Aquatic Biological Diversity Responses to Flood Disturbance and Forest Management in Small, Forested Watersheds

We examined riparian system responses to an extreme rainfall event on 1–4 December 2007, in eleven small watersheds (mean area—13.2 km2) from 2008–2016 at debris flow, high flood, and low flood reaches (all extended overbank flows). Macroinvertebrate responses followed expected outcomes after extreme disturbance including increasing chironomids and other multi-voltine species. A core assemblage of twenty abundant and common species-maintained populations even after debris flow (likely by recolonizing quickly) with total richness during project of 253 including 183 rare species (<0.01 total abundance) supporting an annual turnover of species from 22 to 33%. Primary disturbance changes to habitat were declines in shade and in-channel wood at all reaches, more strongly at debris flow reaches. Macroinvertebrate communities across disturbance intensities became increasingly similar after the storm. Combined effects of the flood reducing channel complexity and previous logging decreasing in-channel wood recruitment from riparian systems, limits habitat complexity. Until this feature of forested watershed streams returns, there appears to be a ceiling on reach scale aquatic biological diversity.

Apparent roughness coefficient in overbank flows

Overbank flows occur in alluvial valleys during flood events when the conveyance of main channel of rivers is exceeded. Once floodplains are inundated and the so-called compound channel flow is observed, the faster flow in the main channel interacts with the slower flow in the floodplain featuring a much more pronounced 3D flow structure compared to single channel flow. These flow mechanisms comprise a shear layer near the interface, lateral momentum transfer and strong secondary currents due to the non-isotropic turbulence. This paper starts by giving an overview of the main flow mechanisms in compound channels pointing out the importance of taking into account the apparent shear stress generated between the main channel and the floodplain flows due to the interaction of these flows. A new simple model was developed to include the apparent shear stress concept as a correction of the Manning roughness coefficient of main channel and floodplains. The proposed method for predicting stage–discharge relationships was calibrated and validated by experimental data from several compound channel facilities. A significant improvement in prediction of the compound channel conveyance in comparison with the traditional methods was achieved.

Can soils act as environmental sponges to help reduce flooding?

&lt;p&gt;Flooding presents a serious socioeconomic challenge to riverine communities across the world, impacting &gt;300 million people each year and causing loss of life, damage to infrastructure, long-term mental and physical health problems, and threatening food security. Across many parts of the globe, including north-west Europe, climate change is projected to increase the magnitude, frequency, and intensity of rainfall events, thus exacerbating future flood risk and increasing the demand for flood alleviation schemes. Historically, flood prevention strategies have focused on constructing hard defences that restrict the overbank flows and aim to convey them downstream. However, as floods become larger and more difficult to predict, the construction of ever-higher defences becomes unfeasible. As such, natural-based solutions are being adopted as a more cost-effective and sustainable approach to managing flood waters through upland attenuation in leaky dams and offline storage in reservoirs in the lowlands. Here we demonstrate the feasibility and efficacy of using agricultural soils as &amp;#8220;environmental sponges&amp;#8221; to retain moisture and reduce downstream flood peaks in a heavily-managed lowland catchment. We use combined field, laboratory, and modelling approach to quantify how increases in soil organic matter &amp;#8211; introduced through cover crops &amp;#8211; can increase soil moisture retention at the field scale and perform groundwater and catchment modelling scenarios to assess how these changes can be extrapolated up to the catchment scale and used to forecast changes in downstream flood risk across a suite of future hydro-climatic and soil management scenarios.&lt;/p&gt;

Hydrological threats to riparian wetlands of international importance – a global quantitative and qualitative analysis

Riparian wetlands have been disappearing at an accelerating rate. Their ecological integrity as well as their vital ecosystem services for humankind depend on regular patterns of inundation and drying provided by natural flow regimes. However, river hydrology has been altered worldwide. Dams cause less variable flow regimes and water abstractions decrease the amount of flow so that ecologically important flood pulses are often reduced. Given growing population pressure and projected climate change, immediate action is required. However, the implementation of counteractive measures is often a complex task. This study develops a screening tool for assessing hydrological threats to riparian wetlands on global scales. The approach is exemplified on 93 Ramsar sites, many of which are located in transboundary basins. First, the WaterGAP3 hydrological modeling framework is used to quantitatively compare current and future modified flow regimes to reference flow conditions. In our simulations current water resource management seriously impairs riparian wetland inundation at 29 % of the analyzed sites. A further 8 % experience significantly reduced flood pulses. In the future, eastern Europe, western Asia, as well as central South America could be hotspots of further flow modifications due to climate change. Second, a qualitative analysis of the 93 sites determined potential impact on overbank flows resulting from planned or proposed dam construction projects. They take place in one-third of the upstream areas and are likely to impair especially wetlands located in South America, Asia, and the Balkan Peninsula. Third, based on the existing legal/institutional framework and water resource availability upstream, further qualitative analysis evaluated the capacity to preserve overbank flows given future streamflow changes due to dam construction and climate change. Results indicate hotspots of vulnerability exist, especially in northern Africa and the Persian Gulf.

FLOOD FLOW CHARACTERISTICS AND BED LOAD TRANSPORT IN NON-VEGETATED COMPOUND STRAIGHT CHANNELS

Floods are the most common natural disasters in Malaysia and have damaged structures, infrastructures, crops and even causes fatalities. It may also lead to erosion and sedimentation in rivers and this will result to complex river behaviour. A hydraulic laboratory experimental study was carried out. Also, flood flow and sediment transport in straight compound channels involving flow resistance, distribution of depth-averaged velocity, stream-wise vorticity patterns, channel bed morphology and bed load transport rate in non-vegetated compound straight mobile bed channels were investigated. The finding showed that the Darcy Weisbach friction factor f increased by 40% and 54% for floodplain and main channel, respectively when relative flood flow depth increase from 0.30 to 0.50. The small bed load transport rates of 0.09 g/s and 0.03 g/s for shallow and deep overbank flows, respectively were measured due to effect of very gentle or mild channel bed slope which was fixed at a gradient of 0.1%.