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Google Summer of Code 2016
Development of a model for woody debris flooding hazard in gvSIG
student: Silvia Franceschi
mentors: Andrea Antonello, Riccardo Rigon
In forested mountain landscapes wood can significantly increment the total flow volume during a flooding event. The presence of large wood on river sides has positive effects on hydrogeomorphological diversity of habitats and represents a source of organic nutrients in the water. Therefore a compromise should be reached in the management of vegetation (within river corridors and on connected unstable slopes) aiming to reduce flood hazard while preserving its function of large wood supply for river ecosystems.
The idea of the project is to develop in gvSIG a model for the evaluation of the hazard derived from large wood transportation during flooding events.
The simplified GIS model should consider:
- the effective presence and characteristics of the vegetation in the inundable river corridors for log requirement
- the characteristics of the flow in the stream network for the transportation of the logs downstream.
With this model the user will be able to make an analysis of the hazard and risk related to the transport of wood during flooding events on a complete river stream with known and commonly used/available GIS data, using directly a GIS for displaying input and outputs.
You can find all the code here: https://github.com/silviafranceschi/jgrasstools
| Date | Programmed Activity | Percentage done |
|---|---|---|
| 23 May - 29 May | Identification of the main activities and possible issues. | 100% |
| 29 May - 5 June | Research a generic allometric function and search of an algorithms for transportation of the logs downstream. | 100% |
| 6 June - 12 June | Participation to the Summer School of Spatio-Temporal Data Analyses Using Free and Open Source Software in Matera (Italy) |
To test the code it is sufficient to compile the JGrassTools and run the algorithms through their internal GUI.
###What did you get done this week? This week I started to research and analyse the literature about the allometric function and the hydraulics relations for the transport of logs in rivers.
Considering a possible implementation of a generic allometric function, there are different approaches in literature based on two main equations:
- height-diameter model
- volume-biomass model.
That should be similar to these if we decide to use linear relations:
H_t = 1.3 + (DBH^2 / (a + b * DBH + c * DBH^2))
and then
VTOT = sum(a + b * DBH^2 * H + c)
All the parameters should be personalized for each tree species or generalized for all the tree species present in the analysed area.
Regarding the transport of logs, the main available models base on a 2D hydraulics model of the river flow while we decided to keep the model very simple and work with a 1D hydraulics model. Following Ruiz-Villanueva et al, 2014 (Large Wood in rivers and its influence on flood hazard) and Ruiz-Villanueva et al, 2015 (Avances en el análisis del material leñoso en ríos: incorporación, transporte e influencia en el riesgo por inundaciones), wood entrainment and incipient motion can be calculated from a balance of the forces acting on each single piece of wood:
- the gravitational force acting on the log: effective weight of the log in a downstream direction
- the frictional force in the opposite flow direction: equal to the normal force acting on the log multiplied by the coefficient of friction between the wood and the river bed
- the drag force (acting in the flow direction): downstream drag exerted on the log by the water in motion.
The movement of wood logs includes two possible transport regimes, floating/sliding and rolling, depending on wood density. When floating both translation and rotation are considered, when rolling only the rotation is considered.
The model should consider also the interaction between the logs and the channel configuration (when a piece of wood reaches the bank it can be entrapped) and among the logs themselves (also if there are logs laying on the river bed).
Additionally the presence of wood influence the hydrodynamics of the river, for this reason it is necessary to include in the flow model a drag force ad an additional term in the Saint Venant equations, similar to roughness.
###What do you plan on doing next week?
Next week I will:
- participate to a summer school in Matera
- go on with literature overview
###Are you blocked on anything? Less time to work on GSoC next week due to the summer school.
###What did you get done this week?
I started this GSoC by setting up my development environment and involving my mentors and my PhD supervisors in the identification of the tools to develop in order to get some support from them specially regarding the scientific background of the code.
The work can be divided into three main parts:
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integration of the position and characteristics of the vegetation (height) in the inundable river corridors extracted from LiDAR data with a SINGLE TREE approach, in general a shapefile containing the position of the trees with in the attribute table the elevation. To do this there are two possibilities either add as input to the submodel LW09_AreaToNetpointAssociator this input as an alternative to the actual inDSM and inStand which are used to evaluate the height and volume of biomass in each pixel connected to the stream network, or create a new submodel dedicated to LiDAR input to be used in case this type of input is available. The issues for using this kind of data are two:
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normally from an automatic extraction of the trees from LiDAR data only the position and height are available, so the problem is: how to calculate the volume of the biomass without the diameter and the species? probably we have to add some additional parameters which are the parameters of a standard allometric function to relate the diameter and the volume to the height
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how can we define if the tree is connected to the network? in this case we have vectors while the connectivity index is a raster, so probably the best solution will be to rasterize the points in which there are the trees and then proceed with the already developed methodology
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evaluation of the hydraulic characteristics of the flow in the process of transportation of the logs downstream: we are looking for some bibliography to study regarding this issue
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integration of the tools in gvSIG via JGrassTools: I followed some webinars at the gvSIG festival about the new development and possibilities given by the gvSIG community, it looks like there are many interesting tools specially for visualizing the data and results.
###What do you plan on doing next week?
Next week I will:
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analyse in deep the problem of the allometric function and the connectivity index
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get some paper to read about the relation between hydraulics parameters (water height and velocity) and the capacity of the flow to transport the logs downstream, so I can start reading them during the week after when I will be at a summer school
###Are you blocked on anything?
No known blocking issues.
I am an environmental engineer specialized in hydrology and hydraulic. I graduated some years ago but only 2 years ago I decided to go on with a PhD. During these two years of my PhD I approached Airborne Laser Scanning data to extract information about the vegetation using a so called single tree approach. I developed some tools to extract the position of the single trees, the height and the volume of the biomass directly from LiDAR data (DSM, CHM and point cloud) with an average precision greater than 70% for all the evaluated variables in different forest structures. My specific scientific background is in hydrology, and in particular in the development of distributed and semidistributed hydrological models. In the past I contributed to the development of the two main hydrological models of the JGrasstools library, Peakflow and NewAge together with several other geomorphological tools.
The last two years I participated in the GsoC program with:
- a model for the estimation of inundated areas and large wood recruitment and transportation during flooding events. The transportation of wood was developed only considering a user defined length of the logs and the width of the sections without a specific evaluation of the available vegetation (position and height) and without any link to the discharge flowing which are all important parameters to consider to evaluate the capacity of the water to carry the logs downstream and to evaluate the amount of biomass clogging the sections
- a simple 1-D GIS based hydraulic model for evaluating the water depth and velocity inside the streams. The model requires as input the water discharge in the upstream section (calculated with an hydrological model), the geometry of the sections (derived from the DTM), and some hydraulics parameters valid in general for the river or specific for each section. JGrassTools is an environmental processing library which started as a hydro-geomorphological processing toolkit and is in development around 10 years now. It is supported by the Department of Environmental and Civil Engineering of the University of Trento (professor Rigon) and by the Department of Forestry of the Science and Technology of the Free University of Bolzano (professor Tonon).
I am a power GIS user, actually I am trying to migrate all my stuff to use gvSIG, but I know also uDig, GRASS and QGIS.
In the last 10 years I actively contributed to the JGrassTools project with some scientific models in the field of hydrology, hydraulic and geomorphology. I also contributed to the development of a fully distributed hydrological model at the university department. I also contributed the design of some tools and functionalities of Geopaparazzi. Recently I approached the gvSIG community to start to understand how to give some contribute in the development of scientific tools.