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. However, what was anticipated less was the evolutionary process by which hydrology within the restored Ranch would evolve, similar to that of vegetation. During planning for the restoration project, 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). This modeling assumed no levees remained and that there was some tidal channel re-creation. The estimated high water extent, which represents slightly less than half of the total Project Area, falls at or 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 feet NAVD88 were predicted to 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 feet NAVD88 (KHE 2006; Figure 1 - 940 KB PDF).
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In terms of average and low tide conditions, approximately 63 acres below the Mean Sea Level elevation of 3.5 feet 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.
The model originally predicted that approximately 10.0 acres or less than 1.8% of the Project Area—representing primarily the bottoms of tidal and stream channels—would be permanently inundated or subtidal (KHE unpub. data; Figure 1 - 940 KB PDF). However, later modeling efforts recognized that low water levels would be driven not only by topography, but by height of gravel bars and shoals in the main creek channel that Giacomini Ranch waters would drain to, Lagunitas Creek. While subtidal areas outside of the Giacomini Ranch fall below 0.49 feet NAVD88, computer modeling predicted that subtidal elevations would range from as high as 3.4 feet 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. These bars increase the volume of water retained permanently under subtidal conditions, with subtidal areas predicted at 28.7 acres, almost three times the area predicted under earlier modeling efforts (KHE 2006).
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During the first few months after breaching, Park Service staff monitored and mapped 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. Additional monitoring was conducted in December 2009 of extreme high tides and low tides during neap tide conditions, with follow-up monitoring of low tides during spring tide conditions expected in April 2010.
During both 2008 and 2009, the upper extent of higher high tides appearedto match very well with that predicted by computer modeling, often mimicking the contour patterns exactly (Figure 2 - 235 KB PDF). In winter 2008, during tides ranging from 6.3 feet to 6.6 feet MLLW (5.8 feet to 6.1 feet NAVD88), tidewaters extended up to the 7.0 to 7.5 feet NAVD88 elevations in the East Pasture (NPS, unpub. data). Computer modeling predictions estimated maximum tidal elevations in the East Pasture of 7.15 feet 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 - 235 KB PDF). In 2009, the acreage of areas subject to high tides was only slightly less (242.5 acres), which probably resulted largely from the fact that mapping was conducted during an approximately 1 foot lower tide level (5.8 feet to 6.3 feet MLLW or 5.3 feet to 5.8 feet NAVD88; Figure 2 -235 KB PDF). Otherwise, there appeared to be good agreement between 2008 and 2009 high tide extent.
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 feet to 6.6 feet MLLW (5.8 feet to 6.1 feet NAVD88), tidewaters extended up to the 5.75 feet to 7.0 feet NAVD88 elevations in the West Pasture (NPS, unpub. data; Figure 2 - 235 KB PDF). During extreme tides, tidal waters were predicted to reach a maximum elevation of 7.13 feet NAVD88 (KHE 2006; Figure 1 - 940 KB PDF). Some attenuation or drop in tidal amplitude would typically be expected as tidewaters crest the banks of tidal creeks and spread across the marshplains or former pastures, although there actually appeared to be amplification relative to the predicted tide level in both the West and East Pastures. The variability in elevation extent in the West Pasture may result from the influences of freshwater, which may "push back" tidal influence where freshwater surface or groundwater flow is strongest. While the southern extent of the West Pasture was surveyed prior to any major rains in 2008, 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 feet NAVD88; 2008 Line in Figure 2 -235 KB PDF). 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 in 2008 (90.7 acres; NPS, unpub. data; Figure 1 (940 KB PDF) and Figure 2 (235 KB PDF)). Interestingly, in 2009, the extent of area subject to high tides actually increased very slightly—91.9 acres—even though the restored wetland was surveyed under lower high tide conditions than in 2008 (NPS, unpub. data).
Where computer modeling predictions and actual water levels diverge substantially 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 feet to -0.4 feet MLLW (-1.2 feet to +0.1 feet NAVD88) showed that water levels do not drop below 4 feet NAVD88 in the East Pasture and approximately 3.75 feet to slightly below 4 feet NAVD88 in the West Pasture (Figure 2 - 235 KB PDF). Predicted subtidal water levels in these areas again were 2.0 feet NAVD88 in the East Pasture and 3.4 feet NAVD88 in the West Pasture (KHE 2006).
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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 feet NAVD88) to 109.4 acres under actual low tide conditions (NPS, unpub. data; Figure 1 (940 KB PDF) and Figure 2 (235 PDF)). 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 (Figure 1 (940 KB PDF) and Figure 2 (235 PDF)). Based on mapping conducted in December 2009, the extent of subtidal areas is actually lower under neap tide conditions—when tidal range between low and high tides is substantially reduced—than under spring tide conditions, when low tides reach some of their lowest levels. In December 2009, subtidal areas totaled 52.9 acres in the East Pasture and 4.5 acres in the West Pasture when tides ranged between 1.9 and 2.7 feet MLLW (1.4 and 2.2 feet NAVD88). This represents almost a 51% reduction in subtidal area with an approximately a 1- to at most 3-foot difference in tidal water elevation. These results suggest that drainage is constrained by the larger volume of water that flows into the newly restored wetland on a spring tide, when high tides are very high, than on a neap tide, when high tides are lower.
Most of the 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 the new side channel for Lagunitas Creek, which drains the new Marshplain Enhancement area in the southwestern portion of the East Pasture.
