Integration of hillslope hydrology and 2D hydraulic modelling for natural flood management

Abstract

Natural flood management (NFM) has recently invigorated the hydrological community into redeploying its process understanding of hydrology and hydraulics to try to quantify the impacts of many distributed, 'nature-based' measures on the whole-catchment response. Advances in spatial data analysis, distributed hydrological modelling and fast numerical flow equation solvers mean that whole-catchment modelling including computationally intensive uncertainty analyses are now possible, although perhaps the community has not yet converged on the best overall parsimonious framework. To model the effects of tree-planting, we need to understand changes to wet canopy evaporation, surface roughness and infiltration rates; to model inline storage created by 'leaky barriers' or offline storage, we need accurate channel hydraulics to understand the changes to attenuation; to model the complex behaviour of the whole network of NFM measures, and the possibility of flood peak synchronisation effects, we need efficient realistic routing models, linked to key flow pathways that take into account the main physical processes in soils and the antecedent moisture conditions for a range of different rainfall events. This paper presents a new framework to achieve this, based on a cascade of the Dynamic Topmodel runoff generation model and the JFlow or HEC-RAS 2D hydraulic models, with an application to the Swindale Catchment in Cumbria, UK. We demonstrate the approach to quantify both the effectiveness of a relatively large 'runoff attenuation feature' in the landscape and the uncertainty in the calculation given model parameter uncertainty.

Towards a methodology for testing models as hypotheses in the inexact sciences

“…For the case of NFM, the requirement is more to compare the responses under past observed flood events, with predictions of the same events with a range of potential implementations of NFM measures (e.g. [88,89]). In part, this is to test of whether those measures will have sufficient benefits to justify the investment (as assessed in terms of potential savings in flood damages) and in part to test whether there might be potential dis-benefits from such schemes.…”

The potential for natural flood management to maintain free discharge at urban drainage outfalls

“…This storm was preferred because (a) it is the largest event experienced in the last decade; (b) it did not cause significant fluvial flooding (such events are not the focus here), and (c) there is anecdotal evidence from local residents that during the event there was nuisance flooding resulting from poor performance of the surface drainage system. The rural response (i.e., upstream of the river level gauge) was characterised by coupling Dynamic TOPMODEL and HEC-RAS (similar to methodologies described in [Hankin et al, 2019;). Dynamic TOPMODEL evolved from TOPMODEL, a long-established semi-distributed, and semi-conceptual hydrological model (Beven & Kirkby, 1979;Gayathri, Ganasri, & Dwarakish, 2015;Lane, Brookes, Kirkby, & Holden, 2004;Metcalfe, Beven, Hankin, & Lamb, 2018).…”

Representing natural and artificial in‐channel large wood in numerical hydraulic and hydrological models

“…Flow resistance, channel geometry, channel slope and discharge inputs are all key uncertainties that need to be considered in model outputs (Bozzi et al, 2015). Applying an ensemble of simulations as previously recommended (Rasche et al, 2019) and applied to LW (Dixon et al, 2016;Hankin, Metcalfe, Beven, & Chappell, 2019) is a logical approach to capture model uncertainty and the range of probable effects at reach or catchment scales. Model performance assessment is also specific to the application and performance criteria used (Refsgaard & Henriksen, 2004); parameters applied to a validated model in one context might not be transferable to another.…”

“…Moreover, morphological change may occur at LW sites independent of their influence due to wider catchment hydro-geomorphic processes. In addition to often documented hydraulic habitat changes that can also be modeled (Bair et al, 2019;He et al, 2009), changes in morphology can alter flood risk mitigation functions of nature-based interventions including LW features (Hankin et al, 2019). Reach scale 2D morphodynamic models provide a useful tool for predicting morphological changes (Williams, Measures, Hicks, & Brasington, 2016).…”

“…However, application of morphodynamic models to LW has so far been limited (Wu et al, 2005) perhaps due to their complexity. Modeling of shear stress combined with knowledge of grain size to predict erosion and deposition responses could be a useful simpler alternative, especially over larger scales (Hankin et al, 2019).…”