**Influences of Catchment and River Channel Characteristics on the Magnitude and Dynamics of Storage and Re-Suspension of Fine Sediments in River Beds**

**Jungsu Park 1,\*, Ramon J. Batalla 2,3,4, Francois Birgand 5, Michel Esteves 6, Francesco Gentile 7, Joseph R. Harrington 8, Oldrich Navratil 9, Jose Andres López-Tarazón 2,10,11,12 and Damià Vericat 2,13**


Received: 14 March 2019; Accepted: 24 April 2019; Published: 26 April 2019

**Abstract:** Fine particles or sediments are one of the important variables that should be considered for the proper management of water quality and aquatic ecosystems. In the present study, the effect of catchment characteristics on the performance of an already developed model for the estimation of fine sediments dynamics between the water column and sediment bed was tested, using 13 catchments distributed worldwide. The model was calibrated to determine two optimal model parameters. The first is the filtration parameter, which represents the filtration of fine sediments through pores of the stream bed during the recession period of a flood event. The second parameter is the bed erosion parameter that represents the active layer, directly related to the re-suspension of fine sediments during a flood event. A dependency of the filtration parameter with the catchment area was observed in catchments smaller than ~100 km2, whereas no particular relationship was observed for larger catchments (>100 km2). In contrast, the bed erosion parameter does not show a noticeable dependency with the area or other environmental characteristics. The model estimated the mass of fine sediments

released from the sediment bed to the water column during flood events in the 13 catchments within ~23% bias.

**Keywords:** bed erosion; catchment area; filtration; sediment accumulation; sediment bed fluidization; sediment re-suspension

#### **1. Introduction**

Fine particles or sediments are considered one of the most important factors affecting the quality and functioning of fluvial environments. For instance, fine sediments are long-lasting sources of toxic substances in catchments; that is, contaminants, such as pathogens, heavy metals as well as nutrients, are transported attached to fine sediments [1–5]. Fine sediment dynamics also have various effects on the health of benthic communities and the overall aquatic ecosystems [6–9].

The suspended sediment yield is affected by various catchment characteristics such as climate, geology, soils, catchment area, and land cover [10–12]. Because of the spatial variability of these catchment characteristics, developing a universally applicable fine sediment transport model remains an important research challenge. Rainfall intensity, erodibility and runoff processes mainly govern the fine sediment dynamics at the basin outlet. Not only these factors but also the catchment sizes cause variations in the sediment supply and transport processes. For example,

Gao et al. [13] suggested that the suspended sediment load (Qs) is dominated by short-time-interval processes in smaller catchments with less-developed drainage density and small capacity to store fine sediment (i.e., drainage area <0.1 km2). As the drainage area increases, the homogeneity of the catchment decreases and drainage density gradually increases, leading to a greater contribution of remobilization of fine riverbed sediments and bank erosion to the overall sediment budget [13].

Suspended sediment concentration (hereafter C) is closely related to flow discharge (Q), but this relationship varies over time, from the flood scale to the annual scale. The C–Q relationship often shows orders of magnitude of scatter [14]. Such variability is explained by the fact that the rising limb of the flood generally shows a different C–Q relationship compared with the falling limb, leading to a hysteresis pattern in the relationship [13,15–19]. The supply of sediment from the channel system is often considered to be a significant source of sediment [18,20–22]. For instance, Klein [17] observed clockwise hysteresis, being mainly driven by the supply of sediments from the channel bed or from highly eroded hillslopes close to the outlet. In contrast, anticlockwise hysteresis can be observed when sediment is supplied from distant upstream sources. Recently, Yang et al. [23] derived a flow and sediment travel time model, verifying that clockwise hysteresis is observed when flow travel time is more extended than the sediment travel time, whereas anticlockwise hysteresis is observed in the opposite. More recently, Juez et al. [24], based in a series of laboratory tests, observed clockwise hysteresis driven by the supply of sediment from the channel bed, whereas anticlockwise hysteresis is observed when upstream supply of sediment has more contribution. These hysteresis patterns are one of the main reasons why single power-law models are generally insufficient to explain the scatter in the relationship between C and Q [14,25]. Seasonality of precipitation and land cover also causes scatter in C for a given Q [15,26–28]. For example, Alexandrov et al. [15] carried out a study in a semi-arid region and observed that autumn–spring convective storms with higher-intensity rainfall often produce higher C than winter frontal storms with lower intensity; much earlier,

Negev [26] and more recently Cantalice et al. [21] have suggested that the first flood in a given water year could have a higher C than subsequent floods of similar magnitude. The reason of these differences was attributed to the re-suspension of deposited sediment from bed during the first flood of the year. Seasonal variations of the flow due to snowmelt may also induce additional sediment supply from the channel bed and cause variations in the functional relationship between C and Q. Stubblefield et al. [29] observed an increase of the sediment supply from the channel bed when the flow

rate was increased by snowmelt in a field study of Lake Tahoe. Interannual variations of the suspended sediment load with water discharge are also caused by larger-scale variations of the environment, such as climatic changes and related variability of discharge [30], or extreme events such as large floods [14].

A conceptual model coupling fine sediment dynamics with bedload transport was presented by Park and Hunt [31] based on systematic analysis of fine sediment and stream bed movement. This study led to the development of a model for the estimation of fine sediment accumulation and re-suspension from the bed [32]. It is worth mentioning that fine sediments are defined as "particles that are transported in suspension in surface waters and can also be accumulated in the sediment beds" [32]. Within this context, the objective of this study is (i) to analyze the applicability and robustness of the model developed by Park et al. [32] and (ii) to study the effect of catchment characteristics (e.g., catchment area, climate) on the performance of the model. The study was carried out based on data of 13 catchments with different drainage area and located in various hydro-climatic environments.
