== WP-I6: A wind throw model for urban trees 
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=== Goals of the project:
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Existing windthrow models from forest meteorology are being investigated with regard to their availability and applicability to urban trees. Such models as GALES or HWIND are based on findings from field trials and wind tunnel experiments on individual trees and are now used for extensive forest areas to estimate the storm risk. Meteorological input variables are always simple information, such as the mean wind at 10 m height outside the tree population. In an urban environment, on the other hand, wind and turbulence vary over a very short distance, and the forces on individual trees strongly depend on the location of the impact. The calculation of the windthrow risk for urban trees is to be carried out with a high spatial resolution of 1 m. Therefore, the detailed vegetation structures and the simulated high-resolution, locally strongly varying wind and turbulence fields are taken into account. In a first step, it will be investigated how the three-dimensional information from PALM-4U regarding the turbulent wind field can be taken into account in the existing windthrow models. Based on the results of these investigations, a windthrow model will be selected and implemented in PALM-4U in order to subsequently conduct sensitivity studies.

For the calculation of the risks of storms, individual parameters regarding the vegetation characteristics and the anchoring in the soil are of great importance. In a further step, the corresponding values from the literature are processed and condensed in tabular form for typical urban trees.  In order to determine the significance of individual input variables on the result, the influence of all these input parameters is calculated individually and in combination in a parameter study that includes the individual variable uncertainties. This results in indications for decision-makers in the municipalities, which data should be kept in the corresponding tree registers in order to be able to use PALM-4U for the investigation of the storm risks of urban trees. Possible extensions of the windthrow model for urban trees, such as the influence of tree vibrations, will be investigated and implemented if applicable.

To prove the realistic results of the implemented windthrow model, it will be applied to well-documented recent storm events. For example, the storm "Ela" in 2014 and storm "Xavier" in 2017 will be simulated, in which a large number of trees were damaged and destroyed, especially in northern German cities. In addition to obtaining the building registers of the corresponding cities (for example Braunschweig for "Xavier"), the tree registers must be expanded to include the necessary input variables determined in WP-I6.2. The results of these simulations are compared to the observed storm damage. Any differences that occur are analyzed, and appropriate modifications are made to the input data, which results in a repeated simulation.


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=== Tasks of the project:
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WP-I6.1: Implementation of a wind-throw model in PALM-4U

WP-I6.2: Extension of wind-throw model and study of parameters

WP-I6.3: Application of wind-throw model
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=== Project structure:
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This work package is conducted at LUH.

=== Deliverables:

DL1 (month 12): Wind-throw model is implemented in PALM-4U

DL2 (month 24): Genauigkeit der benötigten Eingangsdaten für Stadtbäume ist bestimmt 

DL3 (month 36): Werkzeug zur Bestimmung des Windwurfrisikos steht zur Verfügung

Be aware of the shifted schedule, as the work on WP-I6.3 only started in 2021.

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=== Progress so far:
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After a thorough literature review, a wind-throw model was selected based on the model of Peltola and Kellomäki (1993) and Peltola et al. (1999), which is adapted by Groß (2018). It was investigated, how the three-dimensional turbulent wind field in PALM-4U can be applied to this model. A corresponding methodology and the required algorithms were developed. The adapted wind-throw model was implemented and tested in PALM-4U.
Work on collecting and preparing the parameters needed for calculating the storm risk of urban trees started. These parameters are needed to be able to carry out a realistic investigation of the storm risks of urban trees with PALM-4U. 


{{{
#!div style="align:center; width: 800px; border: 0px solid"
[[Image(windthrow.png,nolink,800px,center)]]
}}}

    '''Figure 1:''' The image shows how a strong gust knocks down 32 trees in an idealised setup (7x7 grid, 30s interval). The wake that tree 4 (green) casts on the trees behind it, 25 (blue) and 46 (violet), can also be clearly seen in the representation of the rotational force acting individually on the trees over time.
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=== References:
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'''Peltola, H., & Kellomäki, S.''',  (1993). A mechanistic model for calculating windthrow and stem breakage of Scots pines at stand age: Silva Fennica. 

'''Peltola, H., Kellomäki, S., Väisänen, H., and Ikonen,  V. -P.''', (1999) A mechanistic model for assessing the risk of wind and snow damage to single trees and stands of Scots pine, Norway spruce, and birch: Canadian Journal of Forest Research. 29(6): 647-661. https://doi.org/10.1139/x99-029.

'''Gross, G.''', (2018). A windthrow model for urban trees with application to storm "Xavier": Meteorologische Zeitschrift 27, Nr. 4, 27(4), 299-308.
https://doi.org/10.1127/metz/2018/0905
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=== Contacs:
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gross[at]meteo.uni-hannover.de

knoop[at]meteo.uni-hannover.de