Is there a relationship between flood flows and catchments with thin soils on weathered rock characteristics?

10th Sep 2019

During most rainfall events on the larger parts of the rural hillslope, rainfall flows through soils and rock, and not on the surface.

These throughflows are often assumed to reduce exponentially with depth below the surface, i.e. the rock below the subsoils carries only a small proportion of the flow. 

However, in a recent experiment in a Silurian mudstone, with silty soil up to 500mm deep and the weathered rock horizon (up to 2m deep) infiltration of significant flows has been observed. 

Fractures surfaces lightly pinned together closed-up over a period of hours as the mudstone increased in water content. An unusual feature was that pulses of daily flow took longer each day to infiltrate. This suggested that 

  1. either the pulse exceeded the reservoir capacity of the fractures or 
  2. the hydraulic conductivity was reducing over time. 

Leaks of infiltrating water along the strike of the dipped strata also decayed with time but could be re-invigorated if the daily flows were interrupted. Once again this suggests that hydraulic conductivity may vary over time. 

mudstone fracture flow chart

Figure 1. Reduction in fracture flow in mudstone, various conditions

Hand-sized fractures, fig.2, recovered from the rock excavations showed the effect when tested on the bench. A feature of the soil over the mudstone at the test site is that for 50 years it has never been grazed or farmed and may have similarities with older forest soils. 

mudstone fractures

Figure 2. Mudstone fractures with glass reservoirs

Discussion points:

1) Is there a significant effect on flood flows in upland areas where thin soils cover weathered rock?

2) If weathered rock horizons have variable hydraulic conductivity, is there an argument to support an adjustment of the winter BFIHOST for ungauged catchments?  i.e. throughflows are higher viscosity; evapotranspiration is lower and soil biological activity at a minimum.

3) Do active fractures interacting with roots, mycelia, silt and water flows constitute a weathering mechanism ratcheting open fractures? 

If anyone is interested in taking the above observations further with more detailed research and would like to discuss the work to date in more detail, please get in touch using the contact form below.

Alternatively, click here to read more about how Waterco can help with Natural Flood Management.

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