Estimating the sediment trap efficiency of intermittently dry reservoirs: lessons from the Kruger National Park, South Africa

Abstract

The assessment of sediment yield from reservoir siltation requires knowledge of the reservoir's sediment trap efficiency (TE). Widely used approaches for the estimation of the long‐term mean TE rely on the ratio of the reservoir's storage capacity (C) to its catchment size (A) or mean annual inflow (I). These approaches have been developed from a limited number of reservoirs (N ≤ 40), most of them located in temperate climate regions. Their general applicability to reservoirs receiving highly variable runoff such as in semi‐arid areas has been questioned. Here, we examine the effect of ephemeral inflow on the TE of 10 small (≤ 280 × 103 m3), intermittently dry reservoirs located in the Kruger National Park. Fieldwork was conducted to determine the storage capacity of the reservoir basins. The frequency and magnitude of spillage events was simulated with the daily time step Pitman rainfall–runoff model. Different runoff scenarios were established to cope with uncertainties arising from the lack of runoff records and imperfect input data. Scenarios for the relationship between water and sediment discharge were created based on sediment rating curves. Taking into account uncertainties in hydrological modelling, uncertainties of mean TE estimates, calculated from all scenarios (N = 9), are moderate, ranging from ±6 to ±11% at the 95% confidence level. By comparison, estimating TE from the storage capacity to catchment area (C/A) ratio induces high uncertainty (ranges of 35 to 65%), but this uncertainty can be confined (15 to 33%) when the latter approach is combined with hydrological modelling. Established methods relying on the storage capacity to mean annual inflow (C/I) ratio most probably lead to an overestimation of the TE for the investigated reservoirs. The approach presented here may be used instead to estimate the TE of small, intermittently dry reservoirs in semi‐arid climate regions. Copyright © 2017 John Wiley & Sons, Ltd.

Application of environmental magnetism to estimate source contribution by lithology to Kruger National Park reservoirs

“…One reservoir core was taken at a point of assumed deepest sediment using an Eijkelkamp percussion corer. Reservoir depth information was received on‐site from another research team, data published in Reinwarth et al (2018). The boundary between deposit and reservoir bottom was determined by colour and particle size changes (Baade et al, 2012; Reinwarth et al, 2017; Reinwarth et al, 2018).…”

“…This section describes the field sampling, laboratory, and statistical methods used to determine sediment yields. Catchment area (Baade & Schmullius, 2015) and lithology area percentages (Reinwarth et al, 2018) were established from published literature. Catchment lithologies were the sources identified for the fingerprinting method.…”

TET: An automated tool for evaluating suitable check-dam sites based on sediment trapping efficiency

“…Brune (1953) prepared a standard curve by plotting capacity-inflow (C/I) ratio against sediment trapping efficiency for sites in the USA, and successfully applied it to estimate the percentage of sediment yield based on the sediment trapped. Therefore, the C/I ratio was later selected as the main constituent of the TE index (Romero-Díaz et al, 2012;Parsaie et al, 2018;Reinwarth et al, 2018). In fact, this concept refers to the generalizability of the percentage of inflowing sediment mass that remains permanently in the reservoir.…”

“…The sediment trapping efficiency (TE) index has been widely used to assess reservoir sedimentation efficiency Poesen, 2000, 2001;Eizel-Din et al, 2010;Romero-Díaz et al, 2012;Reinwarth et al, 2018;Parsaie et al, 2018). This index estimates the efficiency of check dams based on upstream watershed area and reservoir capacity (Hadley and Walling, 1984;Siyam, 2000).…”

Fluvial Sediment Yields in Hyper-Arid Areas, Exemplified by Nahal Nehushtan, Israel

“…Errors associated with determination of historic sediment yields require attention, but they are small when considering the spatial distribution of event layers [39,40]. Event sediment layers detected in reservoirs, and their derived specific event sediment yields, are very accurate for long-term assessments [41,42], as long as the trap efficiency is known [43,44].…”