On the magnitude and frequency of sediment transport in rivers.

摘要

What flow or range of flows is most responsible for transporting sediment and maintaining sediment continuity in a river over human time scales? This question has inspired scores of studies analyzing the magnitude and frequency of sediment transport (MFA) in rivers and has been a part of the ongoing debate regarding process vs. form-based approaches to stable channel design. MFA in rivers is of general scientific and management interest as it influences channel form, water quality, aquatic habitat, and channel restoration design considerations.;The research presented in this dissertation asks the following overarching question: What influences how much and how often sediment is transported in a river? In this dissertation, I consider relationships between the drivers of sediment transport at a point in a river (flow regime, sediment size, and channel form) and metrics describing sediment yield, which integrate the relationship between flow regime and transport over time. To study this question, I use theoretical and empirical approaches in a spectrum of stream types from fine bed streams dominated by suspended load transport to gravel and cobble streams dominated by bed load transport. I find that, for example, the frequency of the most effective discharge decreases and the range of flows most responsible for sediment yield increases with increasing flow regime variability. As river bed material becomes coarser, a more narrow range of less frequent flows becomes most effective in sediment transport.;The river management and restoration community has given much effort to predicting the bankfull discharge, Qbf, and associated channel geometry at Qbf for the purposes of channel study, classification, and design. In a study comparing various Qbf predictors in coarse and fine bed rivers, I find that the discharge associated with 50% of cumulative sediment yield based on the flow record---Qs50, the half yield discharge---predicts Qbf better than most other predictors, especially in fine-bed rivers. Other predictors include the most effective discharge, Q eff, and the 1.5-year flood.;Using statistical methods to quantify the uncertainty in the sediment load-discharge relationship as well as the empirical flow frequency distribution, I develop methods to propagate uncertainty in estimations of Q eff and Qs50. In an examination of the influence of flow regime non-stationarity on sediment yield metrics, I find that in urbanizing watersheds with increasing trends in flow variance, estimates of Qeff and Qs50 increase dramatically compared to those based on the entire flow period of record. Finally, I estimate Qeff and Qs50 using empirical, sediment load data-driven models and physically-based models driven by one-dimensional flow-depth relationships evaluated at a cross section. Physically-based models that match the slope of the sediment load-discharge relationship performed well. This is the case with total load models for fine bed sites, but generally not the case for bed load models used on coarse bed sites.;All daily flow records and sediment load data as well as all Matlab RTM and R scripts are contained in the supplementary data.