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Harry Williams, Geomorphology1 SHORELINE EROSION AT MAD ISLAND MARSH PRESERVE. Mad Island Marsh Preserve is a 3,148- acre tract containing the east arm.

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Presentation on theme: "Harry Williams, Geomorphology1 SHORELINE EROSION AT MAD ISLAND MARSH PRESERVE. Mad Island Marsh Preserve is a 3,148- acre tract containing the east arm."— Presentation transcript:

1 Harry Williams, Geomorphology1 SHORELINE EROSION AT MAD ISLAND MARSH PRESERVE. Mad Island Marsh Preserve is a 3,148- acre tract containing the east arm of Mad Island Lake, its associated freshwater and brackish marshes, and surrounding upland prairie and shrub-land habitats. Gulf Intracoastal Water Way Dredge spoils Spartina marsh

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3 3 The Gulf Intracoastal Water Way, constructed in 1941, cuts through the southern portion of the Preserve and is flanked to the south by man-made islands of dredge spoils. GIWW - view west. Preserve Dredge spoils

4 Harry Williams, Geomorphology4 Considerable wave erosion has occurred, and continues to occur, along the Preserve shorelines bordering the Waterway. Shorelines along the Waterway are eroded by waves generated by barge traffic.

5 Harry Williams, Geomorphology Eroded road.

6 Harry Williams, Geomorphology6 This fence was constructed several feet back from the shore one year before this photo was taken. Undercutting has caused this block of soil to fall into the GIWW. It has also destroyed the boat ramp.

7 Harry Williams, Geomorphology7 The erosion is caused by waves cutting a wave-cut notch and causing collapse and cliff formation.

8 Harry Williams, Geomorphology8 A large Spartina marsh borders the GIWW along part of the preserve shoreline. This is a valuable wildlife habitat. It is also being eroded by wave action.

9 Harry Williams, Geomorphology9 There is a sharp cliff here too (hidden by high tide in this photo).

10 Harry Williams, Geomorphology10 The resulting cliff edge is easily identified on air photos. notch

11 Harry Williams, Geomorphology11 The research study was conducted in order to provide information on the location and rate of shoreline erosion along the shores of the preserve over the last fifty years. Long-term rates of shoreline erosion were established by the analysis of sequential aerial photographs taken in 1930, 1943, 1958, 1978 and The shoreline from each photograph was transferred onto a single base map. Movement of the shoreline at selected points was measured and converted to feet (or meters) of erosion per year shoreline 1978 shoreline 80 feet 80 feet in 20 years = 4 feet per year.

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17 Harry Williams, Geomorphology17 Base map.

18 Harry Williams, Geomorphology18 Measurement locations: A…Z = shoreline migration; 1..4 width of GIWW. The main preserve shoreline = A - I.

19 Harry Williams, Geomorphology19 MAD ISLAND PRESERVE, SHORELINE MIGRATION: SITE 1943 ‑ ‑ ‑ ‑ 1991 FEET FT/YR FEET FT/YR FEET FT/YR FEET FT/YR A B C D E F G H I J For the main preserve shoreline, erosion rates for the most recent period ( ) vary from 0 to 6.1 feet per year. WIDTH OF WATERWAY (feet) RATIO: SITE / % % % % The width of the water way increased by an average of 282%.

20 Harry Williams, Geomorphology20 An important finding of the study is that severe erosion is occurring near the entrance to Mad Island Bayou. Bayou entrance

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22 Harry Williams, Geomorphology Views west from the end of the peninsula.

23 Harry Williams, Geomorphology23 The large bend in the bayou was about 24 feet from the GIWW in Breaching of the peninsula at this point would abruptly shorten Mad Island Bayou by several hundred meters and probably increase salt water intrusion into Mad Island Lake. BayouGIWW

24 Harry Williams, Geomorphology24 Partly as a result of this study, the Nature Conservancy of Texas built concrete erosion barriers along this part of the preserve shoreline in so far they appear to be working - there has been no more erosion along this part of the shoreline.

25 Harry Williams, Geomorphology25 UPDATE: In 2005, a grad student (Webster Mangham) did a Masters thesis on erosion at MIMP between 1995 and He used GPS, DOQQs and GIS.

26 Harry Williams, Geomorphology26 The concrete mat still looks pretty good (in places) after 9 years.

27 Harry Williams, Geomorphology27 BUT – in other places it has clearly failed – the soil has been washed out from under it and it has collapsed.

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