UNT GEOG 3350 - MADISLANDEROSIONupdate
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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Harry Williams, Geomorphology 1SHORELINE 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 spoilsSpartina marshHarry Williams, Geomorphology 2Harry Williams, Geomorphology 3The 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.PreserveDredgespoilsHarry Williams, Geomorphology 4Considerable 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.Harry Williams, Geomorphology 51992 1993Eroded road.Harry Williams, Geomorphology 6This 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.Harry Williams, Geomorphology 7The erosion is caused by waves cutting a wave-cut notch and causing collapse and cliff formation.Harry Williams, Geomorphology 8A 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.Harry Williams, Geomorphology 9There is a sharp cliff here too (hidden by high tide in this photo).Harry Williams, Geomorphology 10The resulting cliff edge is easily identified on air photos.notchHarry Williams, Geomorphology 11The 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.1958shoreline1978shoreline80 feet80 feet in 20 years = 4 feet per year.Harry Williams, Geomorphology 121930Harry Williams, Geomorphology 131943Harry Williams, Geomorphology 141958Harry Williams, Geomorphology 151978Harry Williams, Geomorphology 161991Harry Williams, Geomorphology 17Base map.Harry Williams, Geomorphology 18Measurement locations: A…Z = shoreline migration; 1..4 width of GIWW. The main preserve shoreline = A - I.Harry Williams, Geomorphology 19 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 0.7For 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%.Harry Williams, Geomorphology 20An important finding of the study is that severe erosion is occurring near the entrance to Mad Island Bayou.Bayou entranceHarry Williams, Geomorphology 211943 1991Harry Williams, Geomorphology 22199219931994Views west from the end of the peninsula.Harry Williams, Geomorphology 23The 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.BayouGIWWHarry Williams, Geomorphology 24Partly 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.Harry Williams, Geomorphology 25UPDATE: 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.Harry Williams, Geomorphology 26The concrete mat still looks pretty good (in places) after 9 years.Harry Williams, Geomorphology 27BUT – in other places it has clearly failed – the soil has been washed out from under it and it has collapsed.Harry Williams, Geomorphology 28Harry Williams, Geomorphology 29Harry Williams, Geomorphology 302008Harry Williams, Geomorphology 31Harry Williams, Geomorphology 321991Harry Williams, Geomorphology 332013Harry Williams, Geomorphology 34Harry Williams, Geomorphology


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