GRAVITY I: WATER ON HILL SLOPES.

1. Harry Williams, Geomorphology

2. Last Wednesday and Thursday, there were two major rockfall events at Yosemite’s El Capitan, a rock formation extremely popular with climbers. Wednesday’s rockslide killed one person, the first rockfall-related fatality in the park since 1999. Thursday’s released a volume of rock larger than 10,000 cubic meters, about four Olympic swimming pools’ worth of rock. According to Roger Putnam, a climber and geologist at Moorpark College, these rockslides were not unusual from a geologic standpoint. In fact, he said, the quick succession of rockfalls “perfectly shows how a small rockfall happens and then—boom—that causes another rockfall, and another.” Rockfalls are par for the course at Yosemite, where the National Park Service estimates 80 events happen every year. But despite their frequency, there is a possibility that warming temperatures and an unstable climate could cause even more rockfalls at Yosemite and worldwide. Rockslides happen in rock with existing weaknesses, like large cracks, after they become unstable as a result of a trigger. Roy Sidle, a professor of geography at the University of the Sunshine Coast, said in an email that those triggers fall into four major categories: “freeze-thaw action, wetting and drying, temperature changes, and (of course) human disturbances (even rock climbing).” Harry Williams, Geomorphology

3. Putnam said that, until recently, most geologists thought freeze-thaw cycles, also called frost wedging, caused the majority of rockfalls. The idea was that water seeped into cracks during winter rainstorms, then expanded when it froze—the same process that forms potholes. Then, in 2016, Yosemite’s park geologist published a paper in Nature Geoscience that found most rockslides in Yosemite happen on hot, sunny days, not in freezing temperatures. Like sticky wood doors in the summer, rock expands ever so slightly when it’s heated. The more the temperature rises and falls between day and night, or summer and winter, the more rock will expand and then contract, causing cracks to propagate through it. As temperatures rise worldwide, those hot, sunny days will become more frequent, which could mean more rockslides. Harry Williams, Geomorphology

4. GRAVITY 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 rare). The erosion of hill slopes in most areas can be considered a system which links together WEATHERING (breaks down rock in situ), 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

5. The Role Of Water In Hillslope Erosion Most 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). DIFFERENTIAL EROSION CREATES LANDFORMS Harry Williams, Geomorphology

6. Small drainage basin hydrology. Harry Williams, Geomorphology

7. OVERLAND FLOWS: where the rainfall intensity exceeds the infiltration capacity, HORTONIAN OVERLAND FLOW results. R: rainfall rate R > I Slope surface Overland flow = R - I E.g. R=3 inches/hour I=1 inch/hour Overland Flow=2 inches/hour. I: infiltration rate Harry Williams, Geomorphology

8. SATURATION 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

9. Subsurface Flows Water 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). Zone of aeration Zone of saturation Harry Williams, Geomorphology

10. Notice that the water table is usually shallower beneath low-lying areas (e.g. valley bottoms) Normal water table flow stream hills mountains The 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. cliffs ridges Harry Williams, Geomorphology

11. valleys 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 - 7.0 Sandstone 16 - 34 Limestone 22 - 100 canyons deltas AND, their contribution to SATURATION OVERLAND FLOW... Harry Williams, Geomorphology

12. Overland 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

13. Rills 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

14. In 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

15. Significant 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

16. Ray Roberts overflow channel Woodbine sandstone New Mexico Harry Williams, Geomorphology

17. Another 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

18. The Trinity River in Dallas – notice levees. Harry Williams, Geomorphology

19. May 2015. Harry Williams, Geomorphology

20. Rochester Park, downstream from Dallas, flooded in 1989. Harry Williams, Geomorphology

21. Rochester Park. Harry Williams, Geomorphology

22. Two related issues: 1. increasing urbanization affects hydrology – 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. Harry Williams, Geomorphology 22

23. 2. Disturbance of natural surfaces by gas well drilling activities. We will look at an article on sediment runoff from gas well sites in the Denton area. Harry Williams, Geomorphology