Mean Rate Of Spread Research Articles

The effects of junction fire development on thermal behaviour at the field scale

Junction fires, a type of dynamic fire behaviour, involve the merging of two active fire fronts. They have been known to result in extreme increases in rates of fire spread and intensity over short periods of time. When these phenomena occur during wildfires, they pose significant risks to emergency management and communities. Few studies have investigated rates of spread and fireline intensity of junction fires using field scale experiments. This study captured data from 16 junction fires in mallee-heathlands utilising a drone-mounted thermal camera. Mean rates of spread and fireline intensity were found to increase toward the 75 % stage of merging before decreasing, similar to those found in other studies. Although, normalised temperature areas increased continuously for the whole duration of merging. Additionally, a custom-built radiative heat flux device was used to measure temperature and radiant heat at the fireline within the junction fires. Peak radiative heat flux was also found to be highest at the 75 % stage of merging, following a gradual decrease as propagation progressed. Heat flux measurements were found to be similar to those of previous studies conducted in similar vegetation. Due to the many complex interactions between weather, fuel and fire behaviour, further studies need to be conducted to appropriately determine the effects of merging on rates of spread and fire intensity.

Dispersal and coexistence of two non-native crayfish species (Pacifastacus leniusculusandProcambarus clarkii) in NE Portugal over a 10-year period

Key-words: Pacifastacus leniusculus, Procambarus clarkii, dispersal rate, crayfish coexistence NE Portugal is one of the most isolated and low populated regions of Western Europe. Recently, two American crayfish species reached this area: signal crayfish Pacifastacus leniusculus coming from Spain and red swamp crayfish Procambarus clarkii in its northward expansion. The dispersion of both species was followed throughout the last decade in river Macas (Douro catchment). Our results support the hypothesis of faster downstream colonisation. The mean spread rate of signal crayfish was faster for the downstream expansion, 2.8 km·yr −1 , while the upstream rate was 1.7 km·yr −1 . Exceptionally, in one period, the rate of spread reached 6.7 km·yr −1 , which was apparently caused by downstream washing in extreme conditions of flow. The mean rate of spread is higher than the observed in other studies which may also be related to the low current velocity of late spring and summer that provides favourable conditions for a fast upstream colonisation. Coexistence of signal and red swamp crayfish was detected in a large river sector. Signal crayfish strongly dominates in the upper and intermediate sectors which are the colder part of the river and in the lower sectors, species abundances are not significantly different. Winter temperature in the higher altitude reaches seems to moderate the competitive abilities of P. clarkii.

Existing fire behaviour models under-predict the rate of spread of summer fires in open jarrah (Eucalyptus marginata) forest

SummaryFire behaviour data from Project Vesta experimental fires were used to evaluate the fire behaviour relationship on the Forest Fire Danger Meter (FFDM), the Forest Fire Behaviour Tables (FFBT) for Western Australia and the fire spread model of Burrows (1999) for predicting the rate of spread of fires in jarrah (Eucalyptus marginata) forest under dry summer conditions. Experimental fires were conducted at two sites in south-western Australia under conditions of moderate to high forest fire danger with fine fuel moisture contents of 6–9% oven dry weight and wind speeds up to 20 km h−1 (at 10 m in the open). Fuels ranged in age from 2 to 22 y since the last fire and had understorey shrub layers from 0.5 to 2.0 m tall. Mean rates of spread for experimental fires were 392 m h−1 and 405 m h−1 at the low and tall shrub sites respectively, with the fastest fires spreading at close to 1200 m h−1. Fires spread two to three times faster than predicted by the FFDM and FFBT fire spread models, and up to five times faster than predicted by the spread model of Burrows. Both the FFDM and FFBT models predicted fire spread better at the low shrub fuel site and when wind speeds were <12.5 km h−1. All models under-predicted fire spread at the tall shrub site and grossly under-predicted fire spread when wind speeds were >12.5 km h−1. We argue that existing fire behaviour models under-predict because they were developed using data from fires that had not achieved a quasi-steady rate of spread, whereas the line ignition technique used during Project Vesta experiments resulted in fires travelling at a speed closer to their potential rate of spread for the prevailing conditions. Existing guides remain valid for predicting spread during the early stages of fire growth when fires are developing from a point ignition and the headfire remains narrow.

Mechanism of visual perception of movement.

We here present a logical mathematical model for the visual perception of movement in man; it is based on the treatment of signals in the retina in relation to time and distance and on the statistical theory of the separation of signal from noise in the visual system. It shows good agreement with experimental results and has made it possible to determine the comparative level of internal noise of the visual system, the mean rate of spread of excitation from the photoreceptors along the surface of the retina, and many other characteristics.