Filter Drain - design of side drains, bottom drains, and underdrains Filter Drain Category: flow models - saturated zone Filter Drain DescriptionFilter Drain (FD) is a computer program that allows quick and easy design (or review of design) of a filtration and/or an underdrain system of stormwater retention/detention systems. The stormwater retention/detention system can be ponds, lakes, underground trenches, wetlands, canals, rivers, etc., from which water is discharged via perforated pipes embedded in filter material (i.e., clean fine sand, gravel, natural soil, or other filter material). The Filter Drain program has the following capabilities. Input of initial design parameters, and then graphically review the default Filter Drain dimensions which are automatically selected by the program. Calculation of the required length of Filter Drain system to recover (discharge) a specified retained/detained volume of water from the retention/detention system within a specified period of time. Tabular and graphical review of model results. Generation of report-quality input and output data in a tabular and graphical format. Generation of detailed graphics with design parameters and typical construction specification (which can be edited by the user) suitable for insertion into reports or construction documents. Filter Drain is composed of four separate subprograms which allow modeling of side drains with gravel envelop, side drains without gravel envelop, bottom drains/bench drains and parallel underdrains. The program was developed to design and/or review typical filter drains and underdrains for stormwater retention systems where the retained stormwater must be recovered by artificial means while providing water treatment by filtration. The stormwater retention system can be retention/detention ponds, underground exfiltration trenches, wetlands used as retention systems, etc., from which water is discharged via perforated pipes embedded in filter material (i.e., clean fine sand, gravel or other filter material). However, the analytical approach can also be applied to other systems where filter drains or underdrains are used to dissipate water (i.e., lake water level control, agricultural underdrains, wastewater filtration, biofiltration, etc.). The user can input the required design data through an easy-to-use (interactive screen prompts) format and, within a few minutes, analyze various options and/or optimize the most cost-effective filter drain or underdrain system. This design/optimization tool is useful for design, regulatory review or planning purposes. The program allows the user to input initial design parameters for one of four typical filtration systems for which the program automatically selects initial filter trench dimensions for initial review and consideration. The user is allowed to review the initial design parameters graphically on the screen and adjust them as necessary using graphical techniques. Once the configuration of the filter drain system and the design hydraulic parameters have been adjusted and visually verified, the user can execute the calculations to estimate the required length of the filter drain/underdrain system to recover (discharge) a specified retained/detained volume of water from the retention/detention system within a specified period of time. After execution of calculations, the user can review the results graphically and in tabular format. If the results are acceptable, the user can produce a hard copy of the results in the form of a graph which includes design parameters and calculated length of filter drains/underdrains. Also, a detailed graph with design parameters and typical construction specification (which can be edited by the user), can be produced for insertion into reports or construction documents. The hard copy can be produced on a LaserJet printer, Epson/Citizen printer or in ASCII format. Filter Drain Technical Presentation Filter Drains For the three Filter Drain options, Filter Drain utilizes an iterative analytical approach to calculate the hydraulic grade line along the drain pipe. This iterative approach is necessary due to the cumulative effect of the water inflow and the resulting variable hydraulic head produced for the seepage flow through the filter sand. In the filter drain, the total length of drain pipe/trench system is divided into 1.0-foot increments and the calculation starts at the far end of the discharge side of the pipe. The initial hydraulic head in the pipe at the far end of the discharge side is assumed to be slightly below the water level in the pond (about 1% below the water level) and the drop of head along the pipe is then calculated based on pipe characteristics and cumulative inflow into the pipe. The hydraulic head at the discharge point is then compared to the control water level at the discharge point. If the calculated head does not match the control water level, the initial hydraulic head at the far end of the pipe is adjusted and the calculations are repeated. This iterative process continues until the calculated hydraulic head matches the one specified at the discharge point. The pipe flow is calculated using Manning's equation for full pipe flow. The cumulative inflow rate along the pipe is used to calculate the drop in hydraulic grade line for each 1.00-foot increment. The resulting hydraulic head difference between the water level in the pond and the pipe hydraulic grade line is then used as the effective hydraulic head for the seepage flow through the filter sand. The seepage through filter sand is calculated using Darcy's Law and/or a combination of Darcy's Law and equivalent flow net equations. To simplify the calculation process, the seepage through the filter sand was modeled for numerous typical cross sections using the USGS MODFLOW model (The Modular Three-Dimensional Ground-Water Flow Model by Michael G. McDonald and Arlen W. Harbaugh, 1980). From the results of these model simulations, equipotential lines and flow lines were constructed for typical trench cross sections from which equivalent analytical equations were developed based on Darcy's Law and the flow-net theory. The resulting equivalent equations used for each option were compared with the results of various MODFLOW computer model simulations and indicated that the difference in final results are within plus/minus 2%. Taking into account that a factor of safety of 2.0 or higher will be used for calculations of seepage through the filter sand, the potential error of plus/minus 2% becomes insignificant. These equivalent seepage flow equations were incorporated into Filter Drain. For the side drain cross-section geometry, the change of seepage areas of the trench and the change of seepage lengths for each water level increment analyzed are calculated and adjusted by the program using the initial geometry of the trench specified by the user. It is assumed that the side slope of the side drain trench is uniform and the programs apply a linear adjustment process as the water level drops with each increment. Underdrains The Underdrain portion of the model was developed based on the technical publication by the SJRWMD which requires that any artificial dissipation of retained stormwater be achieved by parallel underdrains installed into the native soil beneath the retention ponds. This requirement is based on a research study conducted by the SJRWMD which indicated that stormwater filtered through filter drains achieve much lower treatment than the stormwater filtered through natural soil with all its biotreatment capacities. Filter Drain incorporates all the design requirements of the referenced publication by the SJRWMD and allows calculation of the underdrain spacing and length, minimum pipe diameter and slope, size and length of header pipes, pipe flow velocity and other design parameters through an easy and interactive data input and model execution. In addition to the design guidelines of this publication, Filter Drain has two enhancements: The model incorporates an iterative process to calculate the effective aquifer thickness for parallel drain spacing calculations based on the experimental data by Hooghoudt (1940), included in Groundwater Hydrology, Bouwer, 1978. The model incorporates calculation of drain spacings under submerged pipe conditions thus allowing design of underdrains with high tailwater level conditions. This makes the model more versatile in its application to varied field conditions and design applications. Requirements: Pentium with 8 MB RAM. Filter Drain Detailed Description Data Input Side Drain With Gravel Pack Option - DATA INPUT The Identification Name/No. prompt line requires specification of a name for the simulation. This is for identification purposes only (it is not a file name). This Name will appear on the tabular and graphical printouts and on graphical view screens. The Elevation of Design High Water Level prompt requires input of the actual or relative elevation of the starting water level for drain recovery calculations. This is typically the top elevation of the retained pollution volume. However, this can be any elevation at which the recovery calculations are started. The Elevation of Design Low Water Level prompt requires input of the actual or relative elevation of the ending water level for drain recovery calculations. This is typically the pond bottom elevation (dry bottom ponds) or the normal water level (wet retention system) of the retained pollution volume. However, this can be any elevation at which the recovery calculations are ended. The Elevation of Pond Bottom prompt requires input of the actual or relative elevation of the pond bottom. This elevation is often the same as the "Elevation of Design Low Water Level," but sometimes it can be different. This value is needed as the side drain trench configuration depends on the location of the pond bottom in relation to the design low water level. The Minimum Separation from Slope prompt requires input of the design minimum separation of the gravel pack from the side slope of the Filter Drain trench. Typically, this minimum value is required by the regulatory agencies (2.0 feet in Florida). The graphical design screen will use this separation distance as one of the restrictive lines for trench geometry configuration. The Invert Elevation of Pipe at Discharge prompt requires input of the actual or relative elevation of the invert of the drain pipe at the point of discharge. This option assumes that the drain pipe is placed horizontally with zero (0) slope and that all pipe flow is driven by the hydraulic pressure created by the infiltrating water. Therefore, the drain pipes in the Filter Drain trench system can be installed on a slope or at zero (0) slope. Regardless of the slope of the drain pipe, if any, the specification for this prompt must be at the point of discharge. The Drain Pipe Inside Diameter and Manning's Coefficient of Drain Pipe prompts are required for flow calculations of water discharge from the drain pipe using Manning's equation of full pipe flow. Manning's Coefficient shall be specified for the ultimate condition of the pipe under long term operation (i.e., corroded, with sedimentation, etc.). The Pond Side Slope prompt is required for calculation of the approximate variation of the incremental pond volume when the time of recovery is being calculated. Therefore, this value should be representative of the average side slope of the pond. For ponds with vertical side walls, this value should be very small (i.e., 0.0001), but not zero. The Permeability of Filter Material prompt allows specification of the actual permeability or hydraulic conductivity of the material used in the filter trench to filter the retained water. This value should not be divided by a factor of safety at this point as the next line of data input allows specification of the appropriate factor of safety. The filter material can vary from fine sand to gravel to manufactured filter material and the permeability will also vary (i.e., permeability for fine sand can be 10 ft/day while for gravel it can be > 150 ft/day). The Factor of Safety for Filter Material prompt allows specification of the factor by which the permeability is expected to be reduced under the long term operation of the system. Typically, filter drains are designed to filter polluted or dirty water and, as such, the filter material tends to clog up with time. For this reason, it is necessary to estimate the expected level of clogging and reduction in permeability so that the filter drain system can be designed with a reasonable assurance that it will continue to perform satisfactorily for the duration of the system's design life. This value should be based on experience and expected quality of water to be filtered. For stormwater retention ponds, this value must be at least 2.0 and possibly higher if extremely dirty water is expected to filter through the system. The Pond Area at Design Low Water Level and the Total Volume of Water to be Filtered input data is used to divide the total volume of water in the pond at 0.1 foot increments uniformly proportioned to the incremental volumes based on the area at low water level, the side slope of the pond, and the total volume of water to be filtered. The model linearly prorates the area of the pond at 0.1 foot increments in elevation between the low water level and the high water level and calculates the volume of water for each 0.1 foot increment. These incremental volumes are then used to calculate the time of recovery in relation to the water elevation in the pond. The Elevation of Water Level at Discharge Point prompt requires specification of the actual or relative elevation of the tailwater at the point of discharge. For submerged pipe discharge conditions, this prompt shall be properly specified, as it will affect the overall calculation. If the drain pipe is not submerged at the point of discharge, this value shall be specified a value of invert elevation of the pipe or lower, and it will not be used in the calculations. The Time of Recovery prompt requires specification of the actual time of recovery desired for the retention system. A factor of safety shall not be applied for the time of recovery in this model as it was already applied for the permeability of the filter material. The factor of safety should be applied only once, and if it is applied here, the factor of safety for the permeability of the filter material shall be set to 1.0. The Number of Equal Sections of Drain prompt allows specification of either a single drain system connected to the outfall structure (as is most of the time) or a drain system that is divided into two equal sections and separately connected to the outfall structure. In some cases, when the single drain is excessively long and hydraulically inefficient, it is better to separate the drain into two equal sections and connect them to the outfall structure, effectively decreasing the length to 1/2 and improving the hydraulic grade line of the overall drain system. Therefore, if the drain is to be a single system, 1 shall be specified for this prompt, and if it is to be separated into two equal sections, 2 shall be specified. Side Drain without Gravel Pack Option - DATA INPUT Descriptions are the same as those above. Bottom Drain with Gravel Pack Option - DATA INPUT The explanations for all data input fields except for Minimum Separation from Pond Bottom of the first screen prompt for this option are the same as for the Side Drain with Gravel Pack Option. The Minimum Separation from Pond Bottom prompt allows specification of the distance between the actual pond bottom, or the top of the filter bed or the top of the bench drain and the top of the gravel pack of the drain pipe. This is equivalent to the separation from side slope of the Side Filter Drain systems . Underdrain Option - DATA INPUT (1 of 2) The Identification Name/No. prompt line requires specification of a name for the simulation. This is for identification purposes only (it is not a file name). This Name will appear on the tabular and graphical printouts and on the graphical view screens. The Area of Pond Bottom Input Fata is needed for determination of the approximate dimensions of the pond or other water retention system from which water will be dissipated by the underdrain system. The pond bottom area will be divided by the Average Pond Bottom Length to estimate the average pond bottom width for determination of the total number of underdrains needed given the calculated drain spacing and the design criteria assumed herein. The Total Volume of Water to be Filtered input data is needed to calculate the average pond length and width and to determine the total discharge rate from the system for header pipe sizing purposes. This volume shall be calculated based on the actual volume of the pond between the pond bottom and the design high water level. If the underdrains are for lake control or another system, the volume must be specified for the actual volume to be recovered in the specified period of time (Time of Recovery). The Average Pond Bottom Length input data is used to calculate the average width of the pond for determination of the total number of underdrains needed given the calculated drain spacing and the design criteria assumed herein. The Elevation of Pond Bottom prompt requires input of the actual or relative elevation of the pond bottom or low water level of the lake/wetland system. This value is needed to determine the total stage of water to be recovered by the underdrain system. The Elevation of High Water Level prompt requires input of the actual or relative elevation of the starting water level for drain recovery calculations. This is typically the top elevation of the retained pollution volume or the peak stage of the pond. However, this can be any elevation at which the recovery calculations are started. The Pond Freeboard prompt requires input of the design freeboard of the pond or lake system only if it forms part of the recovery volume. Typically, if a pond is designed with a weir overflow and the underdrain system is to recover only the volume below the weir, the Elevation of the High Water Level would be specified at the invert of the weir and the freeboard would be set at zero (0). However, for retention systems without surface discharge, it may be desirable to include the freeboard as part of the potential volume of water to be recovered. The Time of Recovery prompt requires specification of the actual time of recovery desired for the retention system. A factor of safety shall not be applied for the time of recovery as it will be specified separately in the subsequent prompt. The time of recovery specified herein will be divided by the factor of safety and used to calculate the average vertical recharge rate for calculation of the underdrain spacing. The Factor of Safety for Recovery Time prompt allows specification of the factor by which the Time of Recovery would be reduced for calculation of the rate of vertical recharge and determination of the underdrain spacing. Typically, underdrains are designed for long term operation and some pipe clogging and inaccurate estimation of aquifer hydraulic parameters may result in under-design of the drain spacing. For this reason, it is necessary to estimate the expected uncertainty of the overall design and specify an appropriate factor of safety. This value should be based on experience and the reliability of the data used for design. For stormwater retention ponds, this value should be at least 2.0 and possibly higher. The Average Vertical Hydraulic Conductivity prompt allows specification of the actual hydraulic conductivity (permeability) of the material present between the pond bottom and the top of parallel drains. This value should not be divided by a factor of safety at this point as the next line of data input allows specification of the appropriate factor of safety. The natural or artificial material can vary from natural fine sand to gravel to manufactured filter material and the permeability will also vary (i.e., permeability for fine sand can be 10 ft/day while for gravel it can be > 150 ft/day). The Factor of Safety for Kv prompt allows specification of the factor by which the vertical permeability will be reduced under the long term operation of the system. Typically, underdrains are installed below the pond bottom and the material above the drains acts as a filter for the polluted or dirty water and, as a result, the material tends to clog up with time. For this reason, it is necessary to estimate the expected level of clogging and reduction in permeability so that the underdrain system can be designed with a reasonable assurance that it will continue to perform satisfactorily for the duration of the design life. This value should be based on experience and expected quality of water to be filtered. For stormwater retention ponds, this value must be at least 2.0 and possibly higher if extremely dirty water is expected to discharge into the pond. The F1 = Help for Input Parameters prompt allows the display of a graphical HELP menu for the input design parameters. By pressing the F1 key, the first screen prompt will appear with four small graphics that illustrate the location of various input parameters for better understanding of the overall system set up and assumptions. To display the second page of the graphical help screen, the PgDn key must be pressed. To toggle between the two graphical screen prompts, use the PgDn and PgUp keys. Underdrain Option - DATA INPUT (2 of 2) The Average Horizontal Hydraulic Conductivity prompt allows input of the hydraulic conductivity for in-place soil that exists (or will exist) between the pond bottom and the underlying impervious boundary (i.e., the top of the first poorly permeable soil layer such as hardpan, sandy clay, clay, etc.). This value can be field measured as an average value for the entire effective aquifer thickness or tested in the laboratory using undisturbed tube samples collected at specific depths and then averaged for the total thickness. NOTE: This value and the elevation of the impervious layer will be the two parameters that affect most the resulting underdrain spacing. The Elevation of Top of Impervious Layer prompt allows specification of the actual elevation of the top of the first restrictive soil layer, such as hardpan, sandy clay, clay, rock, etc. The program automatically adjusts the effective thickness of the aquifer (i.e., the depth below the center of the drain pipes), based on research data by Hooghoudt (1940), presented on page 296, Bouwer (1978). The actual elevation of the impervious layer shall be obtained from site-specific soil borings or other tests which identify the depth of the first restrictive soil layer. NOTE: This value and the average hydraulic conductivity will be the two parameters that affect most, the resulting underdrain spacing. The Diameter of Drains, the Selected Slope of Drains and the Manning's Coefficient of Drain Pipes prompts are required for flow calculations of water discharge from the underdrain pipes using Manning's equation of full pipe flow (referenced earlier). The diameter is indicated as Optional input because the program will increase the diameter, if it is necessary, based on the total rate of water that must be discharged through each pipe. If the specified diameter is sufficient (or oversized), the program will maintain the specified diameter without decreasing it. Additionally, the program will set the initial diameter of the headers to be the same as the diameter of the lateral underdrains and increase it as necessary for the design flow through the drains. The capacity of the pipe flow, both underdrains and headers, is calculated using the selected slope of the drains. The specified "selected slope of the drains" is applied to both the parallel underdrains and the headers. Manning's coefficient shall be specified for the ultimate condition of the pipe under long-term operation (i.e., corroded, with sedimentation, etc.). The Separation from Top of Drain Trench to Pond Bottom prompt allows specification of the average distance between the bottom of the pond (or infiltrating surface) and the top of the gravel envelop (trench) of the parallel underdrains. This separation is typically about 2.0 feet for stormwater retention ponds, but could be any value in this model. The second screen of "F1 - Help for Input Parameters" provides a graphical view of this separation distance. The Separation from Mound between Drains to Pond Bottom prompt allows specification of the average distance between the desired groundwater mound (elevation of groundwater surface) at midpoint between parallel underdrains and the bottom of the pond (or infiltrating surface). This separation is typically about 0.5 feet for stormwater retention ponds but could be any value in this model. The second screen of "F1 - Help for Input Parameters" provides a graphical view of this separation distance. The Elevation of Water Level at Discharge Point prompt requires specification of the actual or relative elevation of the tailwater at the point of discharge. For submerged pipe discharge conditions, this prompt shall be properly specified as it will affect the overall calculation. If the drain pipe is not submerged at the point of discharge, this value shall be equal to the invert elevation of the pipe or lower, and it will not be used in the calculations. When this value is set at higher than the center of the underdrain pipe, the calculations of drain spacing assume that the mounding between drains and horizontal flow is controlled by this elevation and not the elevation of the underdrain pipes. The Number of Headers (1 or 2) prompt allows specification of either a single header system that collects all water from the parallel underdrains, or a dual header system, each collecting 1/2 of the total flow from the parallel underdrains and separately connected to the outfall structure. In some cases, the total flow is excessively high and a single header system is hydraulically inefficient, and it is better to separate the header into two equal sections. The two sections can be connected to the same outfall structure effectively decreasing the required diameter of the headers due to the decrease of total flow through each header pipe. Therefore, for a single header system, "1" shall be specified for this prompt, and for two equal sections of headers, "2" shall be specified. CALCULATION Once the input data has been satisfactorily specified and reviewed under the EDIT option (this applies to all drain/underdrain options), the CALCULATE option can be selected. Selecting CALCULATE, the program executes the appropriate calculations and saves the results in a temporary file for viewing. For the Filter Drain options, the calculations can be lengthy and a small window appears showing the Iteration Number, the selected length of drain, and the calculated time of recovery. This window update allows the user to view the progress of the calculations, and if the calculations are taking too long or conversing to an unfavorable result, the user can stop the calculations, by pressing the Esc key. For the underdrain option, the calculations are very brief and a status window does not appear. Once the calculations are completed, the highlighting prompt returns to the EDIT option. The user can now view the results in several forms. The results can be viewed under the DESIGN MODULE via the GRAPHIC option or under the RESULTS MODULE via the VIEW and/or the GRAPHIC options. The details of viewing and printing the result are discussed in the User's Guide. GRAPHICS The GRAPHIC option allows the display of a typical graphical detail with basic construction specifications, which can be reviewed on the screen and modifications can be made to the three-line default construction specifications. The graphical results presented in this option are intended to be used as general guidelines and supporting documentation as to the dimensions and geometric configuration of the filter drains or underdrains analyses designed by the FDModel. Additional construction details and specifications may be needed for proper construction of the designed system. This graphical display of the results is available for all four Filter Drain/underdrain options. RESULTS The VIEW option in the RESULTS MODULE allows screen display of the latest calculated results in a tabular format. This option can be selected before or after execution of the CALCULATE option in the DESIGN MODULE. The RESULTS MODULE always reads the data stored in the latest temporary results data file which is created by the latest run of the CALCULATE option. The GRAPHIC option in the RESULTS MODULE allows screen display and hard copy printouts of the latest calculated results in a graphical format. The GRAPHIC option is available only for the three Filter Drain options as it is not appropriate for the underdrain option. Similar to the VIEW option, this option can be selected before of after execution of the CALCULATE option in the DESIGN MODULE. The GRAPHICS option always reads the data stored in the latest temporary results data file which is created by the latest run of the CALCULATE option. The graphical display presents the same results as the tabular display which can be reviewed and evaluated on the screen and printed on a compatible printer. When the graphic is displayed on the screen, a bar menu appears at the bottom of the graphic for printing options. The bar menu allows selection of three types of printing devices, namely, Epson FX850/Citizen Printers or equivalent, HP LaserJet Printers (must have capacity to print letter type Univers) and Print Screen. The Print Screen option is intended to allow printing with printers that are not compatible with the optional printers provided. For the Print Screen option, it is necessary to first load the appropriate DOS GRAPHIC command prior to starting the FDModel.
Filter Drain Overview |
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