Virtual Case-Study for FRM options
“RAS-Flood” Mapping & Modelling
Seven routine steps below (counted down in reverse order below for emphasis) illustrate the potential for catchment flood alleviation by hydraulic “RAS-Flood” analysis. The goal of each such case-study is to provide evidence for the optimum number and location of CaBA measures. “RAS-Flood“ methodology complements risk-assessment (FRA), and is a useful tool for evaluating community action plans. Essential data and code are free to download. So . . . . .
Each model-run quantifies downstream outcome
Results Step 7: By way of example, the above aerial view shows increased ponding (coloured green) upstream of footbridges adapted for FRM – the blue inundation perimeter being baseline, ie prior to intervention. The downstream hydrograph is indicative of the effect of 50 such interventions spread over the 40 sq km study-area. (Beyond this and as a by-product of “RAS-Flood” analysis, imposition of 1:200 extreme rainfall effectively creates an opportunity map of further potential ‘hydraulic’ intervention sites.)
Alternative ‘virtual’ interventions are overlayed
(individually or in groups – either by form or by roughness)
Penultimate Step 6: Here red ‘X’s are “FRM” footpath crossings, yellow ‘X’s are road crossings, purple are commended locations for landowner/community adaptation. The effect of alternative groups of measures can be compared – overlay by roughness, ie rather than by form, being the easier to impose in software.
Prior to analysis a computational mesh is constructed
Prior step 5: The mesh creation algorithm is constrained by ‘breaklines’ exported from the GIS stream network vectors. (HECRAS 2D code courtesy USACE)
To prepare the mesh, a stream network is developed
GIS step 4: The relief of the watershed is broken down and layered in respect of drainage area and flow accumulation. (Generic GIS, but Globalmapper is hard to beat !!)
Alongside the stream network, smoothed bathymetry is introduced
Critical step 3: 1M grid stream bathymetry is developed for flow continuity from the lidar point-cloud (‘Atrepo’ point-smoothing methodology).
High-res 1m grid Lidar elevation data forms the basis of any ‘RAS-Flood’ virtual case-study
Fundamental data step 2: Enhanced relief data is added. In some locations, flown lidar data has holes which are filled by the radar base layer.
Initially, 10m radar relief is cropped to watershed perimeter
Initial step 1: Basic relief is loaded. Whilst, at present, the interface between radar and lidar elevation data demands some massage, this is seldom at critical locations
Typical Model Results & Output applying steps 1 – 7
(ie minor upstream inundation yielding beneficial downstream flows)
Increasing levels of upstream intervention are tested above for impact on the downstream hydrograph. The most attenuated hydrograph of the illustration is indicative of the ‘multi-measure’ effect of ambitious (five measures per km2) course adaptations within the 40 sq km watershed perimeter. Such adaptation is sometimes described generically as ‘Slow-the-Flow’. In ‘RAS-Flood’ virtual case-study, watercourses are constrained initially at infrastructure-crossings and other hydraulically optimal locations to boost riparian ‘blue-green’ storage – this for community benefit, (always assuming consent and ccreating overbank ‘ponding’ ONLY during extreme events). The calibration event for the flow modelling exercise illustrated above is the 25mm Mid-Wales rainfall (from 06.30-07.30 am on 27 May 2018) on a 40 sq km tributary of the River Severn.
“Every catchment can benefit from a Virtual Case Study“
Essential “RAS-Flood” components are free to download and implement