By John A. Dolan, P.E.
From the October 2018 Issue
There are a number of important factors to consider when evaluating a site for the construction of a new facility or in assessing an existing site for the expansion of an existing facility. These factors include environmental impacts; local, county, state, and federal zoning and planning regulations; availability and adequacy of utilities; and ingress and egress to the site.
While the above factors are of paramount importance, far too often little attention is paid to the critical factors of site grading, drainage, and stormwater management during the key early planning, assessment, and decision stages.
Lack of attention to these key factors in the planning and assessment stages can lead to cost overruns during construction—or worse, difficulties in maintaining the site and facility in the long run. Keep in mind, a key objective of grading, drainage, and stormwater management is to protect property and infrastructure by the safe conveyance of stormwater runoff (both off-site and on-site). The inability to meet this objective can lead to erosion, flooding, and moisture intrusion into the facility.
The first step in most site evaluations is generally a preliminary site layout plan wherein the designer will answer this question: How much impervious surface can we add (buildings, roads, parking, and paving) while still meeting zoning ordinances for setbacks and maximum impervious area? This is a fairly simple drill, but it doesn’t answer the key questions: How do we convey the runoff away from buildings and across paved areas (with swales? catch basins?)? How do we manage the runoff (using detention basins? infiltration/retention basins?)?
The first step in answering these questions is to evaluate the soils on the site. The soil evaluation (beyond providing data for the eventual design of foundation systems) provides important data related to stormwater management to include the porosity of the soil and the elevation of the water table. This data allows a designer to determine which types of storage devices are most feasible (detention or infiltration), and subsequently, how much real estate is required for the stormwater management systems. The storage devices are required to limit the post development runoff from the site to a level that is at, or below, the pre-development runoff (new impervious surfaces increase runoff).
Detention Basins: Dry And Wet
There are two basic types of detention basins—dry and wet. A dry detention basin is a shallow, dry basin with an outlet pipe or orifice at the bottom of the basin. The bottom of a dry detention basin must be above the mean high ground water table, but is otherwise not generally influenced by the native soil types. These basins are generally the cheapest to construct, but they require a significant amount of surface real estate.
Meanwhile, a wet detention basin is a shallow basin that maintains a permanent pool of water by using an elevated outlet control structure. The bottom of a wet detention basin must be below the mean low ground water table to maintain the permanent pool, but is otherwise not generally influenced by the native soil types. These basins are generally more expensive than dry basins; however, they are more aesthetically pleasing in that a water feature has been constructed, and they are easier to maintain. They require approximately the same amount of surface real estate as a dry basin.
Infiltration basins are shallow basins in permeable soils that retain and infiltrate stormwater runoff. These basins are highly dependent on soil characteristics in that they require porous soils to infiltrate stormwater, and the bottom of the basins must be above the mean high ground water table. A major advantage of infiltration basins is that they can be constructed below ground (dependent on the water table), thereby saving valuable real estate. Infiltration basins, whether above or below ground, are generally more expensive to maintain than detention basins in that they must be periodically cleaned of sediment to allow for proper infiltration into the soil.
Once there is adequate information to decide what type of stormwater basin will work most effectively, the next step is determining whether runoff can be effectively routed to the basins. This step requires an evaluation of the site topography. It’s pretty simple physics; water will flow downhill via gravity. The question is: Does the existing topography have enough fall to allow the water to flow from point A (the facility and infrastructure) down to point B (the basin)?
Not addressing this seemingly simple question during the evaluation and assessment stages is one of the most common causes of cost overruns during construction. If there is not enough fall, expensive fill must be imported and graded to allow for proper site runoff, to include ensuring that water runoff flows away from buildings to avoid moisture intrusion, as well as ensuring that runoff on parking lots and other paved areas is conveyed to catch basins and/or swales to allow for safe utilization of these areas during storm events.
When completing site assessments for the construction of a new facility or the expansion of an existing facility, don’t stop short by just answering the question: Can we fit our proposed square peg in a round hole? Make the proper investment in due diligence to ensure that the site can be properly graded and drained to achieve a constructible, maintainable facility.
Dolan is the President of Rimkus Building Consultants in Fort Lauderdale, FL. He leads a team focused on helping clients proactively extend the life of their buildings, industrial facilities, and other infrastructure, by identifying, mitigating and eliminating the complex risks inherent in procurement, design, construction, and operation of large-scale facilities. Dolan has more than 25 years of experience in all phases of planning, design, and construction for projects, ranging from business parks to large scale commercial and industrial facilities. He has also worked with bridge, infrastructure, and roadway projects on the federal, state, and municipal level.
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