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.

Can sea level rise help us restore coastal wetlands? The hydrologic restoration of the Slop Bowl, Brazoria National Wildlife Refuge, Texas

The Slop Bowl marsh in the Brazoria National Wildlife Refuge provides extraordinarily high quality, heavily used bird habitat. Much of this habitat has experienced hypersaline conditions due to both hydrologic alteration by humans and a rapidly and changing physical environment over the past several decades. Oil and natural gas extraction activities have resulted in excavation and channelization along pipelines and hydrologic obstruction by an access road. In addition, subsidence along growth faults has altered hydrologic pathways and lowered surface elevations in the center of the marsh. Our objective was to understand the underlying processes that contribute to hypersaline conditions and to evaluate possible restoration alternatives to reduce the severity of those conditions. Accordingly, we conducted extensive field and hydrologic modeling efforts, and identified the past, present, and future of this marsh habitat under a baseline scenario. We then compared various restoration action scenarios against this baseline. We found that, beginning in about 15 years, relative sea level rise will improve the hydrologic conditions by enhancing tidal flushing. However, if fill material is continually added to elevate the obstructing road as the sea rises, this hydrologic relief may never be realized. Moreover, we found that if a drought occurs during this critical period, a difference of only a few centimeters in the relative water level and road elevation, or changes in marsh surface elevations driven by fault motion and subsidence, may have catastrophic consequences. The modeling also suggests that several potential interventions can bridge this gap over the next 15 years and beyond. Actions that improve tidal circulation, reduce salinity, and enhance marsh accretion are being developed by the project team to enhance and restore habitat in the near term. The most optimal approaches evaluated thus far include the installation of culverts at critical locations, the excavation of a small channel, the modification of flow pathways, and the beneficial use of sediments and vegetative plantings. We conclude that, under specific circumstances or at unique locations such as the Slop Bowl marsh, sea level rise can be leveraged to improve coastal wetland health.

Hydrologic Restoration of the Lac des Allemands Swamp, Barataria, Louisiana

Most of the forested wetlands of coastal Louisiana are in decline, primarily due to impoundment and increased flood duration. The Lac des Allemands swamp of Barataria Basin was a prime example of prolonged inundation prior to hydrologic restoration completed in February of 2018; the swamp had been impounded for over 60 years. To characterize restoration benefits, eight paired 625 m2 permanent sites were established close to and halfway between eight 30 m × 122 m gaps cut into the spoil bank of Bayou Chevreuil. During 2018, canopy closure increased by 20%. In addition, aboveground production of wood and leaves increased over 2017 from 2018–2020. Furthermore, natural regeneration has occurred annually and many of the seedlings are now approximately 1 m tall. In conclusion, hydrologic restoration of impounded wetlands in coastal Louisiana is an extremely cost-effective landscape restoration method.

Expanding catchment-scale hydrologic restoration in suburban watersheds via stream mitigation crediting—A Northern Kentucky (USA) case study

The need for hydrologic restoration is well established in stream ecosystems across the world; however, available funding for catchment-scale restoration typically falls far short of what is required to produce in-stream results. Hydrologic restoration can be particularly important for improving stream integrity in urban watersheds, but implementation can be challenging due to high property values and limited space for retroactive stormwater control measures. This Northern Kentucky (USA) case study summarizes how stormwater mitigation interventions could be, and have already been, used to credit stream mitigation projects via conventional US Army Corps of Engineers crediting protocols. Hydrologic restoration can generate stream mitigation credits by directly improving the flow class and/or by indirectly improving the habitat quality. For example, a stormwater intervention could create a shift from an ephemeral to intermittent flow class, while at the same time facilitating greater substrate stability, lower embeddedness, and other geomorphic improvements, that subsequently improve the categorical habitat rating. The ecological lift of such hydrologic interventions could be further expanded via concurrent in-stream mitigation measures such as re-establishing a jurisdictional stream in place of a drainage ditch or installing habitat structures such as toe wood and log steps, among other activities. Such process-based hydrologic restoration is consistent with the goals of the Clean Water Act and has the potential to be more beneficial to greater portions of stream networks and greater numbers of stakeholders than conventional habitat restoration alone.

Expanding catchment-scale hydrologic restoration in suburban watersheds via stream mitigation crediting—A Northern Kentucky (USA) case study

The need for hydrologic restoration is well established in stream ecosystems across the world; however, available funding for catchment-scale restoration typically falls far short of what is required to produce in-stream results. Hydrologic restoration can be particularly important for improving stream integrity in urban watersheds, but implementation can be challenging due to high property values and limited space for retroactive stormwater control measures. This Northern Kentucky (USA) case study summarizes how stormwater mitigation interventions could be, and have already been, used to credit stream mitigation projects via conventional US Army Corps of Engineers crediting protocols. Hydrologic restoration can generate stream mitigation credits by directly improving the flow class and/or by indirectly improving the habitat quality. For example, a stormwater intervention could create a shift from an ephemeral to intermittent flow class, while at the same time facilitating greater substrate stability, lower embeddedness, and other geomorphic improvements, that subsequently improve the categorical habitat rating. The ecological lift of such hydrologic interventions could be further expanded via concurrent in-stream mitigation measures such as re-establishing a jurisdictional stream in place of a drainage ditch or installing habitat structures such as toe wood and log steps, among other activities. Such process-based hydrologic restoration is consistent with the goals of the Clean Water Act and has the potential to be more beneficial to greater portions of stream networks and greater numbers of stakeholders than conventional habitat restoration alone.

Irruptive White Ibis breeding is associated with use of freshwater crayfish in the coastal Everglades

As avian reproductive success is generally prey limited, identifying important prey types or sizes and understanding mechanisms governing prey availability are important objectives for avian conservation ecology. Irruptive White Ibis (Eudocimus albus) nesting at coastal colonies in the southern Everglades numbered over 100,000 nests in the 1930s. A century of drainage and altered hydrologic patterns reduced aquatic prey availability and eliminated large nesting events; nesting activity in recent decades has been typically less than 5% of historical peaks. Hydrologic restoration is expected to increase ibis nesting activity, but which prey types will support high nesting effort is less clear. In 2017 and 2018, we collected food boluses from White Ibis chicks at coastal colonies in Everglades National Park. We also monitored regional nesting activity from 1999 to 2018. In 2017, the region had 1,075 nests, typical of the past several decades; but in 2018, there were 30,420 nests, representing the highest recorded nesting activity in 87 yr. Prey composition varied between years; estuarine crabs dominated nestling boluses in 2017, while crayfish and fish were dominant prey in 2018. Crayfish, especially Procambarus alleni, were heavily exploited by ibis early in the 2018 breeding season, while fish were used more at the end. Crayfish abundances in wetlands near the colonies were higher prior to 2018, and more crayfish-producing short-hydroperiod wetlands remained available for ibis foraging in 2018. Our results support previous studies indicating that crayfish are important prey for breeding ibises and suggest that unprecedented, extensive flooding of seasonal wetlands promoted crayfish production and initiated the irruptive breeding in 2018. Our observations indicate that rehydration of the southern Everglades could restore ibis nesting activity at coastal colonies, but further investigations of hydrologic variation, crayfish production, and ibis foraging and nesting activity will be helpful to understand these dynamics and the importance of short-hydroperiod wetlands.