Introduction

Debris flows can result from a number of conditions within a watershed. Wildfires remove vegetation which results in increased debris yields and potential for debris flows. The purpose of this procedure is to outline hydraulic analysis methods and considerations for computing debris flow inundation in urban settings in HEC-RAS. These considerations can be applied to both bulked clear water and non-Newtonian debris flows.

Methods to Compute Debris Flow Inundation

  1. To account for the increase in debris yield post-wildfire, inflow hydrographs can be multiplied by a bulking factor to estimate debris flow inundation using HEC-RAS.
    1. In this case, a 2D HEC-RAS hydraulic model was used to compute flood inundation along Gallinas Creek. Unsteady flow hydrographs computed in HEC-HMS were bulked with the estimated debris yield and used as inflow boundary conditions for the 2D HEC-RAS model.
      1. The procedure to develop the 2D HEC-RAS model for Gallinas Creek is not included in this tutorial.
      2. The calculation of a bulking factor is covered in the Debris Yield Modeling Using Erosion Methods in HEC-HMS tutorial.
    2. As the volume of debris within water increases, the fluid begins to behave under the assumptions of non-Newtonian physics. The HEC-RAS modeling software (version 6.0 and later) contains the ability to model non-Newtonian debris flows and compute debris flow inundation.
      1. The debris yield and water volume from the HEC-HMS results can be used to compute the volumetric concentration for the watershed, which is an input parameter required for debris flow modeling in HEC-RAS.
      2. The volumetric concentration can be calculated using the equation below, where both the volume of sediment and volume of water at a given computation point are extracted from the HEC-HMS results: Cv = Vs / (Vs + Vw) 
        1. Where Cv is the volumetric concentration, Vs is the volume of sediment/debris, and Vw is the volume of water.
      3. Non-Newtonian methods can be applied to 1D and 2D HEC-RAS models. The main limitation of the 1D non-Newtonian HEC-RAS model is that the computed water surface is constant across a cross section which does not show any debris mounding or runup that may occur. A 2D HEC-RAS model is able to capture these effects.
      4. A tutorial for modeling a 2D half pipe with non-Newtonian fluid is located here: Modeling a 2D Half Pipe with Non-Newtonian Fluid.

Considerations for Urban Hydraulic Modeling Using HEC-RAS

  1. Culverts and bridges can be modeled in 1D and 2D HEC-RAS.
    1. In this case, bridge structures were included in the 2D HEC-RAS model.
    2. Guidance for modeling bridges inside 2D flow areas is located here: Modeling Bridges Inside 2D Flow Areas.
  2. Floating debris on bridge piers can be modeled in the bridge pier data editor by checking Apply Floating Debris to the Pier and setting a Debris Width and Debris Height.
    1. In this case, floating debris was not modeled.
    2. Guidance for entering and editing bridge data, including floating debris on bridge piers is located here: Entering and Editing Bridge Data.
  3. Debris blockages in culverts can be modeled in the culvert data editor by setting the Depth Blocked to the assumed debris height.
    1. In this case, culverts were not modeled.
    2. Guidance for entering and editing culvert data, including the depth blocked is located here: Entering and Editing Culvert Data.
  4. Modeling restricted or blocked bridge or culvert openings can cause model instabilities. These stability issues can be addressed by reducing the model Computation Interval, adding a Minimum Flow to the flow hydrograph boundary conditions, using a Warm Up Period, and/or modifying hydraulic coefficients to ensure bridge curve transitions are reasonable.
    1. Guidance for addressing model stability problems is located here: Model Stability.
  5. Higher Manning’s n values than would typically be used in a hydraulic model can be applied to achieve reasonable and stable results in a debris flow model. The high Manning’s n values account for the steepness of the watershed, where debris flows typically occur. Increasing the Manning’s n values can also be used to account for additional processes or losses (i.e., large floating debris, log jams, structures, blockages) that are not captured in the model.
    1. A guide for modeling steep reaches (greater than 1% slope) is located here: Modeling Steep Reaches.
    2. A report by Jarrett in 1985 that investigates Manning’s roughness coefficients for steep streams is located here: https://www.hec.usace.army.mil/confluence/rasdocs/hgt/latest/web-resources#WebResources-DeterminationofRoughnessCoefficientsforStreamsinColorado.
    3. A report by Yochum et al. in 2014 that provides photographic guidance for selecting Manning’s roughness coefficients for steep streams is located here: https://www.fs.usda.gov/research/treesearch/46250.