Generic rainfall-runoff boundary brings increased flexibility

The generic rainfall-runoff boundary within Flood Modeller was recently enhanced to provide an expanded range of globally recognised hydrological approaches. It has been redesigned to allow you to select the most appropriate hydrological method for your specific region, providing increased flexibility.

The development of the boundary unit has been approached from two angles. The addition of several methodologies for performing hydrological analysis allows for increased choice of loss and transformation models used within the unit. Alongside this, a multiple sub-basin technique has been implemented, enabling the application of numerous rainfall profiles, loss and transformation models, and baseflow procedures, within a single generic rainfall-runoff unit.

Various scenarios, topographies and land-uses can be accounted for, with selections to allow for any combination of methodologies defined by the user, together with options for impervious areas including Directly Connected Impervious Areas (DCIA). Calculations to evaluate the total flow are completed for each sub-basin, and the total flow from the unit is determined as the summation of these flows over all sub-basins.

These changes provide greater flexibility – the selection of a certain profile type does not restrict the selection of further profiles. Investigation of precise differences between methods can be carried out as a one-click operation, and the resultant hydrographs and hyetographs immediately viewed. Specification of different parameters within methods, from roughness values to time delays, allow you to accommodate for as fine a level of detail as is required, ensuring all physically significant topographic components can be accounted for.

A multiple sub-basin approach

The use of multiple sub-basins within a single generic rainfall-runoff unit has many advantages. It allows the application of an alternative rainfall profile to an area, including the same profile at a delayed time (a rainfall delay can be manually entered for any profile). Different loss and transformation models can be defined and applied to different areas to represent different land-types or flow behaviour. Additional lag can also be applied to sub-basins to account for longer travel times to the outlet, and multiple baseflow profiles can be defined to allow for alternative underlying behaviour of the reach being considered.

Intuitive, simple to use and easy to adjust, the generic unit provides tabs for defining all parameters for the various methodologies chosen, and a ‘Model Summary’ tab to select which profiles to apply. Time-step calculations within Flood Modeller's interface automatically adjust the time interval used to be a common multiple of time intervals within the different methods and profiles applied, without the need for any manual edits by the user. The flexibility of the multiple sub-basin approach allows for combinations suitable for a wide range of hydrological set-ups and scenarios.

Selecting multiple profiles to apply to different sub-basins

The ‘Results’ tab allows for a fast way to view the results calculated by the generic rainfall-runoff unit. View the flow produced by the individual sub-basins and the total summation over all sub-basins, both in tabulated form and graphical plots. A full breakdown of all steps throughout the calculations are provided for reference. The total summation of flows from the generic unit can then be used as input boundary data to existing Flood Modeller models, to determine the effect on water level further downstream in both 1D and 2D models.

Available methodology

The generic unit provides greater flexibility than ever before with multiple globally-applicable methods available. Choose from SCS design storms, user entered profiles and library storm profiles for the gross rainfall entering the system, and calculate the net rainfall based on loss through infiltration through the SCS Curve Number or Green & Ampt approaches. Apply a transformation method to route the net rain through to the outlet; the SCS unit hydrograph and Clarke unit hydrograph methods are both available in the newly designed unit.

Building on the existing methodologies for calculating time of concentration, options are now provided to select from multiple time of concentration methods; user specified, NRCS and TR-55 (as in previous versions of Flood Modeller), but also the Kerby method for calculating overland flow time, the Kerpich method for calculating in-channel flow time, and the Kerby-Kerpich combined method. Further, a time of lag can be specified if more convenient; either manually entered by the user or calculated via the Snyder method.

Recessive and constant baseflow profiles can also be defined to apply to sub-basins. A variety of time delays and lags are provided to accommodate for any additional necessary delays, resulting in versatility over a wide-range of catchments and hydraulic scenarios.

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