Interim Report On RESULTS OF LOXWOOD Flood Survey October 2019

Water Environment:
Model Development

1. The baseline (existing) model has been fully transferred into InfoWorks ICM, form the original strategy to undertake the work in HEC-RAS.
2. The model is run with an upstream flood hydrograph, and direct rainfall runoff on the modelled Loxwood catchment. The model should therefore be the best combination of a fluivial model (like the EA Flood Risk from Rivers mapping) and a pluvial model (like the EA Flood Risk form Surface Water).
3. The updated model is therefore able to replicate the overland surface water flooding of Burley Close.
4. The baseline model is still being improved to include the additional detail including headwalls, private garden walls and surfacing which may affect known property flooding.

 
Initial Assessment of Potential Options

1. Formal option modelling will only be undertaken once the baseline model is fully complete.
2. The model does show floodwater backing up upstream of the Guildford Road culvert, however opening up this culvert in isolation would likely increase flood water levels downstream at Pond Close and Oak Grove.
3. At Oak Grove, the model suggests that there the second twin culverted private crossing to no.6 Oak Grove (which was previously flooding) is an obstruction to flow, as water levels differ by 0.5m either sides of the culverts. One of the two pipe openings is half blocked, so and easy option would be to clear both culverts. The second option would be to remove the existing brick side walls and replace with railings. The replacement of the brick bridge side walls could also be done to the upstream twin culvert crossing to No. 5 Oak Grove. A further option could consider replacing both twin pipe culvert crossings with free span bridges or alternatively a larger box type opening.
4. Improving the drainage through Oak Grove unlikely to negatively impact houses downstream, with the exception of the properties at Burley Close.
5. The model will consider the benefit of opening up the culvert beneath Station Road, however a low flood defence barrier along the back of the properties at Burley Close may be necessary to reduce flood risk. The issue with a formal flood defence is that it may then also block surface water runoff from entering the river, creating additional flooding at Burley Close. The benefit of the latest update to the model is that this can be tested.
6. The model suggests that the source of the overland surface water flooding of Burley Close is from the uphill residential plots, rather than primarily from runoff from the school surfacing. Surface water seen coming through the school fencing, while including some contribution from the school surfacing, is from offsite runoff onto the school site. There may be some options which could be worked with the school to assist in better directing / converting overland flow to the river.