Texas Coastal Wetlands

Transcription

1 Texas Coastal Wetlands Linking Water & Land James McDonald CE394K.3 GIS in Water Resources Fall 2008 Dr. David Maidment Introduction Approximately 40% of the world s population lives within 100 km. of the coast. This can also be seen on the Texas Gulf Coast where 5 million people reside on or near. Coastal wetlands are an extremely diverse and sensitive ecologic setting that links water and land. They provide extremely diverse habitats for plant and animal populations. Thirty percent of US plant species can be found in wetlands. Numerous vital hydrologic and hydraulic functions are simultaneously taking place. Some of these major functions include erosion control, groundwater recharge, flood storage and storm surge reduction. Possibly the most important function are wetlands role in water quality. Coastal wetlands are crucial in filtration and nutrient cycling. It had been found that wetlands are so naturally effective at these processes that they are being designed and artificially created by engineers to serve these functions elsewhere. As the rainforests are often equated with the Earth s lungs breathing and filtering the air, wetlands would most certainly be the Earth s kidneys, filtering its runoff output. It is the objective of this project to determine: are Texas coastal wetlands disappearing, at what rate, and how will this affect us?

2 Project Analysis To address these objective questions Galveston Bay (shown below) and its surrounding area are examined. Figure 1: Galveston Bay Data was obtained from the United States Geological Survey (USGS). Thirty-one 24,000:1 USGS quadrangle maps were utilized. These maps were digitized and developed by the National Wetland Inventory and included habitat classification data. A key factor of these maps is that multiple temporal sets exist. Examined data sets are from 1956 and 1989, the oldest and most recent data sets. Procedure The USGS recognizes a hierarchical wetlands classification scheme developed by Cowardin et al, shown below.

3 Figure 2: Cowardin et al. wetland classification scheme

4 While this figure can be slightly daunting at first, it is quite effective at classifying wetlands based on specific attribute characteristics. These characteristics include: location, salinity, tidal influence, bottom coverage, vegetation, and specific modifiers (water regime, chemistry, soil). This can be seen in the system, subsystem, class and subclass categories. Although these wetland classifications are quite extensive and specific, this was not necessarily helpful in mapping to determine trends. With so many unique values to map it became overwhelming and inconclusive to the eye. As a solution to this problem the field calculator within GIS was utilized. After adding an attribute field, with appropriate programming the field calculator could assign values to the new attribute field that consisted of only system and subsystem of the wetland classification scheme. With this newly organized classification it was possible to map wetland types and display easily determinable changes over time. Figure 3: Field Calculator Re-classification

5 Miles. Figure 4: Original Quadmaps Merged With the 31 quadmaps merged and field calculator categorization, both shown above, wetland habitat maps were born. USGS and US Fish and Wildlife Service (USFWS) mapping and grouping techniques were observed and modeled. Under the Cowardin wetland classification system 2 broad types of wetlands exist. The first is coastal/tidal wetlands which include Estuarine types. These are sometimes referred to as fringe wetlands and are what we typically think of as deltas, marshes and salt flats. Secondly and in contrast to this are the non-tidal freshwater or Palustrine wetlands. These wetlands are distinguished by salinity levels less than 0.5 ppt. Palustrine wetlands vegetation includes forested, scrub-shrub, emergent and moss. These are what we typically think of as swamps and bogs. Natural Palustrine wetlands are often the primary measured wetland type when discussing wetland loss.

6 Results Maps of 1956 and 1989 of wetland habitats in Galveston Bay are shown below Figure 5: 1956 Galveston Bay wetlands 20 Miles.

7 Miles. Figure 6: 1989 Galveston Bay wetlands After initial comparison it is apparent that both Palustrine and Estuarine wetlands are decreasing over the timeframe. This loss is due to multiple factors. Some of these include: development (particularly on Galveston Island), erosion, land change (many wetlands were drained for agriculture in earlier years), and land subsidence. This area is very flat and already at low elevation, just above sea level. Land subsidence can be attributed to the continued mining of oil and natural gas and to the particular geologic soil formation type from previous sedimentation. However, after continued examination the most drastically affected wetland areas appear in the following regions.

8 Miles Figure 7: 1956 Galveston Bay wetlands.

9 Miles. Figure 8: 1989 Galveston Bay wetlands These three locations all happen to be located on the influent rivers bringing in fresh water. Identified from east to west we see the Trinity River delta, San Jacinto River delta, and Bastrop/Chocolate Bayou. In the Bastrop/Chocolate Bayous region we observe a decrease in both wetland categories but a very dramatic loss in the Palustrine wetlands (shown green on the maps). The San Jacinto delta, primarily Estuarine habitats loses almost 100% of its wetlands in the observed period! The Trinity River delta undergoes approximately 50% losses of Palustrine wetlands with some of that area changing into Estuarine. These wetland loss values are shown below.

10 Area (km 2 ) Difference Marine Wetland Freshwater Emergent Wetland Freshwater Forested Wetland Freshwater Shrub Wetland Estuarine Wetland Rivers Lakes Freshwater Pond 6 16 Deep Water Upland Total Wetlands 770 km km 2 Loss of 16% Conclusions The Galveston Bay wetland reduction rate is calculated to be 3.8 km 2 per. Or in American terms, 2.3 football fields per day are lost. However, the most interesting conclusion is the strong correlation of wetland loss and freshwater delta proximity. There are stark differences in all three of the major freshwater influent locations. From this observation we can conclude that wetlands are extremely sensitive to river influents. It is possible that influent patterns changed in a number of components. Influent sediment and corresponding nutrients could be stopped by upstream dams. Water quality could be harshly decreased by upstream discharge (multiple major metropolitan areas exist). Or freshwater volumetric flowrates may have decreased below required levels for continued wetland existence. With all of these factors considered it makes sense to observe a severe change at the deltas particularly within the Palustrine wetlands that are dependent on freshwater to keep salinity levels at bay. We observe a total wetland loss of 127 km 2 over the 33 years. Previous studies by the US Army Corp of Engineers concluded that over each km of wetlands traveled storm surge reduced by 7 cm. This study is based on examined high water marks from multiple events and regression with varying wetland lengths traveled. While this study is somewhat limited, it is intuitive to consider an increased water column drag from wetlands reducing storm surge compared to open water. With the extremely flat topology of Houston, Galveston and the rest of the Texas coast, a minor reduction in storm surge will have an extensive impact on flood wave affected area.

11 Future Detection Remote Sensing This paper has shown historical trends in wetland loss using maps derived from USGS on ground surveys. These trends are useful in that they aid us in predicting the future. However, as technology advances we see updates in our observation and prediction capacity. Satellite radar and aerial photography are remote sensing options that can be used to observe flood inundation levels and determine wetland destruction. Envisat, the largest Earth observation spacecraft, was utilized to visualize flooding extent in along the Gulf of Mexico after Hurricane Ike. The use of lidar has also been beneficial to coastal communities. The comparison of post-storm elevation data to pre-storm lidar data can be used to characterize the nature, magnitude, and spatial variability of hurricane-induced coastal changes, such as beach erosion, overwash deposition, and island breaching (USGS). Major advancements in the area of storm surge prediction have been made through advanced computing visualization. These models were primarily designed for emergency response situations and evacuation planning measures. The knowledge of wetland effects on hurricane events and storm surges is paramount to the accuracy and applicability to these predictions. References Cowardin, L.M., V. Carter, F. Golet, and E. LaRoe Classification of Wetlands and Deepwater Habitats of the United States. U.S. Fish and Wildlife Service. National Wetlands Inventory, USGS - National Wetlands Research Center, Coastal Photos: