FINAL REPORT: Biological Erosion Control: Experimentation and Dissemination to Stakeholders

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1 FINAL REPORT: Biological Erosion Control: Experimentation and Dissemination to Stakeholders Dr. Rusty A. Feagin Spatial Sciences Laboratory Department of Ecosystem Science & Management Texas A&M University 15 Research Pkwy., Ste. B223 College Station, TX This work was supported by National Oceanic and Atmospheric Administration (NOAA) and Texas General Land Office (GLO) grant #6-2.

2 Deliverables: 1. Final Report. 2. Pamphlet Distributed to Stakeholders (in same form as final report). 3. Progress Reports. This work was supported by National Oceanic and Atmospheric Administration (NOAA) and Texas General Land Office (GLO) grant #6-2. Contact Information for Dr. Rusty Feagin (corresponding author): Dr. Rusty Feagin Assistant Professor Spatial Sciences Laboratory Dept. Ecosystem Science & Management/ Forest Science Texas A&M University 15 Research Parkway, Ste. B223 College Station, TX (979) (phone) +1-(979) (fax) ( )

3 Introduction The results of this study challenge our conception of the traditional paradigm that plant roots directly prevent erosion along the coast. Previous studies have focused solely upon the ability of above-ground plant stems and leaves to reduce wave forces in the water column (Roland & Douglass 1985, van Eerdt 1985, Coops et al. 1996, Leonard & Luther 1997, Nepf et al. 1997, Ghisalberti & Nepf 22, Danielsen et al. 25, Smith et al. 27), yet these studies have ignored the physical mechanism that results in the majority of salt marsh erosion - undercutting of the marsh edge by waves (Figures 1 and 2). To investigate marsh edge erosion, we placed extracted marsh cores into a wave flume and sent waves at them. The waves simulated a typical windy 18 hour period in a typical estuary. We tested for differences in erosion rates between cores with plants and without plants, for differences among plant species, and for differences in soil types. Figure 1--- An eroding marsh edge, West Galveston Bay.

4 Methods Our plan was to place marsh plants into a wave flume and test whether different plant species were able to better prevent erosion. In order to correctly scale and quantify the relevant processes, we focused on the effect of a single plant upon erosion. We had initially grown several hundred plants of several species in a greenhouse, in special containers to allow maximum root growth. We were going to open up one side of the rectangular container and expose it to waves. However, after growing the plants and testing out the methodology, we realized that the sediment that we had used was inappropriate. Within a few minutes of placing the containers and plants into the wave flume, the sediment structure fell apart and collapsed in place without having been exposed to wave energy. This learning experience proved that the sediment was too coarse/ sandy with little cohesion. Interestingly, this sediment came from recent deposits on El Jardin beach, in the City of Pasadena adjacent to the Bayport Container Terminal. Thus, we decided to take plant and sediment samples directly from the salt marsh, where sediments were likely more cohesive. Three general salt marsh locations were sampled on the bayside of Galveston Island: Sportsman s Road, Galveston Island State Park, and Old Port Industrial Boulevard. Figure 2--- Do plants help prevent below-ground erosion?

5 We built a sediment corer out of stainless steel. It was 12 inches long and four inches wide, with a gusset on one end and sharpened on the other end. This corer was inserted into the marsh surface through the use of a sledgehammer. The marsh corer was then removed from the ground. The core itself was then extruded using a block of wood. This block of wood was cut to fit the interior of the corer. Once marsh cores were extruded, they were placed into a chamber within a plexiglass box. This box was built with four separate chambers that were 12 inches long and 4 inches wide. The plexi-glass was cut to size, screws were inserted to provide initial structure, and it was then bonded with epoxy. The four chamber design allowed four separate marsh samples to be tested simultaneously. Simultaneous tests lent confidence that all four cores were subjected to the same wave conditions. This four chamber design also gave us the ability to design statistical tests for significance with maximum power. The box was then placed into the wave flume (Figure 3). The wave flume was approximately 4 feet long and the box was placed at one end. At the other end, a small motor-run wave paddle was installed. The water level was set at 6 inches. Salt was added to the water to simulate actual estuarine conditions, leading to a salinity of approximately 12 ppt. Waves were regular, with a significant wave height of approximately 5 inches. This simulated a windy open bay, for example the marshes exposed to the bayside of West Galveston Bay during a winter storm with strong north winds. Figure 3--- A wave in the flume intercepts the plants in the fourchambered box. The two marsh cores on the left are from a natural marsh, the two on the right are from an adjacent restored marsh

6 Marsh samples were subjected to waves for a total of 18 hours. They were initially weighed before being put into the wave flume. They were then taken out of the flume and re-weighed at the following intervals of the test: 1 hour, 6 hours, 12 hours, 18 hours. Weight loss was utilized as the measure of erosion. Using extracted marsh cores, the following questions were investigated: 1. Does the presence of plants reduce sediment erosion? 2. Does an interior marsh soil erode faster than an edge marsh soil? 3. Do some plant species reduce erosion better than others? 4. Do restored marshes erode faster than natural marshes? All tests were replicated 4 times. Data was analyzed for statistical differences using t- tests. In addition to the wave flume tests, the sediment properties of each of the marsh cores were investigated. At each coring site in the field, we extracted a sediment sample immediately adjacent to the core and halfway down the cored hole. The samples were bagged and transported to the lab. In the lab, each sample was weighed, dried in an oven, and re-weighed to obtain a measure of bulk density and % water. Each sample was then burned in a muffle furnace for 18 hours at 44 C and re-weighed to obtain a measure of % organic matter. Each sample was then sieved using sieves ranging from -1 to 6 Φ (from very coarse sand to fine silt or finer). Grain size measures were then converted to cumulative probabilities, and the mean grain size was calculated (Folk & Ward 1957). In order to determine the possible mechanisms of erosion that may exist for marshes, the sediment parameters were then regressed against the erosion rate measurements from the wave flume portion of the study.

7 Results and Discussion The results showed that the presence of plants or live plant roots made no significant difference upon the amount of erosion (Fig. 4a). Differences in plant species morphology did not alter the erosion rates either (Fig. 4b). Figure 4--- Plants do not affect the erosion rate 3 (a) 75 (b) Erosion (g) hours Plant No Plant Spartina alterniflora Batis maritima Rather, the soil type was the master variable that determined the erosion rate. We found that marsh soils from the marsh interior eroded much quicker than those on the edge (Fig 5a). There was also a significant difference in erosion between a restored marsh and natural marsh, again because of the soil (Fig. 5b). Note the scale on Figure 5b after 18 hours of being subjected to waves, the average restored marsh core lost more mass than the average natural marsh core by an exponential factor of approximately 1 2. Figure 5--- The soils at a site affect the erosion rate 5 (a) 1 (b) Erosion (g) hours Edge Marsh Interior Marsh Restored Natural

8 If the plants do not matter, then what soil parameters do matter? We found that bulk density and sediment particle size provided the best predictors for erosion rate (Figure 6). It was clear that in extracted cores with high organic matter, these other soil parameters were strongly correlative. Rather than the living plants and roots, we suggest that it is the indirect input of plant detritus in the form of finely-grained organic particles that lends cohesiveness to the soil, along with the associated changes in bulk density and particle size. As plantproduced detritus becomes incorporated into the matrix, the soil becomes less dense, finer, and more resistant to erosion. Thus, plants do not directly reduce erosion, but do so indirectly through modification of the soil parameters. Figure 6--- Correlation of salt marsh soil parameters with erosion Erosion (g) R 2 = Bulk Density (g / cm3) (a) (c) R 2 = % Silt or Finer (b) R 2 = % Organic Matter (d) R 2 = % Very Coarse Sand

9 Of all the extracted cores that we tested, the dense, coarse, inorganic, and sandy sediment from the terraces at Galveston Island State Park (a restored salt marsh) eroded the quickest (Figure 7). Future studies should follow up upon this work by attempting to corroborate our findings with data from the field. This study is important because it suggests that salt marsh restoration efforts should place the highest priority upon getting the soil right. Figure 7--- Cores extracted from the restored terraces at Galveston Island State Park eroded the quickest. These sediments were dense, had a low organic content, and were coarse-grained and sandy. The presence of plants did not significantly alter the speed at which these restored marsh samples eroded.

10 References Coops, H., Geilen, N., Verhij, H.J., Boeters, R., & van der Velde, G Interactions between waves, bank erosion and emergent vegetation: an experimental study in a wave tank. Aquatic Botany 53: Danielsen, F., Sorensen, M. K., Olwig, M. F., Selvam, V., Parish, F., Burgess, N. D., Hiraishi, T., Karunagaran, V. M., Rasmussen, M. S., Hansen, L. B., Quarto, A., & Suryadiputra, N. 25. The Asian Tsunami: A protective role for coastal vegetation. Science 31: 643. Folk, R. L. & Ward, W. C Brazos River bar: A study in the significance of grain size parameters. Journal of Sedimentary Petrology 27: Ghisalberti, M., & Nepf, H.M. 22. Mixing layers and coherent structures in vegetated aquatic flows. Journal of Geophysical Research 17: Leonard, L.A., & Luther, M.E Flow hydrodynamics in tidal marsh canopies. Limnology & Oceanography 4: Nepf, H.M., Sullivan, J.A., & Zavistoski, R.A A model for diffusion within emergent vegetation. Limnology and Oceanography 42: Roland, R.M., & Douglass, S.L Estimating wave tolerance of Spartina alterniflora in coastal Alabama. Journal of Coastal Research 21: Smith, W.K., Feagin, R.A., Psuty, N.P., Nordstrom, K.F., Carter, G.A., Gemma, J.N., Gibeaut, J.C., Thomas, D.H., Koske, R.E., Martinez, M.L., Whitehead, J.C., & Young, D.R. 27. Coastal barrier islands: Coupling anthropogenic stability with ecosystem sustainability. Frontiers in Ecology and the Environment: In Review. van Eerdt, M.M The influence of vegetation on erosion and accretion in salt marshes of the Oosterschelde, The Netherlands. Vegetatio 62: