Causes and dynamic characteristics of debris flow disaster in Longtangou Watershed, Miyun County of Beijing

ObjectiveSince 2011, several debris flow events occurred in Longtangou Watershed, Miyun County of Beijing, which pose a heavy threaten to the local inhabitants and economic development. After the debris flow event on August 12, 2016, the debris flow in Aiwayu and Damuyu catchments were investigated to examine their formation and dynamic characteristics, aiming to provide theoretical basis for the prevention and treatment of single channel debris flow in the area.

MethodOn the basis of field investigation, laboratory tests, remote sensing maps and rainfall data, the particle composition of debris flow and changes of velocity and peak flow at several cross sections were analyzed.

Result(1) From 2012 to 2016, the area of material source body increased by 11.12 and 4.41 times compared with previous conditions in the Aiwayu and Damuyu catchments, respectively. (2) In 2016, the maximum hour rainfall was 47.0% of the accumulative rainfall. Compared with the debris flow event in 2011, the antecedent precipitation was less and the triggering rainfall intensity was 2.87 times of that in 2011. (3) The debris flow density of Aiwayu and Damuyu catchments was 1.95 and 1.88 g/cm³, and the content of clay particles was 0.02% and 0.05%, respectively. (4) Scale distribution results showed that the R² ranged in 0.853 1-0.959 2, μ ranged in 0.019 1-0.033 8, D_c ranged in 10.31-10.99. (5) Since the confluence from Aiwayu tributary, the flow velocity and discharge increased by 0.21 and 1.67 times, respectively; after flowing through Pinus tabuliformis forest, the velocity decreased by 22.9%, the discharge decreased by 86.2%, and the average particle diameter of the debris flow reduced by 53.0%. The flow velocity and discharge in Damuyu Catchment increased by 0.16 and 7.71 times, respectively after the debris flow passing over the artificial dam.

Conclusion(1) Since 2012, the body area of material source in Aiwayu and Damuyu catchments increased year by year, which is the material source condition causing debris flow here again. (2) The debris flow in this area was mainly triggered by a short-time, high-intensity rainstorm. If the antecedent precipitation was abundant, the triggering rainfall intensity decreased. (3) Both of the debris flow in the two catchments belong to high-density and low-viscous flows with abundant gravels and few clay. (4) The scale distribution had a high fitting degree for Aiwayu and Damuyu catchments. μ < 0.05 verified the conclusion that the debris flow in two catchments had high density. Low viscosity was the main reason for low D_c. (5) In Aiwayu Catchment, the discharge of debris flow reaching peak value in short time was resulted from the confluence of debris flow in tributary to main ditch. Pinus tabuliformis forest in the channel had reduction effects on velocity and discharge of debris flow, and filtering effects on large particulate matter. While in Damuyu Catchment, the discharge of debris flow reaching peak value in short time was resulted from rockfill dam break in the channel.