title: Air Pressures Over Reservoir, Canal, and Water Catchment Surfaces Exposed to Wind
Creators: name: Dedrick, Allen Ray
Publisher: name: Utah State University
Year: 1973

description: Surface air pressure differentials that occur over reservoirs, canals, and water-catchment aprons in high-wind conditions were determined by the use of models in a wind tunnel. Such information was needed concerning the magnitude and location of destructive wind forces on water barriers constructed of exposed flexible membrane liners. Rigid models, without a membrane, were used to measure airpressure differentials. Air-pressure differentials were dependent on geometric configuration and were independent of viccous forces above the critical Reynolds number which was determined experimentally. The geometric factors studied with respect to reservoirs and canals included approach slope, approach slope length, leeward slope, and breadth-depth ratio. Those pertaining to water-catchment aprons included breadth-berm height ratio and berm shape. On the leeward slope of reservoirs, favorable (positive) differential pressures were more apt to occur as the approach and leeward slopes decreased (became less steep), but extreme adverse (negative) pressures occurred under the same conditions near the top edge of the slope. As the leeward slope increased the pressure coefficient values were essentially constant over the entire surface. On the reservoir bottom, adverse pressures did not exist when approach and leeward slopes were at the minimum evaluated (1:4). When the leeward and approach slopes increased the average pressure on the bottom became less favorable. The most adverse pressure coefficients generally occurred at the toe of the leeward slope, while maximum favorable pressures on the reservoir bottom generally occurred at the toe of the downwind slope. In most instances, the pressure on the downwind slope was favorable; however, adverse pressures were recorded near the top edge of the slope. These adverse or negative pressure conditions were more apt to occur as the breadth-depth ratio increased and the leeward slope decreased. Average pressure values on the interior surface of canals were generally less adverse than for reservoirs. Similar to the reservoirs, the most adverse pressures on the leeward slope of the canals were found immediately over the upper edge of the slope. Extreme adverse pressures on the canal bottom were associated with long and steep approach slope conditions. One exception was the occurrence of the most favorable pressure over the canal bottom when the approach slope was short but still steep. In addition to measurement of adverse pressures near the top edge of the downwind slope, adverse pressures were found near the toe of the slope in some instances. The portion of the downwind slope under an adverse pressure influence decreased as the breadth-depth ratio increased. Generally, pressures over a canal were more responsive to the approach slope and approach slope height than to leeward slope arid breadth-depth ratio. For a reservoir, however, pressures were responsive to all geometric factors, depending on which section of the reservoir was being considered. Pressure distributions over water-catchment aprons were independent of breadth-berm height ratios of 50 or greater. Negative pressures were lower over gradual sloped catchment berms by a factor of three than over sine-shaped berms. Each berm shape, on the leeward side of the upwind berm, had a pressure distribution associated with it that was the same as far as shape but was different in magnitude depending on the shape of the windward side of the berm. Pressures were less adverse on the leeward side of the berm, if the windward side were gradually sloped. Pressures were near zero to slightly favorable over a large portion of the water-catchment apron between the berm toes. This near stable favorable condition might change significantly, if site conditions were to change.