Peak Discharge Analyses for Flood Management in Lower Gandak Basin

Retention potential analysis of river restoration and floodplain measures in different catchments of Bavaria, Germany

10.5194/egusphere-egu2020-13199 ◽

2020 ◽

Author(s):

Michael Neumayer ◽

Sonja Teschemacher ◽

Fabian Merk ◽

Markus Disse

Keyword(s):

Spatial Scales ◽

Flood Management ◽

Peak Discharge ◽

Mitigation Strategies ◽

General View ◽

Flood Events ◽

River Channels ◽

Management Decisions ◽

Floodplain Restoration ◽

Restoration Measures

Nature-based solutions are an important component of integrated flood mitigation strategies for improving both the protection against hazardous flood events and the ecological conditions of river-floodplain systems. In order to be able to take these types of measures into account in upcoming flood management decisions, it must be possible to reliably estimate their effects on flood events. Therefore, this study focuses on a more general view on the catchment dependent contribution of combined river and floodplain restoration measures to the strengthening of river retention and flood protection. Furthermore, the importance of considering site-specific circumstances (e.g., the superposition of the flood waves of the main river and its tributaries), is evaluated.The study is based on five investigation areas in Bavaria (Germany) with various topographic properties and different spatial scales (~ 90 &#8211; 560 km2). For each catchment, a physically based hydrological model (WaSiM) was coupled with the two-dimensional hydrodynamic model HYDRO_AS-2D by means of direct and diffuse inflow boundary conditions. Five flood events with various rainfall characteristics (advective/convective) and different return periods (5, 20 and 100 years) were generated with WaSiM. The holistic restoration scenarios are implemented by catchment dependent modifications of river channels and floodplains. As the aim of this study is to analyze the maximum possible efficiency of the restoration scenarios, it is assumed that almost the entire floodplain is available for the implementation of these measures. Highly restricted areas (e.g., settlement & industrial areas, important infrastructure) are excluded from this assumption. First results show that the peak discharge attenuations resulting from the restoration measures are exemplarily dependent on the characteristics of the floodplains (e.g., slope and extent) and the volumes of the flood events. It could be shown that the largest peak discharge attenuations (up to 28 %) and retardation (up to 8 h) occur in catchments with relatively flat and wide floodplains in combination with comparatively small flood volumes. Furthermore, the effectiveness of these measures can be considerably affected by local superposition effects with incoming tributaries. These effects can have site and event specific positive or negative impacts on the peak discharges and may not be neglected when planning restoration measures.Based on these investigations, it is possible to evaluate if catchments are likely to be suitable for river and floodplain restoration in the course of flood management decisions. However, the effectiveness of the measures is always influenced by a combination of many area-specific factors that can only be predicted to a limited extent and therefore requires the modelling of an area.

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Offline Storage Areas as a Natural Flood Management intervention: Evidence from the Evenlode catchment, UK

10.5194/egusphere-egu21-9779 ◽

2021 ◽

Author(s):

James Bishop ◽

Gareth Old ◽

Ponnambalam Rameshwaran ◽

Andrew Wade ◽

David Gasca-Tucker ◽

...

Keyword(s):

Flood Risk ◽

Monitoring Network ◽

Flood Management ◽

Peak Discharge ◽

Storm Events ◽

Capital Costs ◽

Management Intervention ◽

Storage Area ◽

South East England ◽

The Uk

Catchment-based approaches that work with natural processes for fluvial flood risk reduction are currently the subject of much interest both internationally and in the UK, where they are known as Natural Flood Management (NFM). NFM schemes typically seek to replicate, restore, or enhance natural features of the environment so as to store and/or slow floodwaters during storm events. Benefits over traditional hard-engineered flood management approaches include reduced capital costs and carbon emissions, and they can deliver positive outcomes for both water quality and biodiversity. Despite a small number of studies indicating their potential value, the further uptake of NFM schemes is limited by a lack of empirical evidence demonstrating their effectiveness.We present results from an intensive monitoring network within a tributary (catchment area 3.4 km2) of the Littlestock Brook, a lowland agricultural catchment within South East England that presents a flood risk to the downstream village of Milton-under-Wychwood. The catchment forms part of the first NFM scheme of its kind within the River Thames basin, currently being delivered in partnership by the Evenlode Catchment Partnership and the Environment Agency as part of a five-year project (2016-2021). Precipitation, stream discharge, and water level within eight offline storage areas have been continuously monitored since September 2019. High resolution topographic surveys of each storage area enable filling, storing, and drainage dynamics to be determined and compared with downstream hydrograph metrics. A series of storm events between October 2019 and February 2020 have provided a unique dataset for investigating the performance of the NFM scheme.Data from four storms with estimated peak-discharge return periods ranging from 2.7 to 5.5 years demonstrate the potential for reducing peak discharge. During the largest storm, flood volume across the peak of the hydrograph was reduced by 22%, with 64% of total storage capacity remaining unused. Variations in the filling, storing, and drainage characteristics of each storage area have consequences for the overall effectiveness for reducing downstream flood risk and these will be discussed.

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Performance of Retarding Basin in Flood Disaster Risk Mitigation in Welang River, East Java Province, Indonesia

Journal of the Civil Engineering Forum ◽

10.22146/jcef.31938 ◽

2018 ◽

Vol 4 (2) ◽

pp. 109

Author(s):

Prorida Sari ◽

Djoko Legono ◽

Joko Sujono

Keyword(s):

Risk Mitigation ◽

Control Point ◽

Flood Management ◽

Peak Discharge ◽

Flood Routing ◽

Hydrologic Modelling ◽

Hydraulic Simulation ◽

Flood Peak ◽

Tidal Area ◽

Analysis System

Flood phenomenon caused by high rainfall and sea tides on a watershed seat the tidal area, including the Welang River, commonly occur and the number of events is increasing. Construction of retarding basin is one of flood risk mitigation efforts by reducing the flood peak discharge. Assessment of flood management in Welang River was conducted with hydrology and hydraulic approaches, by using the Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) 4.0 and Hydrologic Engineering Center–River Analysis System (HEC-RAS) 5.0.3 software. The hydraulic simulation consists of 4 scenarios. Scenario 1 was the current condition, while scenario 2, 3, and 4 were the retarding basin construction with one side spillway, one on the upstream (River Station (RS) 7400), on the middle (RS 6970), and on the downstream (RS 6590), respectively. The height variation of side spillways are 3 m and 4 m. Flood routing simulation result showed that the existing river channel condition could not accommodate of 2-year flood and 10-year flood, which caused peak discharge of 497.7 m3/s and 794.9 m3/s. At the RS 6590, the maximum runoff height of 2-year and 10-year flood were 0.66 and 1.02 m, respectively. Under the 2-year return period of flood, the discharge reduction caused by the retarding basin at control point RS 5341.4 (Karangketug Village), were 39.63 m3/s, 31.83 m3/s, and 41.93 m3/s, respectively for scenario 2, 3 and 4 with the 3 m side spillway height and 14.71 m3/s, 16.76 m3/s, and 13.74 m3/s, respectively for scenario 2, 3 and 4 with the 4 m side spillway height.

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