EFFECTS OF LIVE & DECOMPOSING TREE ROOTS ON LEVEE SEEPAGE: OBSERVATIONS OF FULL-SCALE FIELD TESTING

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Michelle Shriro, PE, GE with Jonathan Bray, PhD, PE,

1 EFFECTS OF LIVE & DECOMPOSING TREE ROOTS ON LEVEE SEEPAGE: OBSERVATIONS OF FULL-SCALE FIELD TESTING by Michelle Shriro, PE, GE with Jonathan Bray, PhD, PE, Nicholas Sitar, PhD, and Diego Cobos-Roa UC Berkeley Photo by Roy Kroll

California Levee Vegetation Research Project FUNDING AGENCIES AND PARTNERS: (CLVRP) UC Berkeley Team: Professor Jonathan Bray Professor Nicholas Sitar Michelle Shriro Diego Cobos-Roa Field

2 California Levee Vegetation Research Project FUNDING AGENCIES AND PARTNERS: (CLVRP) UC Berkeley Team: Professor Jonathan Bray Professor Nicholas Sitar Michelle Shriro Diego Cobos-Roa Field Construction and Repair URS Corporation (Twitchell Island) with Roy Kroll and Keith Millard (California DWR), American River Flood Control District with Diamond D Construction (Cal Expo), Reclamation District 1601 Thanks to: CLVRP research team and advisors, Peter Buck (SAFCA), Cassandra Musto and Roy Kroll (DWR), Richard Evans (UCD), John Lichter (Tree Associates), Les Harder (HDR), Alison Berry and Shih-Ming Chung (UCD),Dirk VanVuren (UCD), Doug Shields (USDA), Gerald Bawden, Jim Howle and Sandra Bond (USGS), ERDC research team

Full Scale Field Testing Test 1: Parallel Trench Wetting Front Test Smaller scale Decomposing Stump & Control Zone Wetting front captured, but saturated

3 Full Scale Field Testing Test 1: Parallel Trench Wetting Front Test Smaller scale Decomposing Stump & Control Zone Wetting front captured, but saturated flow was not achieved Test 2: Centerline Trench Seepage Test Live tree(s) & Control Zone Wetting and full saturation achieved Captured seepage through levee

4 Test 1: California Exposition and State Fair (Cal Expo) - American River

5 Upper Trenches Stump Trench Control Trench

6 Lower Stump Trench Lower Control Trench Stump

7 Silica Flour Migration

8 Results: First Water

9 Wetting Front: Day 3 Wetting front delayed in front of stump

10 Characterization

core Bark completely decomposed Root completely decomposed Space

11 Annular Space in Decomposing Roots (a) (b) Minor discontinuous annular space found on some decomposing roots between bark and woody core Bark completely decomposed Root completely decomposed Space loosely filled with sand and not continuous Photo (a) by Richard Evans (UCD)

around and through decomposing roots No gap observed between live

12 Live Roots Live roots present from nearby Hackberry tree (base of slope) Small live roots seen frequently growing opportunistically around and through decomposing roots No gap observed between live roots and soil Preferential flow of fluid was not observed around live roots

13 Mammal Holes Line B: Direct Connection Line C: No Outlet Near Line C: Not Connected at Source

through the space when the fluid source has a direct

14 Flow of Silica Flour Discontinuous annular space between decomposed woody core and bark; fluid observed to pass through the space when the fluid source has a direct connection to the gap (i.e., silica flour) Tensiometer Entry Point

15 Water Balance 19,000 gallons 45,000 gallons LOST: 45k 19k 3k = ~23k gallons

16 Summary of Test 1 Observations Mammal holes dominated flow paths Wetting front delayed near stump/roots No preferential fluid flow through roots (live or decaying) during the flow test Fluid observed to pass through discontinuous annular space between woody core and bark when the fluid source had a direct connection Much of water flowed down vertically

17 Twitchell Island Centerline Trench Seepage Test 2

18 Twitchell Island Centerline Seepage Test 2 Site

19 Landside (looking south) Site Layout, cont.

20 Waterside Waterside Vegetation

22 Transition from Silts, Sandy Silts, Silty Sands into Plastic Silt/Clay Sandy Silt to Silty Sand Transition

23 Representative Levee Stratigraphy Silt (ML) Gravel Silt (MH) Silty Sand (SM) Silt (MH) Sand (SP)

24 Control Trench

25 Waterside Oak Tree Waterside Landside 6 to 8 4 to 6 0 to 4

26 Landside Tree Waterside Landside

27 Mammal Burrows Source: jhilmer.com

28 Piezometer Section A Tensiometer Seep

29 Piezometer Tensiometer Seep Crack N

30 Landside Seepage Patterns

31 Day 1: Cracking Between Trenches

32 First Seep at Waterside Oak DAY 1: 10:40pm DAY 2: 6:30am DAY 3: 4:15am

34 Fallen Waterside Oak Angle Change = ~19 degrees

35 Waterside Oak Failure Preliminary LiDAR Courtesy of Gerald Bawden, USGS

36 Observations of Numerous Fallen Trees at Twitchell Island Water side trees near water line are more suseptable to instability

37 Summary of Test 2 Observations As in Test 1, burrowing mammal holes played a big role in determining flow patterns and early seep locations Soil stratigraphy was important in determining seepage patterns Additional soil exploration required to distinguish effect of tree from stratigraphic effects No evidence of decomposing roots No evidence of flow through live roots

38 Observations of the Effects of Trees on Levees Burrowing mammal holes dominated flow paths Soil stratigraphy was also important Decayed tree stump delayed wetting front No preferential fluid flow through live or decaying roots during flow tests Localized flow through discontinuous annular space between woody core and bark when the fluid source had a direct connection to it Regular maintenance is essential to identify and remove precarious trees

California Levee Vegetation Research Program

California Levee Vegetation Research Program Influence of Tree Roots and Mammal Burrowing Activity on Levee Performance: Volume 1. Review of Literature and Case Histories 1.7w x 2.1h Photo 1.5pt border,