I performed construction, maintenance, and/or abandonment inspections on nearly one hundred temporary sediment basins across North Carolina 2015-2017. Most of these basins were skimmer basins, about ten percent were riser basins. Almost all were dysfunctional for one reason or another. A number of basins failed because of incorrect topographic information applied to the temporary sediment basin design.
In addition, basins failed due to improper plan interpretation. Another problem was unskilled or uncaring basin construction.
Temporary Sediment Basin Design and LIDAR
According to NOAA, “LIDAR, which stands for Light Detection and Ranging, is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth. These light pulses—combined with other data recorded by the airborne system— generate precise, three-dimensional information about the shape of the Earth and its surface characteristics.”
“LIDAR systems allow scientists and mapping professionals to examine both natural and manmade environments with accuracy, precision, and flexibility. NOAA scientists are using LIDAR to produce more accurate shoreline maps, make digital elevation models for use in geographic information systems, to assist in emergency response operations, and in many other applications.”
Notice that the list of uses does not include site plan sediment and erosion control plans. LIDAR applications are more consistent with larger expanses, floodplain studies, for example.
The attraction to this type of topographic information is that it is free.
One designer reported that the NC Board of Examiners for Engineers and Surveyors has ruled that the engineers should not use LIDAR for such design. Yet the practice continues.
Designers relying on LIDAR consistently misplaced basins in their plans. Basins built on higher ground than surrounding grade was common. They did not function yet they complied to the plans. Some basins required re-work when relying on this data source. Additional basins were needed on some projects after construction commenced once the design was proved ineffective. One project manager observed that the RE in re-work stands for Really Expensive.
Temporary Sediment Basin Design and Photogrammetry
Topography info gathered by photogrammetry can be better but it also has its drawbacks.
Lewis Graham, President and Chief Technical Officer of GeoCue Corporation is quoted in Commercial UAV News, “Traditional Photogrammetry derived elevation by triangulating the ground from two different images of the same area on the ground,” Graham explained. “The images are displaced by a ‘baseline’, allowing one to use triangulation to discern elevation. The weakness of this was the fact that to measure elevation, a “ray” from each of the two images had to converge at the ground point one wished to measure. This is difficult in environments with overhead obstructions such as trees. Laser scanning (Laser Imaging, Detection, and Ranging, LiDAR) has the distinct advantage of requiring only a single ray to measure elevation.”
Yes, trees are a problem. I have seen this method used to survey the heavily vegetated land. It provided no better design information than LIDAR. In fact, the design of one large project that relied on photogrammetry to gather topographic data resulted in several, completely non-functional basins due to their placement.
Temporary Sediment Basin Design – Budget and Other Problems
Construction budgets drive many decisions. Budget drives the use of LIDAR. LIDAR causes, potentially, a cascade of design, construction, and unforeseen budget problems,
Field run topo is the safest and most accurate method based on my experience. The temptation to over space the survey grid to reduce costs should be avoided in order to ensure accuracy.
Budget reduction initiated the following project.. Management dictated a cost reduction so an attempt to was made to eliminate one basin. The plan showed that six were required. Re-design attempted to eliminate one basin by directing water destined for the eliminated basin to another. The other basin was within one hundred feet of the other but fifteen feet uphill. As a result, the projected savings evaporated. This redo caused design and construction delays and added, even more, costs to the budget.
Designers often ignored actual field conditions and therefore added problems to and decreased the functionality of some basins. A repeated, problematic, basin design had the inlet end of the basin open to run off without constructed slope protection. This design assumed established grass protection. The reality is that it takes time to grow slope cover. These unprotected slopes eroded with the first rainfall. Each successful rainfall contributed to the erosion. Consequently, the assumed grass consistently washed into the bottom of the basin. These conditions required slope repair and/or the installation of unbudgeted slope protection measures.
Design can be the problem in basin design. So can construction. I will illustrate some of those failures in another post.