Building a fluvial hydraulic model

stuart.marshfield@jbaconsulting.com

Representing ‘reality’: the interaction of fluvial and surface water in HEC-RAS

The demand for knowledge to inform flood risk management decisions has never been higher. Gauged data and observations of flooding are essential to verify that hydraulic models are representative of the real world. In Galgate, a village south of Lancaster, a localised rainfall event in November 2017 provided new information that prompted a change in approach to better represent current levels of flood risk.

JBA Consulting was commissioned by the Environment Agency in 2016 to update the 2005 1D model for the River Conder and its tributaries through Galgate.

The new 2D solver in HEC-RAS 5.0 made it possible to update the model to 1D-2D linked within the same software, utilising new in-channel survey, property threshold data and 1m resolution LIDAR. Gauge data, flood reports, photographs and wrack mark levels were available for several historical events. The model was shown to be a suitable representation of fluvial flood risk in Galgate in the context of the documented record.

Flooding in Galgate, November 2017

In November 2017, the River Conder and Whitley Beck catchments were subjected to a short, intense rainfall event, falling on already saturated ground. The result was rapid inundation of the residential area in Galgate, with roads and footpaths becoming conduits for flood water.

Flood warnings were issued after the flooding had started, indicating that the flood response occurred faster than what was recorded at the gauge. The surface water drainage system had reached capacity and flood water ponded behind defensive walls designed to contain water within the channel.

Surface water and urban drainage

The model as it stands is an improved representation of fluvial flood risk, which was verified by the documented record. However, surface water driven flooding and ponding behind the flood defences, as observed in 2017, is not represented and flood risk may be underestimated.

HEC-RAS 5.0.5 allows us to model surface water ponding behind the flood defences, determined by the routes of the surface water drainage system, using Internal Boundary Condition Lines within the 2D domain.

As a result, we can more appropriately represent surface water flow routes within urban areas. This builds on the understanding gained in the fluvial model and accounts for the interaction between fluvial and surface water.

River Conder 1D-2D HEC-RAS model, Galgate Lune Tributaries study area

Flooding of Preston-Lancaster Road (A6), Galgate

Above: Shoothill GaugeMap, GalgateLeft: Wrack mark survey and observed outline, November 2017

1 in 75-year Internal Boundary Condition sensitivity testing