# Conceptual model of debris-flow initiation Ursa sets up and manages simulations of debris-flow runout implemented by [D-Claw](https://github.com/geoflows/dclaw) ([George and Iverson, 2014](https://doi.org/10.1098/rspa.2013.0820); [Iverson and George, 2014](https://doi.org/10.1098/rspa.2013.0819)). D-Claw simulations require the amount and location of debris-flow material to be specified at the begining of the simulation. Ursa relies on multiple empirical models that relate the 15-minute rainfall intensity ($I_{15}$, mm/hr) to volumes of debris and sediment. The remainder of this page steps through each element ursa uses to construct the inintial conditions for simulations. Within each $I_{15}$ considered, multiple runs are set up and combined into an integrated debris-flow simulation index. A user specifies the number of considered $I_{15}$ and the number of runs per $I_{15}$ in the [configuration file](configuration). By default, ursa will consider three permeability values and three debris-flow volumes, for a total of nine scenarios. The contents of the configuration file are specified in the [yaml](https://yaml.org/) format and used internally as a dictionary with the variable name `config`. ## Delineation of source basins Ursa requires delineation of debris-flow source basins and for information about basins, segments, and outlets to be provided in the same format as those produced by [pfdf](https://ghsc.code-pages.usgs.gov/lhp/pfdf/). A user may either provide their own delineation of debris-flow source basins, ensuring that they match the format of those produced by [pfdf](https://ghsc.code-pages.usgs.gov/lhp/pfdf/) or [wildcat](https://ghsc.code-pages.usgs.gov/lhp/wildcat/) or pfdf is run internally. ## Basin probability Ursa will initialize debris flows in basins with a [Staley and others (2017)](https://doi.org/10.1016/j.geomorph.2016.10.019) M1 model probability greater than or equal to the value provided by {confval}`P_THRESH`. Basins that produce debris flows for a specific $I_{15}$ are called "active" whereas those which do not are called "inactive". This calculation is done internal to ursa. Therefore ursa requires that the basin delineation includes calculation of a soil, fire severity, and terrain variable appropriate for use in the M1 model, as is done by wildcat. ## Amount of debris-flow material Ursa will calculate a volume of debris and a volume of water within each active basin. Three options are available for calculation of debris and two for volume of water. The debris and water volume choices are specified in the configuration file as {confval}`sediment_volume_model` and {confval}`water_volume_model`. The sediment volume may either be calculated using the [Gartner and others (2014)](https://doi.org/10.1016/j.enggeo.2014.04.008) Emergency Assessment model, a constant sediment concentration, or a constant erosion depth. The water volume may either be calculated by bulking the sediment based on a constant concentration or by combining the sediment volume with a water volume equivalent to multiplying the 15-minute rainfall depth by the basin area. Multiple additional values are provided in the [initial condition configurations](configuration-IC) to allow the user to scale up and down the sediment and water volumes by constant factors. :::{warning} Setting {confval}`sediment_volume_model` to `constant_concentration` and {confval}`water_volume_model` to `bulk` is not permitted because the volume calculations would be underdetermined. ::: ## Initial location of debris-flow material Once a volume of debris-flow material (the combination of sediment and water) has been determined for each source basin, ursa manages the generation of D-Claw model input files that specify where the debris-flow material is placed in the basin. Two options are available and neither formally represent the typical mechanisms of distributed postfire debris-flow initiation. In both options the same volume of material is placed into each basin. The options differ in the spatial pattern of debris-flow material placement. Refer to the [configuration file](configuration) for additional details and all specification options. ### Reservoir In the first option, material is placed into the basin at a location with a user-specified fraction of the maximum basin drainage area (default is 80%). At this location a small hypothetical reservoir is built into the topography such that the debris-flow material surface has a constant elevation. The debris-flow material within each basin has spatially variable depth. See [`digger.make.reservoir()`](https://ghsc.code-pages.usgs.gov/lhp/digger/src/autodoc2/digger/digger.make.reservoir.html) for additional details. ### Buffered segment In the second option, a constant thickness of material is placed into the basin at a user-specified distance around the provided segments. ## References Gartner, J.E., Cannon, S.H., and Santi, P.M., 2014, Empirical models for predicting volumes of sediment deposited by debris flows and sediment-laden floods in the transverse ranges of southern California: Engineering Geology, v. 176, p. 45–56, . George, D.L., and Iverson, R.M., 2014, A depth-averaged debris-flow model that includes the effects of evolving dilatancy—II. Numerical predictions and experimental tests: Proceedings of the Royal Society of London. Series A, v. 470, no. 2170, p. 20130820, . Iverson, R.M., and George, D.L., 2014, A depth-averaged debris-flow model that includes the effects of evolving dilatancy—I. Physical basis: Proceedings of the Royal Society of London. Series A, v. 470, no. 2170, p. 20130819, . Staley, D.M., Negri, J.A., Kean, J.W., Laber, J.L., Tillery, A.C., and Youberg, A.M., 2017, Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris-flow generation in the western United States: Geomorphology, v. 278, p. 149–162, .