65. USA--LOUISIANA K. Meyer-Arendt D.W. Davis Department of Geography Department of Earth Science Mississippi State University Nicholls State University Starkville, Mississippi 38759 Thibodaux, Louisiana 70301 United States of America United States of America INTRODUCTION Louisiana's 40,000 Inn 2 coastal zone developed over the last 7,000 years by the progradation, aggradation, and accretion of sediments introduced via various courses of the Mississippi River (Frazier 1967). The deltaic plain (32,000 km'), through which the modern river cuts diagon­ ally !Fig , 1), consists of vast wetlands and waterbodies. With eleva­ tions ranging from sea level up to 1.5 m, it is interrupted by natural levee ridges which decrease distally until they disappear beneath the marsh surface. The downdrift chenier plain of southwest Louisiana (8,000 km') consists of marshes, large round-to-oblong lakes, and stranded, oak covered beach ridges known as cheniers (Howe et al. 1935). This landscape is the result of alternating long-term phases of shoreline accretion and erosion that were dependent upon the proximit of an active sediment-laden river, and a low-energy marine environment (Byrne et al. 1959). Since the dyking of the Mississippi River, fluvial sedimentation in the deltaic plain has effectively been halted. Today, most Missis­ sippi River sediment is deposited on the outer continental shelf; only at the mouth of the Atchafalaya River distributary is deltaic sedimen­ tation subaerially significant (Adams and Baumann 1980). Over mos of the coastal zone, subsidence, saltwater intrusion, wave erosion, canalization, and other hydrologic modification have led to a rapid increase in the surface area of water (Davis 1986, Walker e al. 1987). Land loss in coastal Louisiana has been estimated to exceed 100 km 2/yr (Gagliano et al. 1981), Some researchers have attributed up to 40X of the wetland loss to m.an 's activities (Craig et al, 1979). The 500 km long Gulf of Mexico shoreline, which is characterized by barrier islands and beaches in the deltaic plain and by barrier beaches and mud.flats in the chenier plain, is generally transgressive with average shoreline erosion rates in excess of 10 m/yr (Adams et al. 1978, van Bee.k and Meyer-Arendt 1982). The barrier i slands, which function as the first natural line of defense against incoming tropical storms, have lost almost half of their surface area within the last 100 years because of wave erosion and subsidence (Penland and Boyd 1982). 629 H I . Walker (ed.). Arlijiclal Structures and Shorelifll!s, 629-640. e 1988 #Y K/1,wer Acadt!J11ic Publishus. 630 Shorelines in more inland estuarine settings are experienc ng bigh erosion re.tes because of fetch an.d Kave action, and t.be un.consolidated nature of the sbore sediments (Adams et al 1978). Buman settlement in coastal Louisiana was historically confined to beach ridges and natural levees (Davis 1983). However, with e~pansion of urban/industrial land (especially around New Orleans), extension of farmlands, development of the oil and as industry and itB associated demand for improved navigation channels, recreational demands along waterfronts, and continued construction of hurricane-protection levees, man bas extended his activities deep into the wetlands and along the shoreline (Davi and Detro 1980). Three classes of structural modifi­ cation are identifiable: (1) those associated with the harnessing of the Mississippi River, (2) those within the es uarine wetlands, and 3) those associated with the shoreline (Fig, 1), THE MISSISSIPPI RIVER The Mississippi River, confined within a conduit of artificial levees, can no longer inundate its historic floodplain. River leveeing dates from the initial European settling of ew Orleans in 1718; by the 1850s, intermittent dykes flanked most of the lower Mississippi River (Elliott 1932), However, the present comprehensive manipulation of rivet' flow (within levees avere.iing 7.6 m above ground level) is a result of construction made after the 1927 flood that inundated 65,000 kml (USACE n.d,), Extensive revetments ha.ve been laid to reduce ba.nk collapse and stabilize levees, Formerly constructed of willow mats, revetments a.re now made of interlocking concrete mats and ripra.p rubble . Complementing the levees are floodways that can re-direct flood water and reduce river flood stages, Louisiana's floodways (Old River/Atcbafalaya Morganza, Bonnet Carre, and Bohemia), ar-e designed to protect New Orleans, much of which is below sea level. Flow through the Atchafalaya Spillway is ma.inta.ined at aoi of Mississippi River discharge at the Old River Control Structure (Kazma.nn and Johnson 1980). The Bonnet Carre Spillway, 15 km upriver from New Orleans, can divert 7 million liters/second into Lake Pontchartrain, an embayment of the Gulf of Mexico. The structure's 350 moveable gates, completed in 1931 (USACE n,d,) 1 have been opened only 7 times~-the last in ay 1983). In addition to spillways, the United States Army Corps of En­ gineers constructed a freshwater siphon at Violet. lt is designed to stop the advance of saltKater into fresh or brackish areas and preserve the ~arsbes southeast of New Orleans. The project's uccess has led the Corps to investigate the feasibility of diverting water and sediments into the Barataria estuary. Because the Mississippi's east bank south of Bohemia is unleveed, it functions as an additional overflo" spillway. Within the "birdsfoot" portion of the delta, 2 of the 3 major deltaic distributaries are presently jettied. The earliest jetties were constructed in the 1860s, but it "as not until the 1870s that the 631 \•,. ,,,, ',\,, Fountuinhleu ,, Wle Par!. I,,' Lt1I...P l'omd w r1 ,·w,1 0 E-;;, • 1,: - - f rI (w ll ,if ,,t11, ,1 I 1 c-.. .._... n--r. CAlnff!Clda ,..., _ c;. ....... 1 c:,..,,...,..,-.,...,c.,p.,_~ _.. I f,...s;.Wend Fo,.,,worilkill sia,w l"orlr. J f'*1 fiftro;jll...,,.,....., 1.0 ~c.-- ) aGE,.,.O --- ( OO!o!GlnJOte~ (otflOnlL"'4!1k,,oio) -~~1'ICl'V! ~oJo,o lti.,..- ~wt'lbfld dalto - .~.., ""..... bt.lNit.oa Dftd t ~ -~~ 20 .0 ~ 801offl --~frt,ru.dui., HnWil~ Fig. 1. The Louisiana coastal zone and selected locations illustrating a variety of artificial structures. 632 prototype of the modern jetties (the Rads jetties at South Pass) was built (Elliott 1932). Southwest Pass, the major navigation channel today, has been modified by the construction of jetties, bulkheads, and channel-training spur dykes since 1900 (Fig. 1, inset B). WETLANDS DRAINAGE AND RECLAMATION When New Orleans was first surveyed in 1792, each residential block was encircled with canals, and the city's dependence upon a drainage network was established, By 1743, an ordinance required property owners to complete their levees or forfeit their lands (Samuel 1959 ) , As the city expanded into adjacent wetlands, it became even more dependent on levees and artificial drainage systems. One result of these modifications bas been subsidence; portions of the city are now at least 6 m below sea level (Davis and Detro 1980). Presently, New Orleans operates a total of 140 km of canals and 92 k~ of large pipelines. These conduits, along with 21 pumping stations, can remove more than 85 million liters of water/minute from the city streets through at least 50,000 curbside catch basins and 2,024 km of subsur­ face drainlines (Wagner and Durabb 1976). Drainage and reclamation endeavors are not restricted to New Orleans. Since the colonial period, privately funded levees have been built to expand cropland. This reclamation effort was especially intensified during the period between 1880 and 1930, when Louisiana's "alluvial empire" was perceived as a great agricultural frontier. Over 50 projects, which ranged in sizes from 260 to over 14,000 hat were started. Between 2% and 5% of each project's area was allocated to ditches and canals designed to transfer water from a surface that had subsided to 3 m below sea level to surrounding surface levels of one m above sea level (Okey 1914). All but 4 of these projects failed; 2 of them are now residential and industrial centers near New Orleans. Many of these projects failed because of levee breaks and subsequent inundation. Reminders of these ill-fated ventures appear as rectan­ gular waterbodies on maps and aerial images. Impoundment of wetlands for purposes of marsh management (mainly for the maintenance of desired salinities) has been popular since the 1930s, particularly in the chenier plain marshe/i (Gosselink et al. 1979). Present-day reclamation activities are primarily related to the construction of elaborate hurricane-protection levees around several of the more seaward dis­ tributary ridges such as Bayous Lafourche and Terrebonne (USACE 1982a). Drainage of the wetlands has left an extensive canal network that, when combined with other canal types such as those made for trapping, logging, transportation, and hydrocarbon-extraction (Fig. 2), form the most visible structural modification within the coastal zone (Davis 1973). The canalization process has resulted in the loss of at l east 520 kma of surface area. For example, in one oil and gas field there are 68 km of petroleum-related canals, representing the removal of at least 3,2 million m3 of soil (Davis and Place 1983). Canal construc­ tion which is usually accompanied by the breation of spoilbanks on 633 ad.Jae nt 11etlands, mod i fies the bydrologic regime, accel erates sal t­ ater intrusion, and i mpedes over land flow. THE SBORELIN6 tructural modification of the Louisiana shoreline has been conducted ~r Z basic purposes: 1) to maintain entrances to navigation channels, ind 2) to control er osion for protection of development sites, The t cooon structures utilized for the first purpose are jetti es, though offshore break If a ters can be used in conjunction with them.