Reconstructing spatio-temporal patterns of debris-flow activity using dendrogeomorphological methods in the Tangrah Catchment

Introduction
Flash floods are localized hydrological phenomena occurring in small catchments of a few to a few hundred square kilometers, with response times typically being a few hours or less As a result, they represent one of the most significant natural hazards with serious death tolls and economic damage at a worldwide level in general and in Mediterranean mountain catchments in particular.( Villanueva, 2010:383). Dendrogeomorphology is one of the subdivisions of dendrochronology (bah rami, 1389:1). Which is based on the analysis of the annual growth trees of trees for the age of geomorphic processes (Butler,2013:717). Geomorphic processes may influence trees in various ways The impact of rocks and boulders or abrasion processes may cause scars to the stem surface or even decapitate trees. Unilateral pressure of flowing material can result in a tilting of the stem. Finally material may bury the stem base when deposited. Affected conifer trees react to these impacts with callus tissue, tangential rows of traumatic resin ducts  the formation of reaction wood  or with an abrupt growth suppression or release gives an overview of the different impacts of debris flows on trees and their reactions to the disturbances(Bollschweiler,2007:340).
Methods and material
To collect tree samples (dendrogeomorphology), the following is done. Primary visits (from wounded trees in the main bed of the region) were carried out for sampling in April, June and July, 2014 and April, 2016. In field operations, instruments such as GPS to record sample locations, meters are used to measure sample diameter and hand saw for sample sampling. During the sampling operation, 15 samples of damaged trees were selected. During the sampling, the sample was attempted from the main canal to reduce the probable extent of other damage (human injury) and, of course, the damage was caused by the collision of the coarse sediments of the debris flow on the trees . After sampling, they were exposed to sunlight for drying for one month. After drying, each fifteen samples were polished by soft molding and transferred to the lab for examination, and their age was precisely determined using a microscope.
                                          
Results and discussion
The purpose of this study was to determine the aging of floods with debris flow that occurred in pre-flood years in 2001 and 2002 in the region, as well as the reconstruction of debris flows in the catchment area of ​​Tangrah. Due to lack of station in the basin, the dendrogeomorphology method is the best method for accurate estimation of heavy floods in this area. The area under study is located in a mountainous forest covered area. During the years 2001 and 2002, with the advent of a sudden precipitation, the region experienced various currents. Because of this, trees and stones have been damaged by these currents. Due to these injuries, the precise age of these floods and floods that occurred in the past can be determined. Samples taken more than the original bedding, it is certain that these trees were most affected by the flood. Also, using the dendrogeodomorphological method of damaged tree rings and determining the age of wounds in the region, the maximum instantaneous flow of two flows was calculated using Manning method.
 
Discussion and conclusion
 
The results of this study indicate that the most injuries caused by the flow of debris flow in 2001 with Volumetric flow rate 840.68  m³/s. The analyzing of Fifteen samples showed that these flows were related to 2001-2002 debris flows. According to surveys on the gathered samples in the tan drainage basin and questionnaire for determining the debris flow occurrence in previous periods, we concluded that during the past seventy years, the debris flows of 2001 and 2002 are the only large in this debris floods the tangrah catchment.