Mixing of Hillslope, River, and Alluvial Ground Waters in Lowland Floodplains

Abstract

In this paper, we develop and apply a two‐dimensional flow and transport model to simulate the mixing of variously sourced water within a fluvial floodplain during one in‐bank and one out‐of‐bank flood event. The model is applied to two 120 m long transects through the floodplain of the River Severn, Shropshire, United Kingdom, which run to the river from the hillslope bordering the floodplain. For this site, hourly piezometric, river stage, and rainfall data are used to parameterize and validate the flow component of the model. Simulation of the movement of hypothetical conservative tracers driven by these hydrodynamic conditions is then undertaken to confirm and refine an evolving conceptual model of floodplain hydrology. This is based on the bank storage concept, and extended to include the impact of hillslope contributions to the floodplain/riparian zone and the impact of out‐of‐bank inundation on subsurface flow processes. The paper demonstrates that the role of the unsaturated zone has been underestimated in our previous investigations at this site and, in addition, that there is a potentially high residence time for solutes which remain in the unsaturated zone at the end of the flood event. In general, flood events in lowland plain systems are shown to be times of great hydrological and chemical activity. Lastly, the results confirm the complexity of interactions between surface and ground water flows in lowland plain settings and highlight the need for correct treatment of both dynamic boundary conditions and unsaturated zone processes.

Spatiotemporal variability of hyporheic exchange through a pool‐riffle‐pool sequence

“…Claxton et al . [] found that overbank flows can “hold back” hillslope and possibly bank‐retained water due to the steep hydraulic gradient induced by flood waters; so seemingly disconnected hyporheic fluxes could result from this complex behavior. Last, Wroblicky et al .…”

Simple spatially-distributed models for predicting flood inundation: A review

“…If a longer reach is considered or if the reach is relatively far upstream, then the contribution from hydrological processes could potentially be very significant. Such processes include direct precipitation and runoff onto the floodplain (Hughes, 1980;Lewin and Hughes, 1980), evapotranspiration losses (Yao et al, 1996;Boroughs and Abt, 2003), bank-storage effects (Pinder and Sauer, 1971;Hunt, 1990;Whiting and Pomeranets, 1997) resulting from interactions between the river water and alluvial groundwaters contained within the hyporheic zone Ward, 1988, 1993;Winter, 1995;Dahm and Valett, 1996), subsurface contributions to the floodplain groundwater from adjacent hillslopes Claxton et al, 2003;Jung et al, 2004) and flows along preferential flow paths, such as relict channel gravels, within the floodplain (Gillham, 1984;Poole et al, 2002;Burt et al, 2002). Over particular reaches and in certain environments, integration of some or all of these processes within flood inundation model codes may be required and will likely necessitate complex modelling structures (e.g.…”

Multi‐method global sensitivity analysis (MMGSA) for modelling floodplain hydrological processes

“…The model has recently been used to study high resolution floodplain hydrological processes Claxton et al, 2003;Cloke et al, 2003Cloke et al, , 2006aCloke et al, , 2006b). The flow model is described in detail in Renaud et al (2003) and Cloke et al (2006b); only a summary of the model code is given below.…”

“…The model platform used here is ESTEL-2D, the subsurface flow component of the TELEMAC modelling system (Hervouet and Bates, 2000) which solves the Richards equation in saturated and unsaturated porous media with the finite element technique. The model has recently been used to study high resolution floodplain hydrological processes Claxton et al, 2003;Cloke et al, 2003Cloke et al, , 2006aCloke et al, , 2006b). The flow model is described in detail in Renaud et al (2003) and Cloke et al (2006b); only a summary of the model code is given below.…”