CoSMoS (Coastal Storm Modeling System) Southern California v3.0 Phase 2 wave-hazard projections: 20-year storm in Los Angeles County

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 2 data for Southern California include flood-hazard information for the coast from the border of Mexico to Pt. Conception. Several changes from Phase 1 projections are reflected in many areas; please read the model summary and inspect output carefully. Data are complete for the information presented.
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, Los Angeles, 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. 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 I and II grids. To describe and include impacts from long-term shoreline evolution, including cumulative storm activity, seasonal trends, ENSO, and SLR, the DEM was modified for each SLR scenario. Long-term shoreline (Vitousek and Barnard, 2015) and cliff (Limber and others, 2015) erosion projections were efficiently combined along the cross-shore transects to evolve the shore-normal profiles. Elevation changes from the profiles were spatially-merged for a cohesive, 3D depiction of coastal evolution used to modify the DEM. These data are used to generate initial profiles of the 4,802 CSTs used for Phase 2 Tier III XBeach modeling and determining final projected flood depths in each SLR scenario. 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 storm and sea-level rise scenario indicated. 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: Howell, S., Smith-Konter, B., Frazer, N., Tong, X., and Sandwell, D., 2016, The vertical fingerprint of earthquake cycle loading in southern California: Nature Geoscience, v. 9, p. 611-614, doi:10.1038/ngeo2741.
Limber, P., Barnard, P.L. and Hapke., C., 2015, Towards projecting the retreat of California’s coastal cliffs during the 21st Century: in, Wang, P., Rosati, J.D., and Cheng, J., (eds.), The Proceedings of the Coastal Sediments: 2015, World Scientific, 14 p., doi:10.1142/9789814689977_0245
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.
Vitousek, S. and Barnard, P.L., 2015, A non-linear, implicit one-line model to predict long-term shoreline change: in, Wang, P., Rosati, J.D., and Cheng, J., (eds.), The Proceedings of the Coastal Sediments: 2015, World Scientific, 14 p., doi:10.1142/9789814689977_0215.

### 预估灾害：针对给定风暴情景与海平面上升（Sea-Level Rise, SLR）情景，由模型推演得到的有效波高（单位：米）。
### 模型说明：海岸风暴模拟系统（Coastal Storm Modeling System, CoSMoS）可在数百公里级的大范围地理区域内，针对当前及未来海平面上升情景，高精度模拟风暴引发的海岸洪水与侵蚀过程（米级分辨率）。南加州CoSMoS v3.0版本可针对未来气候情景（海平面上升与风暴情景）生成预估结果，为应急响应人员与海岸规划师提供关键风暴灾害信息，助力提升公共安全、减轻实体财产损失，并在复杂海岸区域内更高效地管理与调配资源。
南加州第二阶段数据涵盖从墨西哥边境至康塞普西翁角（Pt. Conception）的海岸洪水灾害信息。相较于第一阶段预估结果，多个区域已更新模型设定；请仔细阅读模型说明并核查输出结果。本次展示的所有数据均完整有效。
### 细节：模型架构说明：CoSMoS模型包含三个层级。
第一层级包含一套Delft3D水动力FLOW网格，用于计算潮汐、水位变化、径流与海流；以及一套SWAN网格，用于模拟陆架区波浪生成与传播。FLOW与SWAN模型采用双向耦合模式：波浪传播与成长过程中会考虑潮流影响，同时波浪产生的轨道流速也会改变潮流。第一层级的SWAN与FLOW模型采用统一的结构化曲线网格，覆盖范围从远海延伸至近岸，分辨率从离岸区域的0.5公里提升至近岸区域的0.2公里。在第一层级FLOW网格的所有开边界处，均加载了空间分布变化的天文潮汐振幅与相位，以及因大尺度效应（如长期海平面上升）引发的水位热膨胀升幅。针对每个模型时间步长，所有网格单元均加载了时空变化的风速场（分解为东向与北向分量）与海平面气压（Sea-Level Pressure, SLP）场，数据来源于CaRD10数据集（美国加利福尼亚州拉霍亚斯克里普斯海洋研究所Dan Cayan博士，2014年私人通信）。针对第一层级SWAN模型的开边界深水波浪条件，采用WaveWatch III数值波浪模型（Tolman等人，2002年）与GFDL-ESM2M全球气候模型（Global Climate Model, GCM）提供的3小时分辨率风速场，推演得到21世纪典型浓度路径（Representative Concentration Pathway, RCP）4.5情景（2011-2100年）下的深水波浪条件。
第二层级可实现近岸区域与物理过程复杂区域（如海湾、港口与河口）的更高分辨率模拟。采用一套嵌套外网格与多组双向耦合区域分解（Domain Decomposition, DD）结构化网格，可在需要的区域实现局部网格加密。第二层级沿南加州湾划分为11个分段，以降低计算耗时并满足计算资源限制。
从第一层级模拟结果中提取的水位与诺依曼（Neumann）时间序列，分别加载至每个第二层级子模型外网格的平行岸线与侧向开边界。部分子模型的侧向诺依曼边界存在不稳定性，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，或保持未赋值状态。开边界时间序列均来自已完成的第一层级模拟结果，因此第二层级与第一层级之间不存在双向数据交互。由于这种单向嵌套模式可能在第二层级边界附近产生错误结果，且任何模型边界附近的数据均存在不确定性，因此第二层级子模型的范围在沿岸方向设置为相互重叠。在向陆方向，第二层级区域分解网格延伸至10米地形等高线；仅在洛杉矶河等河道或其他低洼区域延伸至内陆的情况除外。所有第二层级区域分解网格均加载了与第一层级模拟一致的时空变化风速与SLP场，以模拟风增水效应与局地气压反照效应（Inverse Barometer Effect, IBE，即水位随大气压强梯度变化而升降）。
共计42组河道径流时间序列被加载至第二层级FLOW网格域，以模拟河道向海径流与向内陆传播的沿岸风暴增水交汇引发的回流洪水。时变河道径流要么加载于闭合边界，要么以点源形式分布于相关模型域内。
波浪模拟采用SWAN模型，每个第二层级子模型均配置两套网格：一套与外FLOW网格覆盖范围一致的粗分辨率网格，以及一套更高分辨率的双向耦合近岸嵌套网格。近岸网格的覆盖范围从水深800~1000米延伸至陆上高程8~10米，设置向陆边界以实现高海平面上升情景下的波浪模拟。沿第二层级子模型外SWAN网格的开边界，约每1公里加载一组从已完成第一层级模拟中提取的时空变化二维波浪谱。同时，所有第二层级SWAN网格均加载了与第一层级模拟一致的时空变化风速场，以模拟局地波浪生成过程。
第三层级覆盖整个南加州湾，包含4802条跨岸断面（Cross-Shore Transects, CST），沿岸方向间距约100米。剖面范围从-15米等深线延伸至至少比北美垂直基准面88（NAVD88）高10米的区域；若断面陆侧存在泻湖或其他水道，则截断该断面。从与跨岸断面向海端重合的第二层级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与入射波波峰方向Dp），被加载至每条跨岸断面的开边界。XBeach模型以静水压模式（无垂直压强梯度）运行，包含基于事件的地貌动力变化模拟，可计算每条断面的波浪传播、双向波流相互作用、水位变化与波浪爬高。
CoSMoS模型引入XBeach模拟以考虑亚重力波（infragravity waves）的影响：相较于短波入射波，亚重力波可显著延长波浪爬高的影响范围（Roelvink等人，2009年）。由于长周期涌浪（Tp>15秒）的破碎作用，美国西海岸岸线尤其易受亚重力波影响。
将Delft3D模型（针对高关注度海湾与湿地区域）与开放海岸XBeach模型（跨岸断面模拟结果）得到的最终水位（Water Levels, WLs）进行空间拼接，并插值至10米分辨率网格。将这些水位高程与原始2米分辨率数字高程模型（Digital Elevation Model, DEM）做差值计算，以确定最终洪水淹没范围与淹没深度。
### 模拟事件：模型系统针对预设目标情景运行，如结合海平面上升的1年一遇与100年一遇风暴情景。首先从岸线处的总水位代理指标（Total Water Level Proxies, TWLpx）时间序列中识别风暴。假设总水位由主要影响过程线性叠加得到，针对21世纪大部分时段（2010-2100年）计算得到TWLpx时间序列。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。筛选得到多组100年一遇风暴（有效波高、峰值周期与入射方向各不相同），并开展多组模型模拟，以更全面地考虑区域与方向相关的洪水影响。将模型结果整合并编译为针对特定情景的洪水预估合成结果。
### 数字高程模型（Digital Elevation Model, DEM）：本无缝一体的地形水深数字高程模型主要基于加州海岸地形水深融合项目DEM，并经美国地质调查局地球资源观测与科学中心（Earth Resources Observation and Science, EROS）修改，以纳入最新的高分辨率地形与水深数据集。地形数据来源于2009-2011年加州海岸管理局激光雷达项目采集的裸地激光雷达（Light Detection and Ranging, lidar）数据，水深数据来源于2009-2010年的水深激光雷达数据，以及2001-2013年采集的声学多波束与单波束测深数据。本DEM旨在精准刻画近岸、海滩与崖岸的地形形态，整合了数十套地形与水深数据集，用于构建第一与第二层级网格的大部分区域。为描述并纳入长期岸线演变的影响，包括累积风暴活动、季节变化、厄尔尼诺-南方涛动（ENSO）与海平面上升，本DEM针对每种海平面上升情景分别修改。将长期岸线（Vitousek与Barnard, 2015年）与崖岸（Limber等人, 2015年）侵蚀预估结果沿跨岸断面整合，以更新垂直岸线的地形剖面。将剖面得到的高程变化进行空间拼接，得到连贯的海岸演变三维可视化结果，用于修改DEM。这些数据用于生成第二阶段第三层级XBeach模拟所用的4802条跨岸断面的初始剖面，并确定每种海平面上升情景下的最终预估洪水淹没深度。所有数据均采用北美大地基准面83（NAD83）水平基准与北美垂直基准面88（NAVD88）垂直基准。第二与第三层级数据采用通用横轴墨卡托投影（UTM）第11带投影。
### 输出结果包括：对应指定风暴与海平面上升情景的预估波高。数据覆盖近岸区域，包括风暴增水、海平面异常、潮汐水位与波浪爬高引发海岸洪水的脆弱区域，且与同情景下的风暴与海平面上升模拟结果一致。
### 参考文献：
1. Howell, S., Smith-Konter, B., Frazer, N., Tong, X., and Sandwell, D., 2016, The vertical fingerprint of earthquake cycle loading in southern California: *Nature Geoscience*, v. 9, p. 611-614, doi:10.1038/ngeo2741.
2. Limber, P., Barnard, P.L. and Hapke., C., 2015, Towards projecting the retreat of California’s coastal cliffs during the 21st Century: in, Wang, P., Rosati, J.D., and Cheng, J., (eds.), *The Proceedings of the Coastal Sediments: 2015*, World Scientific, 14 p., doi:10.1142/9789814689977_0245
3. 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.
4. 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.
5. Vitousek, S. and Barnard, P.L., 2015, A non-linear, implicit one-line model to predict long-term shoreline change: in, Wang, P., Rosati, J.D., and Cheng, J., (eds.), *The Proceedings of the Coastal Sediments: 2015*, World Scientific, 14 p., doi:10.1142/9789814689977_0215.