CoSMoS (Coastal Storm Modeling System) Southern California v3.0 Phase 1 (100-year storm) sea-level rise 0.0 m: wave-hazard projections

Projected Hazard: Model-derived significant wave height (in meters) for the given storm condition and sea-level rise (SLR) scenario.
Model Summary: The Coastal Storm Modeling System (CoSMoS) makes detailed predictions (meter-scale) over large geographic scales (100s of kilometers) of storm-induced coastal flooding and erosion for both current and future sea-level rise (SLR) scenarios. CoSMoS v3.0 for Southern California shows projections for future climate scenarios (sea-level rise and storms) to provide emergency responders and coastal planners with critical storm-hazards information that can be used to increase public safety, mitigate physical damages, and more effectively manage and allocate resources within complex coastal settings.
Phase I data for Southern California include flood-hazard information for the coast from the border of Mexico to Pt. Conception. Changes from the initial November 2015 release may be reflected in small areas. Data are complete for the information presented but are considered preliminary; additional changes may be reflected with model improvements made for the Phase II data release in summer 2016.
Details: Model background: The CoSMoS model comprises three tiers. Tier I consists of one Delft3D hydrodynamics FLOW grid for computation of tides, water level variations, flows, and currents and one SWAN grid for computation of wave generation and propagation across the continental shelf. The FLOW and SWAN models are two-way coupled so that tidal currents are accounted for in wave propagation and growth and conversely, so that orbital velocities generated by waves impart changes on tidal currents. The Tier I SWAN and FLOW models consist of identical structured curvilinear grids that extend from far offshore to the shore and range in resolution from 0.5 km in the offshore to 0.2 km in the nearshore. Spatially varying astronomic tidal amplitudes and phases and steric rises in water levels due to large-scale effects (for example, a prolonged rise in sea level) are applied along all open boundaries of the Tier I FLOW grid. Winds (split into eastward and northward components) and sea-level pressure (SLP) fields from CaRD10 (Dr. Dan Cayan, Scripps Institute of Oceanography, San Diego, California, written commun., 2014) that vary in both space and time are applied to all grid cells at each model time-step. Deep-water wave conditions, applied at the open boundaries of the Tier I SWAN model runs, were projected for the 21st century Representative Concentration Pathway (RCP) 4.5 climate scenario (2011-2100) using the WaveWatch III numerical wave model (Tolman and others, 2002) and 3-hourly winds from the GFDL-ESM2M Global Climate Model (GCM).
Tier II provides higher resolution near the shore and in areas that require greater resolution of physical processes (such as bays, harbors, and estuaries). A single nested outer grid and multiple two-way coupled domain decomposition (DD) structured grids allow for local grid refinement and higher resolution where needed. Tier II was segmented into 11 sections along the Southern California Bight, to reduce computation time and complete runs within computational limitations.
Water-level and Neumann time-series, extracted from Tier I simulations, are applied to the shore-parallel and lateral open boundaries of each Tier II sub-model outer grid respectively. Several of the sub-models proved to be unstable with lateral Neumann boundaries; for those cases one or both of the lateral boundaries were converted to water-level time-series or left unassigned. The open-boundary time-series are extracted from completed Tier I simulations so that there is no communication from Tier II to Tier I. Because this one-way nesting could produce erroneous results near the boundaries of Tier II and because data near any model boundary are always suspect, Tier II sub-model extents were designed to overlap in the along-coast direction. In the landward direction, Tier II DD grids extend to the 10-m topographic contour; exceptions exist where channels (such as the Los Angeles River) or other low-lying regions extend very far inland. Space- and time-varying wind and SLP fields, identical to those used in Tier I simulations, are applied to all Tier II DD grids to allow for wind-setup and local inverse barometer effects (IBE, rise or depression of water levels in response to atmospheric pressure gradients).
A total of 42 time-series fluvial discharges are included in the Tier II FLOW domains in an effort to simulate exacerbated flooding caused by backflow at the confluence of high river seaward flows and elevated coastal surge levels migrating inland. Time-varying fluvial discharges are applied either at the closed boundaries or distributed as point sources within the relevant model domains.
Wave computations are accomplished with the SWAN model using two grids for each Tier II sub-model: one larger grid covering the same area as the outer FLOW grid and a second finer resolution two-way coupled nearshore nested grid. The nearshore grid extends from approximately 800-1,000 m water depth up to 8-10 m elevations onshore. The landward extension is included to allow for wave computations of the higher SLR scenarios. Time- and space-varying 2D wave spectra extracted from previously completed Tier I simulations are applied approximately every kilometer along the open boundaries of the outer Tier II sub-model SWAN grids. The same space- and time-varying wind fields used in Tier I simulations are also applied to both Tier II SWAN grids to allow for computation of local wave generation.
Tier III for the entire Southern California Bight consists of 4,802 cross-shore transects (CST) spaced approximately 100 m apart in the along-shore direction. The profiles extend from the -15 m isobath to at least 10 m above NAVD88. The CSTs are truncated for cases where a lagoon or other waterway exists on the landward end of the profile. Time-varying water levels and wave parameters (significant wave heights, Hs; peak periods, Tp; and peak incident wave directions, Dp), extracted from Tier II grid cells that coincide with the seaward end of the CSTs, are applied at the open boundary of each CST. The XBeach model is run in a hydrostatic (no vertical pressure gradients) mode including event-based morphodynamic change. Wave propagation, two-way wave-current interaction, water-level variations, and wave runup are computed at each transect.
XBeach simulations are included in the CoSMoS model to account for infragravity waves that can significantly extend the reach of wave runup (Roelvink and others, 2009) compared to short-wave incident waves. The U.S. west coast is particularly susceptible to infragravity waves at the shore due to breaking of long-period swell waves (Tp > 15).
Resulting water levels (WLs) from both Delft3D (high interest bays and marshes) and open-coast XBeach (CSTs) were spatially combined and interpolated to a 10 m grid. These WL elevations are differenced from the originating 2 m digital elevation model (DEM) to determine final flooding extent and depth of flooding.
Events: The model system is run for pre-determined scenarios of interest such as the 1-yr or 100-yr storm event in combination with sea-level rise. For Phase I, only the 100-year storm is run. Storms are first identified from time-series of total water level proxies (TWLpx) at the shore. TWLpx are computed for the majority of the 21st century (2010-2100), assuming a linear super-position of the major processes that contribute to the overall total water level. TWLpx time-series are then evaluated for extreme events, which define the boundary conditions for subsequent modeling with CoSMoS. Multiple 100-yr events are determined (varying Hs, Tp, Dp) and used for multiple model runs to better account for regional and directional flooding affects. Model results are combined and compiled into scenario-specific composites of flood projection.
Digital Elevation Model (DEM): Our seamless, topobathymetric digital elevation model (DEM) was based largely upon the Coastal California TopoBathy Merge Project DEM, with some modifications performed by the USGS Earth Resources Observation and Science (EROS) Center to incorporate the most recent, high-resolution topographic and bathymetric datasets available. Topography is derived from bare-earth light detection and ranging (lidar) data collected in 2009-2011 for the CA Coastal Conservancy Lidar Project and bathymetry from 2009-2010 bathymetric lidar as well as acoustic multi- and single-beam data collected primarily between 2001 and 2013. The DEM was constructed to define the shape of nearshore, beach, and cliff surfaces as accurately as possible, utilizing dozens of bathymetric and topographic data sets. These data were used to populate the majority of the Tier II grids and generate initial profiles of the 4,802 CSTs used for Tier III XBeach modeling. All data are referenced to NAD83 horizontal datum and NAVD88 vertical datum. Data for Tiers II and III are projected in UTM, zone 11.
Outputs include: Projected wave height for the 100-year storm and 0.0 m sea-level rise scenario. Data correspond to the near-shore region including areas vulnerable to coastal flooding due to storm surge, sea-level anomalies, tide elevation, and wave run-up during the same storm and sea-level rise simulation.
References Cited: Roelvink, J.A., Reniers, A., van Dongeren, A.R., van Thiel de Vries, J., McCall, R., and Lescinski, J., 2009, Modeling storm impacts on beaches, dunes and barrier islands: Coastal Engineering, v. 56, p. 1,133–1,152, doi:10.1016/j.coastaleng.2009.08.006.
Tolman, H.L., Balasubramaniyan, B., Burroughs, L.D., Chalikov, D.V., Chao, Y.Y., Chen H.S., Gerald, V.M., 2002, Development and implementation of wind generated ocean surface wave models at NCEP: Weather and Forecasting, v. 17, p. 311-333.

预估灾害：针对给定风暴情景与海平面上升（sea-level rise, SLR）情景，由模型推导得出的有效波高（单位：米）。

模型概述：海岸风暴模拟系统（Coastal Storm Modeling System, CoSMoS）可在大范围地理尺度（数百公里）上针对当前及未来海平面上升情景，精细化预测风暴引发的海岸洪水与侵蚀过程（空间分辨率达米级）。针对南加州的CoSMoS v3.0版本可输出未来气候情景（海平面上升与风暴）下的预测结果，为应急响应人员与海岸规划者提供关键的风暴灾害信息，助力提升公共安全、减轻物理损失，并在复杂海岸环境中更高效地管理与调配资源。

南加州第一阶段数据涵盖从墨西哥边境至康塞普西翁角沿岸的洪水灾害信息。2015年11月初始版本发布后，部分区域可能存在数据更新。本次呈现的数据内容完整，但仍属于初步成果；2016年夏季第二阶段数据发布时，将结合模型改进进一步更新数据。

模型架构：CoSMoS模型包含三个层级。层级一（Tier I）包含1个用于计算潮汐、水位变化、流场与洋流的Delft3D水动力FLOW网格，以及1个用于计算陆架海域波浪生成与传播的SWAN网格。FLOW与SWAN模型采用双向耦合模式，使潮汐环流在波浪传播与成长过程中得到考量，同时波浪产生的轨道流速也会对潮汐环流产生影响。Tier I的SWAN与FLOW网格采用统一的结构化曲线网格，覆盖从远海至近岸的全域，分辨率从远海的0.5千米逐步提升至近岸的0.2千米。在Tier I FLOW网格的所有开边界上，均施加了空间分布的天文潮汐振幅与相位，以及由大尺度效应（如长期海平面上升）引发的水位热膨胀增量。同时，每个模型时间步长内，均将时空变化的风场（分解为东向与北向分量）与海平面气压（sea-level pressure, SLP）场（源自CaRD10数据集，Dan Cayan博士，斯克里普斯海洋研究所，加利福尼亚圣地亚哥，私人通信，2014年）应用至所有网格单元。针对21世纪典型浓度路径（Representative Concentration Pathway, RCP）4.5气候情景（2011-2100年）的深水波浪条件，通过WaveWatch III数值波浪模型（Tolman等，2002）与GFDL-ESM2M全球气候模型（Global Climate Model, GCM）提供的每3小时风场数据进行预估，并作为开边界条件输入Tier I SWAN模型运行。

层级二（Tier II）在近岸及需要更高分辨率物理过程解析的区域（如海湾、港口与河口）采用更高空间分辨率。通过1个嵌套外网格与多个双向耦合的区域分解（domain decomposition, DD）结构化网格，可实现局部区域的网格加密与高分辨率模拟。Tier II沿南加州湾被划分为11个分区，以降低计算量并在算力限制内完成模拟。

从Tier I模拟结果中提取的水位与诺伊曼（Neumann）时间序列，分别应用至每个Tier II子模型外网格的岸线平行边界与侧向开边界。部分子模型在侧向Neumann边界条件下出现不稳定，针对此类情况，将一个或两个侧向边界转换为水位时间序列，或保留未赋值状态。由于开边界时间序列均来自已完成的Tier I模拟结果，Tier II与Tier I之间不存在反向数据传输。这种单向嵌套方式可能在Tier II边界附近产生误差，且任何模型边界附近的数据均存在不确定性，因此Tier II子模型的范围在沿岸方向上设置为相互重叠。在向陆方向，Tier II的DD网格延伸至10米等深线，仅在洛杉矶河等河道或其他低洼区域延伸至更内陆的例外情况除外。与Tier I模拟中使用的时空变化风场与SLP场一致，所有Tier II DD网格均施加该类场，以模拟风增水与局部逆气压效应（inverse barometer effects, IBE，即大气压强梯度引发的水位升降）。

为模拟高河道向海径流与向内陆迁移的海岸风暴潮叠加引发的加剧洪水，Tier II FLOW域中共包含42个河道径流时间序列。时变河道径流要么施加于闭合边界，要么作为点源分布于相关模型域内。

波浪计算采用SWAN模型，每个Tier II子模型使用两套网格：1个覆盖外FLOW网格范围的粗网格，以及1个分辨率更高的双向耦合近岸嵌套网格。近岸网格覆盖水深约800-1000米至陆上高程8-10米的区域，向陆延伸的设置是为了支持更高海平面上升情景下的波浪计算。从已完成的Tier I模拟结果中提取的时空变化二维波浪谱，沿Tier II子模型外SWAN网格的开边界以约每千米1个的频率进行施加。与Tier I模拟中使用的时空变化风场一致，两套Tier II SWAN网格均施加该风场，以实现局地波浪生成的计算。

层级三（Tier III）覆盖整个南加州湾，包含4802条沿岸间距约100米的跨岸断面（cross-shore transects, CST）。断面从-15米等深线延伸至至少高于NAVD88垂直基准面10米的位置，若断面向陆端存在潟湖或其他水道，则对断面进行截断。从与CST向海端重合的Tier II网格单元中提取的时变水位与波浪参数（有效波高Hs、峰值周期Tp、峰值入射波向Dp），作为每个CST开边界的边界条件。XBeach模型采用静水压模式（无垂直压强梯度）运行，包含基于事件的地貌动力变化过程。每个断面上均计算波浪传播、双向波流相互作用、水位变化与波浪爬高。

CoSMoS模型中加入XBeach模拟，以考量比短波入射波能显著延伸波浪爬高影响范围的亚重力波（infragravity waves）（Roelvink等，2009）。美国西海岸由于长周期涌浪（Tp>15秒）的破碎，岸线区域对亚重力波尤为敏感。

Delft3D模拟得到的水位（重点覆盖海湾与沼泽区域）与开敞海岸XBeach模拟得到的水位（对应CST断面），经空间合并与插值后生成10米分辨率的网格数据。将该水位高程与原始2米分辨率数字高程模型（digital elevation model, DEM）相减，即可得到最终的洪水淹没范围与淹没深度。

模拟事件：本模型系统针对预设的典型情景运行，如1年一遇或100年一遇风暴结合海平面上升情景。第一阶段仅运行100年一遇风暴情景。首先通过岸线处的总水位代理（total water level proxies, TWLpx）时间序列识别风暴事件。TWLpx基于对构成总水位的主要过程进行线性叠加计算，覆盖21世纪大部分时段（2010-2100年）。随后对TWLpx时间序列进行极端事件筛选，以此作为后续CoSMoS模拟的边界条件。共确定多个100年一遇风暴事件（有效波高Hs、峰值周期Tp、入射波向Dp各不相同），并开展多组模型运行，以更全面地考量区域与方向对洪水淹没的影响。将模型结果整合后，生成针对特定情景的洪水预测合成结果。

数字高程模型（DEM）：本研究的无缝岸带地形水深一体化数字高程模型，主要基于加州海岸地形水深融合项目DEM构建，美国地质调查局地球资源观测与科学（Earth Resources Observation and Science, EROS）中心对其进行了部分修改，以纳入最新的高分辨率地形与水深数据集。地形数据源自2009-2011年加州海岸保护局激光雷达（light detection and ranging, lidar）项目采集的裸土激光雷达数据，水深数据则来自2009-2010年的机载激光测深数据，以及2001-2013年采集的声学多波束与单波束测深数据。本DEM旨在尽可能精准地刻画近岸、海滩与崖岸的地形形态，整合了数十套地形与水深数据集。这些数据主要用于填充Tier II网格，并为Tier III XBeach模拟所用的4802条CST断面生成初始剖面。所有数据均采用NAD83水平基准面与NAVD88垂直基准面。Tier II与Tier III的数据采用UTM投影，分区为11区。

输出内容包括：100年一遇风暴与0.0米海平面上升情景下的预估波高。数据覆盖近岸区域，包含该风暴与海平面上升模拟中，因风暴潮、海平面异常、潮汐高程与波浪爬高而易受海岸洪水影响的区域。

参考文献：
1. Roelvink, J.A., Reniers, A., van Dongeren, A.R., van Thiel de Vries, J., McCall, R., and Lescinski, J., 2009, Modeling storm impacts on beaches, dunes and barrier islands: Coastal Engineering, v. 56, p. 1133–1152, doi:10.1016/j.coastaleng.2009.08.006.
2. Tolman, H.L., Balasubramaniyan, B., Burroughs, L.D., Chalikov, D.V., Chao, Y.Y., Chen H.S., Gerald, V.M., 2002, Development and implementation of wind generated ocean surface wave models at NCEP: Weather and Forecasting, v. 17, p. 311-333.