChief of Engineers, GalvestonDist. Corps Engineers, lip. 1866-1912, Vol. I,Rivers and Harbors: House Doc. U. S. Department of Commerce, 1930-1974, Tide tables 740, 63d Cong., 2d sess., p. 762. 1930-1974, East Coast of North & South America: 1916, Arroyo Colorado, Texas: HouseDoc. 1731, Natl. Oceanic & Atmos. Admin. 64th Cong., 2d sess., 6 p. U. S. Department of Interior,NationalPark Service-Region 1919, Brazos Island harbor, Texas: House Doc. Three, 1959, FieldInvestigationReport, PadreIsland, 1710, 65th Cong., 3dsess., 30p. Texas: U. S. Dept. Interior, Natl. Park Service, Field 1928, Annual Report, Chief of Engineers, Im- InvestigationReport, 23 p. provement ofriversand harbors, p. 1028-1031. U. S. Geological Survey, 1927, Surface water supply of 1936, Annual Report, Chief of Engineers, Im- U. S., 1923, pt. VIII (Rio Grande— Roma Station), provementof rivers and harbors,p. 783-785. Western Gulf of Mexico Basin: U. S. Geol. Survey 1939, Brazos Island harbor, Texas: House Doc. Water Supply Paper 568, p. 112-113. 335, 76th Cong., Ist sess., 14 p. 1928, Surface water supply of U. S., 1924 (Rio 1940, Laws of the United States relating to Grande— Roma Station), pt. VIII, Western Gulf of improvement of rivers and harbors from August 11, Mexico Basin: U. S. Geol. Survey Water Supply Paper 1790 to June 29, 1938: House Doc. 379, 76th Cong., 588, p. 189-191. Ist sess., v. 1-3. 1929, Surface water supply of U. S., 1925 (Rio 1941, Annual Report, Chief of Engineers, Im- Grande— Roma Station), pt. VIII, Western Gulf of provementof rivers and harbors, p. 1006-1008. Mexico Basin: U. S. Geol. Survey Water Supply Paper 1950, Annual Report, Chief of Engineers, Im- __608, p. 224-225. provementofrivers and harbors, p. 1177-1180. 1930, Surface water supply of U. S., 1926 (Rio 1958, Review of reports on Gulf Intracoastal Grande— Brownsville Station), pt. VIII, Western Gulf of Mexico Basin: U. S. Geol. Survey Water Supply Waterway, tributary channel to Port Mansfield, Paper628, p. 181. Texas: District, May 29, 53 Galveston p. 1932a, Surface water supply of U. S., 1929 (Rio 1960, Annual Report, Chief of Engineers, Im- Grande— Roma Station), pt. VIII, Western Gulf of provementof rivers and harbors, p. 704-707. Mexico Basin: U. S. Geol. Survey Water Supply Paper 1961, Design memorandumon erosion control at 688, p. 104. inner ends of jetties at Brazos-Santiago Pass, Brazos 1932b, Surface water supply of U. S., 1930 (Rio Island harbor, Texas: Galveston District, April 12, Grande— Roma Station), pt. VIII, Western Gulf of 1961. Mexico Basin: U. S. Geol. Survey Water SupplyPaper 1962a, Annual Report, Chief of Engineers, Im- 703, p. 111. provementof rivers andharbors, p. 782-788. Van Andel, T. H., and Poole, D. M., 1960, Sources of 1962b, Report on Hurricane Carla, 9-12, Sept. Recent sediments in the northern Gulf of Mexico: 1961: GalvestonDistrict, 29p. Jour. Sed. Petrology, v. 30, no. 1, p. 91-122. 1963, Brazos Island harbor, Texas, rehabilitation Wiegel, R. L., 1964, Oceanographical Engineering: of north and south jetties:Design Memo. no.1, lip. Englewood Cliffs, N. J., Prentice-HallInc., 532 p. 1965, Widen Brownsville channel enlargementof Wilson, B. W., 1957, Hurricane wavestatistics for the Gulf fishing boat harbor, rehabilitation and extension of of Mexico: Dept. of the Army> Corps of Engineers, north jetty: DesignMemo. no. 2, 9 p. BeachErosionBoardTech. Memo.98, 61p. 36 Island Net Rate -12.0 9.4- 7.8- 3.1- 3.8- 3.8- 5.7- 9.7- -10.5 -10.0 -11.1 9.7- 8.9- -10.0 8.4- Brazos- Net Dist. 275- 750- 625- 250- 300- 300- 500- 925- -1000 950- -1050 925- 850- 950- 800-

Padre Net Time 1879-80 1960 It II I! II1879-80 1974 11 II south yr segment Rate per 5.0- 5.0- -15.0 ft -20.0 -10.0 -30.0 -10.0 beach Dist. ft 25- -100 50- -150 25- 75- 50-

Time 1969 1974 ii yr Rate per 2.8- -27.8 -25.0 -36.1 ft -25.0 -16.7 -11.1 -16.7

25 A: Changes.Changes Dist. ft - -225 -150 -250 -100 -225 -325 -150

AppendixChanges it DeterminationofShoreline DeterminationShorelvneofShoreline Time 1960 1969 yr per 9.8 4.3 6.5 5.4 2.2 Rate -16.3 -15.2 - - - - -10.9 -19.6 + ft -12.0 -13.0 -14.1 -18.5 -10.9

ft 50 Dist. -275 -375 -350 -225 -100 -150 -125 -250 -300 -325 -450 -425 + -250

Time 1937 1960 I! I! ii ii it ii ii ii ii yr Rate per 6.6- 4.2- 0.4- 3.5- 2.6- 6.6- 8.3- 6.6- 5.3- 4.4- 9.6- 6.1- ft -11.0 -11.4

Dist. ft 0 375- 275- 25 200- 150- 375- 625- 475- 375- 300- 250- 650- 550- 350-

Time 1879-80 1937 ii li v ll li it n II I! v - 1 10 11 12 13 14 15 accretion+ erosion Point 37

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Intensity- minimal minorminimalminimalminimal major minimal minor minor extreme minor minor minimal major minorminimal minor minimalminimalminimalminimal minor extrememinimal minor major minor major minor minimal minor Island coast Bay Island Padre coast Christi Pass southernChristi GrandePassChristi Pass coast coast coast coast coast Arthurcoast Brownsvilleof Padre Island Rio Island Island in. in. UpperUpperMatagorda UpperUpperMatagorda Upper Freeport Corpus Corpus High Sargent Corpus High High in. 00 less Area Galveston CentralMiddle PortLowerGalveston South Beaumont SabineExtreme Galveston SouthPalacios Mouth Aransas Aransas 40 29.40 29. or Pressures 29. to toin. Minimum 01 1940 1940 1941 19421942 1943 1943 1945 1945194619471947 1949 19551957 19571958 1958 19591960 1963 1967196819701970 1973 Central above29.03 28. 28.00 Year 1941 1954 1961 1964 1971 1854-1973 Miller Coast and Intensity major major Dunn extrememinor minimal minor major minimal minorminimalminimal minor extremeextrememinimal extrememinimal minor minor minorminimal minor minor minor minimalminimal minor minimal minor minor B: Texas Winds 74 higher the from 100 135 than to and Appendix to 1854-1973Coast,CyclonesTexasTropicaltheAffecting Affecting Maximum Less 74 101136 Island Bay Cyclones Classification coast coastChristi coast coast coast coast coast coast coast coastPassChristi coast coastPadre coast coast coast coast coast coast Intensity Upper Upper Corpus Upper Corpus O'Connor Freeport Matagorda Rockport Aransas Upper Tropical Area Brownsville Lower VelascoLowerLowerLowerLowerLower Lower Sabine EntireLower SouthLower PortLower Lower BrownsvilleBrownsville Entire PortLower Year 1900 19011.9021908190919091909 191019101912 1913 1915 1916 19181919 19211921 192219251929 19311932 1933 1933 1933193319341934 193619361938 MinorMinimal MajorExtreme Intensity major major major major minimal minimal minor minimalminimalminimal minor extreme ? minimal minor ? minimalminimal minor extrememinimalminimalminimalminimal minor minimal minor minor minimal minor southward southward Isabel Christi Isabel coast Island coast coast coast coast coast coast coast coast coast coast coast coast coast coast coast coast Area Galveston Port GalvestonGalveston CorpusGalvestonGalveston PortIndianola LowerIndianola PadreEntire UpperLower SargentBrownsville LowerEntire Upper EntireLower UpperBrownsville UpperUpperEntireLowerLower UpperUpper Year 1854 1857 186618671868 187118711872 1874 18741875 18761877 1879188018801880 1881 18851886 18861886188618871888 188818911895189518971898 39

AppendixC:MaterialsListofandSources

List ofMaterialsand Sources

List of aerial photographs used in determination of changes in vegetation line and shoreline. *Indicates vegetation line and/or shore- line wasused inmap preparation.

Date Source of Photographs * April 1937 TobinResearchInc. Nov.,Dec. 1954 U. S. Dept Agriculture Nov. 1955 * U. S. Dept. Agriculture Jan.,Feb. 1960 TobinResearchInc. Oct. 1961 U. S. Army Corps Engineers June 1967 U. S. Army Corps Engineers Sept. 25, 1967 Texas Highway Dept. Sept. 28, 1967 (mouthof Rio Grande) Intl. Boundary and Water Commission July 1968 Texas Highway Dept. Nov. 1969 Natl.Oceanicand Atmospheric Admin. Oct. 1970 * Natl.Oceanicand Atmospheric Admin. June 1974 Texas Highway Dept.

List of MapsUsed inDeterminationofShorelineChanges

Date Description Source of Maps

Nov. 1854 Topographic map-453 Natl.Oceanic and Atmospheric Admin. 1867 Topographic map-1045 NatL Oceanic and Atmospheric Admin. July 1879-80 Topographic map-1476a, 1476b Natl.Oceanicand Atmospheric Admin. 1917 Topographic map-3673 Natl.Oceanicand Atmospheric Admin. 1934 15-minute quadrangle U. S. Geological Survey Oct. 1958 Changes inlocationofmouth Intl.Boundary andWater ofRioGrande 1853-1966 Commission

List of 7.5-minute quadrangle topographic maps used in construction ofbasemap. Source of these maps is the U. S. GeologicalSurvey.

Mouthof Rio Grande, Texas NorthofPortIsabelSW, Texas Port Isabel, Texas Northof PortIsabelNW, Texas Port IsabelNW, Texas Southof PortreroLopenoSE, Texas