Case Study: Surveying saltwater marshes

INTRODUCTION AND BACKGROUND

An increasing number of environmental engineering firms need standardized high-quality spatial information across various terrain to aid in environmental planning and design activities as well as to assess their impact.  Precise mapping of both elevation and vegetation disseminations in salt marshes is imperative for conservation objectives.

PROBLEM

Recently an AXIS client desired topographic data to improve their tidal flooding modeling computations for two saltwater marshes encompassing 4,670 acres.Tidal flood modeling based on traditional cross section data conflicted with project wide LiDAR based topography.

SOLUTION

LiDAR and imagery was collected for use as the basis of topographic design.  A flight to capture this data was performed to coincide with predicted mean lower low water (MLLW) to capture a greater percentage of the marsh terrain exposed above the tide level, as water tends to absorb most of the laser used in LiDAR analysis.

The project employed Airborne HD-LiDAR and multi-band Imagery for topographic and vegetation analysis mapping. The LiDAR data was classified into both bare earth and numerous vegetation classes which enabled detailed analysis by biologists assessing the varying heights, density and types of marsh grasses.

Additionally, a detailed surface model was built from the topographic data to support study of the tidal flow and ebb through the marsh, its effect on sediment transport, nutrient loading and groundwater modeling.

Cross sections and random elevation checks proved the high accuracy of bare earth surface derived from the LiDAR data. The LiDAR showed substantial detail and revealed terrain features which allowed a much clearer picture of how the marsh fills and drains with each tidal cycle and proved that bank overtopping was not a regular source of this flow.

LESSONS LEARNED

Airborne LiDAR data provided significant penetration through the existing vegetation and identification of subtle topographic features throughout the work area that would either not be captured using traditional grid topographic methods, or would require significant manpower, effort with greater damage to the marsh.

The sites contain very large areas of gentle slopes, which are flooded during high tide event from many small creeks that wander across the site, some natural and some manmade. The manmade ditches are typically very long and straight, with excavation spoils piled on one side. The piles of soft material have sloughed down to very wide, barely discernable berms that are masked by dense vegetation. These minor surface details would not have been analyzed without the use of HD LiDAR data.

HD-LIDAR Ground Points only

 

BENEFITS

  • More accurate design dataset
  • Substantial cost savings
  • Almost zero intrusion or damage to the marsh and or its inhabitants

 

SERVICES

  • Airborne HD‐LIDAR & Imagery Acquisition
  • Ground control survey NAD83 and NAVD88 datums
  • Digital Aero-Triangulation
  • HD-LiDAR processing
  • 1”=60’ scale planimetric mapping
  • 4-band (R,G,B,IR) digital orthoimagery (Class 1 Accuracy)

 

DELIVERABLES

  • Bare Earth DEM/DTM (non-hydro) 1 meter GSD
  • Mass Points from LiDAR Data
  • Breaklines
  • 1’ Contours
  • DEM metadata
  • 1”=60’ scale planimetric mapping (AutoCAD 2011 and ArcInfo Shapefile formats)
  • 4-band (R,G,B,IR) 0.25’ GSD digital orthoimagery (Class 1 Accuracy) in TIF/TFW format

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