5.1. Background
5.1.1. Using hydrometric data to quantify channel change This thesis started out as an investigation into the potential uses of stream channel measurements to improve our understanding of how the cross- sectional geometry of alluvial river channels changes over time. Until recently, our ability to examine geomorphic change in large numbers of river channels was hindered by the lack of data spanning vast geographic areas and long time- periods.
In the last few years however, the entire historical database of US Geological Survey (USGS) stream channel measurements has been made publicly accessible online (U.S. Geological Survey, 2014e). This database has been gradually built up by hundreds of workers over the past 115 years as a by- product of stream gaging, to define stage-discharge rating curves and estimate how much flow is being carried in a channel at any given point in time (U.S. Geological Survey, 2014d). As alluvial channels adjust their morphology, past rating curves are rendered inaccurate and must be updated with new measurements (Carter and Davidian, 1968). Instead of simply archiving the ‘old’ hydrometric measurements as they become obsolete, this thesis shows how we can use these data to obtain valuable insight into the geomorphic changes that occur in river channels.
Provided one establishes rigorous filtering procedures, the streamflow measurements can be employed to estimate temporal trends in the average velocity, the width, and the average depth of the cross-sectional flow of river channels across a broad range of climatic and geologic contexts. However, quantifying trends in channel geometry puts into question the basic geomorphic assumption that most alluvial channels are “in regime”, i.e. have
achieved a mean, equilibrium channel form and do not exhibit a trend in their geometry over decadal timescales.
5.1.2. The assumption of regime channels
Over the course of the past half-century, attempting to define and quantify the average channel form or ‘hydraulic geometry’ of rivers in different physiographic regions and climates has become one of the dominant research areas in fluvial geomorphology (Leopold and Maddock, 1953; Huang and Nanson 2000; Eaton and Church 2007). Much of the existing work assumes that alluvial stream channels have adjusted to an average cross-sectional size and shape that reflects the mean solid and liquid discharge that is supplied to them from the upstream basin. Alluvial channels may modify their beds and banks to accommodate small temporal changes in flow and sediment, yet they are expected to retain a “mean” channel form over decadal timescales (Schumm and Lichty, 1965).
The assumption that average channel form is stable over time is fundamental to the application of regime theories (Lane, 1955; Griffiths, 1983) which have proven extremely useful in river engineering to estimate the average channel geometry and to design riverside and flood control structures. However, it is important to recognise that we have profoundly modified the average volumes of streamflow and sediment that transit downstream in basins around the world, and that many river channels are still adjusting to these new regimes. It is well known, for example, that the Mississippi valley has transitioned over the past century from a transport-limited (excess sediment) to a supply-limited (sediment-starved) system, due to the proliferation of dams and engineering structures that cut off the sediment supply to the streams (Meade and Moody, 2010).
These anthropogenic modifications to river basins affect the velocity, width, and depth of channel cross-sectional flow, as rivers adjust their geometries to
accommodate the new flow regimes and sediment loads. Channel adjustment processes often exhibit lag effects (Simon, 1994) and so fluvial networks may take many decades to adapt to modifications that occurred upstream. For example, substantial sediment yields generated by landuse changes can be stored throughout river basins in the form of sediment slugs (Nicholas et al., 1995) and may only be flushed or propagated downstream episodically, during high flow events.
In a nonstationary world with a changing climate (Milly et al., 2008), it is becoming increasingly apparent that the assumption of regime channels is inappropriate for channel design. The adjustments and trends in channel geometry that develop over decadal timescales present a real, measurable threat to the stability of riverside infrastructure, navigation and flood hazard. Instead of defining an expected equilibrium form of alluvial channels, it may be more judicious to establish an “envelope of uncertainty” of geomorphic change, based on the potential trends and variability in channel geometry that we expect to see in any given stream channel.
It is essential that we start monitoring these changes in channel form to improve our understanding of the dynamic nature of contemporary fluvial systems. This thesis provides a first attempt to quantify shifts in alluvial channel cross-sectional form using historical stream gaging measurements.
5.2. Results
5.2.1. Magnitudes of trends in channel geometry
One of the primary aims of this work was to develop new methods to investigate the trends and variability in channel cross-sectional geometry at gaging stations across the USA, and to determine whether these trends have been statistically significant over the course of the past sixty years. In chapters 2
to 4, we estimated the rates of change in the discharge, average velocity, width, and average depth (~bed elevation) of the cross-sectional flow.
Results show that significant channel adjustment is expressed at approximately two-thirds of all studied sites, when measured at sub-bankfull levels (the “median stage”). These proportions suggest that the majority of U.S. stream channels are in a nonstationary state over decadal timeframes, thus thwarting attempts at defining a “regime channel” for most stream cross- sections over such short time periods.
The average estimated magnitudes of significant decadal trends in channel form at the median stage were of −8/+7 % for velocity, −4/+4 % for width, and −6/+7 % for depth (see Figure 4.7). In dimensional units, median trends were of −0.03/+0.03 m/s per decade for velocity, −0.8/+0.7 m per decade for width, and −30/+30 mm per decade for depth (chapter 4). Thus, on average, very small dimensional shifts in velocity actually contributed more than width and depth trends to the overall changes in channel capacity. Our findings suggest – at a continental scale – that the cross-sectional capacity of river channels is governed just as much by changes in channel roughness as by changes in the sediment mass balance.
5.2.2. Controls on trends in channel geometry
What controls these shifts in alluvial channel form over time? Throughout this work, numerous controls were investigated, including lithology, drainage density, mean climate, and basin slope. Surprisingly, only two main controls were clearly apparent in all locations.
The primary control on rates of change is the scale at which measurements are made. As the contributing drainage area and the capacity of the river channel increase (chapters 2 and 4), the dimensional rates of change, measured in mm, also tend to increase (chapter 2). However, proportional rates of change (in %), are negatively correlated with the channel size or basin area
(chapter 4), suggesting that absolute and proportional changes in channel size or capacity should be considered separately.
A secondary major control on trend magnitude is the variability (Q90/Q50)
or flashiness (R-B index) of streamflow. Rates of change in alluvial channel geometry were found to increase with Q90/Q50, i.e. the annual variability in the
magnitude of high flows (chapter 2) and with the R-B flashiness index, which describes the average daily fluctuations in streamflow at any given site (chapter 4).
5.2.3. Effects of changes in channel geometry on flood hazard The widespread nature of these changes in channel capacity suggests that geomorphological trends may well alter the frequency and magnitude of flood hazard in many locations independently from hydrological trends. This finding prompted me to investigate the influence of channel capacity on flood hazard. The third chapter thus introduces a new statistical procedure for disentangling the hydrologic and geomorphic drivers of trends in flood hazards. The “flow frequency effect” is quantified as the changes in flood hazard frequency that would arise in the absence of trends in channel capacity, and the “channel capacity effect” as the trend in flood hazard frequency that would arise from the observed shifts in channel capacity at flood stage, if the flow frequency distribution were held constant. This new method allows us to separate the two effects and to quantify the influence of each one on flood hazard independently.
Overall, we found that more than half of the 401 sites showed significant trends in channel capacity and/or flow frequency (in units of flood hazard frequency), suggesting that flood hazard is chiefly nonstationary across the studied locations. Flow frequency trends were consistent with previously documented trends in heavy and extreme precipitation events. Increases in
streamflow frequency were almost twice as frequent as the decreases, suggesting that flood hazard is increasing significantly across the United States.
Trends in channel capacity, surprisingly, contributed to changes in flood hazard on a scale that was of the same order of magnitude as flow frequency trends (medians: +10/−10 % per decade, versus +30%/−64 % per decade, respectively). Perhaps even more importantly, these significant trends in channel capacity were nearly three times more frequent than significant flow frequency trends. Thus although changes in channel morphology may have a slightly weaker influence on flood hazard than changes in hydrological regimes, they appear to be far more common and widespread, and/or easier to detect.
Lastly, this chapter found that channel capacity and flow frequency trends were largely independent (uncorrelated), except in sites that had experienced substantial anthropogenic flow modification. The absence of correlation implies that both geomorphic and hydrologic trends must be quantified if one wishes to determine how flood hazard is changing over time, and whether changes in the channel capacity may intensify flow frequency trends, or on the contrary, lessen them.
5.2.4. Interaction between velocity, width and depth
If changes in channel capacity are so widespread and have a significant influence on the flood hazard frequency, one may wonder precisely how these geomorphic changes are expressed, and whether shifts in flood hazard are due primarily to shifts in the channel width, the average depth or the average velocity of the cross-sectional streamflow.
Chapter four thus investigated the relationship between velocity, width, and depth trends in different channel cross-sections. Surprisingly, the significant majority of US channels were widening and degrading with decreasing velocity, suggesting that flow equilibrium is maintained by a balance between cross- sectional flow area and velocity. Overall, the majority of these velocity, width
and depth trends showed opposite signs, satisfying the concept of dynamic equilibrium.
These relationships between velocity, width and depth varied depending on the type of river, so that in small/flashy streams, channel geometry trends (velocity, width and depth) tended to develop concomitantly (in the same direction), while in large rivers with stable flow regimes, they tended to develop independently (opposite directions). Further, results showed that velocity trends tended to contribute more than width/depth trends to shifts in channel capacity in large channels with stable flow regimes; width trends tended to be the largest in small channels with flashy flow regimes; and depth trends tended to be the largest in large channels. These findings necessitate further investigation in different contexts, to better understand the causes of changes in channel geometry.
5.3. Perspectives and future research avenues
A number of research avenues merit further investigation and will become increasingly practicable now that geo-referenced gaging data are beginning to be published as Open Data around the world.
5.3.1. Using ADCP measurements to analyse changes in channel morphology worldwide
One of the principal challenges of this analysis is to pinpoint the exact location where streamflow gagings are made, so that trends in channel geometry can be measured in one consistent cross-sectional location over time. While most gagings tend to be made in a similar spot near the gage, the exact location chosen by the operator may vary depending on the instrument used (e.g. an ADCP or a current meter, the choice of which may vary over time), the type of ADCP (some need deeper water, some slower water), the presence of obstructions in the river (such as weed growth or debris), the operator (and
their knowledge of the instruments or of the site), and the time of year or meteorological conditions (since low and high flows are not always made in the same location). To increase confidence in transect location, we excluded measurements made at low and high flows (Figures 3.4 and 4.3), and we plotted all measurements to verify channel shape (e.g. Figure 3.5). However, due to the inherent uncertainties in measurement location, we were only able to conduct the analysis for a limited number of carefully-selected historical stream gage records.
To expand these analyses to a wider number of locations, it is vital that the precise geo-reference and meta-description of each gaging transect are provided to the public along with the data. Currently, in both the UK and the USA, the publicly-accessible archives do not contain a precise indication of the location where each measurement was made, since the principal aim of stream gaging is to provide the most accurate measurement of the flow in the conditions present on the day, rather than to record the shape of the channel at a given point in space and time. The lack of adequate georeferencing is also due to the structure of hydrometric data archives, many of which do not contain a parameter for recording GPS coordinates. In the UK, the gaging measurement location is subjectively provided in a column titled “Comments” or “Remarks” as, for example, “measurement made 20 m upstream from bridge”. Automating the data analyses would be far easier if the precise GPS coordinates of these measurements were provided to the users as a numeric value in a sortable column.
Another hindrance to geo-referencing is the lack of adequate instrumentation. For the moment, only a minority of boat-mounted ADCPs around the world are able to geo-locate streamflow measurements with a GPS. Ideally, as the measurement techniques develop, we would hope for consistency in international streamgaging standards. If the processed ADCP gaging files were archived in the same format and published as Open Data online, then one
could conceivably – in the future – automate data downloads from different regions around the world, geo-locate all of the measurements, and quantify changes in stream channels across a wide range of locations.
The procedures described herein serve as a first step for understanding the challenges associated with filtering these data before conducting time-series analyses. One could then determine which rivers of the world are progressively narrowing/widening, aggrading/degrading, how velocity patterns are changing, and how these geomorphic changes are contributing to shifts in flood hazard.
5.3.2. Further separating the different drivers of flood hazards We showed that trends in flood hazard frequency are driven by two principal components: the frequency of flood flows from the upstream basin (“flow frequency”), and the capacity of the river channel to contain those flows (“channel capacity”). If either of these components changes, the resulting flood hazard frequency will be modified. In future work, it would be useful to illustrate, from a statistical perspective, how the total change in flood hazard frequency at any given location (e.g. the trend in the number of days above a fixed value of stage) can be calculated as the sum of the channel capacity and flow frequency effects, as defined in chapter 3 (Slater et al., 2015).
One may further disentangle the drivers of flood hazard by attributing the changes in flood frequency to specific causes. The influence of hydrologic trends on flood hazard frequency can be broken down into (i) the changes in streamflow that originate from atmospheric changes such as precipitation trends (“atmospheric effects”) and (ii) those that originate from changes at the Earth’s surface, such as land use ends (“Earth-surface effects”). The influence of geomorphic change on flood hazard can also be subdivided into (i) the changes in capacity that arise from shifts in the average cross-sectional flow velocity (“flow velocity effects”) and (ii) the changes in capacity that arise from shifts in the cross-sectional flow area (“flow area effects”) (Figure 5.1).
Figure 5.1. Simplified schematic of flood hazard drivers to be integrated in a predictive framework
5.3.3. Upscaling these analyses from the cross-section to the reach
Hydrometric archives of cross-sectional channel measurements are a valuable resource for understanding how our rivers have changed since the end of the nineteenth century. However, these archives only document changes in channel geometry at the scale of the cross-section. To better understand how our channels are changing, we would need to upscale these analyses from the cross-section to the river reach.
Recent technologies such as terrestrial laser scanners and acoustic Doppler current profilers (ADCPs) allow us to map entire reaches of stream channels and to model the distribution of flow velocities and bathymetry with very little error. These new methods are much faster, and sometimes much safer, than the methods that existed in the past, allowing us to repeat measurements with more ease and speed than ever before. It is therefore extremely likely that in the future we will start to use repeat ADCP surveys of river reaches at different points in time to understand exactly how river morphology and hydrology are changing in some of the most critical locations.
5.4. Final words
This thesis illustrates how hydrometric data may be used to understand changes in river channel geometry and flood hazard over time. The cross- sectional measurements of stream channels that have been archived as a by- product of stream gaging since the end of the nineteenth century are one of our most valuable resources for understanding how river channels have evolved in the last hundred years. However, many stream gage records are currently at risk of being discontinued. If we truly wish to understand how our rivers are evolving with climate change and demographic growth, it is essential that we acknowledge the importance of investing in continued hydrometric data collection. Only then will we have records that are long enough to foresee changes in channel morphology, river navigability, aquatic health and flood hazards in years to come.