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APPENDIX B Surface Erosion Module

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APPENDIX B Surface Erosion Module Watershed Analysis Manual B-Surface Erosion APPENDIX B Surface Erosion Module Table of Contents Introduction .......................................................................................... 2 Critical Questions .................................................................................. 4 Assumptions ......................................................................................... 4 Overview of Assessment and Products ...................................................... 6 Qualifications ........................................................................................ 8 Background Information ......................................................................... 9 Analysis Procedure................................................................................11 Surface Erosion From Hillslopes Assessment .........................................................................................14 Surface Erosion from Roads Assessment ...............................................................................................22 Effect on Public Resources ....................................................................................................................37 Surface Erosion Assessment Report ........................................................42 Acknowledgments .................................................................................45 References ..........................................................................................46 Version 5.0 B-1 May 2011 Watershed Analysis Manual B-Surface Erosion Introduction Surface erosion occurs when detachable soils on sufficiently steep slopes are exposed to overland flow and/or the impact of rainfall. Sediments introduced to streams from surface erosion processes are generally fine-grained and can influence water quality and aquatic habitat. Watershed analysis is primarily concerned about identifying locations and activities that deliver sediments to these public resources. Raindrop splash, freeze/thaw, dry ravel, and biogenic processes such as wind throw and animal burrowing are natural causes of soil detachment. Gravity and overland flow of water are natural transport mechanisms of the detached soil particles. Overland flow of water rarely occurs under natural forest conditions because the soil is usually protected by an absorbent, protective layer of organic material resulting from residue of the forest plants. Soil compaction can lead to overland flow and serious erosion consequences. Hillslope angle, soil texture as it affects how well the soil holds itself together, and climate are important influences on the inherent erosion hazard of the site. Any activity that strips the protective duff layer to the bare mineral surface may allow surface erosion. Surface erosion can also occur on compacted surfaces where the capacity of the soil to quickly absorb free water is diminished. The result is that water is readily channelized into surface flows. Among the activities most likely to cause surface erosion are roads, silvicultural practices involving high intensity broadcast burns or mechanical scarification, poor yarding practices, and natural processes such as wildfire. Forest management activities that accelerate soil detachment and transport include: Those that expose bare mineral soil to the weather: • Road construction and maintenance • Yarding techniques that disturb the duff layer such as skidder/tractor yarding, no suspension and one end suspension cable yarding • Site preparation techniques such as burning or scarification Those that compact soil and/or intercept subsurface flow zones, encouraging overland flow include: • Skid trails • Road and landing construction If water bars and other water control measures are neglected, runoff from Version 5.0 B-2 May 2011 Watershed Analysis Manual B-Surface Erosion roads, cut- and fill-slopes, skid trails, etc. can contribute to hillslope erosion. These features actively produce sediment in most watersheds, with construction practices and drainage design influencing how much sediment is delivered to streams. How far material can be transported on slopes, and how it behaves once it enters the stream, are largely determined by the nature of the slope and the texture of the sediment. Factors that influence delivery to the stream system include: Hillslope Erosion • Proximity of erosion to the stream system • Slope angle • Soil texture, reflecting differences in the distance that various particle sizes will travel • Areas where overland flow occurs Road erosion • Amount and condition of road prism area that drains directly into the stream system • Traffic levels on the direct entry area of the road surface • Material used for road surfacing Some of the natural conditions that limit delivery of eroded soil to the stream include vegetated areas along streams that can filter out soil particles, and topographic conditions that prevent eroded material from entering the stream. Management practices that can limit delivery of eroded soil from hillslopes to the stream system include minimizing duff disturbance, water-barring and/or grass-seeding exposed areas near streams, and avoiding compacting the soil. Minimizing the road surface area that delivers directly into the stream, maintaining it according to the traffic levels, and limiting traffic during wet weather are management techniques that may help control the entry of erosion material into streams. Version 5.0 B-3 May 2011 Watershed Analysis Manual B-Surface Erosion Critical Questions The purpose of the surface erosion assessment module is to guide development of information necessary to address key questions critical to understanding erosion processes in a watershed context. Two types of erosion processes are addressed in the module, with the same critical questions for each source: Hillslope Erosion • What is the hillslope erosion potential? • Are contributing activities present? • Is sediment delivered to streams? • What areas are sensitive to forest practices? Roads Erosion • What are the roads' erosion potentials? • Are contributing activities present? • Is sediment delivered to streams? • What roads are sensitive to forest practices? • What is the potential effect of sediment on public resources? • What is the baseline sediment level? • What are the amounts and types of sediment contributions from forest practices? Answering these key questions relies on a combination of maps, aerial photos, and field observations. A series of exercises designed to answer the critical questions, or identify more information necessary to do so, are provided in the module. The module is designed to generate the level of information necessary to introduce sound information into land use decision-making. Assumptions A number of fundamental assumptions underlie the approach developed here. These assumptions dictate a rigorous, yet flexible, framework for the analysis. Our primary assumptions include: Version 5.0 B-4 May 2011 Watershed Analysis Manual B-Surface Erosion Hillslope Erosion Assessment • Sheet erosion of hillslopes is influenced primarily by soil type, hillslope gradient, protective cover, precipitation intensity and human activity (USDA, ARS in press). • Certain soils (easily detachable) and slope conditions (steeper) are conducive to surface erosion (USDA, ARS, in press). • On potentially erodible soils, the primary factors determining whether surface erosion occurs are exposure and compaction of mineral soil. Surface erosion tends to increase with exposure and/or compaction (Packer 1951). • Certain forest practices can expose and/or compact surface mineral soil and significantly increase surface erosion. High-intensity burns, such as those used in site preparation, can expose large areas of mineral soil (Tiedemann et al. 1979). Both ground-based and cable yarding harvest activities have the potential to expose and compact surface mineral soil. The extent of soil disruption tends to be higher on ground-based harvest sites due to the skid trails (Megahan 198 ). Harvest activities that do not expose or disrupt the surface mineral soil are unlikely to increase surface erosion (Bennett 1982). • Rainfall intensity and amount influence whether soils erode; however, since all places in the state of Washington have some probability of intense rainstorms, they also have some probability of surface erosion. • If gullying occurs and the gullies connect to the channel network, then all sediment carried through them will be delivered to the stream system. • Surface erosion may be delivered anywhere in the stream system by dry ravel or overland flow, but is fairly easily disrupted by buffers of slash, duff and other protective soil cover. Therefore, sediment is generally not delivered to the stream system if adequate buffers exist on the hillslopes (Comerford et al. 1992). • Visible evidence of surface erosion is present where surface erosion has occurred in recent years. • Dry ravel is primarily a function of slope gradient, hillslope storage potential, and soil erodibility (Mercereau and Dyrness, 1972). • Most surface erosion occurs within five years of a contributing activity (Mercereau and Dyrness, 1972). • The Forest Practices Rules of the State of Washington (Title 222 WAC) are followed, unless evidence suggests otherwise, and the rules are effective at Version 5.0 B-5 May 2011 Watershed Analysis Manual B-Surface Erosion preventing excessive surface erosion, unless the soils are especially erosive. Roads Erosion • Surface erosion
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