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HomeMy WebLinkAbout2018 Sand Compatability Sieve Analysis GEO/PLAN ASSOCIATES 30 MANN STREET HINGHAM, MASSACHUSETTS 02043-1316 Voice & Fax: (781) 740-1340 Email: GeoPlanAssoc@gmail.com April 5, 2018 Epsilon Associates P.O. Box 700 Maynard, Massachusetts 01754-0700 Attention: Ms. Maria Hartnett Re: Nourishment Sand Compatibility Analysis, Siasconset Beach Dear Ms. Hartnett: I performed size analyses of composite sediment samples from two locations in Reis sand pits from Nantucket taken on March 26, 2018. The purpose of this letter is to evaluate the suitability of these pit sediment sources as mitigation sediment for a segment of beach along Siasconset Beach, Nantucket. The project area is within previously-identified sampling sites designated as sediment sampling transects (Line 15 through Line 19). Extensive sediment sampling of the area (beach, bank, dune) was performed in 2006 along these lines and adjacent areas by Coastal Planning and Engineering, Inc. Other grain size data from this beach area is available from earlier sampling in 1998, 2001 and 2003. Some of these samples I collected and analyzed. The mean sizes of the composite samples and other characteristics are compiled below for comparison. While the methodologies for analysis are consistent, the reporting of the data, the lateral extent of the sampling along Siasconset Beach, and the field sampling methods may vary. This doesn’t affect the documentation of the sand characteristics, and the resulting time-series provide a measure of variability of the natural sands over time. 2 A. Proposed Sand Pit Sources: Reis No. 1 Mean: 0.54 phi (coarse sand) Sorting Coefficient: 1.82 (poorly sorted) 86% sand, 14% gravel gravel <16mm; mud insignificant Reis No. 2 Mean 0.66 phi (coarse sand) Sorting Coefficient: 1.50 (poorly sorted) 88% sand, 12% gravel gravel <8mm; mud insignificant B. Natural Bank Sediments 2001: 2003: 2006: 2.0 phi, (medium - fine sand) includes 8% mud 1.8 phi (medium sand) includes 5.5% pebbles or granules 1.2 phi (0.45 mm; medium sand) includes minor fine pebbles/granules The bank sediments range between medium-fine sand to medium sand, and contain varying amounts of fine gravel and mud. Direct observation of this coastal bank has shown that, although dominantly sand, there is frequently a mud and gravel component and periodic mud layers and clay banks are part of the deposit. The fine or coarse tails and the variation in sizes are typical for glacial outwash sediments in this setting. C. Beach Sediments 1998: 1.5 phi (medium sand) 2001: 1.0 phi (medium – coarse sand) 2003: 0.9 phi (coarse sand) 2006: 0.7 phi (coarse sand) The 2006 samples are coarser than the earlier samples, either due to natural variation in sand sources over time, or cyclic changes in size related to energy. Regardless of the cause, these four sampling intervals indicate that the mean size of the natural sediment on the beach flucturates, but is not coarser than the 0.7 phi 2006 samples. 3 D. Discussion Compatible beach sediment is not sand that exactly matches the existing beach, but rather sediment that is relatively stable and can coexist with the naturally deposited sediment in the coastal setting. If the compatibility of the sediment is evaluated relative to potential stability on the beach (which is generally the case), compatible sediment is equal or coarser than the existing sediment. The proposed source areas are glacial outwash sediments. Both samples have insignificant mud (<1%), which is a plus for compatibility, as mud is quickly lost, and is the most common objection based on aesthetics or water turbidity. Both of the proposed source areas are geologically the same material (outwash sediments) from the same vicinity as the natural bank materials. Both samples contain some gravel. While the gravel does not necessarily match surface beach sediment samples, small gravel is a visible component on these beaches and the adjacent shallow nearshore. Importantly, both samples are coarse sand, which has the greatest likelihood of remaining stable on the Siasconset Beach. While the sizes are reported as means, there are ranges of sizes finer and coarser in all samples. However, both the natural beach sediment and both potential pit sources have very small amounts of sand finer that medium sand. This is the component of the sand that is more likely to be lost from the beach. Therefore, the wave sorting will likely re-sort nourishment sand to have comparable sizes to existing conditions, or coarser, so most of the source material will have as great a probability of remaining within the adjacent beach system as the natural bank material. Both sand pit sediment samples are slightly coarser than both the natural bank and the existing beach sediments. Much of the variation in mean size is due to the differences in gravel content. The differences in gravel content, however, are not significant. Grain size is measured by weight, which is affected by gravel greater than if it were measured by volume, which is how sediment is specified for mitigation purposes. Therefore, both proposed source pit sediments are beach-compatible sediments. Please feel free to contact me if there are further questions concerning the evaluation of these sand samples. Yours truly, Peter S. Rosen, Ph. D. Coastal Geologist 867 Boston Road, Groton, MA 01450 Tel: 978-448-9511 www.pineandswallow.com Principals: Robert Pine, PE, FASLA John Swallow, PhD, LSP Client:Project: P&S Proj.#:Date: Sampled by:Analyst(s): Horticultural Soil Testing Report Sample ID: Sieve Size (US Standard Mesh) % Passing based on whole sample % Passing based on #10 (2mm) sieve 10 91.5%100.0% 18 81.7%89.3% 35 53.5%58.5% 60 15.8%17.3% 140 1.8%1.9% 270 1.0%1.1% Uniformity & %Gravel Spec. Limit D70/D20:2.2 D80/D30: % Gravel:8.5% Spec. Limit 7.5 Bank Sand Mechanical Sieve Analysis Specification Ranges (if provided) Minimum Maximum This report shall not be altered or reproduced without the express permission of Pine & Swallow Environmental. % Organic by loss on ignition. Gradation by washed sieve method. pH 1:1 (v:v) distilled water. Conductivity 1:2 (v:v) distilled water. © 2018 Pine & Swallow Environmental Additional Tests pH: Organic Matter: Soluble Salts (mmhos/cm): USDA Textural Class Read Custom Soils soils Read Custom Soils QA/QC 6/8/18 MA/AC