台风风暴潮影响下潮滩沉积动力模拟初探——以江苏如东海岸为例

Simulation of Tidal Flat Sedimentation in Response to Typhoon-induced Storm Surges: A case study from Rudong Coast,Jiangsu,China

以江苏如东潮滩为研究区,采用沉积动力学垂向二维概念模型来模拟正常天气和台风期间潮滩沉积的空间分布特征,探讨台风风暴潮对潮滩正常沉积层序的改造作用。模拟结果表明,在涨落潮时间—流速对称特征明显的如东海岸,潮汐作用使潮滩沉积呈显著的分带性,且剖面形态向"双凸形"演化,两个"凸点"分别位于平均高潮位和平均低潮位附近。在台风期间风暴增水效应下,开边界悬沙浓度差异将导致潮滩冲淤和沉积分布格局的变化,潮上带和潮间带上部均堆积泥质沉积物,潮间带中下部在风暴过程中普遍遭受不同程度的砂质沉积物侵蚀或之后堆积泥质沉积物,在沉积层序中形成风暴冲刷面。因此,潮滩的风暴沉积记录存在于潮间带上部或更高部位。以此模型为基础,可进一步综合考虑极浅水边界层水动力结构、沉积物粒度分布变化、波—流联合作用、台风降水、互花米草等生物活动、潮沟摆动及人工围垦等因素,从而建立风暴事件在沉积层序中的时间序列,更好地解译沉积记录中的古环境信息。

Tidal flat sedimentation is mainly controlled by sediment supply and hydrodynamic conditions,and the resultant sedimentary records are valuable in the study of the evolution of coastal environments at various time-scales ranging from several days associated with extreme events to millenniums related to climate changes. A strong storm surge event may destroy a relatively thick sedimentary sequences formed over a long period of time. Therefore,it is important to evaluate the influence of extreme events on the continuity and temporal resolution of tidal flat sedimentary records. In the present contribution,an approach to the modeling of the spatial distribution patterns of tidal flat accretion / erosion,under both fair weather and storm surge conditions,is proposed. This model is applied to the Rudong coast,Jiangsu Province,which is exposed to frequent typhoon attack,to illustrate the sediment dynamic processes and the modification of normal tidally-dominated sedimentary sequences associated with storm surges. The model consists of four parts that deal with the current velocity and near-bed shear stress due to tides or combined tide-typhoon effects,suspended sediment transport and vertical( settling and erosion) fluxes,bedload transport and accretion / erosion,and morphological evolution of the bed,respectively. Driven by the tidal water level curve reconstructed by using the harmonic analysis of the observed data from the study area,the model output reproduced the zonation pattern of intertidal flat sedimentation under the fair weather conditions. Furthermore,its prediction about the tidal flat cross-shore profile associated with a small bed slope,strong tidal currents and a weak time-velocity asymmetry,which is characterized by a 'double convexity'shape,is consistent with the observed shape for the study area. According to the model output,the two convexities are located in the vicinity of mean high water and mean low water,respectively. Subsequently,the model was run taking into account both tides and storm surges. Under the condition that a storm surge occurs in association with astronomical spring tides,which occurred over the study area in 1981( i. e.,Typhoon No. 8114),the model prediction is that the bed is subjected to mud accretion over the supratidal zone and the upper part of the intertidal zone,while sand erosion occurs over the lower-middle parts of the intertidal zone. Thus,an erosion surface is formed within the sedimentary sequence,representing the storm effect. This pattern,once again,is consistent with the in-situ observation made following the typhoon event for the area. The model output implies that storm-induced sedimentary record can be found in the upper parts of the tidal flat. It should be noted that the results presented here are only preliminary: a number of detailed morphological parameters about the storm deposit are not available in the model output. In the future,the model may be further improved by taking into account the factors such as the boundary characteristics under extreme shallow water conditions,variability of grain size distribution curves,combined tidal currents and waves,biological activities,tidal creek migration and artificial land reclamation. In combination with inverse methods,the forward modeling will be beneficial to a better interpretation of the formation of tidal flat sedimentary records.