Changes in floodplain inundation under nonstationary hydrology for an adjustable, alluvial river channel

Abstract

Predicting the frequency and aerial extent of flooding in river valleys is essential for infrastructure design, environmental management, and risk assessment. Conventional flood prediction relies on assumptions of stationary flood distributions and static channel geometries. However, nonstationary flow regimes are increasingly observed and changes in flow and/or sediment supply are known to alter the geometry and flood conveyance of alluvial channels. Systematic changes in flows and/or channel geometry may amplify or attenuate the frequency and/or extent of flood inundation in unexpected ways. We present a stochastic, reduced complexity model to investigate such dynamics. The model routes a series of annual peak discharges through a simplified reach‐averaged channel‐floodplain cross section. Channel width, depth, and slope are permitted to adjust annually by a user‐specified fraction toward equilibrium geometries predicted based on each year's peak discharge and sediment supply. Modeled channel adjustments are compared with empirical observations for two rivers in Minnesota, USA that have experienced multiple large floods over the past 6 years. The model is then run using six hypothetical scenarios simulating nonstationary flow regimes with temporal adjustments in the mean and/or variance of the governing peak‐flow distributions. Each scenario is run repeatedly while varying parameters that control the amount of fractional adjustment that channel geometries can make annually. Results indicate that the intra‐annual mean horizontal width of floodplain inundation primarily depends on the governing peak‐flow distribution's coefficient of variation, but the intra‐annual frequency of floodplain inundation (i.e., the fraction of modeled years with inundation) primarily depends on the amount of channel adjustment permitted annually.

Timescale dependence in river channel migration measurements

“…These results appear to contradict the physical explanation for an expected direct relation between flow and channel migration rates; increased flows tend to increase shear stresses along meanders (Schook et al ., ). However, sand and gravel‐bed river channels adjust their geometry and slope to increase uniformity of sediment transport, and thus dampen responses to changing flow conditions (Church and Ferguson, ; Phillips and Jerolmack, ; Call et al ., ) so adjustments may be driven more by sediment supply and transport capacity, rather than flow (Winterbottom, ). Our results also affirm that the timescale dependence for Root River channel migration measurements is not an artifact of biased sampling across differing rates for historical and contemporary data.…”

“…Historical meander migration rates are also used to study if, and to what extent, channel migration rates have changed over time. Rivers respond to climate and land‐use changes via non‐linear adjustments to channel, width, depth, planform pattern, vertical incision or aggradation, and lateral migration (Nanson and Hickin, ; Simon, ; Gaeuman et al ., ; Larsen et al ., ; Swanson et al ., ; Toone et al ., ; Call et al ., ). Ultimately, channel adjustments shape fluvial and riparian habitats and may pose risks for nearby human infrastructure (Wente, ; Allan, ).…”

Near‐Channel Versus Watershed Controls on Sediment Rating Curves

“…However, the southwestern part of this basin (including the North and South Forks of the Crow River included in this study; labeled 21, 36, and 37 in Figure ) is dominated by more fertile Mollisols and is heavily farmed. Stream flows in the UMSB have shown a slightly increasing trend over the 20th and early 21st centuries (Call et al, ; Novotny & Stefan, ). Streams in the UMSB did not experience the downcutting event associated with glacial lake outburst flooding, as did the MRB tributaries.…”

The Power of Environmental Observatories for Advancing Multidisciplinary Research, Outreach, and Decision Support: The Case of the Minnesota River Basin

“…The value of the publicly available hydrology, topography, water quality, imagery, and land cover data sets cannot be overstated and gave the additional data sets here a rich temporal, spatial, and historical context. Many of the data sets were critical in the development of a series of reduced complexity models designed to understand key processes and linkages between processes at the watershed scale (Call et al, ; Cho, ; Cho et al, ; Czuba et al, ; Czuba & Foufoula‐Georgiou, , ; Hansen, Czuba, et al, ), while additional efforts focused on integrating new knowledge into existing mechanistic modeling frameworks like the Soil and Water Assessment Tool (e.g., Kumarasamy & Belmont, ; Mitchell et al, ).…”

“…Hydrologic changes within a watershed can lead to a complex response in the channels and channel networks with potential negative repercussions on erosion, transport of sediment and pollutants, and ecosystem integrity (e.g., Blann et al, ; Konar et al, ; Schilling et al, ; Vörösmarty & Sahagian, ). The MRB is particularly sensitive to hydrologic change, with deeply incised tributaries eliciting a strong geomorphic response to ongoing changes in hydrology (Belmont et al, ; Belmont & Foufoula‐Georgiou, ; Call et al, ; Foufoula‐Georgiou et al, ; Gran et al, ; Kelly et al, ). This makes it a valuable location to systematically investigate linkages between changes in the landscape and river ecohydrologic and morphologic response.…”