Jack A Caldwell
A sediment pond is exactly what its name implied: a place to catch runoff and hold the water while the soil and debris in the water settles out to become sediment. Most sediment ponds are required because discharge of water that contains too high a load of suspended solids violates water quality discharge standards.
I will no spend time in this review talking about those aspects of a sediment pond that are identical to a water retention structure, such as the design of the embankment or spillway. Rather I will concentrate on issues unique to sediment ponds, such as optimum shape to promote solids settlement, retention time, and the requirements of mines as compared to water supply authorities.
As always a Google or Yahoo search will yield a plethora of sites that provide information about sediment ponds of varying type, for different industries, and in different climates. What follows is my personal perspective resulting from past experience and choice of web sites. You may need to undertake your own internet search using keywords specific to your mine and issues; the result will be fun and informative and if you come across anything you think should be here, please let me know so we can share your insight with others.
For guidelines pertaining to sediment ponds see Office of Surface Mining on sediment ponds
The National Resources Conservation Service Practice Design Guide is a good way to start. It provides design guides for sediment ponds that may be applicable to specific mine situations.
As always the Pennsylvania documents are the best there are. The chapter on sediment ponds provides a comprehensive listing of criteria for runoff, erosion, sizing, optimum shaping, and geotechnical design and construction.
The primary regulations that govern the design and operation of a sediment pond are those which set the discharge quality standards. Generally relevant standards set a permissible total suspended solids load that may be discharged.
A sediment pond differs from a dam in that the intention is to retain the water for only as long as is required to settle out the suspended solids; once the water is clear, it may be discharged. While the water is held in the pond, it acts like a water retention structure and so the principles of design of water retention structures come into play and must be honored if failure on first filling is to be avoided. To the extent that the sediment pond constitutes a water retention structure, local regulations may govern the geotechnical design of the embankment and spillway. For example, embankments over 15-ft high will probably be classified as a water retention structure and will have to meet specific requirements for static and seismic stability.
Once the sediment has settled and reached a moisture content where it may be removed and disposed off, you need to consider local regulations that may govern disposal of the sediment. Clearly it is best if the removed sediment can be disposed of on-site in the waste rock dump or tailings impoundment. At a landfill I worked on, we took the sediment and spread it on the top deck to augment the soil, but luckily we had no issues of poor quality sediment.
The HydroCad computer code promises to help you design a sediment pond. . I have not tried to use their code but it appears to be comprehensive and powerful for a wide range of surface water management facility design and operation issues.
Bentley and Halestad Method Water Solutions came up late in my web searches as a potential source of software for use in designing sediment ponds. They state that you can use their PondPack software for design and modeling of detention ponds. You may want to give them a try.
Best Management Practice handbooks are available from the California Stormwater Quality Association. [A brief aside: some organizations use one word, stormwater, and some use two words, storm water. Here we have an echo of that old fight over one or two words for groundwater. You decide, but I try to follow the site’s use.]
An finally as always, the U.S. Navy excels with the Storm Water Best Management Practices Decision Support Tool
Almost any reputable consultant will undertake to design you a sediment pond. A search of the web yielded the following sites where the consultant claims sediment pond experience:
I cannot vouch for any, but this list covers a wide geographic range and is a sampling of what to look for when you seek out your consultant.
Here are some personal notes and observations about sediment ponds that may help in designing, constructing, and operating you pond.
The climate of the upland area of the pond must be defined. From this one may predict the runoff to the pond, the quantity and rate of inflow, and hence size the pond and the spillway.
It is not always necessary to design a sediment pond to as long a recurrence interval event as a water dam. Establishing design criteria for the pond related to runoff, size, and spillway may take into account the site location, downstream facilities, and the consequences of failure or overtopping. I once was part of a design teams charged with building a sediment pond right above a major freeway. We sized to pond for the probable maximum runoff by including generous outlet pipes, secondary emergency outlet pipes, and a spillway big enough to pass the maximum runoff even if the outlet pipes were clogged. We did this simply because it made no sense to take a chance at pouring more water onto what admittedly would be a flooded freeway and let our client be blamed for nature’s caprice.
In a more remote location where nobody lives or moves downstream in a storm, lesser design criteria may be used. It is a balance of cost and risk and required function that will vary from site to site and jurisdiction to jurisdiction.
Data are required to quantify precipitation and runoff patterns. Such data may come from one or more of many sources, including site measurement records, regional databases—usually the local airport, local and national databases accessible on the web, or synthetically generated data that many computer codes produce.
The runoff rate needs be quantified for the range of design events. More important still is quantification of the sediment load. This involves examining the area from which runoff is generated. How much is bare area, how much is vegetated, what is the density of vegetation, what is the erosion pattern & potential. On a mine a sediment pond may be need for only a few years after placing a facility or at lest until vegetation has established and erosion is securely under control.
On one site I worked on we gave up trying to define these numbers: how much soil would the runoff from a 1.4 horizontal to 1 vertical unvegetated slope generate in the event of the 50-year recurrence interval event. Common observations on site suggested that it could be very little if the rain was spread out over a few hours, but if the event was a downpour rivers of mud would appear. So we decided on the pragmatic approach of making the sediment pond as big as there was space and as deep as we could raise the embankment. Beyond that we were at the mercy of nature.
This is not as cavalier an attitude as it may at first sight seem; after all in most instances the sediment pond is there to deal primarily with the low flow event that brings irritating quantities of sediment down that cumulatively affects downgradient receiving waters. In the gully-washers so much water is flowing that any extra-over from the site may be undetectable or statistically insignificant in impacting downstream surface water facilities.
Groundwater flow patterns and rates may be an issue at the site of a sediment pond if seepage from the pond could impact groundwater quality. In most instances however if water seeping from a pond could negatively impact groundwater quality, then we are dealing with a treatment holding pond, not a simple sediment pond, the only sin of the contents of which are that it contains to much sediment for its own good.
I have lined sediment ponds for the good reason that this was a cost-effect way to control seepage that could otherwise impact the stability of the embankment and lead to uncontrolled downstream springs. But liners in sediment ponds are a nuisance for they get in the way of equipment that must be brought in every so often to clear the sediment out of the pond. Avoid them if you can.
You need the topography of the site and its relationship to upstream and downstream facilities to start the design of the sediment pond. It is seldom wise to place the pond directly in the line of flow in a creek or stream. The best operating ponds I have been involved with have been off stream with low flows diverted into the ponds and released when clean to the main-line flow again. Sometimes the water to the sediment pond is collected in a series of pipes and channels and directed to the sediment pond for settling before being directed into the site’s water management creeks and streams.
Geologist and geotechnical engineers will probably have to be involved to characterize the characteristics of the materials of the site and the embankments that make up the sediment pond. They will want to know about the strength of the soils and foundations to establish embankment geometries that will be stable for sudden in-rushes of water and rapid drawdown conditions associated with rapid emptying of the pond.
They will want to know the hydraulic conductivity of the soils so they can quantify seepage rates and patterns and hence control spring and piping that would otherwise undermine and fail embankments.
I have built sediment ponds on municipal solid waste (refuse, garbage, by any other name). We sought to characterize how much the waste could settle over the years—a lot it turned out. And we sought to characterize how this settlement would affect the embankments—also a lot as it turned out. So we opted for lightly compacted embankments with geogrid reinforcing and geotextiles to control seepage and piping along the inevitable soil cracks in the embankments.
One of the most difficult factors to predict when designing a sediment pond is the settlement characteristics of the incoming waters. Sometimes a simple test involving a 55-gallon drum and a surrogate runoff is enough to establish that there is or is not a sedimentation problem. Sometimes there is no option but to build the pond and see what happens. Obviously, the longer you can hold the water before discharge, the more sedimentation you will achieve. And the stiller you can keep the water the better. If this is not possible, it may be necessary to build baffles into the pond or include a series of overflow weirs. Standard practice in your part of the world and industry and with similar runoff from similar disturbed areas may be the best guide.
Sediments may not settle out in the pond as fast as desired. The particles may be too fine to settle. In such a case it may be necessary to add a flocculent or coagulant to the water to expedite settling. The right chemical will depend on the particulars of the runoff source and the shape and operating characteristics of the pond.
The size of the sediment area may be established by regulations; in the coal mining industry some states establish volume as a function of the runoff area served by the sediment pond. Otherwise you may be constrained by the available area and will make the pond as large as there is space available—the bigger the pond the more likely it is to promote settlement. If space is constrained, you may need to incorporate baffles or extend the holding time to achieve discharge criteria.
Once the sediment is dry, it is best to remove it and take it to a suitable disposal area. For this reason a well designed sediment pond will, in my opinion, have ready access for the mechanical equipment that it is nice to drive into the pond to scoop up the dry sediment and load it on to a truck that also should be able to drive down into the pond. Because of the desirability of being able to drive heavy equipment down into the pond, it is best to have a sturdy base and to cover liners, if used, with layers of soil and riprap sufficient to protect them from the traffic and the cleaning process. I lined my sediment ponds with rock on the basis that even the untrained operator could distinguish between soft fluffy sediment and hard, heavy rock.
The above is but a smattering of the literature that is
available on sediment ponds. Practical
experience and common sense are as important as the right document, consultant,
and computer code. Here on the farm in