COASTAL WETLAND ENGINEERING: DESIGNING FOR FUNCTION, CASE STUDIES, AND MODELING TOOLS

Transcription

1 COASTAL WETLAND ENGINEERING: DESIGNING FOR FUNCTION, CASE STUDIES, AND MODELING TOOLS Candice Piercy, PhD, PE Susan Bailey, PE ERDC - Environmental Laboratory TAMU Ocean Engineering EWN Lecture Series March 19, 2018

2 Who are we? Ms. Susan Bailey (BS NCSU, MS LSU, PE) is a research civil engineer with over 15 years of experience working in the Environmental Engineering branch at USACE-ERDC. She works primarily with sediment and dredged material management. She performs laboratory research as well as modeling. Her focus is primarily on modeling dredged material behavior and contaminant transport. Dr. Candice Piercy (BS, PhD Virginia Tech, PE) is a research environmental engineer with 8 years of experience working with the Integrated Ecological Modeling team at USACE-ERDC. Her research focus is on the simulation of feedbacks between ecological and physical processes, primarily driven by vegetation, in a variety of ecosystems including salt marshes, dunes, estuaries, and river floodplains. She also conducts field monitoring studies and helps develop ecologically-informed engineering guidance for coastal ecosystems.

3 Outline Salt marsh dynamics and degradation Thin layer placement Design principles Modeling tools Marsh elevation responses Dredged material settling, consolidation, and compression Case studies

4 Salt marsh dynamics Kirwan & Megonigal 2013 Submergence Edge erosion Pond collapse Kirwan & Megonigal 2013 Mariotti 2016 CWPRA Disruptions to ecological, hydrological, & sedimentation processes can alter the morphology & function of salt marshes Signs of distress in salt marsh can manifest differently depending on the site Dynamic equilibrium

5 Identifying degradation in salt marshes Table 2. Indicators of salt marsh distress at each case study location. Observation Narrow River Avalon Seal Beach Bank Erosion X X Impounded Water X X Pool/Panne Expansion X X Ratio of Water/Mudflat to Vegetation X Low Faunal Use X X Loss of High Marsh Species X X Unhealthy Vegetation X X X Presences of Invasive Species X VanZomeren and Murray (In Review)

6 What are the underlying causes of marsh stress? Potential stressors Hydrological changes Rising sea levels Increased wave energy Physical barriers Reduced sediment loading/deposition Vegetation stress Invasive species Disturbances (drought, storms)

7 So what do we do about it? Remove barriers Restore hydrologic connectivity Facilitate sediment transport into marsh Raise elevation Decrease inundation Alleviates flooding and sulfide stress Expand marsh platform Reverse effects of erosion Remove ditches - filling Restore natural hydrograph (overdrained marsh)

8 How to get more sediment into marsh? USACE moves sediment! Direct vs strategic placement How do we get it there and how much do we add? Thin-layer placement Placement of a thickness of dredged material that does not transform the receiving habitat s ecological function (Wilber, 1992) Has also been used to describe placements ranging in depth from cm to 1 m Developed and commonly used in Louisiana, more recent examples in mid-atlantic and California

9 Data Needs Mapping and survey data Elevation and topography (high resolution and accuracy) Property boundaries, roads, rights-of-way, utilities, benchmarks, structures Detailed topographic hydrological analysis (flow paths, directions) Location of areas of concern (cultural resources, threatened and endangered species, etc.) Geotechnical investigation Soil description/grain size Chemical analysis of soils Soil hydraulic conductivity Some measure of bearing strength for equipment access Foundation sediment consolidation behavior (standard oedometer) Hydrological investigation Tide range, elevations of tidal datums Estimate of wave energy (fetch adequate for small estuaries) Water table elevation Water budget (precip, evapotranspiration, etc.) Vegetation investigation Dominant species Dieback areas, condition, cover Ratio of unvegetated to vegetated area Invasive species (Phragmites) Proposed Dredged Material Core samples from proposed dredging depth Water content Salinity Grain size distribution Organic content Atterberg Limits Column settling test Consolidation tests Self weight Standard oedometer Dredging and placement techniques Dredge size Placement method (nozzle, open pipe, mechanical, etc.)

10 Enriching marshes with DM: where do we put it? Marshes aren t flat and sediment slurry flows Large scale field data collection efforts are time consuming and difficult On-site work can be tricky Gradients hard to determine Soft substrate Ditches/tidal creeks Hidden drainages

11 Selecting areas of the site for sediment addition Low elevations High risk for future marsh collapse Limited hydrologic connectivity Eroding banks Fragmented pool areas Vegetation die back All of the above near the edge higher risk of total loss of area

12 Enriching marshes with DM: how much do we add? Biological vs. construction target elevations Elevation relative to the tide changes over time (short term: settling and consolidation; long term: relative sea level rise, accretion) Construction target elevations greater than biological target elevations should define both Determining biological target elevation Meta-analysis of documented thin-layer sites shows upper intertidal range results in greatest marsh resilience Goals for restoration (low versus high marsh) Relative sea level rise rate Predicted settling, consolidation, and compression Construction target elevation/thickness Biological target elevation/thickness Initial marsh elevation

13 Dominant Marsh Elevation Processes for TLP over Time Elevation, ft NAVD Elevation Vs Time Initial (Days to Months) Solids settle out of slurry solution based on 4.25 zone/compression settling behavior SETTLE model used to predict elevation/state at end of placement Days to ~ 1-2 years Consolidation dominated PSDDF model Time, years 1.30 Fill Foundation > ~ 1-2 years Dominated by marsh equilibrium processes - MEM model Biomass Accretion or Erosion Sea level rise

14 Long-term marsh elevation response to SLR Marsh Equilibrium Model (MEM) projects future conditions based on known interactions between biomass and accretion Developed at University of South Carolina by Dr. James Morris Integrated with new versions of SLAMM to better predict accretion 1-D and 2-D versions available with web, Excel, and ArcGIS interfaces Integrated with ADCIRC in hydromem Outputs include Marsh elevation Above and below ground biomass Soil OM Carbon sequestration

15 Project goal: update MEM to model the effects of TLP Collaboration with ERDC and University of South Carolina (Jim Morris) Key issues When do consolidation processes switch from being dominated by physical processes (modeled by PSDDF) to biologically-mediated processes? How to model TLP-induced mortality proportional to placement thicknesses? How long does vegetation take to recover or recolonize? How to account for replanting/reseeding? Relative contribution to elevation dynamics Years since placement Ecological processes Physical processes

16 Initial functionality added: proof of concept that TLP can prevent collapse With 5 cm thin layer starting in year 40 with 5-yr repeat intervals Without any intervention.

17 Uses for MEM-TLP Estimate marsh recovery time from thin layer placement (initially parameterized for Spartina alterniflora) Sediment layer thickness Position in tidal prism Determine biological target sediment thickness (after initial settling and consolidation) Determine biological target sediment elevation (after initial settling and consolidation) Determine thin layer placement frequency under increasing sea level rise

18 Settling/Consolidation Evaluation to Determine Target Construction Elevation Elevation, ft NAVD Elevation Vs Time Target construction elevation 4.25 Target 3-yr elevation 1.30 Fill Foundation Time, years

19 Predicting Initial Settling and Consolidation SETTLE Models initial behavior during placement & dewatering Uses information from column settling test PSDDF Models longer term consolidation Uses data from laboratory consolidation tests Self weight Standard oedometer Models designed for CDFs. Currently evaluating model optimization to account for wetland processes.

20 1. Propose in situ sediment volume. 2. Run SETTLE to determine dredged material thickness and void ratio at end of placement. Iterative modeling process 4. Repeat Steps 1 3 with new volumes until target elevation is achieved. 3. Run PSDDF to determine marsh elevation after consolidation at target time interval.

21 Column Settling Test Supernatant Settled material Sample ports Sediment - water interface - 6 ft tall, 8-in diameter column - Pour in DM slurry - Record sediment-water interface over 15 days - Sample supernatant for water quality data - Use SETTLE model for data analysis Good Luck Point Kettle Creek Beaver Dam Creek Zone Settling Rate, cm/hr Compression settling curve coefficients* CC = AA DDDDDDDDDD 2 BB A = B = A = B = A = B = * C = concentration of fines at the end of placement (g/l), and DTIME = placement period (days) Information can be used to predict bulking (V final / V in situ )

22 Laboratory Consolidation Tests Self-weight consolidation tests Dredged material only Standard oedometer consolidation tests Dredged material and foundation Higher loadings

23 Conceptual marsh topography changes as a result of DM placement and consolidation Target elevation for marsh function Initial fill elevation Sand mounding Pipe discharge Initial fill thickness at several locations Consolidation at several locations Consolidation of the foundation Post-consolidation (new) marsh surface Pre-placement marsh surface Post-consolidation foundation 1. Existing (pre-placement) marsh surface (solid green line) 2. Place DM slurry to initial fill elevation (solid tan line) 3. Over time, the DM consolidates (dotted tan line) 4. Original marsh surface also consolidates (dotted green line) due to weight of placed DM

24 Results Fines Consolidation Good Luck Point - material placed to +30 cm elevation (dotted lines = compressible foundation) 1.5 Elevation, ft Time after placement, days Preplacement elevation (ft) Fill elevation (ft) Modeled surface Fill Foundation Fill Foundation Fill Foundation Fill Foundation

25 Modeling Scenarios Fines One fill elevation, but four fill thicknesses (0.5 ft 2 ft) Sand 5 ft and 14 ft fill thicknesses (deep ponds in Brick A) Sand mounding at pipe outlet Target elevation for marsh function Initial fill elevation Pipe discharge 15 cm 30 cm 45 cm 60 cm Deep sand fill Pre-placement marsh surface Uniform fill elevation, but variable fill thickness

26 Case Studies

27 USACE Philadelphia District pumped sediment from the navigation channel to ~14 ha marsh Dredging and placement occurred between November 2015 and February 2016 Thicknesses ranged from just a few cm up to ~0.5 m in pools

28 Site monitoring was conducted across project partners Study design Stratified random design: (vegetated vs. pool areas) Before-After/Control-Impact (except for SETs and wells) Thickness of placement spatial variation Elevation over time measuring settling, consolidation, and subsidence Soil properties Physical, chemical, nutrients, and microbial biomass Vegetation species, biomass, stem height, cover Epifaunal macroinvertebrates species, abundance, etc. Nekton species, abundance, etc. Avian surveys species, abundance

29 Dry periods in sediment placement locations are sufficient to grow and establish Spartina alterniflora

30 Blackwater NWR thin layer Same location in 2013 marsh edge is mostly stable and vegetation is still vigorous (Photo from Piercy) During 2002 construction: hydraulic placement of predominantly fine-grained dredged material at Blackwater National Wildlife Refuge Wildlife Drive photos courtesy Bob Blama USACE Baltimore District Same location during plug planting then after 1 growing season

31 Forsythe National Wildlife Refuge Areas within Edwin B. Forsythe NWR considered for marsh restoration via TLP: - Good Luck Point - Brick A - Brick B Multiple channels near each area to be dredged for marsh placement

32 Forsythe National Wildlife Refuge Existing Elevation, ft Average Lowest Target Low Marsh Elevation, ft Good Luck Point Brick A - w/o ponds Target High Marsh Elevation, ft Brick B Modeled placement to +1 ft elevation too high; need to continue iterative modeling: Good Luck Point Brick A (BDC) - w/o ponds Existing Elevation, ft Target Low Target High Elevation 1 year post-placement (ft) Average Lowest Marsh Marsh Average preplacement pre- pre- pre ft -0.5 ft 0.0 ft Elevation, ft Elevation, ft elevation +0.5 ft pre Brick B

33 ERDC Environmental Laboratory ERDC: As one of the most diverse engineering and scientific research organizations in the world, ERDC conducts research and development in support of the Soldier, military installations, and the Corps of Engineers' civil works mission, as well as for other federal agencies, state and municipal authorities, and with U.S. industries through innovative work agreements. ERDC operates more than $1 billion in world class facilities at seven labs located in four states with more than 2,100 employees to administer an annual research program exceeding $1 billion Environmental Laboratory The core of EL s research is focused in the areas of ecosystem science and technology and environmental resiliency. EL conducts cross-cutting research in environmental sensing, ecological modeling and forecasting, and risk and decision science. Emerging research areas include the science of environmentally sustainable material, systems biology, climate change, and environmental security. Student Opportunities at ERDC Pathways Programs Contract Student Internships Human Capitol Office: ERDC-HCO@usace.army.mil