Mapping and Data

Storage - This is the stereotypical dam; reservoirs used for recreation or irrigation impoundment are examples of this type. Since water will be impounded on a "permanent" basis, the design of the dam is complicated by the fact that water will eventually seep through the dam and must be controlled. A typical storage dam may be an embankment dam with an impermeable clay core surrounded by a granular shell material. A typical zoned embankment dam will have chimney and blanket drains, corresponding filter zones, outlet works with gates, valves, and a drain, seep water collection system, cut-off trench and possibly several spillways. The capacity of the spillway(s) is dependent upon the downstream hazard potential (defined later in this publication) and the size of the area tributary to the dam. Generally, an embankment dam is constructed of soil, usually derived local to the dam site, and quality control as well as proper placement of the material is crucial to the success of the dam. Specifications must clearly define what types of materials can be used, how they are to be placed and what compactive effort must be exerted on each "lift." Piezometers are important in embankment dams so that the phreatic surface within the dam can be monitored as the embankment drains and the reservoir is filled. The phreatic surface is crititcal in determining the stability of the dam. The most important difference between the design of a storage dam versus either a tailings dam or flood control dam is the influence of the reservoir on the foundation, abutments and dam due to the long period of saturation. As mentioned previously, a dam that impounds a permanent reservoir will eventually seep, so satisfactory long-term performance of a storage dam mandates that the seep water be controlled. Impervious zones, impervious blankets, cut-off walls and grouting are all means to reduce the rate that water can seep through the dam, foundation and abutments. Drains are also utilized to collect and dispose of seep water safely so that no structural material is transported out of the dam.

Flood Control - Also referred to as a "detention basin," this structure is built upstream or up gradient from a developed area so that an extreme precipitation runoff (flood) is attenuated (reduced) to a manageable level in order to prevent human or economic loss. Due to the nature of its purpose, a detention basin is categorized as a high hazard structure thus; the spillway must be designed to pass the probable maximum flood (PMF). Making the design process more challenging, outlet works are usually required to pass some target flow rate established by a local authority (county, city or town). In many cases, the flow rate is only equivalent to the "25 year" flood. Since a detention basin's primary function is to detain or divert storm flow and reduce downstream flow rates, no gates or valves are allowed on the outlet so that water can never be stored on a long-term basis. Typical retention times in such a basin would be on the order of 24 to 72 hours although some are as long as 5 to 10 days. Because the dam only impounds water when there is a storm event and then only for a few days, a complete and thorough seep analysis is not typically required in the design. In addition, because of the short detention times, the dam is often homogenous with no embankment drains. Normally detention basins are part of a larger comprehensive flood control master plan that accounts for and relies upon attenuated flows between detention basins, flood channels and storm drains to function properly.

Tailings - Although tailings are a mobile material, they are obviously significantly more viscous than water and their physical behavior alters with deposition method and over time. A typical tailings impoundment is lined with high-density polyethylene (HDPE), has a leak detection system and a system of drains beneath the reservoir. Tailings are normally transported as slurry composed of water and tails via pipeline to the reservoir where they are added to the top of previously deposited tails. The water either percolates through the tailings or evaporates, leaving a semi-consolidated mass of tails. Since the inlet is a controlled, the dam is not required to have a spillway, as proper management will not allow the embankment to overtop. A tailings impoundment is designed such that there is enough freeboard to accommodate the probable maximum precipitation storm without overtopping. Most tailings facilities are built in discrete raises or phases on an "as needed" basis. The State Engineer prefers downstream construction for the raises although centerline and upstream raises have been approved as the ability to predict tailings behavior and design methods improve. In order for an upstream raise to be authorized, the State Engineer must be shown that the tails in the foundation area are sufficiently consolidated, not fully saturated and suitable for the size of the raise. Liquefaction and slope stability analysis are required and acceptable factors of safety must be met. Because tailings dams are designed to impound tailings only, no floodwater is permitted to enter the structure by diverting flow around the dam. This permitting of tailings impoundments is done independent of the Division of Environmental Protection, however, the two agencies are concerned about the same items, i.e., liners, leak detection systems, foundation materials, slope stability and finally reclamation. Once a mine has gone into "closure," the mining company is responsible for breaching the dam or otherwise rendering the dam incapable of impounding any mobile material. All monitor wells must also be plugged and abandoned according to state regulations (NAC chapter 534).