Calibration of an estuarine sediment transport model to sediment fluxes as an intermediate step for simulation of geomorphic evolution
Introduction
Modeling estuarine geomorphic evolution addresses concerns that include, but are not limited to, wetland restoration, legacy contaminant resuspension, and estuarine habitat distribution. In light of continued sea level rise and uncertainty of future temperature and precipitation changes, development of appropriate models may assist in preparing for future changes. Sea level rise may increase tidal prism and possibly inundate emergent marshes, thereby altering the sediment transport regime in both channels and fringe areas. Changes in temperature and precipitation will modulate watershed runoff and therefore sediment loads, possibly altering or threatening seaward habitats (Scavia et al., 2002; Pont et al., 2002). Anthropogenic effects on sediment loads have proven to be important (Cappiella et al., 1999); effects from climate change may easily be as large. Future water management practices, which may alter the hydrograph more than climate change, will also effect the timing and magnitude of sediment loads to estuaries.
Modeling estuarine sediment transport with a tidal-timescale model typically involves calibrating to the following hierarchy of data: tidal stage, velocity, salinity, and suspended-sediment concentration (SSC) (e.g., McDonald and Cheng, 1997; Lumborg and Pejrup, 2005). Prior efforts in geomorphic (or boundary flux) modeling of estuaries have used these calibrated tidal-timescale models to simulate bed evolution. Lumborg and Pejrup (2005) predict net fluxes over one year using a 20 d time-series of SSC as the validation parameter. Schoellhamer et al. (in press), however, show that calibration to these parameters does not guarantee accuracy in terms of modeling geomorphic evolution. Uncertainty in input parameters can cause bed evolution to adjust in response to erroneous values; this adjustment will not recognized as a “spin-up” effect, and the simulation of geomorphic evolution will be compromised. For example, Schoellhamer et al. (in press) show that a 10% error in tidal velocity can cause a bed adjustment that requires 10 years to equilibrate.
Some recent efforts to predict morphological development have used more robust approaches: Douillet et al. (2001) adjusted parameters in order to obtain best qualitative agreement between observations of percent mud on the seabed and simulated deposition; Ouillon et al. (2004) further calibrated the Douillet model using satellite-derived estimates of SSC. This two-step approach provides greater confidence than a single-step approach. Hibma et al. (2003) developed an approach for long-term geomorphic modeling, evaluating the results by comparing the development of morphological features within the model to measured morphological features from two estuaries. The lesson from prior and current efforts is clear: a model must be calibrated and validated to the type of data that will be the final product of the modeling effort.
Two types of data provide the most robust calibration information: frequent bathymetric surveys, and continuous cross-sectional sediment flux data. The former gives a snapshot of bathymetric change between survey dates, though the expense and difficulty of these surveys results in large temporal spacing between surveys (∼10 years). This temporal spacing is adequate for decadal-scale geomorphic modeling, but the actual inter-annual and year-to-year mechanics of the sediment transport cannot be verified. In this regard, continuous cross-sectional sediment flux data satisfies multiple goals. The net sediment budget will be correct if the fluxes are modeled correctly, and the tidal and subtidal timescales of sediment transport can be modeled and evaluated. Decadal trends in sediment fluxes will accumulate to alter decadal trends in net erosion and deposition. Therefore, confidence in modeling sediment fluxes generates confidence in modeling net sediment budget trends. However, this is only an intermediate step; the spatial variability of erosion and deposition must still be evaluated using decadal-timescale bathymetric change data.
The two aforementioned data types are available for Suisun Bay, California (Fig. 1, Fig. 2), though their temporal coverage does not overlap. Five bathymetric surveys were performed in Suisun Bay, spanning from 1867 to 1990 (Cappiella et al., 1999). These data show the influence of hydraulic mining on sediment deposition (1867–1887), while the subsequent reduced input of mining debris and decreased freshwater flows (1922–1942) results in net erosion. It would be possible to calibrate a model to these data alone, though the inter-annual and year-to-year sediment transport mechanics could not be evaluated.
An additional data set of cross-sectional sediment flux data, however, is available at the landward and seaward boundaries of Suisun Bay, Mallard Island and Carquinez Strait, respectively (Fig. 1, Fig. 2). These data are available for water years 1997–1998, and 2002–2004. McKee et al. (2006) estimated advective and dispersive loads between the Sacramento/San Joaquin Delta and Suisun Bay at Mallard Island, using continuous SSC data and freshwater flow measurements. Advective loads were estimated as the product of daily averaged point SSC (at the edge of the channel) and daily freshwater flow; the relationship between dispersive and advective flux was determined using point flux data from multiple periods. An error analysis yielded an average uncertainty of ±25% for daily sediment fluxes. Further details can be found in McKee et al. (2006). Ganju and Schoellhamer (2006) developed estimates of advective, dispersive, and Stokes drift flux between Suisun Bay and Carquinez Strait, based on measurements of SSC, longitudinal salinity gradient, and freshwater flow. Continuous point measurements were collected over a two-month period, while cross-sectional measurements were collected over two tidal cycles. The cross-sectional measurements were used to establish the point measurements as surrogates for cross-sectional sediment fluxes; the estimated cross-sectional fluxes were then used to establish long-term continuous data as surrogates for cross-sectional sediment fluxes. Errors in this method ranged from ±22% for Stokes drift flux to ±48% for dispersive flux. Further details of these methods can be found in Ganju and Schoellhamer (2006). The data for both boundaries overlap for five water years which span extremes of freshwater flow and sediment load, and are within the domain of the model.
Our goal for this study is to calibrate the model at the intermediate annual-timescale. We have already calibrated and applied the model at the tidal-timescale, for the simulation of estuarine turbidity maximum (ETM) dynamics (Ganju and Schoellhamer, in press). Tidal-timescale features, such as gravitational circulation, can accumulate to modulate annual-timescale features, such as prolonged landward transport during the dry season. Decadal trends in these features, such as prolonged drought, then accumulate to alter decadal geomorphic trends. Simulating annual sediment fluxes is the intermediate step, before simulating historical decadal bathymetric change, that builds confidence in modeling future scenarios of geomorphic change.
Section snippets
Site description
Our study area is Suisun Bay, the most landward subembayment of northern San Francisco Bay (Fig. 1, Fig. 2). The Sacramento and San Joaquin Rivers deliver freshwater to Suisun Bay, during the winter and spring, due to rain, snowmelt, and reservoir releases. Precipitation is negligible during late spring and summer. Suisun Bay is a partially mixed estuary that has extensive areas of shallow water that are less than 2 m deep at mean lower-low water. Shallow estuarine environments such as Suisun
Calibration and validation
Model performance was varied with respect to the four goals of: (1) representing the inter-annual variability of cross-sectional sediment fluxes, (2) modeling relative year-to-year variability, (3) modeling the net sediment budgets within the error of the estimates, and (4) modeling the magnitude of episodic and inter-annual events as closely as possible. For the calibration years, 1997 and 2004, all goals were met, except for goal 4 in 1997 (matching of peak events). For the validation years,
Delivery phase
The delivery of suspended sediment from the Central Valley to the Delta and Suisun Bay occurs during freshwater flow pulses in the winter and spring. The Delta's depositional capacity is an important feature which must be captured to adequately simulate loads between the Delta and Suisun Bay. The use of an idealized Delta is validated by correct simulation of the episodic sediment delivery phase past Mallard Island, though modeled sediment load past Mallard Island is biased high for the two
Conclusions
Developing an estuarine geomorphic model requires appropriate calibration data beyond tidal stage, currents, salinity, and SSC. Optimally, frequent bathymetric surveys can be used for evaluating the capabilities of a model. However, the time and cost-intensive nature of these surveys dictates typical intervals of at least a decade. This is sufficient to evaluate the performance of a geomorphic model; however, decades of sediment transport must be modeled up-front. An accompanying data set of
Acknowledgments
This study was supported by the US Geological Survey's Priority Ecosystems Science Program, CALFED, and the University of California Center for Water Resources. Use of the ROMS model was supported by the US Geological Survey's Community Sediment Transport project, with assistance from John Warner, and from the entire ROMS community. The constructive comments of the two anonymous reviewers greatly improved the focus and detail of this manuscript. This work was conducted as part of “CASCaDE:
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