No abstract

The concept of “passive” river restoration after dam removal is to allow the river to restore itself, within constraints such as localized bank erosion defense where infrastructure or property boundaries are at risk. This restoration strategy encounters difficulties in an urban environment where virtually the entire stream corridor is spatially constrained, and stream-bank protection is widely required. This raises the question of the meaning of river restoration in urbanized settings. In such cases, the sedimentary record can document paleohydrologic or paleogeomorphic evolution of the river system to better understand long-term response to the removal of the dam. Secor Dam was a low-head weir on the Ottawa River flowing through the City of Toledo, Ohio, and its outlying suburbs. The dam was constructed in 1928 and removed in 2007 to enhance aquatic ecosystems, improve water quality, and avoid liability concerns. Predam removal feasibility studies predicted the hydrological and sedimentological responses for the dam removal and determined that reservoir sediments were not significantly contaminated. Postdam removal studies included trenching, sediment coring, geochronology, and surveying. The buried, pre-1928 channel was located and showed that watershed urbanization resulted in channel armoring. Incision in the former reservoir exhumed a woody peat layer that was subsequently shown to be a presettlement hydromorphic paleosol currently buried beneath 1.7 m of legacy sediments, mostly deposited since ca. 1959. Today, the river flows through an incised channel between fill terraces composed of legacy sediments. Additional coring and survey work documented that the channel lateral migration rates averaged 0.32 m/yr over the past ∼80 yr, and that the meander wavelength is increasing in response to dam removal. Using sediment budget concepts, significant channel bank erosion and lateral channel migration should be expected until this river system reworks and removes accumulated legacy sediments currently in floodplain storage. In this dam removal project, “active” restoration practices, such as riparian wetland restoration, would have been more in accord with scientific understandings. That did not happen in this case because of disagreements among different constituencies and because of limitations of funding mechanisms.

No abstract

<p>Land use and management changes and landscape modifications, including urbanisation and agricultural intensification, have resulted in significant increases in flood risk across the UK in recent decades. To combat this, a shift towards catchment-based flood risk management has seen a marked rise in Natural Flood Management (NFM) schemes applied across the UK. These schemes largely represent mitigation strategies that work with natural processes to restore and augment hydrological and morphological catchment features for enhancing downstream flood resilience through the slowing, storing and filtering of runoff and flow. This has been implemented through the introduction of woody debris, afforestation of floodplains and runoff attenuation features. However, despite growing evidence highlighting their potential benefits, the function of these structures in the landscape and their effectiveness for flood risk reduction is still highly uncertain.</p><p> </p><p>To address this knowledge gap, this study evaluates the effectiveness of a range of larger-scale floodplain and in-channel NFM features for flow attenuation and flood risk reduction.  To achieve this, a two-year field campaign was conducted in Somerset, South West England, involving the collection of continuous discharge, storage volume and local rainfall data at four sites in the Tone and Parrett catchments. The sites contained NFM structures including offline and online storage ponds and in-channel woody debris. Using these data, filling, storing and spilling capabilities were characterised through the utilisation of field-scale DEMs from Structure from Motion (SfM) and manual surveys. Storm events were separated, and key hydrograph characteristics analysed, to determine the effect of NFM structures on high flow events and the potential for flow attenuation.</p><p> </p><p>The results indicate an increase in storage and flow attenuation as a result of the inclusion of NFM. Increases in flow lag time downstream of in-channel features were identified, relative to an upstream gauge. Longer recession limbs were also recorded downstream of storage ponds, illustrating the buffering influence of upstream structures and the consequential slowed water release downstream. Floodplain-based storage structures were found to only function optimally during the largest events, where pond filling could occur directly from the channel and flow is temporarily stored on the floodplain. These results will provide vital evidence for both local and national NFM applications.</p>