A survey of a residential lot* on the east side of San Antonio, Texas,
illustrates challenges to floodwater drainage, rainwater collection, and irrigation.
A survey of a residential lot* on the east side of San Antonio, Texas,
illustrates challenges to floodwater drainage, rainwater collection, and irrigation.
The survey data is available as an interactive map. Note: you must have an SVG plugin installed to view it.
This residential lot is 100′×375′, about 0.86 acres, and oriented with the primary axis due North. A road* runs along the north edge of the property, and there is an easement along the south edge for a future street.
The soil is primarily black clay, and not far beneath the surface (3-15′) is a confining layer of caliche; water flows above it to seasonal springs feeding Salado Creek about ½ mile to the West.
Overall, the lot slopes steadily, but very gradually downward from North to South (from front to back) at a rate of about 1::257, less than 1/20″ per foot, and does not have an overall slope East to West.
It was not possible during this survey to obtain lines of sight out of this lot due to dense foliage and privacy fences. The overgrowth, primarily hackberry trees, also interfered with access to the perimeter of the property. Notably, the southeast corner was inaccessible, where there is an old mound of roofing waste. The branches of a large monkeyball tree located in an adjacent lot droop down to cover this mound. As it was, establishing the lines of sight such as those shown in the photos at right required the removal of intervening branches and some small trees. This might be less of a problem if the land is surveyed during the winter.
Concrete chunks from the demolition of sidewalks are piled in the back of
this lot to provide material for stonemasonry improvements to the property.
The elevations of these piles were not determined to avoid damage to survey equipment and personnel,
so the topographic information should reflect the ground on which this material was dumped.
The 145 points of elevation data collected show the N/S ordinate determines most of the elevation variability (r²=0.875), and the E/W ordinate almost none (r²=0.013). The t statistic for E/W correlation is -1.385 and falls within the acceptance region for the null hypothesis that the slope is zero. On the other hand, the N/S correlation is significantly linear since the t statistic is 31.704, far greater than the 1.65 required for an α of 10% (or even the t=3.17 required for α=0.1%, but we're not surveying Chartres Cathedral here). Rejecting the null hypothesis of zero slope implies significant linear correlation between the N/S ordinate and elevation. The 90% confidence interval is a slope between 0.00369 and 0.00410 (1/20″ to 1/23″/ft).
After removing the N/S correlation, the residuals correlate (t=5.48) to a very gradual slope of -0.00226 (1/36″/ft), downward from West to East. But this correlation accounts for very little (r²=0.173) of the residual's variability along the E/W ordinate.
Hence, this parcel slopes gradually, but very consistently, downward from North to South at less than 1/20″ per foot, and negligibly downward from West to East.
The minimum slope required for water to flow reliably is ¼″ per foot, far steeper than the slope of this land. Further evidence of the lack of surface runoff is the standing water present around the house persisting after extended periods of rainfall saturate the soil above the caliche layer not far from the surface. Some of this water, perhaps a significant amount, does not run off the surface at all but flows a few feet underground to feed seasonal springs along Salado Creek.
Raising topsoil and walkway surfaces 6-18″ with permeable fill can provide areas and paths free of standing water. Although placing a berm along building foundations may help prevent water running off the eaves from splashing onto exterior walls, it will do little to drain water away from the structures; a french drain is necessary for this. But since the land does not slope enough to allow existing foundations to have subsurface drainage to daylight, digging a dry well, pond, or tank will be necessary to provide a below-grade destination for runoff.
Excavation below the caliche layer is prohibitively expensive and so can not be large enough to provide more than a temporary destination for runoff, a dry well. Since the land does not slope steeply enough for surface runoff, rainwater will need to be directed to one or more dry wells by arroyos or subsurface drain pipes before it is pumped into a storage tank holding the water at a level above grade.
The location of the storage tank determines the minimum water level needed to direct irrigation water to any part of the lot by gravity alone. If it is located at the northern property boundary, the water surface needs to be at least 6½′ above grade, 9½′ high at the southern edge, and midway only 4′ above ground level. From the storage tank, aqueducts feed water to acequas, small ditches used to distribute water to the orchards and gardens.
Water management is a challenge on a flat piece of ground. In addition to berming the foundation of the house to drain water away from the dripline so it cannot splash back onto exterior walls, walkways need to be raised above grade to provide dry ground on which to walk. Rainwater collection is made even more difficult by the shallow subsurface caliche layer, as the water needs to be carefully channeled to flow into locations where it can be pumped quickly to storage above grade. Then, to distribute irrigation water by gravity, aqueducts need to be built starting high enough so they can slope down to ground level and provide water to any part of the yard.
The survey data is available as an interactive map.
Note: you must have an
SVG plugin
installed to view it.
* Specific location masked for privacy.
©2005, David E. Wagner II. All rights reserved.
