Subsidence Reversal through Marsh Re-establishment on Twitchell Island
Motivation: Due to the diking and draining of marshes for farming in the Sacramento-San Joaquin Delta, the land has severely sub-
sided, forming bowl-shaped islands up to 7 m below sea level (1). As the land subsides, it creates a greater threat to the region’s freshwater supply, since a levee breach could allow saltwater to intrude from San Francisco Bay into the Delta. One proposed method to reverse
this subsidence is to create marshes by flooding areas to shallow depths suitable for marsh vegetation and thereby, restoring natural
processes. Below a threshold, marshes naturally keep pace with sea level rise through the growth of below-ground biomass, anaerobic
(i.e. slow) decay of vegetation, and incorporation of incoming sediments.
D
F
NW Boardwalk
BF
CE
B
C
A
only pipe base located
Project Background: In 1997, a 3-hectare experimental plot was created by the USGS on Twitchell Island in the Sacramento-San
Joaquin Delta. The area was flooded to 25 cm. Within two growing seasons, freshwater wetland vegetation, primarily tule (Schoenoplectus spp.) and cattail (Typha spp.), had filled in the area (2). One aspect of this experiment was to measure the change in surface elevation,
which was performed using feldspar markers and SETs (Surface Elevation Table). Measurements were taken by USGS until 2008.
The goal of our project was to update the surface elevation measurements and determine if subsidence reversal had continued.
E
100 m
Letters are SET pipe base locations.
Methods: We constructed an SET-like device that consisted of a vertical quad-strut lowered into the estab-
lished pipe base. It attached to two parallel single struts with vertically aligned holes. A thin copper tube with a 5
cm diameter “foot” was then threaded through the two single struts. This arrangement ensured that the rod was
perpendicular to the horizontal crosspiece when lowered to the marsh surface. A bull’s-eye bubble level was attached to verify all measurements were level. One rod was used multiple times to obtain 15 point elevations.
Results: Surface elevation Only one of the original eight SET pipe bases could be located at the site.
Results from Miller et al. 2008 report an accretion rate of 4.18 cm/yr from 1997 to 2005 at this one location. Our results show an increased rate of 7.96 ± 1.1 cm/yr from 2005 to 2013.
-467.6 cm NAVD88
Surveyed from local benchmark (PID: JS1725)
SET pipe base located (Point C)
-495.4 cm, October 2013
(Current study)
100
OLD PEAT
SET pipe base
driven to refusal
during construction in 1997
-594.4 cm = Project Datum
(19.5 ft) below MSL
Laser levelled in 1997
110
Relating to previous measurements Measurements had to be tied into a
90
Relative Surface-Elevation (cm)
NEWLY ACCRETED
Measurement setup (October 2013)
nearby benchmark in order to be compared the initial elevation in 1997. This surveying
was performed using a Topcon AT-G6 Auto Level over a total distance of 1 km, creating a
large uncertainty. Note, it is possible that the SET pipe base has shifted over time and that
these measurements are not being related to the original data set correctly.
80
70
60
Conclusions: The increased rate of elevation change may be due to the increased
*Area in green is
reprinted from
Miller et al. 2008
07 08 09 10 11 12 13 14 15
Date of Measurement
Filled circles are from SET measurements. Bars indicate IQR.
Open diamonds are from feldspar measurements.
Madeline Fostera (madeline@berkeley.edu), Bronwen Stanfordb, Micheal Levyc, Evan Varianoa
a
Environmental Engineering, UC Berkeley; bEnvironmental Studies, UC Santa Cruz; cBiophysics, UC Berkeley
40
Water Level (cm)
Water depth maintained
at 25 cm from 1997-2008
Variable from 2008 - present
-563.7 cm, March 2005
(Miller et al., 2008)
Known Water Levels Since 2008
Water levels Throughout the USGS experiment (1997-2008) the site’s water level was
maintained at 25 cm. Since that time, the water levels have varied. Data was pieced together to investigate this variability. The water level is important, as it controls the oxygen
availability for decay processes. For the available data in 2013, there were seven times the
water table was at or below the surface of the marsh for 8 hours or longer, most likely creating aerobic conditions and thus, increasing the rate of decay.
Dense vegetation in the
marsh (October 2013)
25 cm
20
0
−20
−40
−60
Jul 21Dec 3,
2010
Jan 1Oct 20,
2011
Apr 25Dec 31,
2012
Jan1Nov 6,
2013
Jul 21 − Dec 31,Jan
2010
1 − Oct 20,Apr
2011
25 − Dec 31, Jan
2012
1 − Nov 6, 2013
2010-2011 Source: USGS
2011-2012 Source: UC Berkeley, Biometeorology Lab
water levels observed in 2010 and 2011. The available data shows a median water level of 47 cm, which is closer to the conditions in
a second experimental pond that is not discussed here. This deeper pond (55 cm) had higher rates of elevation change with a maximum of 9.15 cm/yr in one area with emergent vegetation from 2001-2005 (2).
Even with the large uncertainty, our measurements show that the surface elevation continued to increase. The positive change that
occurred despite the lack of constant inundation is a promising result for new restoration projects in the Delta’s islands.
Works Cited:
1. Miller RL, Fram MS, Fujii R, Wheeler G. 2008. Subsidence reversal in a re-established wetland in the Sacramento-San Joaquin Delta, California, USA. San Francisco Estuary and Watershed Science 6(3): article 1.
2. Miller RL and Fujii R. (2011). Re-establishing marshes can turn a current carbon source into a carbon sink in the Sacramento-San Joaquin Delta of California, USA. In D. A. Contreras
(Ed.), River Deltas: Types, Structures and Ecology (pp. 1–34). Nova Science Publishers, Inc.