An
Application of Microtopography: Using UAS LiDAR to Quantify Burrowing Shrimp
Mounds in Willapa Bay
Willapa Bay tidal flats in Washington
state contain aquaculture operations (oyster/clam farming). Native burrowing
shrimp form burrows and mounds in the mud, liquifying it in the process. This can
cause shellfish to sink in the mud and die, posing disruptions to aquaculture. There
is a need to better understand the extent and distribution of burrowing shrimp
populations in the bay in order to assess the impacts of their presence and/or
removal. Quantification of shrimp mound density serves as a proxy for shrimp
population. This research investigates methods for quantifying shrimp mounds
through UAS LiDAR-derived terrain analysis. High resolution true-color imagery
(1.33cm) and LiDAR data (~4cm point spacing) was acquired for mudflats in
Nahcotta. Using a sink-fill algorithm on a 5cm DSM derived through kriging
interpolation along with a corresponding hillshade allowed for reasonable
qualitative identification of shrimp mound regions. Using SAGA GIS’s Vertical
Distance to Channel Network tool along with a threshold, a mound area
percent cover of 37.5 was identified. This was compared to percent cover values
from 100 quadrat samples. Accuracy of the LiDAR-based quantification shows high
sensitively to the average mound radius used for the quadrat samples. Radii
values from 3 – 5cm produce percent errors that range from -53 to +30.5, with
3.5cm showing less than 5 percent error. Overall, mound density quantification proved
more viable than individual mound identification. Classification methods
employing true-color and/or hyperspectral imagery can likely supplement this
method and provide a means for further validation.
