Chapter 2 Surficial erosion

Biotechnical and soil bioengineering slope stabilizationA practical guide for erosion control

by D. H. Gray and R. B. Sotir

Prepared by Walter Chen, Dept. of Civil Engineering, NTUTFor classroom teaching purpose

Prepared by Walter Chen ( 陳偉堯 )

Erosion vs. movement

Surficial erosion 表面侵蝕 Detachment and transport of individual particles 小顆粒 Wind and running water 風和水是主要的動力

mass movement 土體運動 movement of relatively large, initially intact masses of

soil and/or rock along critical failure surfaces 大土體的沿著臨界破壞面滑動

gravity --> limiting force equilibrium and limit analysis 極限平衡和極限分析

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Type of erosion

Rainfall erosion 雨水的侵蝕 Wind erosion 風的侵蝕

Prepared by Walter Chen ( 陳偉堯 )

Erodibility 可侵蝕性

The susceptibility of a soil to erosion 土壤受侵蝕的可能性 Some soils (silts) are inherently more erodible than others

(coarse, well-graded gravels) 有些土壤 ( 如沉泥 ) 本質上較其他土壤易受侵蝕

Increasing the organic content and clay size fraction of a soil decreases erodibility – why? 增加有機物含量和粘土成分降低可侵蝕性，為什麼？

Prepared by Walter Chen ( 陳偉堯 )

Erodibility 可侵蝕性

No simple and universally accepted erodibility index for soils 沒有簡單通用的侵蝕性指標

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Topographic variables 地形變數

Influencing rainfall erosion 影響降雨侵蝕的因子 Slope angle 邊坡角度 Slope length 邊坡長度 Size and shape of watershed 集水區大小形狀

The influence of length tends to increase as slopes become steeper For a 6% slope, increasing length from 100 to 200 feet

increases soil loss by 29% For a 20% slope, increasing length from 100 to 200 feet

increases soil loss by 49%

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

From wikipedia

Unified Soil classification system

Prepared by Walter Chen ( 陳偉堯 )

Unified Soil classification system

From wikipedia

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Wind erosion

Wind can only pick up and carry in suspension dry soils with particle sizes primary less than 0.1 mm (fine silt size)

Most of the soil particles transported by wind occurs near the ground surface (< 3 feet)

Using low barriers or windbreaks to prevent wind erosion is effective

Prepared by Walter Chen ( 陳偉堯 )

Role of vegetation 植生的角色

As low barriers or windbreaks 低屏障物、防風物 Increases surface roughness 增加表面粗糙度 Slowing and deflecting the wind 降低或使風轉向 Binding soil particles together 固結土壤顆粒

Prepared by Walter Chen ( 陳偉堯 )

Vegetation and wind erosion

Vegetation is very effective in combination with fencing to trap drifting sand and build up dunes along beaches

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )

Types of water erosion

Raindrop splash 雨滴飛濺 Sheet erosion 片狀侵蝕 Rilling 蝕溝 Gullying 蝕渠 Stream channel erosion 河流渠道的侵蝕 Groundwater erosion 地下水侵蝕

Prepared by Walter Chen ( 陳偉堯 )

Raindrop splash 雨滴飛濺 Impact of water drops falling directly on exposed soil

particles or thin water surfaces covering the ground 雨滴直接打擊裸露地表

As much as 100 tons/acre can be splashed into the air in a heavy storm 一場暴雨中可能被打到空中的土壤重量 1 acre = 0.404685642 hectares

Splashed particles may move more than 2 feet vertically and 5 feet laterally 顆粒彈跳的距離

Prepared by Walter Chen ( 陳偉堯 )

Sheet erosion 片狀侵蝕 The removal of soil from sloping land in thin layers or sheets

Dry ravel – surface layers of coarse-textured soil dry out and lose their apparent cohesion

Slope wash – rainfall erodes without causing rilling or gullying

An important mechanism of slope retreat and source of sediment in cut slopes in granitic and andesite soils

Highway cuts in these soils often give the impression of being very stable yet discharge tons of soil

Prepared by Walter Chen ( 陳偉堯 )

Rill erosion 蝕溝

Removal of soil by water from very small but well-defined, visible channels or streamlets where there is concentration of overland flow

Rilling is the form of erosion during which most rainfall erosion losses occur

Prepared by Walter Chen ( 陳偉堯 )

Gullying 蝕渠

Intermittent stream channels larger than rills Tend to form where large volumes of runoff are

concentrated and discharged onto steep slopes with erodible soils

Common in grasslands and is likely the principal form of erosion in steep, forested watersheds

Prepared by Walter Chen ( 陳偉堯 )

Stream channel erosion 河流渠道的侵蝕 Soil removal from stream banks and/or sediment

scour along the channel bottom Stream channel erosion should be considered

separately from the rainfall-associated types of erosion discussed previously

Prepared by Walter Chen ( 陳偉堯 )

Groundwater erosion 地下水的侵蝕

Removal of soil caused by groundwater seepage or movement toward a free face

Commonly referred to as piping

Prepared by Walter Chen ( 陳偉堯 )

Prepared by Walter Chen ( 陳偉堯 )