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Shore Perpendicular Structures Shore Parallel Structures
Residential and industrial development of Indiana's coastline has required protection from damaging storms. Two types of structures are generally constructed to provide this protection: structures situated perpendicular to the shoreline, such as jetties, groins, or large breakwater structures; or structures situated parallel to the shoreline, such as seawalls, rock revetments, and bulkheads. As sand moves along the shoreline from both easterly and westerly directions toward Gary, large structures can restrict or even block the movement of sand. Shore Perpendicular Structures If a structure perpendicular to the shoreline extends far enough out into Lake Michigan, reaching beyond the lakeward boundary of the breaking wave zone (18 to 20 feet deep), the structure may block virtually all of the sand from passing that point. The sand becomes trapped on the updrift side of the structure and is no longer available to replace sand eroding on the downdrift side. Erosion of downdrift beaches and dune-bluffs supplies sand to the littoral drift to make up for the sand trapped on the updrift side and to replace sand that is lost to deeper water offshore during intense storm events.14 Examples of shore perpendicular structures along Indiana's coastline include: (1) Michigan City Harbor jetties; (2) Port of Indiana; (3) breakwalls at the U.S. Steel-Gary Harbor complex; and (4) the Indiana Harbor and Ship Canal complex.15 Smaller structures which do not extend out beyond the lakeward boundary of the breaking wave zone might only block and retain 25% to 75% of the sand moving along the coast. In this case, sand leaks around the lakeward end of the structure. Some extremely small structures may affect less than 25% of the littoral drift. On the updrift side of these smaller shore perpendicular structures, erosion may stop as sand accumulates and forms a widening beach in response to sand being trapped. If sand accumulation continues over a long period of time, wind transport of dry sand to the back beach area can begin to create new sand dunes. This blowing sand is usually trapped and stabilized by native dune grasses which contribute to dune height growth. This process occurs at three locations along Indiana's shoreline: east of Michigan City, east of the Port of Indiana in Portage, and east of the U.S. Steel lakefill breakwater in Gary. In response to sand accumulating against the east side of the U.S. Steel breakwall due to net westward sand transport, new vegetated dunes have grown 117 feet lakeward and beach widths have grown 170 feet lakeward between 1967 and 1979 in this accretional area. Shore Parallel Structures Shore protection structures, oriented parallel to the shore, tend to increase erosion rates on adjacent property by creating a non-eroding coast protected by sheet steel, concrete, and wooden walls or rock revetments. While these structures do not stop sand from moving along the beach and lake bottom like shore perpendicular structures do, they prevent erosion which normally would have contributed sand to the littoral drift necessary to maintain protective beaches and offshore sand bars. This lack of sand contribution creates a "sand-starved" condition in front of the erosion protection structure. Reduction of this "sand deficit" is usually accomplished at the expense of the adjacent erodible coast.16 Examples of shore parallel structures along Indiana's shoreline are: (1) rock revetment at the Town of Beverly Shores; and (2) sheet steel breakwall system at the Town of Porter, Dune Acres, and Ogden Dunes.17 The longest shore parallel structures are found west of Gary along the USX steel mill facilities. On the downdrift side of man-made structures, if sand (littoral drift) is not abundant enough to maintain wide beaches and broad off-shore sand bars at a particular location, erosion rates may be higher there compared to other parts of the coast, even though the same wave energy and lake levels are present at both sites. The deficit of sand may be due either to natural or man-made conditions. Erosion rates usually increase dramatically on the downdrift side of a new structure as a result of severe sand-starved conditions created by sand being retained on the opposite (updrift) side of the littoral barrier. When no input of sand is available to replace sand that continues to move away from the structure in the downdrift direction, beach widths become narrow and the offshore sand bars lose height and width. This allows more wave energy to reach the shoreline, increasing erosion of the erodible beach and dune-bluffs. 
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