Donald K. Stauble and Bill Birkemeier Coastal and Hydraulics Laboratory US Army Corps of Engineers

Transcription

1 Donald K. Stauble and Bill Birkemeier Coastal and Hydraulics Laboratory US Army Corps of Engineers

2 Define the Problem Navigation Shore Protection Environmental Political So what is the problem? Management (improper maintenance, Inadequate dredging, material dumped offshore) Engineering/structural (old project falling apart) Changing perceptions (public expects a cure all, not require maintenance, last forever) Changing conditions (change in storm climate, sediment supply, circulation)

3 How does local sponsor need help at this site? What are his perceived problems? Why is there a problem? What is happening? What processes are at work?

4 Many coastal engineering failures can be traced back to inadequate initial project assessment When characterizing a site: Include all components of system Recognize the temporal variability of Most physical and biological processes Be aware of the spatial variability in processes, landforms and underlying geology

5 What data are needed to address the questions? Historical Contemporary from Field Data Collection Study Model or Computer Simulation Combination of Data Sources

6 Historic Resources Previous reports and studies Aerial photography Topography and bathymetric surveys Shoreline change maps Wave, water level and circulation information Surface sediments and subsurface geology

7 Example: Chatham s ~140 Year Historic Cycle of Inlet Formation 1740 Breach 1846 Breach 1871 Breach Spit Breach S. Barrier Island Migration to S.W. 3. Spit Re-growth to South 4. Spit Breach 1987 Breach Chatham Light Monomoy Light (After Giese 1988)

8 Evaluate what to look for in a field study data gaps Identify local geologic processes and geomorphic indicators regional sediment management big picture Evaluate prevailing coastal processes wave climate, tidal circulation, prevailing winds, storm impacts Environmental concerns Talk to locals - local knowledge Identify local competing issues

9 Data Analysis and Project Monitoring Bathymetry and Topographic Surveys morphodynamic change analysis Coastal Processes Analysis waves, tides, wind, longshore currents Aerial Photography geomorphic assessment, shoreline change analysis Sediment Analysis fill suitability, sediment transport Geographic Information System (GIS)

10 Coastal Processes Analysis Wave gage or WIS data (Stauble, 1993) Tidal elevation/velocity data

11 Coastal Processes Analysis Wind Currents (ADCP)

12 Usually not available before the late-1930 s Various film types Black & White, Color, Color IR Scales vary between photo dates function of aircraft altitude and camera focal length May have distortions tilt, roll, & elevation changes of aircraft lens distortions use only center 1/3 of photo May have few common reference points between data sets Tidal fluctuations and flight window may limit use

13 Geomorphic Assessment Evaluate and measure features (inlet width, shoreline, shoals) Look for indications of sediment transport Be wary of: tide level recent storm impact Other errors Measure change in inlet width over time

14 Maps may be based on : Bathymetric survey mean low water shore or mean lower low water shore Topographic surveys - mean high water or mean sea level Aerial Photographs wet/dry line visible on photo Field Survey using GPS or traditional survey mean high water tidal datum based on profile elevation or visible debris line of last high water Published Shoreline Change Map USE WITH CAUTION May have inaccuracies or used incompatible definitions Check original sources shoreline definitions and assumptions

15 Need common vertical and horizontal datums Need common scale Accuracy of some source maps questionable May be based on up to 60 year old data Loss of accuracy via publication Physical change to source map media use milar if possible Shorelines based on field surveys What line was mapped? Shorelines based on aerial photography Was common line used?

16 Sediment Analysis Surface samples to tell sediments on bed for sediment transport and sediment distribution studies Sampling: on water - dredges, grabs on land - hand samples Sampling locations

17 Sediment Analysis Subsurface sampling to tell layering and underlying geologic structure Sampling: on land auger drill, vibracores on water vibracores, rotary drills.

18 Sediment Analysis Lab Analysis Grain size distributions Sediment composition/layering - cores

19 High-resolution subbottom profiling Define layers, structure Use to locate sand bodies, plan coring

20 bottom bedforms sediment texture

21 Geologic Framework: for project planning and design Indicator of processes Geophysical and sediment analysis

22 Regional Management Considerations The bigger picture! Littoral Cells, Southern CA. Coastal Compartments, Middle Atlantic Bight Inman and Frautschy (1966)

23 Use to organize all geographic data types May be too sophisticated for some studies

24 Use GIS as a tool to: Geo reference all available geographic data to common horizontal and vertical coordinate system on a base map Digitize and mosaic aerial photography to a common scale Digitize Inlet/coastal features from screen Measure area and calculate volumes of features Identify coastal evolution patterns Develop management plan

25 Base Map Theme Layer Air Photo Mosaic Theme Layer ArcView Image Analysis Create base map theme Make photo mosaic Register photos in coordinate system AutoCAD Base Map Control Points

26 Line Features Shorelines wet/dry line Flood and Ebb Shoal Edge Channel Centerlines

27 Shoreline Change Breach N N

28 North Flood Shoal Shoreline Area - North Spit On Screen Measurement Polygon Features 1982 pre inlet 1987 post inlet