The USGS long-term debris model, established by Gartner, Cannon, and Santi in 2014, draws upon the complete database of 344 volumes of sediment deposited by debris flows and sediment-laden floods with no time limit since the most recent fire.
The USGS long-term debris model has showcased its effectiveness in continuously simulating debris yield without temporal restrictions, covering areas smaller than 30 km².
The best model developed from the multiple linear regression analyses of this database predicts volumes of sediment based on the following equation:
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\begin{aligned}
& \ln V=6.07+0.71 \times \ln i 60+0.22 \times \ln B t-0.24 \times \ln T+0.49 \times \ln A+0.03 \times \sqrt{R} \\\\
& V=\text { Volume of Sediment }\left(\mathrm{m}^3\right) \\
& i 60=\text { Peak } 60 \text {-minute Rainfall Intensity }(\mathrm{mm} / \mathrm{h}) \\
& \text {Bt }=\text { Total Area of Watershed Burned by Most Recent Fire }\left(\mathrm{km}^2\right) \\
& T=\text { Time since The Most Recent Fire }(\text { year }) \\
& A=\text { Watershed Area }\left(\mathrm{km}^2\right) \\
& R=\text { Relief }(\mathrm{m})=\mathrm{h}_2-\mathrm{h}_1 \\
& \quad h_2=\text { Highest Elevation in the Watershed }(\mathrm{m}) \\
& \quad h_1=\text { Lowest Elevation in the Watershed }(\mathrm{m})
\end{aligned}
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Required Parameters
Parameters that are required to utilize this method within HEC-HMS include the maximum 60-minute precipitation [inches or millimeters], watershed relief [feet or meters], and watershed area burned by the most recent wildfire [mi² or km²] .
A Note on Parameter Estimation
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.
