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Analysis of benefits of debris flow control projects in southwest mountain areas of China

CHEN Ning-sheng1,ZHOU Hai-bo1,2,LU Yang1,2,YANG Ling 3,Lü Li-qun1,2 1.Institute of Mountain Hazards and Environment,Chinese Academy of Sciences;Key Lab. of Mountain Hazards and Surface Process,CAS,Chengdu 610041,China; 2.Graduate School of Chinese Academy of Sciences,Beijing 100049,China; 3.Sichuan Institute of Nuclear Geology,Chengdu 610041,China  
The main features of most small watersheds in the southwest mountain areas of China include special landform,abundant rainfall and rich loose solid materials,thereby debris flows in these areas are always high frequency characteristics and natural calamities are extraordinary serious.To combine geotechnical engineering with bioengineering is a main control measure.The direct effectiveness of geotechnical engineering is to retain sediments and reduce the debris flow magnitude,and the indirect effectiveness of geotechnical engineering is to reduce bed erosion,keep the slope stable,reduce gully gradients,the debris flow velocity and peak discharge,the debris flow density,and increase the basal level of erosion,etc.By analyzing the protection effectiveness of the check dams of 15 gullies in these areas,it is found that the loose solid materials reduce by 1.5%~48%,the indirect effectiveness of stabilizing gully bed increases by 0.13%~35.39%,and that to stabilize the slope increases by 0.01%~65.01%.Because of check dams,the loose solid materials reduce by 0%~5%,the indirect effectiveness of stabilizing gully bed increases by 0.04%~6.08%,and the effectiveness to stabilize the slope increases by 0.06%~33.98%.According to the above analysis results,the amount of directly detained solid materials by sediment storage dam is several times to hundreds of times larger than that by check dams,and the indirectly reduced amount is twice to ten-fold.Bioengineering has indirect effectiveness of detaining solid materials by means of increasing vegetation coverage,reducing surface runoff,reducing runoff erosion and improving soil structure.Because of increasing 19%~60% vegetation coverage in 12 debris flow gully basin,the amount of debris flow solid materials indirectly reduces by 4.7%~34.6%.For example,the bioengineering of the Heisha river basin in the Xichang City have received a significant effect.The vegetation coverage of this area increases by 60%,and the highest average value of erosion modulus is 9000 t/(km2·a),and the erosion type changes from extremely strong to a mild,and the solid materials reduce by 31.1%(5.894×106 m3).
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