Potential landslides in California and the ongoing crisis in Hunza, Pakistan

Amongst the (appropriate) focus upon Haiti at the moment, we must of course remember that landslides do not stop elsewhere. So, here are two important ongoing stories:

1. Potential landslides in California this week:
The US National Weather Service has issued a series of warnings for very heavy rainfall in California this week, with the pontential for landslides, flash flood and debris flows. For example: ...Flash flooding and debris flows possible for recent burn areas In southwestern california late monday morning through monday night... .the first in a series of powerful pacific storms is expected to Move into southern california tonight into monday. Initially...rainfall Is expected to be fairly light...with rainfall coverage and intensities Expected to increase significantly by late monday morning through Monday evening. Rainfall rates between one half inch and three Quarters of an inch per hour will be possible monday afternoon Into monday evening. Local rates over one inch per hour will be possible Near thunderstorms and across favored south facing slopes...including The station and morris burn areas. Rainfall totals with this first Storm system through monday night are generally expected to range Between one and three inches across coastal and valley Areas...with three to five inches in the foothills and Mountains...except local amounts up to 6 inches possible across Favored south facing slopes. For the station burn area...rainfall Totals of 3 to 6 inches can be expected through monday night. Additional periods of heavy rain will be possible later in the Week as future storm systems move into the area...which may Require additional flash flood watches to be issued.

And:
...Wet weather to continue through much of the week... A series of storms...no fewer than six...are lined up across the Pacific all the way to asia. The first of these systems was moving Through the central california interior this afternoon...and will Spread light to moderate rain to the central and southern san Joaquin valley and the adjacent sierra foothills through tonight. Rain also will spread into the high deserts of kern county by late Afternoon and tonight. A second system is developing west of the California coast...and will move into california monday. It is This system that will spread occasional rain...heavy at Times...into the region. For the rest of the week...it is still too early to say when and How much each individual system will impact central california. But there will be periods of rain...heavy at times. Preliminary Indications are for another strong surge of moisture during the Day tuesday...again wednesday...and yet another thursday. Some Decrease in activity is now seen by the weekend...or at least Becoming more showery at times.

There is particular concern about the potential for debris flows in the Station Fire burn zone in the San Gabriel Mountains, as shown in this NASA satellite image:


However, as the La Conchita landslide showed five years ago, many areas in California have the potential for a landslides during very heavy periods of rainfall. It will be an interesting week.


2. The Attabad landslide in Hunza
The blockage of the Karakoram Highway by the Attabad landslide in northern Pakistan continues to cause major problems to that area. Meanwhile, the lake is continuing to fill. The Pamir Times continues to provide excellent coverage of this serious event. Over the weekend they posted a series of images of the slide. First, the lake:


Second the contact between the water and the dam sediment:


And finally the flow path once over-topping is achieved:

In some places the materials look worryingly fine grained and even muddy, but it is impossible to get a proper idea from these images. The report says that the water level is currently 10-15 m from the dam crest, which is of increasing concern. Various reports suggest that there is little government action to date in terms of creating a channel, although clearly this is unverified. It would be unwise to allow the water to flow over the top of the dam without being controlled. However, a report from the ICRC in Pakistan, dated 14th Jan, states that "According to the local government, a dam burst is very highly unlikely to occur."

Australian wildfires and risks of increased erosion rates

(Updated 10th Feb to include latest casualty numbers)

The extraordinary wildfires in Australia are dominating the headlines in the UK, half a world away. Wildfires are quite common events, but the number of fatalities that this particular episode has caused is really quite unusual. Below in Figure 1 I have plotted the recorded worldwide recorded number of deaths from wildfires for the period since 1980, using data from the CRED EM-DAT database . I have added the (updated to) 173 reported deaths from this event so far as an extra column, although note that reports suggest that this total may rise substantially:

Figure 1: Global numbers of reported fatalities from widlfires, based upon the CRED EM-DAT database. The 2009 value is the reported number of deaths from the Austrlian wildfires.

The average annual global total number of deaths is 59.5 fatalities per annum. Care is needed in the interpretation of the above as CRED only record events that kill ten or more people, thus these values consistently underestimate the true toll, but nonetheless the unusual impact of these events is clear.

In the context of this blog it is also interesting to think through the likely long term impact of these fires in terms of erosion and landslides. A recent paper by Smith and Dragovitch (2008) looked at the long term consequences of wildfires in SE. Australia. These two researchers have published extensively on sediment production and erosion in Australia, so have a strong pedigree.

The study focused on a fire that occurred in January 2003 during a drought in the Snowy Mountains near to Thredbo (Fig. 2), a sub-alpine environment. The study used erosion pins to monitor surface level change on both burnt and unburnt hillslopes over a period of 795 days after the fire.

Figure 2: Google Earth perspective image of the area around Thredbo, the location of the study reported by Smith and Dragovich (2008)

The study showed that after the fire the areas that had been burnt suffered a net loss of soil representing an average of 3.8 mm of material, with the most intense erosion occurring on the lower slopes (Fig. 3). On the other hand, the unburnt areas saw a net accumulation of soil of an average of 2.6 mm, again with the greatest accumulation at the lower slopes.

Figure 3: Mean net soil loss and gain for burnt and unburnt areas as reported by Smith and Dragovich (2008).

Thus, the burnt areas clearly suffered a net loss of material in the aftermath of the fires. The study showed that this loss of soil declined with time after the fire, with a slight increase again during snow melt, presumably as vegetation re-established. However, these values are perhaps surprisingly low compared with those recorded in other environments, especially in N. America, given the steep slope angles seen in Figure 2. Interestingly, Shakesby et al. (2007), who studied post-fire erosion in Eucalyptus forests in SE. Australia, came to similar conclusions, stating that "except under extreme post-fire rainfall conditions, present-day wildfires affecting south-east Australia seem to be less potent in geomorphological terms than might be expected given the severity and frequency of the wildfires". They attribute this to the rapid rate of plant growth in the aftermath of fires plus the resistance of the soil to erosion.

The conclusion is therefore that although the fires have devastated vast areas, and made thousands homeless, there should not be a serious increase in erosion in the burnt areas. This will help greatly in the post-fire recovery of the burnt areas.

References:
H SMITH, D DRAGOVICH (2008). Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia CATENA, 73 (3), 274-285 DOI: 10.1016/j.catena.2007.11.003

R SHAKESBY, P WALLBRINK, S DOERR, P ENGLISH, C CHAFER, G HUMPHREYS, W BLAKE, K TOMKINS (2007). Distinctiveness of wildfire effects on soil erosion in south-east Australian eucalypt forests assessed in a global context Forest Ecology and Management, 238 (1-3), 347-364 DOI: 10.1016/j.foreco.2006.10.029