The Impact of Urbanization on Temporal Changes in Sediment Transport in a Gravel Bed Channel in Southern Ontario, Canada

Abstract

A field investigation has been undertaken to characterize the event‐based bed load transport dynamics of a highly urbanized gravel bed stream. A combination of direct bed load and tracer particle measurements were taken over a 3 year period during which time approximately 30 sediment mobilizing events occurred. Sediment transport measurements were used to calibrate a fractional bed load transport model and combined with hydrometric data which represent four different land use conditions (ranging from rural to highly urbanized) to analyze the differences in discharge magnitude and frequency and its impact on sediment transport. Fractional transport analysis of the bed load measurements indicates that frequent intermediate discharge events can mobilize sand and fine gravel to an approximate equally mobile condition, however, the transport rates at these discharges exhibit greater variability than at discharges above the bankfull discharge. Path lengths of the coarse fraction, measured using tracer clasts, are insensitive to peak discharge, and instead transport at distances less than those reported in other gravel bed channels, which is attributed to the shorter duration discharge events common to urban streams. The magnitude‐frequency analysis reveals that the frequency, time, and volume of competent sediment mobilizing events are increasing with urbanization. Variability in effective discharges suggests that a range of discharges, spanning between frequent, low magnitude events to less frequent, high magnitude events are geomorphically significant. However, trends in the different land use scenarios suggest that urbanization is shifting the geomorphic significance toward more frequent, lower magnitude events.

The impact of hydrograph variability and frequency on sediment transport dynamics in a gravel‐bed flume

“…The frequency of the hydrographs per year was derived from hydrometric time series described in Plumb et al ().…”

“…The Froude number in the experiments ranged from 0.4 to 0.8 (estimated based on measured flow depths and known discharges), which is within range of the prototype streams and previous experiments studying unsteady flow (Lee et al, ; Mao, ; Waters and Curran, ). Peak sediment input rates were established based on measured bedload transport rates in Mimico Creek and scaled to the flume by means of Froude similarity; see Annable et al () and Plumb et al () for details on field sampling methods and results. The sediment input rating curve was approximated into two input rates, such that the input rate was increased partway through the rising limb, at 70% of the peak discharge ( Q peak ) and then decreased part way through the falling limb.…”

“…The adjustment of channel form in a gravel‐bed channel is concordant with the magnitude and frequency of discharges capable of transporting bed material (Wolman and Miller, ; Poff et al, ). Although it is common to characterize fluvial changes by a single discharge (Leopold et al ., ), a channel is ultimately formed by the range in competent flows capable of performing work on the erodible boundaries (Lenzi et al, ; Surian et al, ; Plumb et al, ). These competent flows, combined with the amount and sizes of transported sediment, interact with the bed and banks to determine an equilibrium condition in terms of hydraulic resistance and amount of sediment transported.…”

“…TTP is time-to-peak and t h is the threshold duration. SeePlumb et al (2017) for more details.B. D. PLUMB ET AL.…”

Using high-resolution bedload transport tracer measurements to investigate the characteristics of bedload transport over a large urban flood event

“…The objective of this paper is to develop representative sediment transport models of a river reach where intensive in-situ event based and inter-event sediment transport investigations have been conducted (including a large magnitude storm event) and compare the results to pre-and post-event observations. Mean travel distances of bed material will be calculated using the volumetric rate expression of bed material transport and compared to bed material particle tracking measurements obtained before and after each event [3].…”