Monday, January 6, 2014

Pond Mapping: Why Wait Until Spring?

At this writing, a "Polar Vortex" is invading the Upper Midwestern US.  Temperatures are peaking in the Twin Cities of Minnesota at -11 F give or take a few degrees.  Although this temperature is a wee-bit extreme for even the hardiest of souls, Momentum Environmental recently explored how they could capitalize on the long winter months in Minnesota to rapidly map municipal stormwater basins.  Local municipalities across the US are looking for cost-effective ways to rapidly inventory their stormwater basin infrastructure to ensure compliance with new State and Federal mandates.

Equipped with a tape measure, an ice auger, a jug of water, a Lowrance HDS-5, and their best thermal ice-fishing garb, they set out to test the mapping capabilities of Lowrance and ciBioBase through 11.5" of ice on an stormwater detention pond in Blaine, Minnesota.
Lance Hoff of Momentum Environmental prepares one of five sites for ground truth measurements of sonar through ice, sonar through a drilled hole, and hand measured depth
Water was poured onto the ice prior to logging .sl2 sonar data in order to achieve bottom acquisition through the ice
Aerial view of pond and GPS Tracks.  At numerous locations along the track, measurement sites were prepared and the transducer gathered bottom data through the ice.  Data were later uploaded to ciBioBase and processed for depth, vegetation biovolume, and bottom hardness.
Hoff and Dan Murphy, also from Momentum trudged back and forth through the snow while the sonar and GPS was continuously logging.  Hoff and Murphy cleared snow and watered the ice every several feet.  This was enough for the HDS skimmer transducer to penetrate the ice and get a bottom lock (Lowrance also manufactures an ice transducer that may be a better long-term solution than the skimmer which is designed for mounting on a boat transom).

After about an hour of data collection in this manner, the SD card with the logged pond data were uploaded to ciBioBase. Approximately 10 minutes later contour, vegetation biovolume, and bottom hardness maps were processed, complete with water volume summary statistics.  The data were ready for export and comparison with the other test datasets!
ice, pond mapping, stormwater, lowrance, .sl2, ciBioBase, Momentum
One foot bottom contours produced with ciBioBase processing of Lowrance .sl2 data collected through the ice.  An ice thickness and water elevation offset was applied to the data.
ice, pond mapping, stormwater, lowrance, .sl2, ciBioBase, Momentum
Vegetation biovolume (% water column filled with vegetation) and bottom hardness maps (red = hard, tan=soft)were simultaneously created along with bottom contours
The table below shows some interesting results.  First, depths from the Lowrance HDS-5 were remarkably accurate and precise compared with actual depth measurements taken with the tape measure. The largest difference found was approximately 3" (which may have been due to the tape measure sinking that far into the muck). These findings are consistent with other verification research published in this blog showing Lowrance HDS producing highly accurate and precise depth measures in a variety of environments.

Results from 5 verification sites where holes were drilled and depth was compared with a tape measure (hand measure), sonar through open water, and sonar through the ice.  Differences between the measures (subtraction) are also presented
Another noteworthy finding that Momentum and Contour Innovations will be exploring further is the effect of ice depth and composition on through-ice depth measurements.  Because sound travels quicker through ice than fluid water, we expected to find shallower than actual depths through the ice.  We plan follow up studies with higher sample sizes at a range of depths and ice conditions to determine the range of this bias and proper offsets to apply to the contour data in ciBioBase.  We'll keep you posted here.

These findings demonstrate the power and flexibility of the ciBioBase system for rapid inventorying and monitoring of stormwater retention basins.