DURMM: THE DELAWARE URBAN RUNOFF MANAGEMENT MODEL

Transcription

1 DURMM: THE DELAWARE URBAN RUNOFF MANAGEMENT MODEL A USERS MANUAL FOR DESIGNING GREEN TECHNOLOGY BMPs TO MINIMIZE STORMWATER IMPACTS FROM LAND DEVELOPMENT William C. Lucas INTEGRATED LAND MANAGEMENT, INC. Prepared For Delaware Department of Natural Resources And Environmental Control Division of Soil And Water Conservation JANUARY 2004

2 Preface TABLE OF CONTENTS PREFACE ii PAGE CHAPTER 1 URBAN RUNOFF HYDROLOGY AND HYDRAULICS 1.1 DURMM MODEL OVERVIEW DURMM LAND COVER/AREA INPUT TIME OF CONCENTRATION- SHEET FLOW TIME OF CONCENTRATION- OVERLAND SWALE FLOW STORAGE ROUTING CHAPTER 2 BMP POLLUTANT REMOVAL 2.1 RUNOFF POLLUTANT ROUTING REDUCTION KINETICS FILTER STRIP BMPS BIOFILTRATION SWALE BMPS BIORETENTION BMPS INFILTRATION TRENCH BMPS CHAPTER 3 DURMM EXAMPLE- PRE-DEVELOPMENT CONDITIONS CHAPTER 4 DURMM EXAMPLE- POST-DEVELOPMENT CONDITIONS LIST OF FIGURES FIGURE 1: DURMM MODEL RUNOFF AND BMP ROUTING DIAGRAM. 1-3 FIGURE 2: EXAMPLE SITE- PRE-DEVELOPMENT CONDITIONS FIGURE 3: EXAMPLE SITE- POST-DEVELOPMENT CONDITIONS 4-2 APPENDIX A USERS GUIDE APPENDIX B STEP BY STEP GUIDE APPENDICES JANUARY 2004 i

3 Preface PREFACE This Manual is written as the companion document to Green Technology: The Delaware Urban Runoff Management Approach, a Technical Manual for Designing Nonstructural BMPs to Minimize Stormwater Impacts from Land Development. The intent in this Users Manual is to provide guidance on how to use the DURMM Model that is referenced in the Technical Manual. It is strongly recommended that the reader refer to the Technical Manual for background and discussion on the elements addressed in this Manual. In this way, the designer will gain a better appreciation of the issues and processes underlying the model. In this manner, the user will have a better understanding of the implications of design parameters, and thus be prepared to utilize Green Technology BMPs most effectively. The Users Manual starts with a condensed summary of the processes involved in urban runoff hydrology, hydraulics, pollutant loading, and their removal by BMPs. It is essential that the user understand these concepts, and how DURMM applies them, in order to use the model effectively. For this reason, the user must read these sections before going directly to the Chapters on model input parameters and output results. JANUARY 2004 ii

4 Chapter 1 CHAPTER 1 URBAN RUNOFF HYDROLOGY AND HYDRAULICS 1.1 DURMM MODEL OVERVIEW The Green Technology approach incorporates the DURMM spreadsheet model to assist in BMP design. This overview is intended for users of the DURMM spreadsheet model that are unfamiliar with the technical background set forth in the Green Technology Technical Manual. Since DURMM explicitly incorporates elements of conservation design not addressed by normal engineering practices, the user must become familiar with the concepts and computations underlying DURMM. It is essential that these concepts be understood so DURMM can be used properly to generate the most appropriate designs. The fundamental elements of DURMM examined in the Technical Manual are briefly discussed in Chapters 1 and 2. The perceptive designer will realize that Green Technology can both save time and construction costs, while providing considerable environmental benefits when compared to typical practices. The hydrological processes incorporated into the DURMM Model recognize the infiltration/interception contributions of soils as affected by soil type, land cover characteristics, as well as how runoff volumes respond to rainfall amounts. The hydraulic design component routes runoff through swales, and partitions overland discharge from infiltration components. DURMM disaggregates different combinations of land cover and soil type, based on the Curve Number (CN) method set forth in TR-20, the most widely used hydrology/hydraulics program. As TR-20 is required in Delaware for the design of structural BMPs, DURMM uses the same algorithms included in TR-20 for projecting runoff from larger storms. However, for modeling hydrology of small urban watersheds under smaller storm events, the TR-20 method has shortcomings that substantially under-predict the hydrological contribution from lawns and landscaping that comprise urban pervious areas. For these reasons, TR-20 is only used for large magnitude runoff events over 5.0 inches. For smaller events, TR-20 is modified in DURMM to use an infiltration approach, similar to that incorporated in the SLAMM model. Described in more detail in the Technical Manual, this relationship provides more accurate results. The DURMM routines are applied to all graded pervious areas that are disturbed during site development. These areas are classified as graded pervious areas. To better address the potential of conservation design that minimizes the footprint of development; TR-20 routines are used to generate the runoff volumes from those areas where an intact soil profile is not disturbed during site development. When planted in woodland or meadow, these areas will generate very little runoff compared to lawns or landscaping planted in disturbed areas. These areas are classified as natural pervious areas. An important point also raised by the SLAMM model is that impervious runoff CNs are by no means exactly 98, as assumed in TR-20. Impervious surfaces have differing amounts of infiltration and depression storage, depending upon edge/area ratio, surface roughness and slope. The latter is particularly important in the case of roofs. Pitched roofs or extensive paving such as parking lots (which permit little infiltration at the edges) have a high CN of 99.7, while flat roofs have a CN of 98.1, and narrow streets with average roughness have a CN of (Smooth streets have a higher CN, and old rough streets have a lower CN.) When calculating the relative JANUARY

5 Chapter 1 contributions of runoff from these differing types of impervious surfaces, fine differences between these values are important at low rainfall depths. DURMM accounts for these differences and eliminates the rounding error introduced by using a uniform value of 98 for impervious surfaces. Many design manuals and proprietary TR-20 software programs average the CN over both the pervious and impervious surfaces, unless routed as separate subareas during design. However, this approach can lead to substantial errors in runoff volumes, especially during small rainfall events. In contrast, DURMM not only has more accurate algorithms for calculating pervious and impervious runoff as a function of rainfall, it calculates runoff volumes from pervious and impervious surfaces separately. Segregating the runoff from pervious and impervious surfaces permits DURMM to project the effects of impervious area disconnection. DURMM allocates runoff from disconnected impervious areas as excess precipitation onto the receiving pervious surfaces. Upon entering the percentage of impervious surfaces that are disconnected, and entering the wetted area and CN of receiving pervious surfaces, DURMM then calculates the excess precipitation onto, and the resultant runoff volumes from, the receiving pervious surfaces. Where enough pervious surfaces are available, runoff from these pervious surfaces can be substantially less than the sum of runoff from the pervious area and impervious areas without disconnection. Receiving wetted pervious surfaces must be located below flow spreaders or in the conveyance path. In low-density sites where receiving pervious surfaces have highly permeable soils, disconnection can substantially reduce runoff during the quality storm. This feature of DURMM provides a quantitative process-based approach to project the effects of disconnection, which has hitherto been lacking in any design approach. As the design basis for the impervious area disconnection BMP, it provides the designer with a powerful tool to quantify the benefits of integrating site planning with drainage design. Combined with measures to decrease CN by minimizing impervious areas and increasing afforestation, and thus reducing runoff volumes, conservation site design becomes an important nonstructural BMP. The potential for incorporating impervious area disconnection is a key element in the design of nonstructural BMPs. By using overland conveyance of runoff wherever possible, not only does overland flow reduce volumes and peak rates, it also permits explicit methods for designing BMPs by disconnecting impervious areas and providing infiltration as an integral part of the design process. Furthermore, there is substantial potential for removal of pollutants when runoff is conveyed through properly designed swales and filter strips. DURMM also incorporates disconnection routines to compute the reduction in runoff from the wetted area of filters, bioretention and bioswale BMPs. After accounting for disconnection, the resulting total runoff volume is used for generating event peak flows according to TR-55 routines. Figure 1 displays the how DURMM routes runoff over pervious surfaces to reduce volumes. Note how DURMM provides for several different BMPs within the routed drainage area. This is necessary since concentrated flow from roofs and streets may not be amenable to filtering by filter strips, but would be amenable to filtering in bioswales or bioretention facilities. JANUARY

6 Chapter 1 Figure 1: DURMM Model Runoff and BMP Routing Diagram JANUARY

7 Chapter DURMM LAND COVER/AREA INPUT Developed sites are a complex mosaic of pervious and impervious surfaces. For pervious cover input, the individual area of each disturbed land cover/soil group complex is entered into the graded pervious columns. Land cover is entered by keying in a cover code according to the table shown in the spreadsheet, along with its corresponding hydrologic soil group letter. Once all entries are complete, DURMM integrates the area and curve number of various complexes to develop the weighted average curve number for the graded pervious areas. Note that most BMPs will be located in the graded pervious areas, since they are either located close to the impervious source areas, or involve grading in their construction. A BMP can be allocated as being in natural pervious area only where no disturbance is involved in their construction. Conservation Site Design principles can be used to minimize the area of disturbance. Where the design includes a mechanism to ensure that meadow or woodland will be established in undisturbed areas, such areas can be classified as pervious natural areas. As above, the individual area of each natural land cover/soil group complex is entered into the natural pervious columns, and DURMM develops the weighted average curve number for the natural pervious areas. For impervious surfaces, DURMM requires that the user calculate the extent of more distant disconnected impervious surfaces as opposed to the impervious surfaces that discharge directly to the BMP(s). If they comprise a substantial proportion of the site, the disconnected impervious areas will control the Tc that establishes peak flows. These surfaces are usually roofs, streets and parking lots, from which runoff can be filtered as it passes over lawns and is conveyed through bioswales. As shown in figure 1, DURMM segregates runoff from these disconnected impervious areas so it can be routed over disconnected pervious surfaces during conveyance to the BMP. The percentage of impervious surfaces that are disconnected is entered separately, along with the dimensions and curve number of the receiving pervious area, along with the percentage of the flow path that is disconnected. DURMM incorporates disconnection routines that compute the reduction in runoff from these disconnected impervious areas according to the wetted area and curve number of the pervious area. More detailed explanation of the individual area entry cells are described in the example reviewed in Chapters 3 and TIME OF CONCENTRATION-SHEET FLOW Time of Concentration (Tc) determines the peak runoff rate for a given runoff volume. DURMM allocates Tc as a combination of sheet flow and shallow concentrated flow through swales. Explicit calculations of channel flow are omitted since its travel time is typically negligible on the smaller sites for which DURMM is intended. Sheet flow is calculated according to the equation employed in TR-20, using a Manning s Roughness coefficient n of 0.45 for woodland or meadow, 0.24 for dense turf, 0.15 for short turf/or fair conditions, for gravel or stone, and for pavement. However, unlike TR- 20, the precipitation depth P that establishes sheet flow velocity is based on the event depth, not just the two-year event. Therefore, sheet flow Tc decreases as P increases. This provides a more JANUARY

8 Chapter 1 accurate representation of the dynamics of the runoff event. DURMM provides for two consecutive segments of sheet flow to model complex flow paths; for instance, where sheet flow from a parking lot then passes through a filter strip. Since pervious sheet flow travel time has such an influence on Tc, and the corresponding peak flow rates, designers often use pervious sheet flow paths as a mechanism to increase Tc so as to reduce peak rates, even where the relevant pervious area contribution to total runoff may be minimal. By differentiating the runoff contributions from various source areas, DURMM routines determine whether the contribution from impervious surfaces exceed that from the pervious areas. In this event, the path should originate from the impervious area, since its runoff controls the runoff response. Tc paths originating as sheet flow over pervious areas would clearly be inappropriate in this case. More detailed explanation of the individual sheet flow entry cells are described in the example reviewed in Chapters 3 and TIME OF CONCENTRATION-OVERLAND SWALE FLOW While Tc determines the peak rate, the shallow concentrated conveyance velocities that contribute to Tc are determined by peak runoff rates. Complex interactions between flow depth, channel shape and vegetative retardance affect flow velocities. As a result of this complexity, current practice using TR-20 requires the user s best judgment for manual entry, and/or applies default slope/velocity relationships to estimate shallow concentrated flow velocities. Under typical conditions, the default equation used in TR-20 for unpaved shallow concentrated flow generates velocities faster than that encountered in most swales, giving a smaller Tc than circumstances warrant. While this may partially offset the error of using a longer sheet flow Tc that is also incorrect (see above), compounding such errors do not necessarily make the results more accurate. Because the sheet flow, swale flow, and channel flow velocities that determine peak flow rates are rarely verified in most designs, inaccurate input assumptions will substantially alter results. Where the geometry of the Tc paths differ between pre-development and post-development conditions, inaccurate use of the TR-20 method can introduce substantial errors in the design results. To address this issue, DURMM provides for a more precise method to estimate Tc based upon the swale flow routines described in the Technical Manual. For shallow concentrated flow (or swale flow), the worksheet simultaneously solves for peak flows as a function of Tc, while solving for Tc as a function of peak flows, as affected by the conveyance parameters. The equations from TR-55 are used to project peak flow. Given the runoff volumes and an initial estimate of Tc, the worksheet calculates the peak flow. The worksheet then applies routines to calculate the velocity of swale flow as a function of conveyance channel geometry, slope, vegetative cover, and flow depth. Iteratively applying differing flow depths to the swale, swale geometry is analyzed to develop total discharge and velocity, which provides the total Tc when added to sheet flow Tc. Flow depths are adjusted until the flow velocities used to generate the Tc at the flow discharge matches the discharge rate from TR-55 using the Tc from the swale flow calculations. JANUARY

9 Chapter 1 DURMM requires entry of the appropriate values for the surface codes of the swales. Dense turf corresponds to Surface Code 4 (or retardance "D", the Biofiltration Function in Horner s method). The short grass curve corresponds to Surface Code 3 (or Retardance "E"), representing retardance before grass is well established (the Stability Function). To account for higher retardance due to poor maintenance, the thick brush curve of Surface Code 5 represents retardance "C" (the Capacity Function). Incorporating values entered for swale geometry, length, slope, and surface code, DURMM determines the flow velocity and depth through the contributing swales. Two consecutive segments of shallow concentrated flow are provided to model more complex flow paths. To better model field conditions, DURMM also requires entry of the proportion of total flow conveyed by the upper and lower segments according to their percentage of total area involved. Details of the hydraulics involved in the swale flow calculations are set forth in the Technical Manual. As in the case of sheet flow, this approach results in decreasing Tc as P increases. However, the relative contribution of swale flow to total Tc becomes greater than that of sheet flow. This reduces the typical dominance of Tc by often over-estimated, and generally unreliable, sheet flow computations. By generating a different Tc for each storm event, the worksheet thus generates the Tc needed for calculating peak flows for the bank protection, conveyance and flooding events. It should be noted that this results in higher peak flows for the 10 and 100 year events when compared to TR-20, since TR-20 calculation of Tc is based only upon the 2 year precipitation depth. By designing the conveyance systems to maximize the Tc, the designer can reduce peak flows substantially. This not only reduces the size of, or even requirements for, detention storage, it also provides for the conveyance channels to filter runoff prior to discharge to the BMP(s). If necessary, swale flow routines can approximate channel flows by using the surface code for pavement and a steep sided geometry. More detailed explanation of the individual swale flow entry cells are described in the example reviewed in Chapters 3 and STORAGE ROUTING Where check dams are used, biofiltration swales can provide the required storage for conveyance and flooding detention without the need for a separate pond structure. The simplified TR-55 equation is used to project the required storage volume, based upon the increase in runoff peak rates over pre-development conditions. Based upon the specified number of dams, DURMM generates the spacing, height, and stage/storage volumes provided in the swale, which can be used as input into a separate routing program. By default, height is the drop between the dams. Based upon the specified check dam length and stone size, DURMM then generates the stage/outflow through the check dams to be used as input into the routing program. Note that tailwater will occur once flows overtop the check dams, so there is minimal increase in flow rate once the dams fill up. Separate routing software is required to precisely route flows through the biofiltration swales with check dams. JANUARY

10 Chapter 2 CHAPTER 2 BMP POLLUTANT REMOVAL 2.1 RUNOFF POLLUTANT ROUTING Pollutant mass loads are based upon Event Mean Concentrations (EMCs) from each type of pervious and impervious land cover, as determined by the literature. DURMM integrates the area and EMC of individual land covers to develop the area-weighted average EMC for the graded pervious areas. A similar process is used to develop the area-weighted average EMC for natural pervious areas, as well as for the impervious areas. The simplification of using areaweighted averages is close to, but not as accurate as, flow-weighted averages. However, since most loads originate from impervious areas where the runoff volumes are very similar among the cover types, this simplification is acceptable. The average EMCs for each area category are then multiplied by their respective runoff volumes to develop mass loads from impervious, pervious graded and pervious natural areas. These mass loads are then summed to develop the total load, which is then divided by the total runoff volume prior to disconnection. This develops the average EMCs into the BMP(s). While substantial filtering will occur during conveyance through the wetted area involved in disconnection, these losses are omitted from DURMM routines. The reduced runoff volume after disconnection is then multiplied by these EMCs to develop mass loads into the BMP(s). The runoff from the entire area can be allocated into different filter strip, bioretention and bioswale BMPs, depending upon the site design. The volume of runoff in each BMP is reduced according to disconnection routines, with EMCs reduced as discussed below. To develop the extent of wetted area for the disconnection routines, bioretention BMPs have two widths: the actual width of the facility itself, and the wetted sheet flow width into the bioretention facility from adjacent impervious surfaces. The bioswale routine calculates the wetted area as the side slope width over a 0.25 feet flow depth, plus the bottom width. Filter strip wetted area is the product of length and width. Resulting runoff volumes from each of these BMPs is multiplied by its reduced EMC to develop the mass load leaving each BMP. Mass loads are then summed across all BMPs to develop total loads, which are divided by the resulting sum of runoff volumes to develop EMCs and mass loads from filtering BMPs, along with the percentage of load reductions. Further reductions in loads from filter strips and bioswales are possible with an infiltration trench. Infiltration losses from bioretention facilities are also modeled as if it were an infiltration trench. In these events, the loads are reduced according to infiltrated volume. Figure 1 provides a graphical display of runoff routing and EMC reduction approaches. 2.2 Reduction Kinetics BMPs function to remove pollutants from urban runoff through five major pathways: infiltration, filtration, adsorption, settlement and transformation. Depending on the BMP, some, or even all, of these processes can occur simultaneously. Note that the following discussion examines the reduction in pollutant EMCs by BMPs, not removal rates in terms of pollutant loads. This is due to the fact that runoff volume losses are explicitly accounted for in the JANUARY

11 Chapter 2 disconnection routines, so it is the EMCs of pollutants in the remaining surface runoff that become the parameters of interest. The literature on BMP removal rates shows great variability in the efficiency of various BMPs to reduce pollutant loads. As a result, reported reduction efficiencies can range over an order of magnitude, and negative reduction efficiencies are often reported. To attempt to resolve the factors involved, the Technical Manual examined three variables that control much of the variability in BMP performance. These variables are: 1.) input EMC, 2.) minimum irreducible EMC, and 3.) potential maximum reduction efficiency. Note that input concentrations are independent of BMP design, and minimum irreducible EMC and potential maximum reduction are generic to the type of BMP. However, the final value for potential reduction efficiency is related to effectiveness in the design of the BMP itself. Maximum reduction efficiency is thus dependent on the pollutant involved, the type of BMP, and how well it is designed. The reduction processes and the particulars involved in optimizing BMP design are briefly summarized for each type of BMP in the following sections. Relevant factors such as hydraulic residence time, hydraulic loading rates, slope, length, depth of flow and other parameters are examined for each BMP to develop a quantitative approach to project reduction rates for each fraction. The reader is referred to the Technical Manual for a detailed discussion of these factors. The various equations and coefficients discussed in the Technical Manual are incorporated into DURMM. 2.3 FILTER STRIP BMPs Filter strips are a BMP in which runoff passes as sheet flow through vegetation. Filter strip BMPs provide reductions of pollutant loads through filtration by vegetation and infiltration. While turfgrass is quite acceptable, a better filter media is a dense stand of native grasses, since they have a much deeper rooting depth to promote infiltration. Grass filter strips provide a greater reduction in particulate fractions than swales in a given length, since sheet flow is restricted to the densest thatch and blades. Four factors affect the performance of filter strip BMPs: hydraulic loading rate, filter strip length, filter strip slope and vegetation cover. Hydraulic loading rate is the ratio of connected runoff volume into the filter strip divided by filter strip length. A filter strip width of at least 10 feet is required for adequate results. Increasing width to 20 feet provides the best results, with minimal increases in removal in widths over 30 feet. Surprisingly, slope effects are fairly minor, reducing maximum reduction rates by some 10% at slopes of 25% for narrow strips. Variations in vegetative cover are also less important. These parameters are used to develop EMC reductions and mass load removals within the filter strip. An imperative feature in the design in all filter strip BMPs is the provision of sheet flow conditions. Filter strips are thus best suited for situations where runoff has not been concentrated, as is found from small parking lots, driveways, sidewalks and uncurbed streets. When used in median strips, the paving itself is an effective level spreading device. If entering flows are conveyed by pipe or swales, or if the contributing area is large, a level spreader device is necessary for filter strips to function properly. Flow from roof downspouts can also qualify, if there is no defined swale below the downspouts. Where filter strips are used after flow has become channelized, their reduction rates are not nearly as effective. Therefore, an engineered JANUARY

12 Chapter 2 level spreader is necessary to ensure sheet flow discharge to the filter strip. A detail of the typical level spreader is displayed in the Specifications Manual. The length, width, slope, vegetative cover number and underlying curve number parameters of the filter strip are entered into the appropriate cells in DURMM. The filter strip routine then computes the infiltration of runoff, and the reduction in EMCs due to the filter strip. More detailed explanation of the individual filter strip entry cells is described in the example reviewed in Chapter 4. The designer can see the effects of manipulating these variables by observing how the EMCs change in response to varying input values. Filter strips are also the integral component of terrace BMPs. In this case, the side slope entering the terrace becomes a filter strip. While the terrace itself provides for detention routing when check dams are used, filtration in the conveyance channel is assumed to be minimal, since flow depths are relatively deep, and runoff is already filtered. Given that the side slope provides such substantial filtering, further reductions along the channel would provide minimal additional removal. 2.4 BIOFILTRATION SWALE BMPs Biofiltration swales represent a class of BMPs in which filtering occurs as flow travels through vegetation along a defined channel. The filter media is usually turfgrass, although warm season grasses, sedges and rushes are preferred since they require less maintenance. Biofiltration swales are well suited for concentrated flow situations, where runoff has already been collected by piped conveyance systems. However, they are also very effective as a conveyance system in themselves, and they can also be designed to provide detention to meet the discharge criteria set forth in the regulations. While the typical flow depth in bioswales is deeper than filter strip BMPs, it should still be well below the top of vegetation in properly designed swales. It is very important that vegetation not approach submergence, since this causes the vegetation to bend over with the flow, which reduces roughness. Resulting flow velocities will be much greater, and the opportunity for contact filtering is less. There is a clear trend for a reduction in efficiency and increase in the minimum concentration with increasing depth of flow. For these reasons, biofiltration swales should be designed so that the average flow depth does not exceed 0.25 feet during the quality storm event. Average flow rate is typically assumed to be 40% of the peak flow rate, based upon Horner (1992). It is essential that the swale soil profile promote vegetative growth and infiltration, if located above the water table. Therefore, bioswales should be constructed into uncompacted soils where possible. However, construction involves mass grading, reconfiguration of underlying drainage patterns, and substantial compaction, often deep into native soils. Therefore, it may be necessary to excavate the compacted subgrade along the bottom of the swale, and replace it with a blend of topsoil with sand to promote infiltration and biological growth. At the very least, organic topsoil should be deep plowed into the subgrade to penetrate any compacted zones. Details of typical bioswale sections are displayed in the Specifications Manual. JANUARY

13 Chapter 2 For design, the curve number, length, slope, surface code and geometric parameters of the bioswale are entered into the appropriate cells. More detailed explanation of the individual bioswale entry cells are described in the example reviewed in Chapter 4. After entering the relevant data, pressing the swale button calculates the depth and velocity and residence time. From these results, the routine then calculates the reductions in EMCs and mass loads. The designer can see the effects of manipulating input variables by observing how the EMCs change in response to varying input values. A typical detail for the biofiltration swale cross-section is displayed in the Specifications Manual. An important benefit of biofiltration swales is their ability to provide detention storage for events larger than the quality event. By installing check dams at regular intervals, a bioswale several feet deep can provide enough storage for controlling peak flow of the 100 year event. Check dams also absorb the energy of high flows, reducing the potential for resuspension of previously deposited sediments in larger storms. Since the flow from the dams enters the pool created by the next check dam, flow velocities remain quite low throughout, even in extreme events. Using check dams, many of the benefits of an off-line layout can be provided without the need for flow-splitting devices and the loss of space required for an off-line facility. A typical detail for the check dam elevation and profile is displayed in the Specifications Manual. 2.5 BIORETENTION BMPs The preceding vegetative filtering BMPs are the first line of defense in the Green Technology BMP approach. If thoughtfully incorporated into the site design, filter strip and biofiltration swale BMPs can reduce pollutants to acceptable levels by themselves. However, in intense urban development where runoff EMCs can be quite elevated, such as from convenience stores and gas stations, bioretention BMPs are a very effective development in BMP design. These living filters comprise an organic loam at least 2.5 feet deep, covered with a layer of mulch and vegetation. Generally located off-line in small depressions designed to intercept the quality storm volume, filtering occurs as runoff percolates through the mulch and soil matrix into an underdrain. Bioretention BMPs are particularly effective for removing metals and TSS, and phosphorus to a lesser extent. Nitrogen reduction is more variable and less effective. Since much of the captured runoff is released by evapotranspiration, bioretention facilities provide mass reduction rates even greater than that removed by their reduction in EMCs. Underdrain effluent from bioretention facilities can be up to 12º C cooler than the temperature of incoming runoff, providing excellent thermal protection to the receiving waters. These aspects of bioretention facilities make them particularly attractive as a nonstructural BMP. Bioretention facilities are typically sized to handle a hydraulic loading rate of 1.5 feet per inch of runoff. Prorated up to a 2-inch quality event, runoff from a largely impervious area generates a loading of some 2.75 feet. At an infiltration rate of at least 1 inch per hour, this translates to a 33 hour drawdown time. Since hydraulic loading rates control facility sizing, it is important that the facility be located to capture the most polluted runoff from impervious areas, with as few contributions from pervious areas as is possible. Depending upon the CN of the contributing watershed, these factors suggest that bioretention facilities be sized at roughly 5% of JANUARY

14 Chapter 2 the drainage area. Where underlying infiltration rates are high, exfiltration from the facility can replace the need for underdrains. If the landscaping is not flood tolerant, surface ponding should be restricted to less than a foot at most, and surface drainage should occur within hours after the rainfall ends. This is necessary to ensure that such landscaping will bear the occasional immersion. Therefore, bioretention BMPs using upland plants should be located off-line so extended detention surcharge from larger storms is not a potential problem. However, if facultative wetland species are used, a greater range of flooding regimes is acceptable. Not only can such plants tolerate greater depths for longer times, they can also function in hydrologic conditions that approach constant saturation. For these reasons, the Specifications Manual recommends facultative plants for bioretention facilities. Filtration, immobilization and adsorption are the main mechanisms of pollutant reduction in bioretention. Adsorption is particularly effective in removing the soluble pollutants that are not susceptible to filtration. As such, the proper composition of the soil matrix of the facility is very important. Sand has a very low cation exchange capacity (CEC), so its adsorption potential is low, but its infiltration rates are very high. On the other hand, organic soils have some 15 times the CEC of mineral soils on a weight basis, but bioretention media using organic soil with too much silt and clay fail with regularity due to clogging. Laboratory experiments suggest that sandy loam topsoil can be an excellent medium, since that its organic matter and fines fraction is less than 20%. If the topsoil has too much fines, the organic soils must be augmented with sand so the final mix is less than 20% organic matter, silt and clay. Other researchers have shown that a proportion of sand as high as 90% will work well, if there is adequate (at least 5%) organic matter to provide the CEC. This organic matter can be produced by the plants once they are established. This Manual recommends a mix of 1/3 sand, 1/3 peat moss and 1/3 mulch to ensure excellent infiltration and adsorption capacity. For best results, the ph should be close to neutral so metals adsorption is maximized. Analysis of bioretention facilities indicates that mulch retains very high concentrations of metals compared to the soils. This suggests that regular replacement of the mulch layer would improve performance and useful life. Accumulation rates in the soils indicate a useful life of nearly 60 years before the soil matrix becomes saturated, or longer if the mulch is replaced regularly. Unlike filtration facilities, however, there do not seem to be any design factors that provide incremental changes in reduction rates. Instead, two thresholds should be met in all designs: a minimum depth of 2.0 feet (or 2.5 feet for optimal phosphorus reduction), and a maximum loading volume of 2.75 feet during the quality event. The reduction in EMCs by bioretention BMPs is a function of input concentration and maximum removal rate. Additional research may provide information to ascertain if flushing effects occur at higher rates. The length, facility width and media width parameters of bioretention BMPs are entered into the appropriate cells in DURMM. The bioretention routine then computes the infiltration of runoff from the contributing area over the side slope areas of the bioretention area, defined as facility width minus media width, times length. A separate routine for infiltration trenches described below is then used to calculate infiltration volumes. This volume is subtracted from the total runoff volume, since it does not contribute to the outflow hydrograph. More detailed JANUARY

15 Chapter 2 explanation of the individual bioretention entry cells are described in the example reviewed in Chapter 4. A typical detail of the bioretention facility is displayed in the Specifications Manual. 2.6 INFILTRATION TRENCH BMPs Trenches are the preferred infiltration BMP. For proper operation, infiltration facilities must be designed to ensure that they do not clog over time. This requires that filtering BMPs are used to provide adequate pretreatment, designed according to the principles set forth above. Where EMCs of dissolved pollutants are elevated, pretreatment by these BMPs is essential in any event. While it is thought to be clean, runoff from roofs can have substantial amounts of nutrients from airborne sources and TSS from decaying roofing materials; if connected directly to an infiltration facility, there is a real potential for it to clog eventually and subsurface nitrate loading will occur. For these reasons, direct connection of downspouts to infiltration facilities is not recommended unless measures can be established to ensure adequate maintenance. Otherwise, the wetted area must be increased to handle the extent of clogging by the inevitable accumulation of fines. Infiltration basins are structural BMPs that require considerable area and maintenance, and they often fail after several years of operation. This Manual thus focuses on infiltration trenches as the primary nonstructural subsurface infiltration BMP. Infiltration trench design parameters are discussed in the Technical Manual. Note that ratio of trench width to depth to seasonal high water table (SHWT) and surface to volume relationships tend to favor trenches that are long and narrow over shorter wide trenches. This is another reason to avoid dry wells and other rectangular/circular subsurface infiltration structures. However, this preferred geometry can be easily incorporated into linear BMPs such as biofiltration swales, which can also serve to provide the required pretreatment. A subdrain is not required, since infiltration rates should be high enough to completely discharge the trench between storm events. Generally, infiltration trenches can be filled directly by surface infiltration from filter strips or swales. Construction details for infiltration trenches are provided in the Specifications Manual. With infiltration trench BMPs, there is no reduction in EMCs in runoff that is not infiltrated. Surface runoff loads are simply reduced in proportion to the amount of runoff infiltrated during the quality storm event. The inflowing EMC is multiplied by remaining surface runoff volume to determine mass loads leaving the site in surface runoff. More detailed explanation of the individual infiltration trench entry cells are described in the example reviewed in Chapter 4. DURMM computes the amount of storage and infiltration as the trench fills up and discharges during the storm. Details of the typical bioswale incorporating an infiltration trench are displayed in the Specifications Manual. JANUARY

16 Chapter 3 CHAPTER 3 DURMM EXAMPLE: PRE-DEVELOPMENT CONDITIONS DURMM is written as an Excel spreadsheet workbook. In any project, the procedure is to open the DURMM workbook. Enable macros at the prompt when opening the workbook, save it by the project name and begin to enter the relevant data. The display convention is for user input cells to be displayed in green. These are the only cells that the designer can change. The results of computations are displayed in blue. Cells that refer to input/operational assumptions and coefficients are displayed in orange. Cells that are revised during computations are shown in pale yellow. Check prompt cells that highlight a faulty result are displayed in bright yellow. The macro operation buttons are grey. In this particular example, a hypothetical site was created to simplify graphical and computational display. This is a site in New Castle County, south of the Chesapeake and Delaware Canal, so the Delmarva hydrograph is to be used. Existing uses are largely agricultural, with a house and a small stand of woods. Figure 2 displays the site, along with the relevant land cover/ soil group complexes, as well as the parameters of the flow paths. It is assumed that the boundaries of the site correspond to the drainage divides, so offsite impacts and/or contributions are excluded from consideration. The hydrologic soil group (HSG) over the entire site is B. The procedure for data entry is as follows: First, open the PRE-DEVELOPMENT Worksheet in DURMM, and hit the CLEAR ENTRIES button. This clears all entries and enters default values between 1 and 0 through the worksheet. Then, enter the project data in the green cells. These cells are unlocked to permit data entry, and are automatically copied into the subsequent worksheets. DURMM ANALYSIS PRE-DEVELOPMENT SUBAREA PROJECT: DURMM USER'S MANUAL SAMPLE PREPARED BY MUNICIPALITY: PENCADER COUNTY: NEW CASTLE INTEGRATED LAND MANAGEMENT, INC. SUBAREA: SAMPLE SITE HYDROGRAPH DMV DATE: BMP: ALL BMPs October 31, 2003 Enter the relevant project parameters into the green cells of the PRE- DEVELOPMENT Worksheet. The County and hydrograph cells are very important- DURMM will not function without the proper entries in these cells. Note how the orange cells in the lookup table for the counties return the proper rainfall amounts for the design events, as shown to the right: DELAWARE RAINFALL DEPTHS USED NEW CAST. KENT SUSSEX QUALITY-2.0" BANKFULL- 2 YR CONVEYANCE- 10 YR FLOODING- 100 YR JANUARY

17 Chapter 3 Figure 2: EXAMPLE SITE- Pre-development Conditions JANUARY

18 Chapter 3 Next, enter the land cover/hsg complexes in the appropriate category cells. Since design software such as AutoCAD returns polygon areas in square feet, areas from the design can be entered directly. The HSG is also entered directly. Land cover type is entered by entering the proper code according to the look-up table shown below. Once the code is entered, the land cover name automatically is entered into the worksheet, the proper CN is entered into the area weighted CN routine, and the area is converted to acres. This table is displayed in the worksheet to simplify looking up the proper cover. This procedure avoids errors from typographical mistakes. Any pervious entries over 12 or impervious entries less than 21 or greater than 26 will result in an INVALID! surface prompt, and enter an impossibly high CN value. Such entries will have to be corrected for the macros to run. This table is displayed in the worksheet next to the entry cells to facilitate looking up the proper cover. Note that impervious surfaces have the same CN, regardless of underlying soil, as would be expected. However, the CN values vary, instead of the uniform value of 98.0 used in TR-20. SURFACE A B C D 1 WOODS-GD WOODS-FR MEADOW PASTURE-GD PASTURE-FR CROPS-NT CROPS-MT CROPS-CT LANDSCAPE-GD LANDSCAPE-FR LAWNS-GD LAWNS-FR PARKING LOT SLOPE ROOF FLAT ROOF RES.STREET RES.DRIVES SIDEWALKS 96.0 The GD suffix represents good conditions, while the FR suffix represents fair conditions. Crops are allocated as conventional tillage (CT), minimal till (MT) or no till plus cover crop (NT). Landscape is equivalent to Woods-FR in terms of CN, but it has higher phosphorus loading rates, hence its own category. Landscape-FR is applied to landscaping located in compacted areas around buildings. Lawns-FR is applied to ball fields and other high intensity park areas. The criteria for pre-development conditions assume existing land cover. Regardless of actual practices, agricultural crops should be classified as Crops NT, as if they were crops using the best possible management practices. No-till (NT) crops are thus entered in the natural pervious category, along with pastures, meadows and woodlands, and all other codes up to 6. Minimum till (MT) and conventional till (CT) crops, landscaping and lawn codes from 7 to 12 JANUARY

19 Chapter 3 are classified as graded pervious, due to their extensive management inputs and proximity to structures. These cover classifications should not be entered into the natural pervious category. Note that impervious areas can be entered as the product of length and width, or as an area by itself, times one. This expedites entry for simple sites. In more complex sites, individual polygon areas generally need to be summed up separately and entered into an available cell. A scratch worksheet is included to enter the raw site data in this event, so the user can summarize data for entry into the available cells. Note how the weighted average CN of the individual HSG/land cover polygons is displayed at the bottom of each category. A summary of the entire area is included, along with the impervious percentage. DURMM INPUT DATA: GRADED CODE HSG AREA AREA(ac) IMPERVIOUS CODE LENGTH WIDTH/ PERVIOUS /AREA NO. AREA(ac) LAWNS-GD 11 B 16, SLOPE ROOF RES.DRIVES CN & ACRES OF GRADED PERVIOUS NATURAL PERVIOUS CODE HSG AREA AREA(ac) CN & ACRES OF IMPERVIOUS WOODS-GD 1 B 5, TOTAL ACREAGE 5.44 % IMPERV. 2% PASTURE-GD 4 B 26, DISCONNECTION ADJUSTMENTS CROPS-NT 6 B 184, PERCENT IMPERVIOUS 100% WETTED AREAS WETTED LENGTH 560 OVERLAND 0.06 WETTED WIDTH 5 FLOW PATH PERCENT FLOW PATH TOTAL 0.06 CN & ACRES OF NATURAL PERVIOUS PERVIOUS CN 61.0 %IMPERV. 60% Enter the land cover code, hydrologic soil group, and area in the proper pervious category cells as indicated above. Note that surface codes 7 through 12 should only be entered into the graded pervious category. Enter impervious covers in the available impervious cover cells as shown. For disconnection, the required data includes the percent of impervious disconnected, which is 100% in this case, since no structures discharge into pipes. The wetted length is the equivalent of the perimeter of the house and drive. A wetted width of 5 feet is used as an average- it is larger (roughly 25 feet long) below downspouts, while it is minimal between downspouts. It is roughly 4 feet from the driveway, resulting in an average of 5 feet overall. Since the Tc path is below the wetted area, and is dominated by upgradient runoff, it is excluded from the disconnection computations. The pervious CN is chosen to match the adjacent lawns. In a more complex site with differing HSGs, a different CN could apply. The wetted area cells show the area of overland and flow path wetted areas, and their combined percentage of the impervious source area. Enter the pertinent disconnection data in the cells as indicated above. Do not omit CN, or the default value of 0 will freeze the computations. JANUARY

20 Chapter 3 For each design event, the worksheet enters the rainfall volume from the lookup table for New Castle County, as entered above. The worksheet computes the runoff volumes from the natural pervious, graded pervious and impervious areas, as well as the reduction in runoff due to the disconnection of impervious areas. Note that the disconnection reduction is relatively insensitive to the precise value of the wetted area. However, it is quite sensitive to the percentage of impervious area that is disconnected. EVENT PRECIPITATION IMPERV. GRADED NATURAL ROUTED REDUCTION RUNOFF(in.) QUALITY ,777 4,321 9% 0.22 BANKFULL 3.3 1, ,568 16,064 3% 0.81 CONVEYANCE 5.2 1,962 2,045 37,230 40,822 1% 2.07 FLOODING 7.3 2,781 4,007 66,727 73,139 1% 3.70 Values in the Routed column represent the total runoff volume after the disconnection routine has been applied. Note that for each storm event, the sum of the values in the Impervious, Graded and Natural columns does not equate to the value in the Routed column, due to disconnection. The Reduction column shows the reduction in total runoff volume as a result of disconnection. When the volume reduction due to disconnection is considered as a percentage of impervious area runoff, rather than total runoff, disconnection effect is more apparent. For instance, the total runoff volume for each storm without disconnection is: Quality 4,761 cubic feet Bankfull 16,597 cubic feet Conveyance 41,237 cubic feet Flooding 73,515 cubic feet These values are obtained by summing the values in the Natural, Graded, and Impervious columns. The difference between these values and the Routed totals represent the reduction in impervious runoff volume due to disconnection: Quality 440 cubic feet Bankfull 533 cubic feet Conveyance 415 cubic feet Flooding 376 cubic feet When compared to the Impervious runoff volume and expressed as a percent reduction, this yields: Quality 62% reduction Bankfull 44% reduction Conveyance 21% reduction Flooding 14% reduction The overall volume reductions due to disconnection would be much more significant on sites with a higher percentage of impervious area, as will be demonstrated in the postdevelopment example in the next Chapter. JANUARY