Po River planimetric adjustment and degree of anthropogenic confinement (1954-2021)

We examine the planimetric evolution of the Po River mainstem via multi-temporal (1954/55-1975/78-1988/89-1998/99-2004/05-2021) mapping of the active channel bed including the secondary channels. The active channel is the portion of a river comprised between the main margins (i.e., the banks) that includes the low-flow channel bed, as well as unvegetated to sparsely vegetated bars/islands and excludes vegetated bars/islands characterized by homogeneous forest (and/or shrub) cover. In this work, we use the active channel width as a proxy of transport capacity in relation to in-channel sediment storage, as well as upstream and lateral sediment supply from the banks and the tributaries. Additionally, because wide channels generally evolve where there is a lot of lateral motion due to active bar deposition, we use the active channel width as a qualitative indicator of local sediment deposition (or the divergence of the sediment flux).
To quantify anthropogenic confinement, we map the active channel bed on the six sequential airphoto sets and historical topographic maps, and then classify the relevant margins into natural (i.e., mobile) and artificial (i.e., immobile/confining) ones.
The active channel width and the number/area of secondary channels are analyzed with respect to the concurrent degree of anthropogenic confinement. To control for the effects of local anomalies associated with discrete water and sediment inputs (e.g., tributary confluences) and barriers (e.g., dams) that may confound underlying spatial patterns of channel adjustment, the Po River mainstem is subdivided into six valley segments. This segmentation is based on the location of main confluences, dominant channel bed texture (gravel or sand), channel width, slope, degree of anthropogenic confinement as evaluated in photo year 2021, and presence/typology of secondary channels. In particular, the segments’ upper ends were set: (1) upstream of major tributary confluences (i.e., Stura di Lanzo in segment S1, Dora Baltea in S2, Sesia in S3, Ticino in S4 and Oglio in S6); (2) at major dams (i.e., Isola Serafini in S5). The lower end of segment 6, i.e., the downstream limit of the study area is defined at the Po River delta. This morphological approach aims to reduce in-segment heterogeneity while maximizing between-segment differences, so that channel adjustment may be evaluated in relation to different channel typology.

本研究通过多时相（1954/55-1975/78-1988/89-1998/99-2004/05-2021）的测绘，考察了沱江主河槽的平面演化过程，包括次级河道。主河槽系指河流中由主岸（即河岸）所围成的部分，包括低流量河床、未植被或稀疏植被的河滩/岛屿，并排除以均质森林（及/或灌木）覆盖的植被河滩/岛屿。在本研究中，我们将主河槽宽度作为运输能力的一个代理指标，与河床内沉积物储存、上游及河岸和支流侧向的沉积物供给相关联。此外，由于宽阔的河道通常在活跃的沙洲沉积作用导致的横向运动较多处形成，因此，我们使用主河槽宽度作为局部沉积物沉积（或沉积通量的发散）的定性指标。
为了量化人为限制，我们对六组连续航空照片集和历史地形图上的主河槽进行了测绘，并将相关边缘分类为自然边缘（即移动性边缘）和人工边缘（即固定/限制性边缘）。
在分析主河槽宽度和次级河道数量/面积时，我们将其与同时期的人为限制程度相结合。为了控制与离散水和沉积物输入（例如，支流交汇处）以及可能干扰河槽调整潜在空间模式的障碍（例如，大坝）相关的局部异常效应，我们将沱江主河槽划分为六个河谷段。这种分段基于主要交汇点位置、主要河床质地（砾石或沙子）、河槽宽度、坡度、2021年摄影年份中评估的人为限制程度以及次级河道的存在/类型。特别是，各段的起始端设置为：（1）主要支流交汇点上游（例如，S1段的Stura di Lanzo，S2段的Dora Baltea，S3段的Sesia，S4段的Ticino和S6段的Oglio）；（2）在主要大坝处（即S5段的Isola Serafini）。第6段的下端，即研究区域下游限界，定义为沱江三角洲。这种形态学方法旨在减少段内异质性，同时最大化段间差异，以便可以根据不同的河槽类型评估河槽调整。