(See also Stormwater Management Basics 1)
Goals of Stormwater Management
A first underlying goal of virtually all stormwater management is to calibrate post-development water flows from one’s property to closely (if not exactly) match pre-development water flows. Doing so preserves the status quo, and thereby, one hopes, diminishes any impact and eliminates any unforeseen consequences.
A second underlying goal of virtually all stormwater management is to maintain if not improve the quality of water flowing from one’s property. Here too the goal is to ‘do no harm’, by maintaining the status quo. However, as mankind increasingly alters the character of the Earth’s freshwater, however subtly, it may be a better goal to consistently improve the quality of water flowing from one’s property above its initial quality.
A third underlying goal of virtually all stormwater management is to properly modulate the flows of stormwater off-site to best meet local needs. For example, an agricultural field downstream may have different needs of flowing stormwater than those of a residential subdivision, or a golf course, or an office park, or an industrial facility, or a harbor. One must bear in mind that we are all linked in connecting chains along the water cycle; one person’s outflow is another person’s inflow, and so on. As freshwater — as well as the cycling stormwater from which it is continually replenished — is a scarce and expensive resource to all life, we must pursue the highest quality stewardship of that resource.
Hydrology and Hydraulics
The field of hydrology is generally considered to deal with the predominately natural water processes, such as precipitation, snowmelt, infiltration, percolation, evapotranspiration, surface water flows, runoff and other natural water channeling. The field of hydraulics is generally considered to deal with water processes such as groundwater flow, and the typically human-designed open channel flow, and flow through piping (often under pressure) and other structures. These two fields clearly overlap and intermesh, as both our built environment and our water-handling infrastructure increasingly penetrate and affect pre-existing natural settings. Stormwater management must therefore draw to some degree on the disciplines and bodies of knowledge of both hydrology and hydraulics.
Water Resources Management
Stormwater management can also be seen as a subset of the larger field of water resources management. Water resources management encompasses the acquisition, design, development, control and maintenance of water resources of all kinds upon which humans, animals, plants and crops depend. It will therefore concern itself not only with the purity of municipal drinking water, but also with flood control at a dike or levee, or the environmental reclamation of fouled wetlands. Stormwater management, though, is typically limited to the gathering, manipulation, control and quality primarily of rainfall, snowmelt and related surface waters and groundwater.
The design of any stormwater management system begins with the collection and assessment of a great deal of initial data. Stormwater management systems can be quite complex, and their optimization must be conditioned on a great many variables particular to the site and case at hand. Following is merely a partial listing of the information that must typically be gathered prior to designing a stormwater management system:
1. A survey and detailed topographic map of the site of the proposed system.
2. Area maps, local drainage patterns, and jurisdictional drainage standards or criteria.
3. Complete layout of any proposed development, construction or improvements, including all structures, impervious surfaces, roadways, gradient slopes, wooded or landscaped areas, wetlands, drainageways, etc., including elevation information.
4. Locations, sizes and depths of all existing and proposed subgrade utilities.
5. Information on both upstream and downstream drainage patterns, flows and/or restrictions.
6. Complete local weather and storm data.
7. Soil test data, soil boring analysis, soil chemistry and soil strength assessments.
8. Seasonal water table, aquifer and/or well information.
9. An understanding of any local, regional, state or Federal regulatory, submission, review and approval process and related requirements.
Over time, organizations such as the American Society of Civil Engineers (ASCE) and the United States Army Corps of Engineers (USACE) have developed rough parameters within which the typical small- to moderate-size stormwater management system — say, for example, a proposed 25-to-40-acre development on an exurban or semi-rural site without severe topography — can likely be designed.
1. The system is designed to operate completely by means of gravity flows, through unpressurized piping, with no need for pumping or lift station infrastructure.
2. The system is arranged in a dendritic layout, resembling the branching from a tree trunk.
3. Limits are placed on minimum water flow velocity and pipe fullness (to insure sewer piping remains flushed and clear), as well as on maximum water flow velocity (to minimize erosive scouring).
4. Sewers are placed sufficiently far below ground surface to prevent disruption or crushing due to traffic or other site loads, freezing, or water penetration of basements or low site areas.
5. Sewers are of readily available sizes and types of piping, with pipe diameters never decreasing downstream.
6. Sewer junctions and connections are designed to insure proper through-flow.
Clearly, topography is an initial determinant of the nature of any stormwater management system, for it determines where and how and how fast surface water will flow across the ground. A relatively broad and flat plain may hold water, whereas one traversed by channels may drain. Steeply pitched ground will tend to dry out quickly, unless, that is, it is highly impermeable or has no ready outfall. If building within a hollow, surrounded by much higher ground on all sides, one has a much different stormwater management task than when building atop a mountain.
And neighboring topography — beyond the limits of one’s own property or planned development area — also matters, for one should no sooner flood an adjacent lower-lying property than one would wish to be flooded by an adjacent higher-lying one. In fact, an underlying principle of stormwater management in most cases is to replicate, to the greatest degree possible, the pre-existing water flow and drainage conditions of any site with the final post-development conditions.
Continue with Geology next article.
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