So what’s the attraction with gels or, water enhancers, in Australia? Gels have been used operationally for the last couple of seasons in NSW and South Australia. And, Victoria has recently awarded a contract to trial gels. What are the implications for our Air Crews and Mixing Crews integrating these products into mainstream Fire Bombing activities?
Perhaps we should review the composition of gels and the mechanism that they utilize to suppress a fire. When used as a fire suppressant, gel solutions contain less than 1% of a Super Absorbent Polymer (SAP) and more than 99% water. The SAP is an organic polymeric powder with the ability to absorb at least 300 times its weight of water. In some cases, colour pigments, suspending oils and other components are mixed with the SAP to transform it from a powder into a liquid suspension that may be easier to handle and mix with additional water. When the SAP composition absorbs water its character changes to a viscous, gelatinous mass or a thickened solution dependent on the amount of water that has been absorbed.
The fire suppressing ability of gel solutions depend totally on their contained water. When applied in sufficient quantities, the gel solutions’ contained water cools the fire sufficiently to reduce flame length and slow its rate of spread so that ground troops can more easily gain control of it. Both a gel solution and a Class A foam have essentially the same fire suppressing ability as their contained water. In view of this, it is important to recognize that water, Class A foam and gel solution should only be used in direct attack or immediately adjacent to the fire so that water evaporation is minimized prior to the appearance of flaming combustion.
This is not, of course, true for long-term (LT) retardants that contain retardant salts. Long-term retardant reacts with the vegetation to chemically change its’ decomposition mechanism so that it will not support flaming combustion. The contained water in LT retardant solutions is present primarily to assist in forming a continuous coating on the applied vegetation. Studies by the US Forest Service demonstrate LT retardants reduce fire intensity and spread 1.5 to 2 times more effectively than water when vegetation is still wet from application. After this water evaporates, a series of reactions occur which alter the vegetative decomposition so that flaming combustion is minimized.
So, why are gel solutions considered superior to water and sometimes as replacement for LT retardant in bush fire fighting?
The average depth of the solution over the aircraft footprint is called coverage level (CL), and is usually expressed in a number ranging from 1 to 10 (gallons per 100 square feet). Modern aircraft drop systems utilise ‘constant flow’ technology and compensate and adjust flow rates exiting the aircraft to achieve the requested CL. The impact of CL was derived from years of experimental work conducted by the US Forest Service to determine the long term capability of retardant solutions and relating that information to different fuel models. Long-term retardant is proven to be highly effective between coverage levels 1 and 10. A single engine airtanker (SEAT) or crop duster style aircraft full-salvo drop can deliver and average coverage level 4, or an average of 1.6mm of solution depth, over a drop pattern roughly 100 metres long by 10 metres wide. Large Airtankers (LAT’s) have the ability to reach coverage levels of 10 (4-mm of depth).
Recovery rate is defined as the volume of liquid that reaches the fire ground expressed as a percentage of the volume of liquid delivered over the fire.
It has been shown that thickening fire retardant solutions increases their recovery rate within the fire ground. This is, of course, true for water suppressants (Class A foams and gels), as well. Fire retardants and suppressants are generally delivered to remote bush fire incidents in fixed or rotary wing aircraft in order to reduce their rate of spread until ground forces arrive to extinguish the fire. Gel solutions have been shown to hold together more effectively than Class A foam or water and can have recovery rates approaching LT retardant solutions during free fall from the delivery aircraft to the fire.
Between 2005 and 2008 the California Department of Forestry (now Calfire) collected a large amount of data during aerial drop testing of gum-thickened Phos-Chek® solutions during initial attack of active bush fires – up to 90% recovery when applied during less than 10-mph wind speed conditions. (Both wind speed and wind direction relative to the aerial drop have an impact on recovery rate of all solutions.) Many years of water drop tests – often the ‘control’ to compare the recovery rate of LT retardant solutions – show no statistical difference between a water drop and a foam drop, weighing in around 30 to 70%. It therefore appears to be a ‘no brainer’ to switch from Foam to Gel and benefit from around perhaps a 20%-40% increase in suppressant reaching the fire ground.
Gels range in cost, but sit somewhere around $0.30 per mixed Litre depending on mix ratio approved. Foam is more like $0.01/Lt and LT retardant $1.00/Lt. Agencies and media often report astronomical rates associated with the cost of air drops. But let’s put things in perspective, Aircraft (fuel, standby hire cost, flying hourly rate) are the most expensive part of an aerial program. Sure, gels may be cheaper than LT retardant per litre, but if you have to fly more missions or if the fire escapes, it is false economy.
Foam and gels are effective as direct attack tools only. Both of these tools rely only on the water they encapsulate or absorb to cool and insulate the substrate/vegetation on which they are applied. On “bad fire days” – low relative humidity, high winds and temperatures – gels applied aerially simply won’t retain the cooling properties of water for longer than 10 – 30 minutes because their absorbed water will rapidly evaporate. “But what about the blow torch applied to a handful of gel” I hear you say? It is possible to build up layers of up to a least 25mm (or 1 inch), particularly with the liquid gels containing oils which appear to impart adhesive qualities along with a gentle application technique. But not aerially. Aircraft can’t fly slow enough nor can a load be released fast enough to generate a deep enough layer (see box Coverage Level). Gel will also be intercepted and coat every stage of the vegetation ladder (canopy, mid-story and ground fuels). I don’t think the blow torch test would work so well if your hand was coated with less than 1mm of gel. Can gel be applied with slip-ons or Tankers to protect structures – you bet! And, when applied in this manner thick gel coverings can be built up on even vertical walls and sloping roofs.
When is it a waste of time to use aerial assets in bombing the fire or attempting to build retardant lines around it? The elephant in the room. The limit of the Fire Authorities ability to hold a fire line is about 3kMW. Above this intensity, fires are subject to spot beyond the width of conventional fire lines (hand crew, bulldozer). On the other hand, data collected from the Victorian mega fires of 1983 and 2009 showed peak intensities of around 100kMW. Clearly, aircraft usage for line building is limited on what are now considered to be “extreme” or “catastrophic” fire days, simply because of spotting or embers thrown ahead of the fire. On such days, fire bombing is usually limited to Asset Protection. That is, dedicating resources to the assets that matter most to us: saving the lives of people and their property that is being impacted.
It may be harsh to say this, but sometimes, the best thing we can do is keep a cool head and wait for a change in weather conditions, topography or fuel type. Then we can resume the line building process, safely and effectively.
Notwithstanding public relations benefits, the main advantage of aircraft in delivering fire additives (foam, gel or LT retardant) is rapid initial attack to contain the fire to a small size. When the fire cannot be contained by direct attack, there is only one option available. That is to build a control line around the fire. Again, aircraft can be of assistance in the speed of line construction. Often there is a race against time for the Fire Manager and a balancing act. A dozer might work one flank of the fire and be removing vegetation ahead of the fire at say 1km per hour. To complete the control line, aircraft can assist laying LT retardant lines. This is where only LT retardant is of benefit, slowing the advance of the fire by reducing the ability of the vegetation to function as fuel.
Right to Operate
It is worth remembering that LT retardant was developed in the 1960’s by Industry in co-operation and working collaboratively with the US Forest Service and the California Department of Forestry and Fire Protection to assist in controlling conflagrations while putting the fire fighters in reduced harm. The U. S. Forest Service (USFS) has tested Water Enhancers (gels), written a specification and maintains a Qualified Products List (QPL) for those products that meet the Specification. Indeed, Australasian Fire Authorities (AFAC), of which each State Fire Authority in Australia is a member, adopted a policy position stating that they would use only fire fighting additives that appear on the USFS QPL. It must be recognized that this means that the identified additive concentrate should only be used only at the mix ratios at which they were tested and approved.
With such a strong history and research program, it is interesting to note that the USFS has chosen to have a ‘long term Retardant only’ strategy for use in fixed wing aircraft. Time will tell where gels place settles in the history of Australian bush fire fighting.
For more information, go to www.phos-chek.com.au