Iowa State University, Department of Natural Resource Ecology and Management, 2310 Pammel Dr., Ames, IA 50011, USA.
Iowa Geological Survey, University of Iowa, 340A Trowbridge Hall, Iowa City, IA 52242, USA.
Sci Total Environ. 2019 Apr 10;660:1015-1028. doi: 10.1016/j.scitotenv.2019.01.038. Epub 2019 Jan 7.
Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of suspended sediment (SS) and total phosphorus (TP) to floodplain storage within the 52.2 km Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Cross sectional area of the 25 in-field channel cross sections increased by a mean of 17% over the 16 year study period (1998-2014), and field data indicate a general trend of degradation and widening to be present along Walnut Creek's main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Estimated annual fluxes of SS and TP to floodplain storage decreased by 61 and 62% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 9 and 18%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses and floodplain storage opportunities continue to decline. In addition to loss of storage, higher discharges confined to the channel may have greater stream power, resulting in further enhancement of SS and TP export through accelerated bed and bank erosion. These increased contributions to watershed loads may mask SS and TP reductions achieved through edge of field practices, thus making it critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale.
冲积平原储存通常代表泥沙收支中最大的泥沙通量之一。在响应大规模干扰的流域中，随着河道演化的进行和河道几何形状的相关变化，漫滩-河道侧向连通性可能会随时间而变化。本研究通过 25 个现场河道横断面测线、水力模型（HEC-RAS）以及流量站衍生的水质和水量数据，调查了河道几何形状变化对美国爱荷华州 52.2 公里长的核桃溪流域（美国爱荷华州）漫滩淹没频率以及悬浮泥沙（SS）和总磷（TP）向漫滩储存通量的影响。25 个现场河道横断面的横截面积在 16 年的研究期间平均增加了 17%，现场数据表明，核桃溪干流存在退化和变宽的趋势。产生侧向漫滩流动所需的估计流量增加了 15%，研究期间的漫滩淹没体积减少了 37%。估计的 SS 和 TP 向漫滩储存的年通量分别减少了 61%和 62%。研究期间，向漫滩储存通量的估计减少量分别使 SS 和 TP 向流域出口的排放量增加了 9%和 18%。随着河道演化的进行和漫滩储存机会的持续减少，SS 和 TP 向流域出口的贡献可能会继续增加。除了储存损失之外，限制在河道内的更高流量可能具有更大的水流动力，从而通过加速床面和岸坡侵蚀进一步增强 SS 和 TP 的输出。这些对流域负荷的增加贡献可能掩盖了通过田间边缘实践实现的 SS 和 TP 减少，因此在解决流域尺度的泥沙和磷负荷时，必须考虑河道演化的阶段和进展。
