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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Harry Williams, Geomorphology 1GRAVITY I: WATER ON HILL SLOPES.Introduction Outside of river channels where fluvial processes operate, all land surfaces can be considered HILL SLOPES (completely flat areas are very rare, and even they can be considered hill slopes with a slope of 0o). The erosion of hill slopes in most areas can be considered a system which links together WEATHERING (breaks downrock in situ), HILL SLOPE PROCESSES (mass wasting)( material - solid rock, regolith, soil - is moved down slope either by gravity or by running water) and EROSION (usually by rivers in valley bottoms).Harry Williams, Geomorphology 2DIFFERENTIAL EROSION CREATES LANDFORMSThe Role Of Water In Hillslope ErosionMost rain falls onto the valley sides. Whether or not the water infiltrates depends on 2 factors:1. the infiltration capacity of the surface = rate of infiltration allowed by surface (depends on surface conditions).2. rainfall intensity (e.g. inches per hour).Harry Williams, Geomorphology 3Small drainage basin hydrology.Harry Williams, Geomorphology 4OVERLAND FLOWS:where the rainfall intensity exceeds the infiltration capacity, HORTONIAN OVERLAND FLOW results. R: rainfall rateI: infiltration rateR > IOverland flow = R - ISlope surfaceHarry Williams, Geomorphology 5SATURATION OVERLAND FLOW occurs mainly at the base of slopes and in concavities, which become saturated during prolonged rain (by the combination of infiltration, interflow (flow down slope within the soil) and groundwater flow - once the soil is saturated its infiltration capacity is zero, so any additional rain will not infiltrate - it will be stored on the surface or become overland flow. The rain does not have to be very intense – just prolonged enough to saturate the soil. Saturation overland flow is generated by parts of the watershed, which can expand and contract with the seasons – see diagram.Harry Williams, Geomorphology 6Subsurface FlowsWater that INFILTRATES the ground becomes either SOIL MOISTURE (films clinging to particles in the AERATION ZONE) or GROUNDWATER (the SATURATED ZONE, the top of which is THE WATER TABLE). Just above the water table there is the CAPILLARY FRINGE where water is drawn up from the water table by capillary action -> discontinuous saturation. Zone of aerationZone of saturationHarry Williams, Geomorphology 7mountainshillscliffs ridgesThe water table is not level, it follows the shape of the surface - higher under hills, lower in valleys; because of this, both soil moisture and groundwater can flow from high elevation to lower elevations - down slope as interflow and groundwater flow. Normal water tablestreamflowNotice that the water table is usually shallower beneath low-lying areas (e.g. valley bottoms)Harry Williams, Geomorphology 8canyonsvalleysdeltasAND, their contribution to SATURATION OVERLAND FLOW...These subsurface flows are usually very slow - a typical flow rate for clean sand is around 10 m/day; their main contribution to hill slope erosion is the removal of material in SOLUTION eg Rock Type Ground Lowering (mm/1000 years)Igneous/metamorphic 0.5 - 7.0Recent (Tertiary) sandstone 16 - 34 Limestone 22 - 100Harry Williams, Geomorphology 9Overland Flows Hortonian and saturation overland flows move sediment down slope. This results in sheetwash, rills and gullies. Definitions: sheetwash = sheet of water flowing across surface; rills = sheetwash concentrates into small channels; gullies = sheetwash and/or rills concentrate into larger channels on hill slopes. Sheetwash is aided by rain splash erosion - raindrops detach particles from the surface - most effective in dry regions which lack protective vegetation. In all cases, the movement of soil and rock particles by flowing water is EROSION.Harry Williams, Geomorphology 10Rills formed from concentrated sheetflow (left); further surface flow concentration results in gullies (below). Cultivation of the field promoted overland flow and gullying (below left).Harry Williams, Geomorphology 11In this region, Hortonian overland flows do occur - especially on Woodbine sandstone during intense rainstorms. Very intense rain may only last a few minutes, but a lot of erosion can occur.Steep Man-made slope on sparsely-vegetated Woodbine sandstone. Sheetwash occurred during an intense thunderstorm.A small “alluvial fan” of sandy soil washed down slope by sheetwash.Harry Williams, Geomorphology 12Significant overland flows occur where infiltration is low and rainfall intensity is high. Infiltration is affected greatly by the presence of vegetation, which promotes infiltration by maintaining an open soil structure. For this reason, Hortonian overland flows occur mainly in arid regions with poor vegetation cover, which are subject to rare but intense thunderstorms. eg. Big Bend area, West Texas, Southwest U.S.In humid vegetated regions, overland flow is less common, except where the surface vegetation is disturbed (e.g. construction, fire, cultivation, compaction…. etc).Harry Williams, Geomorphology 13Woodbine sandstoneNew MexicoRay Roberts overflow channelHarry Williams, Geomorphology 14Another exception would be urbanized areas. Concrete has effectively zero infiltration. It generates Hortonian overland flows that drain rapidly into storm sewer systems. These in turn feed the water very quickly into local streams. The result is that floods may be more likely following urbanization of a watershed, unless there is adequate protection by levees, retention ponds and/or other flood control structures.If flood protection is inadequate, flooding usually occurs downstream from the main urban area.Harry Williams, Geomorphology 15The Trinity River 1989 - full from levee to levee.Harry Williams, Geomorphology 16Rochester Park, South Dallas, doesn’t have such good levees.Harry Williams, Geomorphology 17So, if you live in this neighborhood, get a boat...Harry Williams, Geomorphology 18Harry Williams, Geomorphology 1919Two related issues:1. increasing urbanization – DFW is a great example. As watersheds become more urbanized, less rural, their hydrological behavior changes – possibly resulting in more floods, erosion, and channel adjustments.We will take a closer look at this in an article on hydrologic changes in White Rock Creek watershed. 2. Disturbance of natural surfaces by gas well drilling


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