Hydrodynamic modeling for river delta salt marshes using lidar

topographyA Cell-Edge technique for bathymetric processing

Ben R. HodgesDept. of Civil, Architectural, and Environmental

EngineeringUniversity of Texas at Austin

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The lidar challenge for hydrodynamics

• Processed lidar at 1x1 m resolution• Hydrodynamic models at 10x10 m or coarser.• Simple averaging loses information.• Ho do you handle subgrid data?• Today we’ll look at an example of

topographical processing for hydrodynamic representation.

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Where we are working...

Texas coastimage from Google Earth

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Where we are working...

Corpus Christi

Houston

image from Google Earth

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Where we are working...

Nueces River Delta

City of Corpus Christi

image from Google Earth

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Nueces River Delta

image from Google Earth

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1 x 1 m ~ 120 x 106

10 x 10 m ~ 1.2 x 106

30 x 30 m ~ 130,000

data processed by J. Gibeaut, Texas A&M Corpus Christi

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Summary of Cell-Edge technique

• Use median filter on fine grid data with Matlab medfilt2() function for baseline coarse grid.

• Identify coarse grid cells with a large fraction (45%) of subgrid cells that are a defined increment (0.2 m) higher than coarse grid median and use 75th percentile for coarse grid elevation.

• In each coarse grid cell, begin at median elevation and sweep upwards in elevation increments (0.1 m) using Matlab bwboundaries() function to identify obstacles.

• Find highest obstacle that crosses entire cell on X axis, and create an edge feature in X direction.

• Repeat for Y direction.

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Conclusions• Image processing techniques (available in Matlab) provide

powerful tools for bathymetric processing.• Advantage of Cell-Edge method: Creates a coarse grid array

with cells and edges that create hydraulically blocking features.

• Open issue: How to confirm that the edges are continuous?• Open issue: 75th percentile elevations on high cells tend to

slightly underestimate blocking elevation.• Open issue: How to handle connectivity of subgrid channels?• Disadvantage: Median filter is computationally expensive for

large arrays.

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Acknowledgements• Coastal Bend Bays and Estuaries Program• U.S. National Science Foundation• Texas Water Development Board• City of Corpus Christi• U.S. Army Corps of Engineers

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