狗牙根和牛鞭草的消浪减蚀作用

Role of Cynodon dactylon L. and Hemarthria altissima in wave attenuation and erosion control

剧烈的土壤侵蚀和库岸坍塌是三峡水库消落带的主要生态环境问题之一，而植被恢复是治理消落带的关键措施之一，植物的消浪固土能力是消落带适宜植物配置的重要依据。为了探讨三峡水库消落带典型草本植物狗牙根（Cynodon dactylon （L.） Pers.）和牛鞭草（Hemarthria altissima （Poir.） Stapf et C. E. Hubb.）的消浪减蚀效应，进行了植物消浪和波浪冲刷的水槽概化模拟试验。结果表明，2种植物的消浪效应有较大差异，在周期较长（1.2～1.5 s）时狗牙根平均消浪系数为0.54，明显高于牛鞭草（0.37）；而在周期较短时（0.8～1.0 s）则2种草本消浪性能相当，消浪系数均介于0.15～0.30之间。在同样的试验条件下，狗牙根、牛鞭草平均侵蚀模数分别为17.63、57.28 g/（m2＆#183;min）。2种草本的减蚀效应均在89%以上，最高可达98.59%。植物的减蚀效应是其地上部分和地下根系的综合作用结果，其中牛鞭草根系的固土贡献率在91%以上，而地上部分仅为7.4%～8.6%；狗牙根根系固土贡献率（71.2%～76.2%）显著低于牛鞭草，但其地上茎干部分贡献率（23.8%～28.8%）要显著高于牛鞭草（P＜0.05）。可见2种植物都有较好的消浪减蚀效应，总体上狗牙根的消浪能力和综合减蚀效果均要略强于牛鞭草。波浪大小和周期、植被盖度、植物地上部分和根系形态对消浪减蚀效应有重要影响。该研究结果可以为三峡水库消落带的植被恢复和水土保持工程提供一定的参考依据，并可适当的推广到其他水库或湖泊消落带。

Soil erosion and bank degradation is a major post-dam concern regarding the riparian zone of Three Gorges Reservoir （TGR）. The development and succession of vegetation are the main countermeasures, especially to enhance bank stability and mitigate soil erosion. Thus, the role of forCynodon dactylon（L.） Pers.andHemarthria altissima（Poir.） Stapf et C. E. Hubb. inwave attenuation and erosion control was measured via the flume experiments. The results showed that the two grass species play an important role in wave attenuation and erosion control. The effects of wave attenuation and erosion control capabilities for the two species were different. The mean wave dissipation coefficient （MWDC） for Cynodon dactylon（L.） Pers. andHemarthria altissima （Poir.） Stapf et C. E. Hubb. were 0.37 and 0.30, respectively, with the wave period of 1.2 s to 1.5 s. Nevertheless, the values of MWDCfor Hemarthria altissima（Poir.） Stapf et C. E. Hubb.was similar toCynodon dactylon（L.） Pers.（ranged from 0.15 to 0.30） when the wave period was between 0.8 s to 1.2 s. The MWDCvalues of 0.43 and 0.30 were responded forCynodon dactylon（L.） Pers. under 80% and 40% area coverage, respectively. In contrast, the MWDCvalues forHemarthria altissima （Poir.） Stapf et C. E. Hubb. were 0.35 and 0.24, respectively, under two different coverage. The MWDCvalues forCynodon dactylon（L.） Pers. with a wave height of 8 and 4 cm were 0.34 and 0.39, respectively; whilst, the ones forHemarthria altissima （Poir.） Stapf et C. E. Hubb. were 0.15 and 0.32, respectively. The MWDCvalues showed generally increasing trend with increasing wave period. Species studied also significantly reduced wave induced soil erosion. The results of washout tests indicated the soil erosion modulus decreased with increasing eroded time. The largest erosion modulus was measured for bare land （799.40 g/（m2＆#183;min））, followed byHemarthria altissima（Poir.） Stapf et C. E. Hubb. （57.28 g/（m2＆#183;min））,Cynodon dactylon（L.） Pers.（17.63 g/（m2＆#183;min））. When the wave height was 8 cm, the mean erosion modulus was 84.81, 224.82 and 1 746.67 g/（m2＆#183;min） forCynodon dactylon（L.） Pers. samples,Hemarthria altissima （Poir.） Stapf et C. E. Hubb. samples and bare land, respectively; whereas, the mean erosion modulus for wave height equal to 4 cm was 13.54, 22.67 and 656.8 g/（m2·min）, respectively. The percentage erosion control effectiveness of the two species studied ranges from 89.0% to 98.6%. Erosion control effect forCynodon dactylon（L.） Pers. was found a bit higher as compared to that for Hemarthria altissima （Poir.） Stapf et C. E. Hubb. under the same test conditions, but not significantly different. Erosion reducing is a combined consequence of above-ground and root system. The relative contribution rates of roots for Hemarthria altissima （Poir.） Stapf et C. E. Hubb. to prevent soil from erosion was as high as 91%, while its shoots only contributed the proportion of 7.4% to 8.6%; Roots of Cynodon dactylon（L.） Pers. afforded a smaller soil conservation contribution rate thanHemarthria altissima （Poir.） Stapf et C. E. Hubb., whilst its aboveground contribution rate, ranged from 23.8% to 28.8%, was significantly higher thanHemarthria altissima （Poir.） Stapf et C. E. Hubb. In summary, the capability of wave attenuation and erosion control forCynodon dactylon（L.） Pers. was better as compared to the Hemarthria altissima （Poir.） Stapf et C. E. Hubb, which recommendingCynodon dactylon（L.） Pers. as a better selection for soil conservation in the riparian zone of TGR. The efficiency of wave dissipation and erosion reducing capacity for the investigated species were influenced by the vegetation coverage, the morphological features of aboveground and underground root system, the wave size and period. Combined with the knowledge of vegetation dynamics, ecological site properties, and species resistance to erosion, the observations made in this experiment would be useful for species selection in the context of ongoing and future revegetation activities in the riparian zone of TGR and similar riparian areas in the Yangtze River Basin.