Because most landfill sites in the United States are exposed to precipitation, owners/operators must take the initiative to manage stormwater and the leachate that it generates.
The Subtitle D regulations require that owners/operators of municipal solid waste landfills design, construct and maintain:
* A run-on (stormwater coming from the surrounding areas) control system to prevent flow onto the active portion of the landfill during peak discharge from a 25-year storm; and
* A run-off (stormwater from the landfill surface) control system from the active portion of the landfill to collect and control at the least the water volume resulting from a 24-hour, 25-year storm.
Developing a stormwater management strategy requires an understanding of the hydrologic cycle in nature (see Figure 1). The water holding elements of the cycle are: atmosphere; vegetation; snowpack and ice caps; land surface; soil; streams, lakes and rivers; aquifers; and oceans.
The liquid-transport phases of the hydrologic cycle are: atmospheric precipitation onto land surface; throughfall from plants; melting snow and ice; land surface run-off to streams, lakes and rivers, and from streams, lakes and rivers to oceans; infiltration from the land surface to the soil; exfiltration from the soil to the land surface; interflow from the soil to streams, lakes and rivers and vice versa; percolation from the soil to the aquifers; capillary rise from the aquifers to the soil; groundwater flow from streams, lakes and rivers to aquifers and vice versa; and groundwater flow from aquifers to oceans and vice versa.
Vapor-transport phases of the hydrologic cycle are: evaporation from the land, streams, lakes, rivers and oceans to the atmosphere; evapotranspiration from plants to the atmosphere; sublimation from snow and ice to the atmosphere; and vapor diffusion from the soil to the land surface.
Four things can happen to precipitation at landfills (see Figure 2 on page 42). It can become surface run-off, evaporate, infiltrate into the cover material and be extracted by plant transpiration or infiltrate into the refuse, where it eventually may become leachate.
In order to minimize leachate, surface water must be re-moved quickly, before it infiltrates the landfill. The longer that surface water remains on the landfill, the more the infiltration.
Landfill Hydrology Landfills are usually located in a watershed or catchment. The owner/ operator should have a thorough understanding of the site's hydrologic conditions, which can be disturbed by the landfill; removal of existing vegetation and alteration of the natural slopes and soils can cause hydrologic changes. Owners/operators should understand climate, land uses, cover type, slopes and soil types at the site. Anticipating the hydrologic effects of the landfill will help in estimating the stormwater or surface water run-off quantities.
Owners/operators should use daily landfill cover in order to prevent the refuse chemical constituents from entering stormwater run-off. Land-fills accelerate erosion and increase sediments which are eventually transported to streams and rivers by stormwater, thereby affecting the water's physical quality. Owners/operators should implement an erosion and sediment control plan.
A common method to estimate stormwater run-off is the rational formula, which estimates peak discharge from small drainage areas or watersheds. This method traditionally is used to size storm sewers, channels and other drainage structures which handle run-off from areas of less than 200 acres.
The rational formula is: Q=cia, where Q is the peak rate of run-off in cubic feet per second; c is the run-off coefficient (an empirical coefficient representing a relationship between rainfall and run-off); i is the average intensity of rainfall for the time of concentration for a selected design storm; and a is the drainage area in acres.
The rational method has some limitations that affect its accuracy:
* Watershed characteristics should be fairly homogeneous - otherwise another method should be selected;
* The method is less accurate for larger areas and is not recommended for use with watersheds that are larger than 200 acres in size;
* The method becomes more accurate as the amount of impervious surface increases; and
* The method assumes that a rainfall equal to the time of concentration results in the greatest peak discharge.
Time of concentration is the time required for run-off to flow from the most hydraulically remote part of the watershed to the point of analysis. There are three types of flow: overland or sheet flow, shallow concentrated flow and channel flow. Recommended maximum length is 300 feet for sheet flow and 1,000 feet for shallow concentrated flow. Chan-nel flow occurs when flow converges in gullies, ditches and natural or manmade water conveyances (in-cluding pipes not running full).
There are four common methods to calculate time of concentration and its components: the seelye method, the kinematic wave method, the soil conservation service method and Manning's method. These are explained in most hydrology books.
The accuracy of the rational method depends upon the judgment and experience of the user. The user must select the appropriate run-off coefficient(s) and determine the time of concentrations based on the development plan (including proposed hydrologic changes) and experience with landfill development and hydrology.
Typical run-off coefficients for developed landfills range between 0.4 to 0.6 depending on cover soil permeability, degree of compaction, slopes, vegetation type and density and the design storm chosen.
Another common way to estimate stormwater run-off is the graphical peak discharge method developed by the United States Department of Agriculture-Soil Conservation Service (SCS). It is described in SCS Technical Release No. 55 (TR55), Urban Hydrology for Small Watersheds. This method of run-off calculation yields a total run-off volume as well as a peak discharge. It takes into consideration infiltration rates of soils, as well as land cover and other losses to obtain the net run-off.
As with the rational formula, the graphical peak discharge method is an empirical model and its accuracy depends upon the user's judgment. This method is widely used by engineers and is available in a computer program.
First, the drainage area is measured. Then a curve number (similar to a run-off coefficient) is calculated. Curve numbers depend on soil type and hydrologic group. Subsequently, run-off depth and volume is determined for a certain design storm, after which time of concentration is calculated. Then initial abstraction is determined to account for all losses that occur before run-off begins. Following abstraction estimates, the unit peak discharge is calculated and then adjusted based on swampy conditions in the watershed. Typical curve number values for landfills range between 77 and 94, assuming no vegetation.
Other hydrologic techniques to estimate run-off are available, and users must evaluate the assumptions and limitations of each. Users should apply at least two methods of run-off measurement and compare the results to check the accuracy of the estimates.
With the information provided by the run-off estimates, a stormwater management system can be de-signed. A complete system includes run-off control, run-on control and erosion and sediment control. Both run-on and run-off can be controlled similarly.
Ditch Construction Drainage ditches should be constructed to divert run-on water away from the active landfill. Run-off needs to be routed to drainage ditches and then toward a sedimentation basin. When the entire landfill liner system is installed at one time, the stormwater falling on inactive cells must be diverted. When only run-off from the landfill must be removed, drainage facilities should minimize the distance that the surface water travels. In many cases, interception ditches are used to route water to a larger main ditch located along the perimeter of the site.
Ditches running over the landfill should have low base slopes (10 percent maximum) to minimize erosion. The following formula, known as Manning's Formula, is used to design a ditch section:
V = 1.486/n X r [superscript]2/3s[superscript]1/2
where v is the mean velocity; r is the mean hydraulic radius in feet (ob-tained by dividing the cross-sectional area by the wetted perimeter); s is the slope of the energy line (approximately equal to the slope of ditch for small slopes) in feet per feet; and n is roughness coefficient (dimensionless).
The greatest difficulty in applying Manning's Formula is choosing the proper roughness coefficients. However, typical values can be found in most hydraulics books. Applying Manning's Formula involves assuming a typical ditch section (usually triangular or trapezoidal), checking its ability to carry the design flow and checking whether the velocity is within the maximum permissible value of the ditch's natural lining material. A freeboard of three to six inches is usually added to the ditch depth to account for future reduction of ditch hydraulic capacity.
If velocities of flow exceed permissible values, designers can install liners to protect the drainage ditch. Linings can be rock riprap, erosion matting, vegetative lining and concrete, depending on the velocity of flow and the cost effectiveness of each liner.
In addition to drainage ditches, culverts and stormwater basins are used to manage stormwater. Culverts, which mostly are used to drain water below access roads, can be circular or rectangular and are made from concrete, metal or poly-ethylene plastic, depending on the cover height, desired service life, cost effectiveness and hydraulics.
For the long term, concrete culverts are preferred over metal or plastic culverts, which need to be replaced more often. A culvert should be overdesigned, because in most cases long-term maintenance is not expected.
A culvert can flow full or partly full. Flow depends on inlet geometry, slope, size, roughness, approach, tailwater condition, etc. The use of nomograph solutions is recommended for high design flow. However, 18- to 24-inch circular section culverts with a minimum slope of 1 percent can be used safely for flows up to 10 cubic feet per second.
Basin Planning Stormwater basins contain the diverted flows and minimize downstream flooding. They are designed to reduce the peak run-off discharge of post-landfill development by providing storage and sizing the principal spillway pipe or outlet and allowing the pre-development discharge to flow out slowly through the principal spillway. Stormwater basins have emergency spillways (usually rock-lined, earthen weirs) that can pass the peak discharge of a 100-year storm.
Stormwater basins reduce the total suspended solids or sediments from the surface water before they enter natural drainways. To estimate the storage volume needed, the owner/operator must calculate expected soil loss from the landfill. Musgrave's equation or the universal soil loss equation can be used to estimate soil loss from the site, while Stoke's equation can be used to estimate settling velocity of soil particles and basin area. A minimum length to width ratio of 2:1 should be used to size the basin area; otherwise baffles will have to be installed.
If the basin has a self-dewatering device, it must provide a minimum detention time (40 hours for most sediments) for stormwater. Longer detention times may be required if chemical pollutants or fine sediment particles are in the surface water.
In non self-dewatering basins, accumulated sediment must be cleaned out periodically to maintain the basin's hydraulic capacity. This process can be incorporated into the overall stormwater management maintenance program. Standard procedures are available in most hydrology books.
Many of the techniques used to control run-off and/or run-on also assist in erosion and sediment control. For example, a stormwater basin reduces the ability of water to carry suspended materials by de-creasing flow velocity, thus enhancing deposition. Other devices use the inherent energy of the sediment-laden flow to separate the sediment. Controls such as silt fences also reduce flow velocity and affect deposition. Decreasing the erosion of topsoil or cover materials by compaction, chemical stabilization, revegetation and mulching also have proved effective for erosion and sediment control.
In general, the following recommendations can help control erosion and manage stormwater erosion effectively:
* Avoid, if possible, siting landfills in wetlands or in the 100-year flood plain of adjacent rivers or streams;
* Define existing and intermittent flow channels and the area and characteristics of the contributing watershed;
* Schedule landfilling operations to minimize disturbed areas (i.e., phase-in operations);
* Attempt to limit the handling of topsoil or cover materials to only one operation;
* Construct and stabilize controls in advance of landfilling;
* Stop flowing water from entering the active fill area with permanent perimeter diversions;
* Establish a complete sequence of controls (i.e. interception, conveyance, transportation, energy dissipation and sediment disposition);
* Use stormwater detention to improve the quality and reduce the intensity of stormwater;
* Plan for the National Pollutant Discharge Elimination System (NP-DES) stormwater permit by keeping good operations records and monitoring discharges off the site;
* Avoid, if possible, combining leachate and stormwater management systems;
* Use a surface water collection/ removal (SWCR) system immediately above the hydraulic barrier in the final closure cap. Geonets, geocomposites and/or granular material can be used for this purpose;
* Inspect and maintain controls after each significant storm; and
* Integrate run-off/run-on and erosion and sediment control into every phase of the landfill operation.
Optimizing The System Landfill operators should perform each stormwater management task at the lowest possible cost. Simple day-to-day tasks such as maintaining an access road culvert can be high-cost items when viewed over a facility's life and post-closure care period. For example, installing an access road culvert at its ideal grade line can achieve the shortest length, make replacement simpler and cut down on siltation and downstream scour.
The owner/operator should evaluate the initial cost of a stormwater control against its long-term maintenance cost. For example, both rock riprap and geosynthetic erosion matting are used to line drainage ditches. Rock riprap costs more to install than erosion matting; however, riprap costs less to maintain.
When possible, the owner/operator should install less expensive temporary control products instead of more expensive permanent control products, particularly when the landfill is developed in phases with changing drainage patterns and changing access routes. For example, a less expensive corrugated metal pipe or polyethylene pipe instead of a reinforced concrete pipe can be installed underneath a temporary landfill access road. This is especially true when the pipe may not be reused, when the application service life is short and when the cover on the pipe is not excessively high.
The owner/operator needs to keep good inspection and maintenance records and review them from time to time to evaluate the performance and cost effectiveness of drainage products before purchasing them again.
New Regulations With the 1990 passage of the EPA's final stormwater rule under the National Pollutant Discharge Elimination System, stormwater management and best available control technology became crucial for owners/operators. Both operating and closed landfills may need to be reviewed for permit application needs, so site owners/operators should:
* Plan stormwater control strategies;
* Install stormwater controls such as diversions well ahead of actual operations so that vegetation can be established;
* Keep all stormwater management activities up to date;
* Expose and work as small an area as practical;
* Control water quality and quantity on the site;
* Use good engineering practice in designing stormwater controls; and
* Implement an aggressive inspection and maintenance program.