Harry Williams, Geomorphology1 SHORELINE EROSION AT MAD ISLAND MARSH PRESERVE. Mad Island Marsh...

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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 WayDredge spoils

Spartina marsh

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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

Dredgespoils

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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.

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1992 1993

Eroded road.

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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.

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The erosion is caused by waves cutting a wave-cut notch and causing collapse and cliff formation.

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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.

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There is a sharp cliff here too (hidden by high tide in this photo).

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The resulting cliff edge is easily identified on air photos.

notch

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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 1991. 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.

1958shoreline

1978shoreline

80 feet

80 feet in 20 years = 4 feet per year.

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1930

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1943

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1958

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1978

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1991

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Base map.

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

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MAD ISLAND PRESERVE, SHORELINE MIGRATION: 1943-1991SITE 1943‑1958 1958‑1978 1978‑1991 1943‑1991 FEET FT/YR FEET FT/YR FEET FT/YR FEET FT/YRA 84 5.6 53 2.7 49 3.7 186 3.9B 88 5.9 44 2.2 53 4.1 186 3.9C 27 1.8 62 3.1 53 4.1 142 2.9D 35 2.4 41 2.0 18 1.4 94 2.0E 0 0.0 88 4.4 80 6.1 168 3.5F 0 0.0 53 2.7 62 4.8 115 2.4G 0 0.0 35 1.8 71 5.4 106 2.2H 0 0.0 31 1.5 71 5.4 102 2.1I 0 0.0 44 2.2 27 2.0 71 1.5J 0 0.0 35 1.8 0 0.0 35 0.7

For the main preserve shoreline, erosion rates for the most recent period (1978-1991) vary from 0 to 6.1 feet per year.

WIDTH OF WATERWAY (feet) RATIO: SITE 1943 1958 1978 1991 1991/19431 150 230 425 469 313%2 212 336 504 531 250%3 212 292 513 593 280%4 195 354 531 558 286%

The width of the water way increased by an average of 282%.

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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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1943 1991

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19921993

1994

Views west from the end of the peninsula.

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The large bend in the bayou was about 24 feet from the GIWW in 1994. 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

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Partly as a result of this study, the Nature Conservancy of Texas built concrete erosion barriers along this part of the preserve shoreline in 1996 - so far they appear to be working - there has been no more erosion along this part of the shoreline.

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

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The concrete mat still looks pretty good (in places) after 9 years.

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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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1991

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