Aerial oblique photograph of Giacomini Wetlands taken after levee breach. Northern portion of East Pasture or Shallow Shorebird area. Photograph: Courtesy of Robert Campbell Aerial Photography.
This change was predicted—at least in part. Computer hydraulic modeling conducted as part of planning for the restoration project estimated that, based on existing and proposed elevations, 256 of the 550 acres in the East (area adjacent to Point Reyes Station) and West (area adjacent to Inverness Park) Pastures of the Giacomini Ranch would be inundated by tides daily or close to daily (KHE 2006; see attached graphic). This modeling assumed that no levees present and some tidal channel recreation. The estimated high water extent, which represent slightly less than half of the total Project Area, fall below Mean High Water elevations ranging from 5.0 to 5.1 feet NAVD88 (KHE 2006). During higher high tides, which occur roughly 6.5% of the time, approximately 310 acres below elevations ranging from 5.7 to 5.8 ft NAVD88 would be inundated by Tomales Bay waters (KHE 2006). Very infrequently, during extreme high tides, as has been observed in mid-December and early January, approximately 382 acres might be expected to be underwater where elevations fall below 7.0 to 7.1 ft NAVD88 (KHE 2006).
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In terms of average and low tide conditions, approximately 63 acres below the Mean Sea Level elevation of 3.5 ft NAVD88 are estimated to be inundated 65% of the time, only being exposed during lower low tide conditions (KHE 2006). This represents approximately 11.5% of the Giacomini Wetlands area.
Approximately 10.0 acres or less than 1.8% of the Project Area, representing primarily the bottoms of tidal and stream channels, would be expected to be permanently inundated or subtidal (KHE 2006). While subtidal areas outside of the Giacomini Ranch fall below 0.49 ft NAVD88, computer modeling predicted that subtidal elevations would range from as high as 3.4 ft NAVD88 in the West Pasture to 2.0 ft NAVD88 in the East Pasture (KHE 2006). These differences result from the fact that two separate gravel bars control minimum water surface level elevations or the extent of subtidal areas for the East and West Pastures by acting as natural "weirs" (KHE 2006). One gravel bar system is north of the Giacomini Ranch in Lagunitas Creek just at and slightly north of the former North Levee in the West Pasture. This bar, which tops out at around 2 feet NAVD88, controls minimum water surface elevations and, therefore, subtidal conditions in Lagunitas Creek and the East Pasture. A separate gravel bar just north of where the culvert for Fish Hatchery Creek used to be controls minimum water levels or subtidal conditions in the West Pasture: the top of this bar is approximately 3.4 feet NAVD88.
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During the first few months after breaching, Park Service staff has been monitoring and mapping tide levels during higher high tide and low tide events to determine how well water levels in the Project Area corresponded with those predicted by hydraulic modeling.
The upper extent of higher high tides appears to match very well with that predicted by computer modeling, often mimicking the contour patterns exactly (see attached graphic). During tides ranging from 6.3 ft to 6.6 ft MLLW (6.8 ft to 7.1 ft NAVD88), tidewaters extended up to the 7.0 to 7.5 ft NAVD88 elevations in the East Pasture (NPS, unpub. data). Computer modeling predictions estimated maximum tidal elevations in the East Pasture of 7.15 ft NAVD88, with the area exposed to tidal influence at extreme tides being close to 282 acres (KHE 2006; Figure 1 - 940 KB PDF). However, while the maximum tidal elevations were sometimes greater than predicted, the amount of area in the East Pasture subject to extreme tidal influence turned out to be slightly lower than that predicted (251.7 acres), a difference of 11 percent (NPS, unpub. data; Figure 2 - 1,015 KB PDF).
In the East Pasture, tidewaters often follow the approximate contours of grading of the Marshplain Enhancement area, a 16-acre area in the southwestern portion of the East Pasture that was lowered 1 to 2 feet in elevation. In addition, they extend just slightly upstream of the area where Tomasini Creek was rerouted out of its formerly leveed alignment to create Tomasini Slough. Figure 3 (52 KB PDF) shows minimal water fluctuations in upper Tomasini Slough prior to breaching of the levee and the corresponding change in water levels and water level fluctuations with the accidental breaching of the levee in early July 2008 and early November 2008, after the levees were purposefully breached. As designed, during the extreme high tides, tidewaters extended up to the very base of the berm for the Tomasini Triangle Marsh.
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The maximum elevations reached by extreme high tides were a little more variable in the West Pasture. During tides ranging from 6.3 ft to 6.6 ft MLLW (6.8 ft to 7.1 ft NAVD88), tidewaters extended up to the 5.75 ft to 7.0 ft NAVD88 elevations in the West Pasture (NPS, unpub. data; Figure 2). During extreme tides, tidal waters were predicted to reach a maximum elevation of 7.13 ft NAVD88 (KHE 2006; Figure 1). Some attenuation or drop in tidal amplitude would be expected as tidewaters crest the banks of tidal creeks and spread across the marshplains or former pastures, although there was little attenuation in the East Pasture. The variability in elevation may result from the influences of freshwater, which may "push back" tidal influence. While the southern extent was surveyed prior to any major rains, some of the surveying in the central and northern portions of the West Pasture near Fish Hatchery Creek did take place after some rain, and it was near Fish Hatchery Creek that maximum tidal elevations were lowest (5.75 ft NAVD88; Figure 2). Extreme high and low tide lines were mapped using salinity of waters as a guide, particularly after rains and groundwater flow increased freshwater influence. Despite the variable and generally lower maximum tidal elevations in the West Pasture, there was only an 8 percent difference between predicted extent of area inundated at maximum tides (98 acres; KHE 2006) and actual area (90.7 acres; NPS, unpub. data; Figures 1 and 2).
Where computer modeling predictions and actual water levels diverge comes with low tide conditions. While mapped high tide water levels correspond pretty closely with predicted levels, water levels during lower low tides are much higher than predicted by computer modeling. In other words, more area is remaining subtidal than predicted. Mapping of the water levels during predicted lower low water conditions ranging from -1.7 ft to -0.4 ft MLLW (-1.2 ft to +0.1 ft NAVD88) showed that water levels do not drop below 4 ft NAVD88 in the East Pasture and approximately 3.75 ft to slightly below 4 ft NAVD88 in the West Pasture (Figure 2). Predicted subtidal water levels in these areas again were 2.0 ft NAVD88 in the East Pasture and 3.4 ft NAVD88 in the West Pasture (KHE 2006; Figure 1).
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In the East Pasture, this increase in minimum water level represents more than a substantial increase in the areal extent of permanent inundation from 26.5 acres using predicted subtidal elevations (2.0 ft NAVD88) to 109.4 acres under actual low tide conditions (NPS, unpub. data; Figures 1 and 2). This difference constitutes an increase in subtidal areas from approximately 7.6% to 31% of the 350-acre East Pasture. The discrepancy is not quite so great in the West Pasture, where predicted extent of permanent inundation (2.2 acres; KHE 2006) or 1% of the West Pasture is only slightly lower than the actual extent of subtidal area (7.4 acres; NPS, unpub. data) or 3.7% of the West Pasture (Figures 1 and 2).
Most of this subtidal ponding occurs in the northern portion of the Project Area, where elevations are the lowest, typically because of subsidence or a drop in elevations due to construction of levees and drying out and subsequent compaction of soils. In the West Pasture, persistent ponding occurs in Fish Hatchery Creek and West Pasture Old Slough, as well as in the northeastern portion of the Project Area where the borrow ditch was removed despite efforts to deconstruct in such a way as to not create a depressional basin. Some areas in the southern portion of the West Pasture are functioning more as residual basins of higher salinity waters that flood during high tides and subsequently pond and are, therefore, not necessarily representative of "subtidal" conditions.
In the East Pasture, flooding is particularly persistent in the Shallow Shorebird area adjacent to the Point Reyes Mesa, which is the lowest pasture or marshplain area in the Project Area. The elevation gradient in this area is such that tidal overflow from Tomasini Slough drains eastward to the lowest elevations in the East Pasture adjacent to the Point Reyes Mesa and the levee of the former Tomasini Creek channel. Here, waters pond, because there are no channels or outlets that would allow drainage on low tides.
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So, what is causing these higher than predicted subtidal water levels, particularly in the East Pasture? One factor is that, while levees have been removed, the undiked marsh that had developed on the outboard of the levees is, in many cases, higher in elevation than the marshplains or former pastures. These marsh shelves, then, are acting as mini "levees" and not allowing tidal waters to drain westward into Lagunitas Creek, particularly in the northern portion of the East Pasture. The model assumed that there would be no levees present. Because of these mini-levees, waters are being funneled exclusively through the two primary tidal channel outlets that were created—the Tomasini Slough, which flows into Lagunitas Creek near Railroad Point in the northern portion of the East Pasture, and, to a lesser extent, the new side channel for Lagunitas Creek, which drains the new Marshplain Enhancement area in the southwestern portion of the East Pasture.
Extremely strong tidal velocities at the mouth of Tomasini Slough indicate that tidal channels are still adjusting to the volume of tidal waters that enter during high tides, with velocities expected to drop somewhat when the width of the channel becomes large enough to fully accommodate flows. Because channel width is not large enough currently to fully accommodate flows, waters are not fully draining on the low tide, leaving the amount of open water area in the East Pasture larger than would be expected based on elevation alone. This also accounts for the large lag observed in the peak low tide conditions, which often are delayed beyond predicted low tide times at Inverness by as much as 2 hours or more.
Over the coming years, existing and created channels will increase in size to accommodate flood flows, and new tidal channels will potentially develop, increasing exchange between the restored wetland and Lagunitas Creek and creating more of an equilibrium between tidal inflow and outflow. In addition, some portions of the higher elevation undiked marsh outboard of the levees may continue to erode (as they have been doing prior to restoration), allowing more tidal waters to sheetflow across the marshplain back into Lagunitas Creek. Some of these changes may be accelerated during flood events, although storms so far have not been of sufficient magnitude to have altered any of the wetland landscape.
Figures:
Figure 1 - Predicted Tidelines (940 KB PDF)
Figure 2 - Post-Restoration Tidelines (1,015 KB PDF)
Figure 3 - Change in Tidal Regime (52 KB PDF)
-- Content for this page was composed by Lorraine Parsons, Project Manager, Giacomini Wetland Restoration Project, Point Reyes National Seashore
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