Post-Fire assessment onf
Transcript of Post-Fire assessment onf
Post-Fire assessment usingSentinel-2 images in French
Mediterranean area
Office National des Forêts
Yvon Duché, Jean-Luc Kicin, Benoît Reymond, Rémi Savazzi
ONF presentation
ONF (National Forestry Board) is a State Public Body, under the joint supervision of the Ministries of Forestry and the Environment.
ONF is responsible for forest management for both the state and local government, and has a public service remit to help the State and local authorities in protecting forests against fires.
ONF has a specialized agency in the Midi-Mediterranean area to deal with wildfire issues that performs the following tasks:
• Operational - monitoring, detection, first response, support to control activities
• Development and maintenance work - roads, water points, areas cleared of undergrowth
• Expertise support - hazard and fire mapping, fire risk management plans, equipment mapping …
• Project management
Calculation method usingSentinel-2 data
Within these missions, ONF establishes wildfire maps. Sentinel-2 images have been used from 2016 using differenced Normalized burnt Ratio (dNBR) calculation method. This method is based on the different spectral responses of Near InfraRed(NIR) and Shortwave Infrared bands (SWIR) for unburnt/burnt areas.
So
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e :
US
DA
Fo
rest
Se
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Calculation method usingSentinel-2 data
Example : Rognac fire, 10th of August 2016, 2 655 ha
Post-fire image of 13th of AugustSentinel 2A © ESA 2016 © CNES 2016
Pre-fire image 3rd of AugustSentinel 2A © ESA 2016 © CNES 2016
NBR = ������
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NBR post fireNBR pre fire
dNBR = NBR(pre-fire) - NBR(post-fire)
Fire outline mapping
Result of the dNBR calculation
(raster format) Value > 0.1
Vector conversion
No pixels
shape
generalizing
The dNBR calculation allows a quick fire outline mapping.
Fire outline mapping
• Flat terrain
• Maximum outline difference < 20 m (i.e. pixel size)
• 0.4 % surface difference between GPS record and dNBR map
Aix-en-Provence, 14th of June 2016 Châteauneuf-les-Martigues, 14th of July 2016
• Steep terrain
• 6 % surface difference between GPS/manual cartography (unattainable areas) and dNBR map
Accuracy of fire outline mapping has been validated on different kinds of situations.
Fire outline mapping
Red = false spot / Green = true spot
• dNBR mapping seems accurate for “big” (size/intensity) fires / in most summer conditions in French Mediterranean conditions equivalent to GPS survey
• For big fires it allows a quick mapping of the fire outline
• In steep terrain, dNBR is more accurate than traditional mapping
• dNBR can also contribute to check and map small fire spots (20x20) more rapidly and more efficiently
These tests have shown that :
Fire outline mapping
Detected Undetected
But depending on burn severity and vegetation type, some burnt areas can be missed by the dNBR calculation.
This kind of situations often occur during the winter period when burn severity is lower than during summer time.
Moustiers-Ste-Marie, 22nd of October 2017
Moustiers-Ste-Marie, 22nd of October 2017
Roquefort des Corbières, 6th of September 2017
Fire outline mapping
Therefore, fire outline mapping during winter period must complete dNBR calculation with GPS survey for instance.
Detected
Undetected
Moustiers-Ste-Marie, 22nd of October 2017
In 2017, availability of Sentinel 2B images has reduced the fire mappingdelay.
In Mediterranean area, all fires of 50 ha and over have been mapped. The first one occurred on 24th of March, the last one on 30th of November.
44 fires from 50 to 2263 ha have been mapped using Sentinel images (16280 ha overall i.e. 83% of all burnt areas).
Fire outline mapping
Assessing Post-fire vegetationdamages using severity index
The vegetation burn severity is defined as being the loss of aerial and subterranean organic material due to burning, by combustion or mortality.
The classes of the severity index are defined from US fires, but can be used as first approximation to interpret the dNBR in Mediterranean conditions.
Assessing Post-fire vegetationdamages using severity index
Fields measures on the Rognac fire indicate that :
• For a same severity index, impacts can vary with the type of vegetation in place before fire :• Wooded type (forest) vegetation height > 3m
• Brush type (moor) vegetation height < 3m
• Grass type
• For a same severity index impacts on the vegetation vary with the vegetal cover density
• Test of a composite index mixing both the type of vegetation and of severity index Densité /
couvert de
la
végétation
Sévérité du feu sur la végétation (dNBR)
Faible Moyen Fort
Forêt
Houppier vert ou
légèrement roussi sur la
partie inférieure
(Sous étage et litière
brulée)
Houppier totalement roussi
(présence de quelques
sujets encore vert possible)
Etage arborée
totalement ou
partiellement brulé
(feu de cime)
Arbustif
Faible Végétation totalement brulé
Moyenne Strate arbustive roussie Végétation totalement brulée
Dense Strate arbustive verte et
roussie (en mélange)
Strate arbustive brulée et
roussi (en mélange)
Végétation
totalement brulé
Herbacée Végétation rase
totalement brulée
Végétation haute
totalement brulée
Assessing Post-fire vegetationdamages using severity index
Sentinel-2 images can also be used to qualify the type of damaged vegetation, analyzing pre-fire situation.
Pre-fire image
1 : Incombustible (Minéral, Eau)
2 : Herbacee
3 : Arbustif Dense
4 : Résineux
5 : Feuillus
Image classification
External GIS
databases
1 : Non combustible (mineral eau)
2 : Herbacée
3 : Arbustif dense
4 : Résineux hors IFN
5 : Feuillus hors IFN
6 : Vigne
7 : Verger
8 : Haie
41 : Résineux indifférenciés
42 : Pin Alep
43 : Pin Maritime
44 : Pin Laricio ou Noir
45 : Pin Pignon
51 : Feuillus indifférenciés
52 : Chêne liège
53 : Chêne vert
Pre-fire land cover
Assessing Post-fire vegetationdamages using severity index
Finally, the use of Sentinel-2 images allows a vegetation post-fire damages assessment. They represent a quick and accurate tool to evaluate the risk of potential secondary effects of wildfires.
Pre-fire land cover
Densité /
couvert de
la
végétation
Sévérité du feu sur la végétation (dNBR)
Faible Moyen Fort
Forêt
Houppier vert ou
légèrement roussi sur la
partie inférieure
(Sous étage et litière
brulée)
Houppier totalement roussi
(présence de quelques
sujets encore vert possible)
Etage arborée
totalement ou
partiellement brulé
(feu de cime)
Arbustif
Faible Végétation totalement brulé
Moyenne Strate arbustive roussie Végétation totalement brulée
Dense Strate arbustive verte et
roussie (en mélange)
Strate arbustive brulée et
roussi (en mélange)
Végétation
totalement brulé
Herbacée Végétation rase
totalement brulée
Végétation haute
totalement brulée
Non Combustible
Vignes parcourue
Verger parcourue
Haie brulée
herbacée brulée
Arbustif dense vert en mélange avec roussi
Arbustif dense roussi
Arbustif dense totalement brulé
Chêne liège parcouru encore verts
Chêne liège roussis
Chêne liège totalement brulés
Post-fire
damages
assessment
Arbustif dense vert en mélange avec roussi
Feuillus Indifférenciés parcouru encore verts
Feuillus Indifférenciés roussis
Feuillus Indifférenciés totalement brulés
Feuillus hors IFN parcouru encore verts
Feuillus hors IFN roussis
Feuillus hors IFN totalement brulés
Pin Alep parcouru encore vert
Pin Alep roussis
Pin Alep totalement brulés
Pin Maritime parcouru encore vert
Pin Maritime roussis
Pin Maritime totalement brulés
Pin Pigon parcouru encore vert
Pin Pignon roussis
Pin Pignon totalement brulés
Résineux Indifférenciés parcouru encore vert
Résineux Indifférenciés roussis
Résineux Indifférenciés totalement brulés
Résineux hors ifn parcouru encore vert
Résineux horsIFN roussis
Résineux hors IFN totalement brulés
Burn severity
Conclusion
Sentinel-2 has brought a great change in extended wildfire mapping thanks to good :
• revisit delay
• resolution
• available bands
It doesn’t replace draft operational cartography but allows to draw the final map accurately with limited field checking.
Some post-fire damages can also be come up to.
This assessment would need complementary data bases (soil) to evaluate the erosion and floods risk.
Further work on images has to be made from now on to evaluate vegetation regrowth within 2017 burnt areas.