Restoring Geomorphic Integrity in Urban Streams via Mechanistically Based Stormwater Management: Minimizing Excess Sediment Transport Capacity

Stream channel erosion, enlargement, and habitat degradation are ubiquitous in urban watersheds with conventional stormwater management. Hydrologic-based restoration aims to discharge a more natural flow regime via stormwater management interventions. Whether such interventions facilitate geomorphic recovery depends, in part, on the degree to which they restrict discharges that would otherwise contribute to channel erosion. Erosion potential (E), the ratio of post-developed to predeveloped sediment transport capacity, provides a simplified, mechanistic framework to quantify the relative influence of stormwater interventions on the geomorphic effectiveness of the flow regime. This paper compiles ca. five years of data following stormwater-based interventions in three distinct settings in the United States and Australia to demonstrate how the E framework can be used to elucidate the role of hydrologic restoration interventions in helping to facilitate trajectories of geomorphic recovery (or lack thereof). In a previously developed watershed with unstable streams, substantial reductions in E coincided with a trajectory of geomorphic recovery, whereas our case study that did not reduce E between the study periods exhibited continued instability. Furthermore, a greenfield study site that used the E framework to optimize their SCMs to match the sediment transport capacity of the predeveloped regime (E = 1) was able to maintain a recovery trajectory in a legacy-impacted setting that is otherwise highly susceptible to hydromodification. Although available space and funding will limit the ability to fully reduce E in previously developed watersheds, these case studies underscore the mechanistic value of using stormwater controls to maximize reductions in E if geomorphic stability is a goal of stormwater interventions. Streambed material size and channel evolution stage also likely affect the level of E reduction necessary to promote geomorphic recovery, with coarser-grained and/or over-widened streams potentially needing less reduction than finer-grained and/or more entrenched channels.

Ephemeral Gully Erosion Advancements within the AnnAGNPS Watershed Simulation Model

<p>Concentrated runoff increases erosion and moves fine sediment and associated agrichemicals from upland areas to stream channels. Ephemeral gully erosion on croplands in the U.S. may contribute more of the sediment delivered to the edge of the field then from sheet and rill erosion. Typically, conservation practices developed for sheet and rill erosion are also expected to treat ephemeral gully erosion, but science and technology are needed to account for the separate benefits and effects of practices on each of the various sediment sources.</p><p>Watershed modeling technology has been widely developed to aid in evaluating conservation practices implemented as part of a management plan, but typically lacks the capability to identify how a source, such as sheet and rill erosion, ephemeral gully erosion, or channel erosion, is specifically controlled by a practice or integrated practices. The U.S. Department of Agriculture’s Annualized Agricultural Non-Point Source pollutant loading model, AnnAGNPS, has been developed to determine the effects of conservation management plans on erosion and provide sediment tracking from all sources within the watershed, including sheet and rill, ephemeral gully, and channel erosion. </p><p>This study describes the ephemeral gully erosion capabilities within the AnnAGNPS model and discusses research needs to further improve these components for integrated conservation management planning.  Conservation management planning by agencies within the U.S. and by international organizations requires a systematic approach when determining the extent of ephemeral gully erosion impacts on a field, watershed, or national basis, and/or to predict recurring or new locations of ephemeral gullies prior to their development.  This technology provides the capability to separate the impact of ephemeral gullies on erosion from other sources and then evaluate the impact of targeted practices to control erosion at the source and subsequent downstream resources.</p>

Quantifying long term evolution of fluvial channel in glacier forefield

<p>Alpine rivers have experienced considerable changes in channel morphology over the last century. Human disturbance and natural factors are the main drivers of changes in channel morphology that modify natural sediment and flow regimes at local, catchment and regional scale. River sediment loads are likely to increase due to increasing snow and glacier melt runoff, facilitated by climate changes. Additionally, channel erosion and depositional dynamics and patterns are influenced by sediment delivery from rock walls, hillslopes, and sediment in the forefields of retreating glaciers. Land cover changes may facilitate or obstruct runoff and soil degradation.</p><p>In order to reliably assess the magnitudes of the channel changing processes and/or their frequencies due to recent climate change, the investigation period needs to be extended to the last century, ideally back to the end of the Little Ice Age. Moreover, a high temporal resolution is required to account for the history of changes of channel morphology and for better detection and interpretation of related processes.</p><p>The increasing availability of digitised historical aerial images, together with advancements of digital photogrammetry, provides the basis for reconstructing and assessing long-term evolution of the surface, both in terms of planimetric mapping and generation of historical digital elevation models (DEMs). This work presents the temporal evolution of fluvial channel morphology in Kaunertal, Austria, spanning twenty periods from 1953 to 2019. Here we use photogrammetric analysis of recent and historical images, together with LiDAR and drone-based photogrammetric DEMs, to quantify the river changes in terms of channel incision, riverbank erosion, as well as the spatial patterns of channel erosion and deposition and the amounts of mobilized sediment. We show that geomorphic changes are mainly driven by deglaciation, i.e. glacier retreat, and sediment delivery from recently deglaciated steep lateral moraines. Overall, this work contributes to better understand the links between channel changes and climatic factors and highlights similarities and differences in the evolutionary trajectories of the main rivers in the catchment.</p>

Geomorphological Parameters as Correlates of Channel Erosion in River Ajilosun in Ado-Ekiti, Ekiti, Nigeria

The paper examined the relationship between geomorphological parameters and channel erosion in River Ajilosun in Ado-Ekiti, Nigeria. Data on geomorphological parameters and channel erosion of the River Ajilosun were generated through direct field measurements using tape measure, ranging pole and a piece of flat board. Some of the variables were also derived through simple linear mathematical relationships. Analysis of the various data was done with both descriptive and inferential techniques. The result of the analysis showed that channel erosion exhibited spatial variation across and along the longitudinal profiles of River Ajilosun and also between the concrete-channelised and the alluvial reaches of the river. The result of the analysis also revealed both positive and negative correlations between the geomorphological parameters and channel erosion in the river. The measures suggested for controlling channel erosion in River Ajilosun included effective channel and slope management, improvement of the channel capacity retention of channel bank vegetation and protection of the valley side vegetation among others.