Air-Excavation Procedure

Transcription

1 T E C H N I C A L M E M O R A N D U M Date: To: Alan Chiu, Mercertech International, LLC From: Lucas Vannice, PLA, Project Number: Project Name: Mercer Island Chiu Subject: Mercer Island Chiu 5-lot Subdivision (SUB ) This memo is intended to summarize the findings of the air-excavation analysis by Davey Tree Expert Company (Davey) on March 12 14, The purpose of this analysis was to air-excavate along the limits of disturbance adjacent to five trees (821, 736, 827, 828, and 823) mentioned for retention in City Arborist Comment #2 in the January 25, 2018 letter, SUB Request for Information_1 from the City of Mercer Island, and identify the location of roots. The decision to retain or remove these trees was determined based on the findings of this study as described below. Air-Excavation Procedure Davey was retained by the developer to perform exploratory air excavation within the driplines of trees 821, 736, 827, 828, and 823 (Figure 1). Air excavation occurred along the limits of work for proposed development based upon site plans provided by the project engineer, CHS Engineers on May 8, The specific location of work for air-excavation was laid out in the field prior to the arrival of Davey by project developer. Trench location was demarcated on the ground with spray paint. Davey air-excavated within these boundaries. Davey air-excavated to an approximate trench depth of eighteen inches, and used the air-excavation tool to probe for roots to approximately twenty-four inches below grade. Watershed Arborists were on-call during this air-excavation work. On May 13, 2018, all trenches were completed and Lucas met John Kenney, Mercer Island City arborist, on-site to evaluate findings. Air-Excavation Results Tree #821 Air excavation trench for tree #821 was located at the 50% dripline threshold, which was seventeen feet from the trunk and encircled approximately 280 degrees (95 feet) of the dripline circumference. Davey s exploratory trench for tree #821 revealed three smallsized structural roots, and a significant amount of feeder roots throughout the trench (Figure 2). The three small sized roots consisted of a 1 inch cluster of roots along the west side, a 2 inch root along the northwest side, and a 3 inch cluster of roots along the northeast side. Along the northeast and east side, determining which roots belonged to

2 Page 2 tree #821 (subject tree), and which roots belonged to tree #823 (adjacent tree), was difficult. Tree #736 Air excavation trench of tree #736 was located at the 50% dripline threshold on the south and west, which was six feet from the trunk, and totaled 20 feet in length. Davey s exploratory trench for tree #736 revealed one small-sized, 2 inch root to the southwest and feeder roots throughout the trench (Figure 3). Tree #827 Air excavation trench of tree #827 was located along the east side of the tree, with the trench located at the 50% dripline threshold at its closest point. This trench totaled 30 feet in length. Davey s exploratory trench for tree #827 revealed one small-sized 2 inch root and feeder roots throughout the trench (Figure 4). Tree #828 Air excavation trench of tree #828 was located along the north and east side of the tree, with the trench located at the 50% dripline threshold, and totaling 40 feet in length. Davey s exploratory trench for tree #828 revealed two small-sized roots in the trench, one two inches and one one inch in diameter. There appeared to be feeder roots throughout (Figure 5). Due to the proximity to adjacent cherry laurel shrubs, it was difficult to differentiate between roots coming from tree #828 and roots from the cherry laurel. Based on the minimal amount and size of roots discovered, it was determined that tree #828 would be viable for retention. Tree #823 Air excavation trench of tree #823 was located along the north side of the tree, with the trench located at the 50% dripline threshold at its closest point, while following the limits of excavation. This trench totaled 40 feet in length. Davey s exploratory trench for tree #823 revealed one small-sized root and moderate feeder roots throughout the trench (Figure 6). Again, it was difficult to tell if the observed feeder roots were from tree #823, or from adjacent shrubs. Based on the minimal amount and size of roots discovered, it was determined that tree #823 would be viable for retention. Project Recommendations Tree #821 Although only three roots ranging between one and three inches in diameter were discovered as part of the air-excavation study, Tree 821 will require removal due to the impact of the findings of the updated health assessment for Tree 820. See Tree Retention Justification Memo prepared by on for additional information.

3 Page 3 Tree #736 Based on the minimal amount and size of roots discovered, it was determined that tree #736 would be viable for retention. Tree #827 Based on the minimal amount and size of structural roots discovered, it was determined that tree #827 would be viable for retention. Tree #828 Based on the minimal amount and size of roots discovered, it was determined that tree #828 would be viable for retention. Tree #823 Based on the minimal amount and size of roots discovered, it was determined that tree #823 would be viable for retention. Trees Suitable for Retention All trees suitable for retention should be retained with careful attention to minimizing disturbance to the tree. Tree protection fencing should be installed per the Tree Retention Plan prepared by and should be maintained throughout the life of the project. Any damages to these trees which result from demolition or construction will be considered a violation of the tree protection plan and could be enforced by the City Arborist. If dead limbs or limbs which conflict with proposed development exist, a certified climbing arborist should remove these limbs from these trees by climbing without the use of spurs.

4 Figure 1. Site overview map showing locations of air-excavation work in green. Page 3

5 Figure 2. Air excavation site location and photos for Tree 821. Page 5

6 Figure 3. Air excavation site location and photos for Tree 736. Page 6

7 Figure 4. Air excavation site location and photos for Tree 827. Page 7

8 Figure 5. Air excavation site location and photos for Tree 828. Page 8

9 Figure 6. Air excavation site location and photos for Tree 823. Page 9