Background

The very heavy and persistent rain that was experienced over the winter of 2023/24 caused a number of localised flooding incidents in Maulden which have been highlighted in a recent report published by the Parish Council   [1].

The report piqued my interest and I thought that it would be an interesting topic to explore further with the objective of identifying other locations in the village that might be at risk from flooding.

I have used the Topographic Wetness Index (TWI)   [2], [3], [4], which is a commonly used geographical metric for identifying flood vulnerability areas. TWI quantifies the propensity of water accumulation on inclined surfaces and is generated from a mathematical analysis of the surrounding topography. For my base topography, I used the 1m resolution LiDAR elevation data available from DEFRA in order to compute the TWI for the extent of the parish.

TWI over the parish extent

The map below is the TWI for the entire parish. The value of TWI varies from about 0.39 in the driest locations, to over 14.5 in the most wettest.

The top five percent wettest areas

From the base TWI dataset above, I refined the analysis by computing the 95th percentile of the data in order to identify areas that were in the top five percent in terms of wetness, and hence most likely to be vulnerable to flooding, or a least increased ground water flow. The 95th percentile of the region occupied by the parish has a TWI index value of 9.22.

The southern edge of the village has the highest TWI values, the areas occupied by The Moor and Water End, with more sparse regions in the Northern section where the centre of the village is located.

Locations identified in the Maulden Parish Council report

Below are my maps showing some of the locations cited in council report. I was not able to identify all of the locations in the report as no exact position was given for some of them. Whatever the case, there is a clear correlation between the flooding event locations and high areas of TWI.

In the map below, note the TWI extent in the field behind the bend in George Street, suggesting that this area was possibly a “feeder” for the significant flooding event that happened there. The junction of The Brache and Ampthill Road is a well know area where water accumulates frequently, perhaps being driven by the TWI regions on the recreation ground and the upper section of The Brache. Also the bend in Ampthill Road, near The Knoll, frequently produces large water pooling after heavy rain.

The Water End area which has highest TWI regions in the village, and the close proximity to the A507 and River Fitt. Possibly, these areas might contribute to elevated water flow into the river and the surrounding land.

Conclusion

I think that it is clear from the above analysis that there is a direct link between flooding incidents in the village and elevated regions of Topographic Wetness Index. I hope that the above might be a useful adjunct to any flood mitigation strategy implemented in Maulden, and to residents concerned about property damage and road safety issues caused by excessive rain induced water flow.

References

[1] Maulden Parish Council, FLOODING REPORT MAULDEN, 2024, Available online.

[2] Fitra, Joni, Semangat Marudut Tua Debataraja, and Lismawaty Lismawaty. Identification of Flood Vulnerability Using the Topographic Wetness Index Method in Pantai Labu Baru Village, Deli Serdang, North Sumatera. E3S Web of Conferences 483 (2024): 01014. https://doi.org/10.1051/e3sconf/202448301014.

[3] Riadi, B, B Barus, Widiatmaka, M J P Yanuar, and B Pramudya. Identification and Delineation of Areas Flood Hazard Using High Accuracy of DEM Data. IOP Conference Series: Earth and Environmental Science 149 (May 2018): 012035. https://doi.org/10.1088/1755-1315/149/1/012035.

[4] Ballerine, Clayton. Topographic Wetness Index Urban Flooding Awareness Act Action Support, Available online.