The Los Angeles District Debris Method - Equation 1 (Gatwood, Pedersen, and Casey, 2000) is employed to simulate events in watersheds ranging from 0.1 mi² to 3 mi² in size, where peak flow data is unavailable .
This equation was derived from a comprehensive dataset comprising 349 observations collected across 80 watersheds in Southern California. All the factors included in this equation demonstrated statistical significance at a confidence level of 0.99. It is worth noting that the LA Debris Method Equation 1 exhibits its highest efficacy in arid or semi-arid regions, precisely the same geographical area where it was originally developed.
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\begin{aligned}
& \log D_y=0.65(\log P)+0.62(\log R R)+0.18(\log A)+0.12(F F) \\\\
& D_y=\text { Unit Debris Yield }\left(y d^3 / \mathrm{mi}^2\right) \\
& P=\text { Maximum 1-Hour Precipitation (inch) } \\
& R R=\text { Relief Ration }(\mathrm{ft} / \mathrm{mi}) \\
& \quad R R=\frac{h_2-h_1}{L} \\
& \quad \quad h_2=\text { Highest Elevation in the Watershed }(\mathrm{ft}) \\
& \quad \quad h_1=\text { Lowest Elevation in the Watershed (ft) } \\
& \quad \quad L=\text { Maximum Stream Length (mi) } \\
& A=\text { Drainage Area (ac) } \\
& F F=\text { Non-Dimensional Fire Factor } \\
&
\end{aligned}
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The Fire Factor (FF) can be approximated using the Factor Factor Curve (watersheds ranging from 0.1 mi² to 3 mi²) provided below, which illustrates a scenario of 100% combustion. An illustration of how to calculate the Fire Factor in cases of partial combustion can be found in the Los Angeles Debris Method Manual (Gatwood, Pedersen, and Casey, 2000).
Required Parameters
Parameters that are required to utilize this method within HEC-HMS include the maximum 1-hour precipitation [inches or millimeters], relief ratio [ft/mi or m/km], and non-dimensional fire factor.
A Note on Parameter Estimation
HEC-HMS initially assigns a default value of 1.0 to the Adjustment-Transposition (A-T) factor. However, it's essential to fine-tune and verify this value by taking into account the disparities in geomorphological characteristics between the specific watershed under consideration and the original watershed (San Gabriel Mountains, CA) from which the regression equation was originally derived.
The Flow Rate Threshold parameter was introduced as an independent variable to segment storm events for continuous simulation. It establishes the lower boundary for direct runoff flow rate, marking the commencement of a debris flow event when the direct runoff exceeds this threshold. Conversely, the event concludes when the direct runoff drops below the specified threshold. This parameter assumes particular significance in the calibration process, especially for continuous simulations.
