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Shading under Engineered Experimental Pier Units
Shading under Engineered Experimental Pier Units
In the summer-fall of 2006 we evaluated shading impacts by two piers constructed (Fig 1) using a traditional design with board decking. Board spacing was 0” and 1.5” (max surveyed); pier heights ranged from 2’ to 6’, and widths from 4’ to 8’. Light data was collected under the center of the pier simulators every 5 minutes and daily means were calculated (Fig 2, 3). In the spring-summer of 2007 we evaluated shading impacts by two additional piers, one constructed in traditional design with board spacing of 0.75” and the other decked with a new composite grid (Fig 4). The grid had gaps of 3”x0.5” and was manufactured by FiberGrate; pier heights ranged from 2’ to 6’ and widths from 4’ to 8’ as in the first round of simulations. Light data was collected every 5 minutes and daily means calculated as above. The results of these four pier simulations are reported in Table 3; 6 of the 30 structures were found to cause sufficient shading to kill both Juncus roemerianus and Spartina alterniflora plants based on the shading experiments. Ecograte or similar grid decking provided sufficient light transmission to allow marsh plants to survive under all combinations tested (Table 3).
Figure 1 - Pier segments used to determine shading profiles by changing height and width.
Figure 2 - Data from the selected piers and light profiles taken during Jul-Oct 2006. Similar data were collected on two additional designs tested from Mar - June 2007.
Figure 3 - Data from the Line Quantum Sensors taken during Oct 2006 (top panel) and spring 2007 (bottom panel). The grated decking had more light transmission under all scenarios of height and width.
Figure 4 - Two new piers constructed in Mar 2007. Structure on left is ¾” spaced deck boards, structure on right is Ecograte. Note the increased light penetration through the Ecograte compared to decking timbers.
Table 3 - Shading levels under the various pier combinations tested. Shading was calculated from the PAR readings on the bottom center sensor as a % of the light at the top center sensor, integrated over the full daylight hours and standardized to a 12 hour day to correct for different daylengths. Shade levels (in red) greater than 90% cause the mortality of marsh plants, and between 55-90% are in the “stressed” zone (yellow).
Conclusions from the Engineered Experimental Pier Units were that:
Taller piers provided more opportunity for higher light levels entering along the sides and towards the center of the pier.
Narrower pier widths provided less overall shading impacts as less area of the marsh is covered over.
Wider board spacing (where safe) allows more light to penetrate down through the decking, but grate materials are even better.
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