Designing a Flood Storage Option on Agricultural Land: What Can Flood Risk Managers Learn from Drought Management?

The increasing probability of loss and damage to floods is a global concern. Countries are united by an urgent need to reduce flood risk to households, businesses, agricultural land, and infrastructure. As natural and engineered protection erodes with climate change and development pressures, new approaches to flood risk management delivered at the catchment scale that work with nature hold promise. One nature-based solution that aligns with this Special Issue on river flooding is the temporary storage of floodwaters on the floodplain. In many countries, this would involve controlled flooding inland low-lying agricultural land. Designing schemes that farmers and irrigation districts will adopt is essential. To inform future floodplain storage options, we review farm-centred drought management, specifically, agreements that transfer agricultural water to municipalities through fallowing in California, USA and an Australian farm exit scheme. These initiatives reveal underpinning principles around the need to: balance the multiple objectives of the parties, share the benefits and responsibilities, address local impacts and practical guidance on incentive design including the consideration of conditional participation requirements and responding to farmer and public preferences. In terms of funding there is opportunity for blended financing with flood-prone communities, insurers, and conservation charities.

Physical controls and a priori estimation of raising land surface elevation across the southwestern Bangladesh delta using Tidal River Management

The Ganges-Brahmaputra-Meghna delta in Bangladesh is one of the largest and most populated deltas in the world and threatened by relative sea level rise (RSLR). Renewed sediment deposition through tidal river management (TRM), a controlled flooding with dike breach, inside the lowest parts of the delta polders (so-called “beels”) can potentially counterbalance RSLR. The potential of TRM application in different beels across southwestern Bangladesh, however, still remains to be determined. We used a 2D morphodynamic model to explore the physical controls of five variables on total sediment deposition inside the beels during TRM: river tidal range (TR), river suspended sediment concentration (SSC), inundation depth (ID), width of the inlet (IW) and surface area of the beel (BA). Non-linear regression models (NLMs) were developed using the results of 2D models to quantify how sediment deposition inside the beels depends on these variables. The NLMs have an average coefficient of determination of 0.74 to 0.77. Application of the NLMs to 234 beels of southwestern Bangladesh indicates that TRM operation in beels located closer to the sea will retain more sediment as a result of decreasing SSC further inland. Beels in the western part retain more sediment because of lower average land surface elevation. Smaller beels have higher potential to raise land surface elevation due to nonlinear increase of sediment deposition per day (SPD) with beel area. Compartmentalization of larger beels may increase their potential to raise land surface elevation. Thus, the length of time of TRM application in cyclic order will need to vary across the delta to counterbalance RSLR, depending on current beel land surface elevation and local TRM sediment accumulation rates. We found that operating TRM only during the monsoon season is sufficient to raise land surface in 96 % and 80 % of all beels by more than 3 and 5 times the yearly RSLR, respectively. Applying TRM only seasonally offers huge advantages as to keeping the land available for agriculture during the rest of the year. The methodology presented here applying regression models based on 2D morphodynamic modeling may be used for the low-lying sinking deltas around the world to provide an a-priori estimation of sediment deposition from controlled flooding to counterbalance RSLR.

Root System of Olive Trees ( Olea Europaed L.) Based on Runoff Harvesting System During Dry Period

This paper presents a study of a field trial experiment at olive orchard irrigated by runoff harvesting system under a dry climate which was carried out on 5-year-old olive trees (Olea europaea. L, cv. Barnea) in the middle of Negev desert, starting right after the floods, onwards during the summer growing season. The beginning of the experiment occurred after 2 years with little rain and no run-off events. The olive trees were under severe drought stress when we first initiated controlled flooding in 2017. In the second research year (2018), a massive natural flood had occurred at the end of April. Results show that the water distribution within the soil was highly inhomogeneous even under flood conditions. Soil water loss rate, due to transpiration was mainly correlated with the total amount of soil water and not atmospheric conditions. The relative root water uptake from shallow soil layers (0.3-1.5m) gradually reduced along the season, while the relative water uptake from the deeper layers (1.5-4m) became more pronounced.

Irrigating Wastewater using RWIS for Treating in Soil-Plant System

In Sweden, large areas of willow plantations designated for energy purposes have been established. Pilot tests, using various waste products from society for fertilisation and/or irrigating purposes, have been carried out at Kagerods wastewater plant since 1992. These tests have clearly shown that recycling of municipal wastewater in Salix plantations, can replace a large part of the conventional wastewater treatment, due to natural purification processes in the soil/plant system. During the last few years, several full-scale treatment plants based on the soil/plant system have been set up. Rosenqvist Mek.Verkstad has developed the irrigation system "RWIS" for distribution of wastewater into the plantation. The system uses a controlled flooding technique, combined with computer operation controlled electric valves placed in the fields. By measuring the flow and having knowledge about the contents of the wastewater, it is possible to have total control over the wastewater that is distributed in a specific area. The irrigation system has to be able to withstand trying conditions, without major maintenance in between harvests.

Identification and analysis of geotechnical risks in the conditions of flooding of the remaining gaps

Controlled flooding of the remaining gaps of former quarries (especially lignite quarries) and the formation of lakes that can take different uses is one of the ways of recovering and rehabilitating degraded mining lands which offer opportunities for sustainable development of a mining region. The advantages of such a solution are multiple, starting from the restoration of biodiversity, to economic and social benefits. Basically, several flooding techniques are known, but depending on the geological, geotechnical, hydrological and hydrogeological conditions characteristic for a particular region, these techniques must be adapted to the risks that may arise before, during and after flooding the remaining gap. Most of the times, the risks are of geotechnical nature and arise as a result of the interaction between the rocks and the mixture of loose rocks that form the final slopes and the dump with water. The paper is part of an extensive research, which is carried out under the concrete conditions of the remaining gaps in the Rovinari mining basin, and essentially aims at: identifying the main categories of geotechnical risks that can occur under the given conditions, determining the triggering factors, to produce negative geotechnical phenomena and their effects, as well as to identify the main measures to avoid them.

Sediment management in tidal river: A case study of East Beel Khuksia, Bangladesh

The widespread construction of coastal embankments limited the natural deposition on the floodplain and accelerated the silt deposition in river channels. It resulted in drainage congestion and large-scale waterlogging problem. The temporary de-poldering is one of the effective methods to solve this issue. During high tide, muddy water enters the selected tidal basin depositing a major portion of sediment and at low tide, relatively clearer water erodes the riverbed. This paper presents a twodimensional numerical model to simulate the mechanism of sediment transport and deposition during the process of controlled flooding. The model was applied to three different scenarios of the embankment cuts in East beel Khuksia, Bangladesh. The study recommends operating single embankment cut at a time if the tidal equilibrium is fulfilled by the opening size of that embankment cut. The developed model can be used to assess the land heightening in sediment-starved tidal basins and ultimately plan the rotation of tidal basins for sustainable sediment management.