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1 Chapter 5 Table of Contents 5-1 Introduction Stormwater Quality Considerations Stormwater Volume Considerations Downstream Impacts Requirements General Requirements Detention Requirements for Redevelopment Detention Requirements for Expansion of a Developed Site Types of Detention Facilities Scale of Implementation Regional Detention Local-area Detention Individual Site Detention Outlet Configurations Full Spectrum Detention Sizing Methodology Full Spectrum Detention Sizing of Full Spectrum Detention Volumes Simplified Equations Design Worksheets Release Rates Year Allowable Release Rate WQCV Release Rate EURV Release Rate Design Considerations General Layout and Grading Volume Embankments Emergency Spillways Soil Riprap Spillway Baffle Chute Spillway Outlet Structure Flush Safety Grate Raised Grate with Multiple Vertical Openings Raised Safety Grate with Vertical Opening Raised Grate with Offset Vertical Openings Outlet Pipe Hydraulics Trash Racks and Debris Blockage Inlets March 2017 City of Durango 5-i

2 Chapter Vegetation Retaining Walls Access Geotechnical Considerations Linings Environmental Permitting and Other Considerations Orifice and Weir Hydraulics Orifices Weirs Additional Configurations of Detention Facilities Water Reservoirs Upstream of Railroad and Highway Embankments Side-Channel Detention Basins Designing for Safety, Operation, and Maintenance Examples References Tables Table 5-1. Facility Sizing Methods Table 5-2. Applicability of Full Spectrum Sizing Methods Based on Watershed Area Table 5-3. City of Durango Predevelopment Peak Unit Flow Rates, q (cfs/acre) for Various Precipitation Return Frequencies, Soil Groups, and Slope and Shape Factors Figures Figure 5-1. Example Configuration for Regional Detention (Adapted from: Arapahoe County)... 5 Figure 5-2. Example Configuration for Local-Area Detention (Adapted from: Arapahoe County)... 6 Figure 5-3. Example Configuration for Individual Site Detention (Adapted from: Arapahoe County)... 8 Figure 5-4. Extended Detention Basin Combined with Full Spectrum Detention Figure 5-5. Stand-Alone Bioretention with Downstream Zone 3 Basin Figure 5-6. Stand-Alone Bioretention with Downstream Zone 2 and Zone 3 Basin Figure 5-7. Flush Grate (Sloping Drop Box Shown) Figure 5-8. Grate with Vertical Openings (Horizontal Drop Box Shown) Figure 5-9. Grate with Vertical Opening (Sloping Drop Box Shown) Figure Grate with Offset Vertical Panels (Horizontal Drop Box Shown) Figure Typical Grate Configurations for Outlet Structures Figure Sloping Broad-Crested Weir Figure Embankment Protection Details and Rock Sizing Chart (Adapted from: Arapahoe County) Figure Trash Rack Sizing ii City of Durango March 2017

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5 5-1 Introduction facilities including detention basins, retention ponds and constructed wetland ponds are used to manage stormwater quantity by attenuating peak flows during flood events. facilities can also be designed to enhance stormwater quality by incorporating design components to promote sedimentation, infiltration, and biological uptake. Combining flood control detention functions for managing the rate of runoff with water quality practices such as bioretention, sand filters, etc. results in multifunctional storage facilities that address both stormwater quantity and quality. This chapter provides criteria for the analysis and design of storage facilities that are implemented independently or in combination with stormwater quality facilities. Additionally, this chapter provides criteria on full-spectrum detention designs that help to mitigate the effects of increased flow volumes on downstream channels. The City of Durango requires the site runoff from a range of storms up to the 100- yr event to be captured and released at an allowable release rate representative of undeveloped conditions. To provide additional water quality and stream stability benefits the controlled release of the Water Quality Capture Volume (WQCV) (see Water Quality Chapter) and the Excess Urban Runoff Volume (EURV) will also be required. This chapter references the Chapter of the UDFCD Manual for additional background, design parameters, and sizing methods. Criteria presented in the Chapter of the UDFCD Manual shall govern except as modified or added to herein. In addition to guidance on design of detention storage facilities, this chapter recognizes that detention facilities can require significant land area and be a prominent feature within land development plans. Detention storage planning and design should be performed during the initial phases of a site design to incorporate features that serve multiple purposes and that are integrated functionally and aesthetically into the land plan. facilities must also be safe and maintainable. When properly designed and maintained, storage facilities can be viewed as community assets rather than liabilities Stormwater Quality Considerations Detention facilities can be designed to provide both flow control and water quality benefits by providing extended detention and/or infiltration of small, frequently occurring events to enhance stormwater quality as a part of the flood control facility. Water quality treatment may also be provided through runoff reduction techniques that have the potential to also affect detention storage facility and downstream conveyance sizing. Early in the planning process, opportunities to provide runoff reduction, stormwater quality management and flood control should be evaluated so that a comprehensive and coordinated approach can Photograph 5-1. facilities should be considered during be developed. Extended detention and other the early stages of site design. This detention facility doubles as open space amenity for passive and active recreational activities. water quality best management practices and runoff reduction practices are discussed in Water Quality Chapter and Volume 3 of the UDFCD Manual, which is a resource for stormwater quality March 2017 City of Durango 5-1

6 Chapter 5 planning and design in The City of Durango Stormwater Volume Considerations In addition to the increase in peak flow rates, stormwater runoff volume increases with urbanization as a result of increased impervious areas, particularly impervious areas that are directly connected to the drainage system. The increase in runoff volume, especially for more frequent storm events, has the potential to disturb the downstream receiving stream s equilibrium and cause channel instability. Therefore, detention basin designs that help to mitigate the effects of increased runoff volumes, in combination with the controlled release of the EURV and treatment of the WQCV, are preferred. Additionally, volume reduction practices upstream of detention facilities can be used in conjunction with flow control facilities to optimize mitigation and reduce costs Downstream Impacts Even with comprehensive management of stormwater runoff, the effects of urbanization, including excess irrigation, increased snow melt runoff, reduced sediment loads (i.e., sediment hungry clear water runoff), and significant increases in stormwater runoff volumes are very difficult to fully mitigate. Therefore, some downstream channel instability can be anticipated due to development. This will require attention to channel and right-of-way improvements that may need to extend downstream beyond project limits. 5-2 Requirements General Requirements Detention storage facilities are critical elements in the management of stormwater and shall be required for all new development, redevelopment, expansion or improvement projects. Detention facilities shall be designed to treat the WQVC and manage stormwater runoff for storms up to and including the 100-year event Detention Requirements for Redevelopment Redevelopment of a site occurs when a change in the property use and/or function is desired, resulting in physical changes to the site. Unless detention is provided via a regional facility, the redevelopment of a site shall require that on-site detention be provided for the entire site, including those areas that previously had not provided detention due to the site being developed prior to the adoption of this Design Manual Detention Requirements for Expansion of a Developed Site Expansion of a site occurs when additional area on the site is to be developed. The expansion of a site shall require that the current standards for detention for the entire site are met. There are two conditions that may arise for site expansion, depending upon whether or not detention has been provided for the existing site prior to expansion, including: Detention has been provided for the existing developed area. The new expansion shall require that additional detention be provided to accommodate the expanded development or that the existing facilities be modified to serve the full site development. Detention has not been provided for the existing developed area. Detention will be required for the 5-2 City of Durango March 2017

7 full expansion and to the extent possible, for the existing site area that has not been previously detained. A reasonable attempt to provide detention storage will be required for the previously developed, un-detained portion of the site or the release rates from the expansion area must be less than the allowable release rates to compensate for the un-detained area. 5-3 Types of Detention Facilities Detention storage facility designs can generally be characterized based on scale of implementation and outlet configuration, as discussed below Scale of Implementation Typical development-related detention facilities can be classified as regional, local-area or individual site. Regional detention facilities typically serve a broad purpose within a watershed and manage flows from multiple parcels and, possibly, multiple development projects. Local-area ponds, which by definition serve multiple parcels, may also provide benefits in terms of attenuating flows from developed areas, but typically serve only one development project. Regional and local-area detention facilities typically require a commitment for maintenance by a public entity or a legally-binding maintenance agreement. Individual site facilities typically only serve a single parcel, have localized benefits, and are maintained by the property owner or private entity. Regional or local-area detention is the typical approach used at a subdivision or project scale instead of providing individual site detention at the time each lot is developed. For large subdivisions, regional or local-area detention should be implemented by the first sub-divider rather than passing on the responsibility for detention to owners of individual filings. The coordination of development phasing with the construction of detention facilities should be addressed within the basin plans. Additional information is provided below for regional, local-area and individual site facilities. Each of these types may include water quality features or be used in conjunction with separate water quality features or runoff reduction techniques. When water quality treatment is included within a detention facility, its effect on the required flood control storage varies with the type of facility Regional Detention Regional detention typically refers to facilities that are owned and maintained by the City that serve multiple development projects or multiple phases of a development project. A primary function of regional detention facilities is to mitigate the effects of developed runoff so that downstream natural channel features and floodplains can be preserved. The location of these facilities can also differentiate minor drainageways from major drainageways. Under natural conditions, significant drainageways tend to develop when the contributing area grows beyond about 100 acres. Therefore, regional detention facilities will typically be located with a contributing area of about 100 acres or more. Figure 5-1 illustrates a typical regional detention concept. The following are required for regional detention facilities: 1. Regional detention facility shall be designed to accommodate the fully developed condition flows from the upstream watershed. 2. Regional detention facilities are required to be owned and maintained by a public entity, with March 2017 City of Durango 5-3

8 Chapter 5 ownership and maintenance responsibilities clearly defined to ensure the proper function of the facility in perpetuity. 3. Drainage easements or tracts for the facility, including access from a public street, shall be provided. 4. An approved Development Agreement is required. See the Principles and Policies Chapter for information on maintenance requirements. 5. The creation of a jurisdictional dam, according to the State Engineer s Office definition, is discouraged. 6. The facility shall be permitted under applicable environmental permits and clearances. 7. Construction of the regional facility must be coordinated with development in the upstream watershed. If the regional facility has not been constructed, temporary on-site detention (and water quality) shall be required to be provided with development projects until the regional facility is available. 8. The drainageways upstream of a regional facility shall be designed to convey fully-developed 100- year flows to the regional facility. 9. Roadway embankments may be used to create the storage volume. When this is the case, roadway embankment design should consider the implications of hydrostatic and hydrodynamic forces on the embankment, including the potential for piping and scour. Roadways under the jurisdiction of other agencies, such as CDOT, may be prohibited from being used as detention basin embankments or require special consideration and permission. 5-4 City of Durango March 2017

9 Figure 5-1. Example Configuration for Regional Detention (Adapted from: Arapahoe County) March 2017 City of Durango 5-5

10 Chapter Local-area Detention Local-area detention refers to facilities located upstream of a major drainageway (generally having a drainage area between 1 and 100 acres) and serving more than one parcel. Local-area detention facilities are typically constructed during the development process to serve several landowners in the upstream watershed or by a single landowner. For a local-area facility to be recognized in the determination of flow rates for downstream major drainageways, the facility owner must provide easements and a legally binding maintenance agreement. Figure 5-2 illustrates a typical local-area detention concept. The conditions listed previously for regional facilities shall be adhered to for local-area facilities. Photograph 5-2. Local-area storage facilities can serve as amenities for a residential development when properly designed and maintained. Figure 5-2. Example Configuration for Local-Area Detention (Adapted from: Arapahoe County) 5-6 City of Durango March 2017

11 Individual Site Detention Individual site detention refers to facilities serving one parcel, generally commercial or industrial sites draining areas less than about 10 acres. On-site detention is best suited for infill and redevelopment projects and in other areas where regional or local-area facilities are not able to be implemented. A primary function of individual site detention facilities is to reduce developed condition flows so that downstream storm drain and/or channel capacities are not exceeded. In cases where downstream capacity is limited, it may be necessary to detain flows to below allowable release rates to avoid adverse impacts downstream. Individual site detention also provides opportunities to provide water quality treatment features. Figure 5-3 illustrates a typical individual site detention Photograph 5-3. Landscaped areas located throughout a site provide opportunities for localized individual site detention as shown in this bioretention facility. concept. Individual site detention facilities will not be recognized in the determination of flow rates for downstream major drainageways because of difficulties with ensuring that these facilities are maintained and continue to function in perpetuity on private property. General guidelines for individual site detention facilities include: Integrating Detention and Site Landscaping Requirements. Locating detention basins in areas reserved to meet site landscaping requirements is generally encouraged. Incorporating detention into landscaped areas generally creates detention facilities that are easier to inspect, are relatively easy to maintain, and can enhance the overall aesthetics of a site. Further discussion regarding landscaping improvements in detention facilities is provided later in this chapter. For some small sites, detention and water quality can often be provided using practices such as bioretention that provide for infiltration, storage and regulated outflow. Parking Lot Detention. Parking lot detention may be acceptable on commercial and industrial sites and can offset some of the storage volume that needs to be provided on landscaped areas. Parking lot detention will be allowed on a case-by-case basis. Underground Detention. Underground detention shall be considered only in cases where other alternatives using surface storage cause the proposed development to not comply with other criteria, such as public safety or other City development codes, and will only be allowed through the variance process provided in this Design Manual (i.e. as approved by the City Engineer). Rooftop Detention. Rooftop detention is prohibited, except as may be allowed through the variance process provided in this Design Manual (i.e. as approved by the City Engineer). March 2017 City of Durango 5-7

12 Chapter 5 Figure 5-3. Example Configuration for Individual Site Detention (Adapted from: Arapahoe County) Outlet Configurations Outlet structures that are designed to better attenuate the full range of storm events are considered fullspectrum outlets. These outlets release an outflow hydrograph that more closely represents the undeveloped condition hydrograph. They also provide some mitigation of increased runoff volumes by releasing them over an extended period of time Full Spectrum Detention Full Spectrum Detention (FSD) is a design concept introduced by UDFCD in 2005 (Urbonas and Wulliman 2005) that provides better control of the full range of runoff rates that pass through detention facilities than the conventional multi-stage concept. This concept also provides some mitigation of increased runoff volumes by releasing a portion of the increased runoff volume at a low rate over an extended period of time (up to 72 hours). This concept can be applied for any size drainage basin up to 640 acres and can be integrated into individual site, local-area or regional detention facility designs. 5-8 City of Durango March 2017

13 By providing an Excess Urban Runoff Volume (EURV) in the lower portion of the facility storage volume with an outlet control device similar to a Water Quality Capture Volume (WQCV), frequent and infrequent inflows are released at rates approximating undeveloped conditions. The EURV is based on the incremental difference between the developed and undeveloped runoff volume for the range of storms that produce runoff from impervious land surfaces. Additional information on FSD is provided in the Chapter of the UDFCD Manual. Designing a detention basin to capture the EURV and release it slowly (at a rate similar to WQCV release rates) means that the frequent storms, smaller than approximately the 2-year event, will be reduced to very low flows near or below the sediment carrying threshold value for downstream drainageways. Additionally, by incorporating an outlet structure that limits 100-year runoff to the allowable release rate or to the undeveloped condition rate, the discharge hydrograph for storms between the 2-year and 100-year storm event will approximate the hydrograph for undeveloped conditions. This reduces the likelihood that runoff hydrographs from multiple detention facilities will combine to increase downstream discharges above undeveloped conditions and helps to more effectively mitigate the effects of urbanization. 5-4 Sizing Methodology The methodology for estimating the total volume of runoff that is required to be captured by the storage facility can vary depending on the contributing area, type of facility, and its intended function in the drainage system. To determine the appropriate methodology, the following questions should be answered: 1. What is the size of the watershed area contributing to the facility? 2. Will the facility be regional, local-area or individual site? 3. Will the facility include the WQCV/EURV or will the WQCV and/or EURV be provided at alternative locations in the basin area? Considering these factors, the storage characteristics including location, volume, allowable release rates, multi-use opportunities, and other design features can be determined. Determining final detention storage facility characteristics typically requires an iterative process to achieve the design goals with the minimum storage requirements. The Chapter of the UDFCD Manual describes a design procedure that can be applied for various types of detention storage facilities. This Design Manual provides approximate, simplified methods (empirical equations) that are adequate for smaller basins. More complex methods, are available for larger, regional facilities. Use of the more complex methods may reduce the calculated required volume of the facility. Table 5-1 summarizes the types of storage facilities and acceptable methods for determining their size and allowable release rates. March 2017 City of Durango 5-9

14 Chapter 5 Table 5-1. Facility Sizing Methods Facility Type Regional Localarea Individual site Tributary Area 100 to 640 acres Less than 100 acres Less than 10 acres Developed Inflow Hydrograph Estimate Hydrograph routing (CUHP/SWMM) required to develop 100-yr inflow hydrograph into storage facility. Hydrograph routing (CUHP/SWMM) required to develop 100-yr inflow hydrograph into storage facility. Hydrograph routing (CUHP/SWMM), or use hydrographs built into Durango-FSD workbook. Sizing Calculations Input design inflow hydrograph into Durango-FSD Spreadsheet for sizing storage facility. Empirical equations can be used to estimate EURV and WQCV (EURV includes WQCV). Input design inflow hydrograph into Durango-FSD Spreadsheet for sizing storage facility. Empirical equations can be used to estimate EURV and WQCV (EURV includes WQCV). Durango FSD Spreadsheet for sizing storage facility. Empirical equations for 100-yr detention volume, EURV and WQCV (EURV includes WQCV) Allowable Release Rate Undeveloped runoff rates Undeveloped runoff rates Undeveloped runoff rates Full Spectrum Detention Full spectrum detention is designed to be combined with one of the five types of WQCV treatment stormwater controls (SCs), these include: Extended detention basins, Retention ponds, Constructed wetland ponds, Sand filters, and Rain gardens (bioretention). The 100-year FSD volume described in this chapter is consistently expressed as the total storage volume including EURV; also, EURV consistently includes the water quality volume (e.g. the WQCV is a sub-set of the EURV, and the EURV is a subset of the total storage volume). Therefore the WQCV and the EURV are both included in the 100-year FSD volume and the City of Durango does not recommend adding any part of the WQCV to either the EURV or the calculated 100-year volume. Figure 5-4 illustrates an extended detention basin combined with FSD. In Figure 5-4, Zone 1 represents the water quality portion of the facility. Zone 2 represents the difference between the EURV and Zone 1. Zone 3 represents the difference between the 100-year volume and the EURV City of Durango March 2017

15 Figure 5-4. Extended Detention Basin Combined with Full Spectrum Detention A single facility may include a combination of these storage elements or the storage elements may be segregated into separate facilities, as shown in Figures 5-5 and 5-6. Segregating the storage elements may be beneficial if a project is being phased or when adequate land is not available to combine all of the elements in one facility. Figure 5-5. Stand-Alone Bioretention with Downstream Zone 3 Basin Figure 5-6. Stand-Alone Bioretention with Downstream Zone 2 and Zone 3 Basin March 2017 City of Durango 5-11

16 Chapter Sizing of Full Spectrum Detention Volumes Three methods for sizing full spectrum detention storage volumes are as follows: 1. Simplified Equation (developed based on UDCD equations with adjustments made for Durango Rainfall note that only using the 100-yr simplified equation may result in an overestimation of the required storage volume), 2. Durango-FSD workbook (developed based on UD-FSD workbook from UDFCD with adjustments for Durango rainfall), 3. Hydrograph routing using CUHP and SWMM (see Hydrology Chapter). The recommended range of application for the methods based on upstream watershed area is shown in Table 5-2. FSD facilities may be sized using any of the methods shown in the table for the ranges of watershed area; however, the Durango-FSD workbook more accurately represents input variables than the simplified equation, and the hydrograph routing approach provides the most accurate approach. The hydrograph routing approach is recommended to evaluate the function of multiple FSD facilities arranged in parallel or series in a watershed. The three sizing methods are described in the following sections. Table 5-2. Applicability of Full Spectrum Sizing Methods Based on Watershed Area Sizing Method Watershed Properties Simplified Equations Durango-FSD Less than 10 acre X X CUHP/SWMM Hydrograph Routing 10 to 100 acres X X 100 acres to 1 mile X X Greater than 1 mile X X Multiple detention facilities in parallel or series X Simplified Equations Simplified equations are provided in this section for determining full spectrum detention design volumes and 100-year release rates. Once these values are determined, a FSD facility may be designed according to the technical guidance described in Section 5-6. Three different volumes are associated with the design of a full spectrum detention facility, as illustrated in Section These are: 1. WQCV (Zone 1), 2. EURV (Zone 1 plus Zone 2), year volume (sum of Zones 1, 2, and 3). Within the ranges identified in Table 5-2, these volumes may be determined using simplified equations, as described below City of Durango March 2017

17 WQCV. See the Water Quality Chapter for the simplified equation to calculate the WQCV. EURV. Equations 5-1, -2 and -3 may be used to find EURV in acre-feet for specific soil types (1.48i ) EURV A Equation 5-1 A (1.28i ) EURV A Equation 5-2 B (1.16i ) EURV A Equation 5-3 C/D 12 Where: EURV soil type i A = Excess urban runoff volume in acre-feet, = % Imperviousness expressed as a decimal, = watershed area in acres. Apply the equations above for each of the soil types found in the watershed and then calculate a weighted average value based on the relative area proportion of each soil type. Whenever NRCS soil surveys are not available for a catchment area, soils investigations are recommended to estimate equivalent soil type. These equations apply to all FSD facilities, and the EURV need not be added to the flood control volume or to the WQCV. If hydrologic routing is used to size the flood control volume, the EURV remains the same as calculated by these equations and is included in the facilities stage/storage configuration for modeling. 100-Year Volume. A simplified equation can be used to determine the required 100-year full spectrum detention volume for tributary areas less than 10 acres. This volume includes the EURV (and the EURV includes the WQCV). The City of Durango does not recommend adding additional volume above that provided in Equation 5-4. If a more detailed analysis is desired, see Table 5-2. Where: i 0.013i ) A% (0.033i 0.035i ) B% V 100 P100A Equation 5-4 (0.028i 0.037i ) CD% V 100 P 100 A i = 100-yr detention volume (ac-ft), = 100-yr one-hour rainfall depth (inches), = Area of watershed (acres), = imperviousness percentage expressed as a decimal, March 2017 City of Durango 5-13

18 Chapter 5 A%, B%, and CD% = indicates percentage of each NRCS soils type (expressed as a decimal) Design Worksheets The Full Spectrum Worksheet in the Durango-FSD workbook performs all of these calculations for the standard designs. 5-5 Release Rates Year Allowable Release Rate The maximum allowable 100-year release rate for a full spectrum detention facility is equal to 90 percent of the predevelopment discharge for the upstream watershed. This release rate for full spectrum detention basins has been shown to be effective in controlling future development peak discharges in a watershed to levels below predevelopment conditions in the 2-, 5-, 10-, 25-, 50-, and 100-year events downstream of multiple detention basins. The predevelopment 100-year unit discharge (cfs/acre) for specific soil types per acre of tributary catchment varies based on the ratio of the flow length squared to the watershed area as well as the watershed slope and is provided in 5-14 City of Durango March 2017

19 Table 5-3. Definitions of the slope factor and shape factor are provided as follows: Slope Factor = average overall watershed slope along the flow path length (ft/ft) Shape Factor = L 2 A (where L = watershed flow path length (ft) and A= total watershed area (ft 2 )). The values in these tables must be multiplied by 0.9 to determine the allowable 100-year release from a watershed. When the calculated slope or shape factor for a specific basin fall outside the range shown in March 2017 City of Durango 5-15

20 Chapter 5 Table 5-3, the maximum or minimum factor shall be used to determine the pre-development as appropriate (e.g., if a watershed was calculated to have a slope factor of or greater a slope factor of 0.04 shall be used for calculation purposes). For calculated slope or shape factors which fall between the ranges shown in 5-16 City of Durango March 2017

21 Table 5-3, interpolation should be used to calculate the pre-development peak flow rate. When multiple soil types exist in the watershed, use the table values for each soil type and calculate a weighted average value relative to the area proportion of each soil type. Use Equation 5-5 to calculate the allowable unit discharge from the basin. Where: Q 0. 9aq Equation 5-5 Q = Allowable 100-year release rate (cfs) a = Area of watershed (acres) q = weighted average unit release rate based on relative proportions of watershed soil types (cfs/acre) (Table 5-3) March 2017 City of Durango 5-17

22 Chapter 5 Table 5-3. City of Durango Predevelopment Peak Unit Flow Rates, q (cfs/acre) for Various Precipitation Return Frequencies, Soil Groups, and Slope and Shape Factors HSG A Factor 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year Slope/Shape <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 < <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 < <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 < <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 < HSG B Factor 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year Slope/Shape HSG C/D Factor 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year Slope/Shape WQCV Release Rate The WQCV release rate is determined based on a 12- to 40-hour drain time for extended detention basins. The purpose of this slow release rate is to provide time for pollutants to settle and for the rate of discharge to be less erosive. Equation 5-6, provided in the section below, can be used to develop an estimate of the size of the drain opening area (orifice area) on the outlet control plate for the WQCV. The WQCV and release rates are discussed in more detail in the Water Quality Chapter EURV Release Rate The EURV release rate is determined based on a 72-hour drain time. The purpose of this slow release rate is to reduce the potential for downstream erosion and to release flows that more closely mimic undeveloped conditions. This is accomplished by a control plate placed on the outlet structure with the appropriate orifice sizes and spacing. UDFCD has estimated that the area of the openings/orifices in the control plate based on Equation 5-6. (0.95 H0.085 A o = 88V ) (S /T 0.09) d H ( ) Equation City of Durango March 2017

23 Where: A o = area per row of orifices spaced on 4-inch centers (in 2 ), V = design volume (WQCV or EURV, acre-feet), T d = time to drain the prescribed volume (hrs) (i.e., 12- to 40-hours for WQCV or 72 hours for EURV), H = depth of volume (ft), S = slope (ft/ft). The Durango-FSD Spreadsheet performs these calculations for a set of standard outlet structure designs. 5-6 Design Considerations The design of a detention facility entails detailed hydraulic, structural, geotechnical, and civil design. This includes a detailed site grading plan, embankment design, spillway design, hydraulic and structural design of the outlet works, safety grate design, maintenance access, consideration of sedimentation and erosion potential within and downstream of the facility, liner design (if needed), etc. Collaboration between geotechnical engineers, structural engineers, hydrologic and hydraulic engineers, land planners, landscape architects, biologists, and/or other disciplines is encouraged during the preliminary and final design phases. It is beyond the scope of this Design Manual to provide detailed dam design guidance. There are many excellent references in this regard, such as Design of Small Dams (U.S. Bureau of Reclamation 1987). The City urges all designers to review and adhere to the guidance in such references as failure of even small embankments can have serious consequences for the public and the municipalities downstream of the embankment. As discussed in Section 5-4-1, FSD facilities are configured together with one of five types of water quality basins. The following guidelines for the design of full spectrum detention facilities apply to Zones 2 and 3 as shown in Figures 5-4 through General Layout and Grading facility geometry and layout are often best developed in concert with land planners and a landscape architect. Whenever desirable and feasible, multiple uses of a basin should be considered, such as creation of riparian and wetland vegetation, wildlife habitat, paths, and other passive or active recreation opportunities. If multiple uses are being contemplated, it is recommended that the inundation of passive recreational areas be limited to one or two occurrences a year and of active recreation areas to once every two years. Generally, the area within Zone 1 and Zone 2 is not well suited for active recreation facilities such as ballparks, playing fields, and picnic areas, but may be suitable for passive recreation such as wildlife habitat and some hiking trails. It is desirable to shape the water quality portion of the facility (Zones 1 and 2) with a gradual expansion from the inlet and a gradual contraction toward the outlet, thereby minimizing short-circuiting. Maintenance is also an important consideration with respect to layout and grading. Consider how lower areas of the basin, such as the forebay and micropool will be accessed, and with what equipment. March 2017 City of Durango 5-19

24 Chapter Volume The total 100-year storage volume determined using one of the three methods described in Section 5-4 shall be provided, along with additional basin storage and depth necessary to contain emergency flows and provide freeboard as described in Section Embankments Embankment should be designed to not catastrophically fail during the 100-year and larger storms that the facility may encounter. The following criteria apply in many situations (ASCE and WEF 1992): Side Slopes: For ease of maintenance, the side slopes of the embankment should not be steeper than 3(H):1(V), with 4(H):1(V) preferred. The embankment s side slopes should have full vegetated coverage, with no trees or shrubs on the upstream or downstream of the portion of the embankment which is or is anticipated to be retaining water. Trees or shrubs may be installed in the basin floor. Soil-riprap protection (or equivalent) may be necessary to protect it from wave action on the upstream face, especially in retention ponds. Settlement and Compaction: The design height of the embankment should be increased by roughly 5 percent to account for settlement. All earth fill should be free from unsuitable materials and all organic materials such as grass, turf, brush, roots, and other organic material subject to decomposition. The fill material in all earth dams and embankments should be compacted to at least 95 percent of the maximum density based on the Modified Proctor method of ASTM D698 testing. Freeboard: The elevation of the top of the embankment should be a minimum of 1 foot above the water surface elevation when the emergency spillway is conveying the maximum design or emergency flow. When the embankment is designed to withstand overtopping of the undetained peak flow without failure, freeboard requirements may be reduced or waived. Anti-seepage measures may also be required. This topic is covered in detail in FEMA s Technical Manual: Conduits through Embankment Dams (2005) and NRCS s National Design Construction and Soil Mechanics Center Technical Note Filter Diagrams for CO-1 Structures (2003). Construction of a filter diaphragm will be adequate for most scenarios covered in this chapter. If the storage facility is determined to be jurisdictional per the criteria of Colorado Division of Water Resources (DWR), also known as the Office of the State Engineer, the embankment shall be designed, constructed and maintained to meet DWR s most-current criteria for jurisdictional structures. The design for an embankment of a stormwater detention or retention storage facility should be based upon a sitespecific engineering evaluation Emergency Spillways An emergency spillway will generally be needed to convey flows that exceed the primary outlet capacity or when the outlet structure becomes blocked with debris. When the storage facility falls under the jurisdiction of the DWR, the spillway s design storm is prescribed by the DWR (DWR 2007). If the storage facility is not a jurisdictional structure, the size of the spillway design storm should be based upon the risk and consequences of a facility failure (e.g., avoidance of a critical facility). Generally, embankments should be fortified against and/or have spillways that, at a minimum, are capable of conveying the 100-year peak runoff from the fully developed tributary area (prior to routing flows though the detention basin). However, 5-20 City of Durango March 2017

25 detailed analysis and determination of downstream hazards (such as critical facilities) should be performed and may indicate that the embankment protection and/or spillway design needs to be designed for events larger than the 100-year design storm. An emergency spillway also controls the location and direction of the overflow. The emergency spillway and the path of the emergency overflow downstream of the spillway and embankment should be clearly depicted on the construction plans. Structures (such as utility boxes) should not be permitted in the path of the emergency spillway or overflow. It is recommended that the limits of the flow path be shown on the construction drawings. Soil riprap is the most common method for providing embankment protection on a spillway. Although not preferred, baffle chute spillways may also be considered on a case by case basis. Further discussion regarding these two types of embankment protection is provided below Soil Riprap Spillway Soil riprap embankment protection should be sized based on methodologies developed specifically for overtopping embankments. Two such methods have been documented by Colorado State University (USNRC, 1988) and by the US Department of Agriculture (ASAE, 1998) and designers are referred to these publications for a complete description of sizing methodology and application information. Figure 5-13 illustrates typical rock sizing for small (under 10-feet high) embankments based on these procedures that may be used during preliminary design to get an approximate idea of rock size. Final design should be based on the more complete procedures documented in the referenced publications. The thickness should be based on the criteria identified in the Open Channels Chapter for steep channels. For spillway design, it is critical that the soil riprap has an adequate percentage of well-graded rock. The invert of the emergency spillway is set at or above the 100-year water surface elevation. A concrete wall is recommended at the emergency spillway crest extending at least to the bottom of the soil riprap located immediately downstream. The top of the crest wall at the sides should extend to the top of the embankment, at least one foot above the spillway elevation Baffle Chute Spillway The USBR has developed design standards for a reinforced concrete chute with baffle blocks on the sloping face of a spillway, commonly referred to as baffled chute drop spillway. The primary reference that is recommended for the design of these structures is Design of Small Dams (1987). In addition, Design of Small Canal Structures (Aisenbrey, et al. 1978) and Hydraulic Design of Stilling Basins and Energy Dissipaters (Peterka 1984) may provide useful information for the design of baffle chute spillways. The hydraulic concept behind baffle chute spillways involves flow repeatedly encountering obstructions (baffle blocks) that are of a nominal height equivalent to critical depth. The excess energy is dissipated through the drop by the momentum loss associated with reorientation of the flow. Design of Small Dams provides guidelines for sizing and spacing the blocks. Designing for proper approach velocities is critical to structure performance. One advantage of this type of spillway is that it does not require any specific tailwater depth. However, the designer does need to consider local flow and scour patterns in the transition back to the channel. For safety reasons and considerations of appearance, a baffle chute spillway is not recommended for use as a grade control structure in a stream. Caution is advised when using a baffle chute spillway in a high debris March 2017 City of Durango 5-21

26 Chapter 5 area because the baffles can become clogged, resulting in overflow, low energy dissipation, and direct impingement of the erosive stream jet downstream. A step by step procedure for the design of a baffle chute drop spillway is provided in Design of Small Dams Outlet Structure Outlet structures control release rates from storage facilities and should be sized and structurally designed to release flows at the specified rates without structural or hydraulic failure. Sizing guidance is provided earlier in this chapter with additional guidance in Volume 3 of the USDCM. The most common design consists of a configuration that releases the WQCV (Zone 1) and the balance of the EURV Figure 5-7. Flush Grate (Sloping Drop Box Shown) (Zone 2) through an orifice plate (typically a steel plate containing a vertical column of small, equally-spaced orifices. The 100-year volume (Zone 3) above the EURV is then controlled by an orifice at the bottom of the outlet vault structure, or drop box, after spilling over the crest of the drop box. The crest of the drop box acts as a weir and its length, as well as the size of the drop box opening, needs to be oversized to account for flow area reduction by the safety grate bars and blockage by debris. Figure 5-14 provides guidance for determining initial minimum trash rack sizes for an outlet structure. Values from this figure account for clogging and metalwork losses through the safety grate. In addition to using Figure 5-14, also ensure that the velocity through the grate unhindered by debris blockage does not exceed 2 feet per second. Additional discussion regarding safety grates and debris blockage can be found in Section Design procedures for analyzing drop box hydraulics and accounting for debris blockage are described in Section The hydraulic capacity of the various components of the outlet works (orifices, weirs, pipes) can be determined using the Durango- FSD Workbook, or other standard hydraulic equations. A rating curve for the entire outlet can be developed by combining the rating curves developed for each of the components of the outlet and then selecting the most restrictive Photograph 5-4. Example of an FSD outlet structure. Note the WQCV and EURV orifices have been incorporated into the concrete outlet structure. A steel orifice plate can also be used (Photo Credit: UDFCD). element that controls a given stage for determining the composite total outlet rating curve. The following sections describe procedures to generate a rating curve for four example types of 100-year drop box outlet structures. The Durango-FSD workbook uses the procedures described below to automatically size and 5-22 City of Durango March 2017

27 generate an outlet rating curve based on the input provided by the user. See Volume 3 of the USDCM for additional information on sizing the water quality orifices and incorporating water quality features into the outlet structure Flush Safety Grate A flush grate drop box is a grate, either bar or close mesh, that is flush with the top of the box opening. The box opening may be horizontal or constructed with the slope of the embankment (as shown in Figure 5-7). The top of the outlet box should be evaluated for both weir (A) and orifice (B) flow at increasing water depths. The lesser of the two calculated flow values will indicate which controls for a given depth. Detailed discussion regarding weir and orifice hydraulics can be found in Section The net weir length and orifice open area, considering blockage by grating and potential debris, must be applied as discussed in Section UDFCD contracted with Bureau of Recreation (USBR) to construct a physical model to refine weir/orifice calculations for a sloping drop box Raised Grate with Multiple Vertical Openings A raised grate with multiple vertical openings offers improved flow capacity and resistance to debris blockage. It has vertical openings (open bar or close mesh) on two to four sides. See Figure 5-8 for a graphical representation of this grate configuration. This outlet must be evaluated for the two separate flow conditions (listed below and shown in Figure 5-8) to determine which controls at each incremental depth: A. Weir Flow: Calculate weir flow using the drop box interior perimeter reduced for the vertical grate supports and a 10% perimeter reduction for clogging. Figure 5-8. Grate with Vertical Openings (Horizontal Drop Box Shown) B. Orifice Flow: Calculate orifice flow using the smaller of the interior drop box area or the total grate area reduced for metalwork and debris clogging Raised Safety Grate with Vertical Opening A grate with one vertical opening may also offer improved flow capacity and resistance to debris blockage. Figure 5-9 provides a graphical representation of this grate configuration. March 2017 City of Durango 5-23

28 Chapter 5 This outlet must be evaluated for the two separate flow conditions (listed below and shown in Figure 5-9) to determine which controls at each incremental depth: A. Weir Flow: Calculate weir flow using the average of the net perimeter length (i.e., reduced for metalwork and clogging) calculated from the condition shown in Figure 5-7 and Figure 5-9. B. Orifice Flow: Calculate orifice flow using the smaller of the interior drop box area or the total grate area reduced for Figure 5-9. Grate with Vertical Opening (Sloping Drop Box Shown) metalwork and debris clogging. To account for clogging, the vertical grate net opening area should be reduced by 10%, while the horizontal grate net opening should be reduced by 50%. To simplify orifice calculations at various stages for a vertical or sloping grate, use the Durango-FSD workbook Raised Grate with Offset Vertical Openings A grate with offset vertical openings is a bar or close mesh grate that is elevated and extends beyond the sidewalls of the concrete outlet structure. This results in a vertical and horizontal gap between the grate and the walls of the drop box on all four sides of the structure and provides grate area below floating debris similar to a micropool design. Figure 5-10 shows a horizontal grate configuration. The grate could also be sloped. This outlet must be evaluated for two separate flow conditions (listed below and shown in Figure 5-10) to determine which controls at each incremental depth: Figure Grate with Offset Vertical Panels (Horizontal Drop Box Shown) A. Weir Flow: Calculate weir flow over the walls of the drop box using the smaller of the unclogged drop box perimeter or the grate perimeter reduced for metalwork and 10% debris clogging. B. Orifice Flow: Calculate orifice flow using the smaller of the interior drop box area or the total grate area reduced for metalwork and debris clogging Outlet Pipe Hydraulics Once the hydraulics of the top of a drop box are evaluated using the procedures discussed in Sections through 5-6-9, the capacity of the outlet pipe and its orifice plate flow restrictor must be determined for increments of increasing water depth. For standard outlet designs, the Durango-FSD workbook can be used 5-24 City of Durango March 2017

29 to determine the correct location of the orifice plate flow restrictor in order to control the 100-year discharge to the required discharge rate. The discharge pipe of the outlet works should be evaluated to ensure it is not under outlet control as a culvert at the 100-year (or design) discharge, and the orifice plate covering the opening of this pipe in the bottom of the drop box should be evaluated to ensure it limits flow to the required release rate. The stage-discharge relationship of the outlet pipe and orifice is then compared to the controlling stagedischarge relationship for the top of the drop box plus flow through the water quality/eurv orifices may also be added. The ultimate control of the outlet is the smaller value of the flow through the top of the drop box plus water quality/eurv orifices, and the flow through the outlet pipe orifice over the range of stage. The design goal is that the outlet pipe orifice controls flow for the 100-year event, and the grate controls for more frequent return periods. Determining the final hydraulics of the outlet structure becomes an iterative process. A final stage discharge curve is determined by completing the steps outlined above. This stage-discharge curve and the basin geometry are then input into the Durango-FSD workbook, or a SWMM model to evaluate hydrograph routing and the associated maximum stage, storage volume, and release rate. Often times it will be necessary to adjust the dimensions of the outlet box or the restrictor plate and orifice area of the outlet pipe to achieve the desired outflow from the basin. The goal is to have the 100-year orifice at the bottom of the box in front of the outlet pipe control the 100-year release rate at the maximum stage, not the hydraulic condition at the top of the outlet box. A final check on the overall safety of the outlet should be made to ensure that the velocity of flow through the grate open area reduced for metalwork but not for clogging does not exceed 2 ft/s Trash Racks and Debris Blockage Trash racks should always be installed as part of an outlet structure to reduce safety concerns. Consider maintenance of the structure and potential access by the public when selecting the type of trash rack. For example, a close mesh grate will be more appropriate in high pedestrian traffic but will require more frequent maintenance as it will catch smaller debris. Trash racks of sufficient size should always be provided on an outlet structure so that they do not interfere with the hydraulic capacity of the outlet. See Figure 5-14 for the minimum open area based on the outlet size. Typically, outlet structure safety grates consist of either a bar grate, a close mesh grate or an open grate as shown in Figure Close mesh and bar grates can be used for horizontal, sloping or vertical surfaces. Open grates are typically only used along vertical openings, as shown in Figures 5-7 through Figure 5-11 provides typical dimensions for the three aforementioned grates as well as appropriate open area values. The open area includes a reduction for both the grate metalwork as well as an assumed 50% potential debris blockage. In some cases it may be appropriate to increase or decrease this value based upon the potential for debris at a specific site. March 2017 City of Durango 5-25

30 Chapter Inlets Figure Typical Grate Configurations for Outlet Structures Inlets should provide energy dissipation to limit erosion. They should be designed in accordance with drop structure or pipe outlet criteria in the Hydraulic Structures Chapter of this Design Manual, or using other energy dissipation structures as appropriate. Additionally, forebays or sediment traps are recommended to provide a location to remove coarse sediment from the system prior to it being deposited in the vegetated area of the basin. Forebays need regular monitoring and maintenance Vegetation The type of grass used in vegetating a newly constructed storage facility is a function of the frequency and duration of inundation of the area, soil types, and the other potential uses (park, open space, etc.) of the area. The City of Durango Engineering Division recommends use of native grasses to reduce frequency and cost of maintenance and help maintain infiltration rates. See the Revegetation Chapter for detailed information on establishing vegetation, including soil testing and amendments, seed mixes, and plantings. A planting plan should be developed for new facilities to meet their intended use and setting in the urban landscape. Trees and shrubs are not recommended on dams or fill embankments. However, use of trees immediately outside of detention basins will not interfere with their flood control operation or increase maintenance needs significantly. Also, sparse planting of trees basins may also be acceptable as long as they are not located near inlets and outlet or on the emergency spillway(s) and will not interfere significantly with maintenance or create clogging problems with the water quality screen. On the other hand, use of shrubs on the banks and bottom, while not affecting the flood routing, can increase maintenance significantly by providing traps for a source of debris and obstructing maintenance procedures. Because storage facilities are frequently wet, they are ideal nurseries for invasive and undesirable plants such as siberian elms, russian olives, tamarisk, etc. This unplanned vegetation should be removed annually Retaining Walls The use of retaining walls within detention basins is generally discouraged due to the potential increase in long-term maintenance access and costs as well as concerns regarding the safety of the general public and maintenance personnel. The design of all retaining walls shall conform to the City of Durango Land Use Development Code (LUDC). Where walls are used, limit the length of the retaining walls to no more than 5-26 City of Durango March 2017

31 50 percent of the basin perimeter. Also, consider potential fall hazards associated with pedestrians, cyclists, and vehicles in determining the appropriate treatment between a sidewalk, path, or roadway and the top of the wall. Considerations include distance from the public to the wall, curvature of the path or roadway, single or terraced walls, and volume of traffic. Potential solutions include dense vegetation, seat walls, perimeter fencing, safety railing and guardrail. In some cases walls less than 2 feet will warrant a hard vertical barrier; in other cases a 3-foot wall may be the point at which this barrier is appropriate. Any retaining wall 30 inches in height or greater shall require a retaining wall permit from the City of Durango. The City of Durango recommends providing a hard vertical barrier in any location where walls exceed 3- feet. Retaining walls shall not be more than eight feet tall. Where taller retaining walls are needed, a series of terraces or benches shall be used. Terraces and / or benches shall be at least five feet wide and shall be planted with approved landscaping. Adequate horizontal separation between terracing walls should be provided to ensure that each wall is loaded by the adjacent soil, based on conservative assumptions regarding the angle of repose. When determining the separation between walls, consider the proposed anchoring system and the required equipment/space needed to repair the wall in the event of a failure. Ensure that failure and repair of any wall does not impact loading on adjacent walls. Separation between adjacent walls should be at least twice the adjacent wall height, such that a plane extended through the bottom of adjacent walls would not be steeper than a 2(H):1(V) slope. Slope of finished grade between walls should not exceed 4 percent. Also consider a drain system behind the wall to ensure that hydrostatic pressures are equalized as the water level changes in the basin. Retaining walls shall be designed and stamped by a Colorado Registered Professional Engineer, specializing in Structural Engineering, under the following conditions: Access 1. Any wall six feet or greater at its highest point, as measured from the top of the wall to the lowest adjacent ground level, not including its foundation depth; or 2. Any wall, four feet in height or greater, where its height exceeds its distance from a right-ofway line or a common property line. All weather stable maintenance access shall be provided to elements requiring periodic maintenance Geotechnical Considerations The designer must take into full account the geotechnical conditions of the site. These considerations include issues related to ground water elevation, embankment stability, geologic hazards, seepage, and other site-specific issues. It may be necessary to confer with a qualified geotechnical engineer during both design and construction, especially for the larger detention and retention storage facilities Linings Sometimes an impermeable clay or synthetic liner is necessary. Stormwater detention and retention facilities have the potential to raise the groundwater level in the vicinity of the basin. Where there is concern for damage to adjacent structures due to rising ground water, consider lining the basin with an impermeable liner. An impermeable liner may also be warranted for a retention pond where the designer seeks to limit March 2017 City of Durango 5-27

32 Chapter 5 seepage from the permanent pool. Note that if left uncovered, synthetic lining on side slopes creates a serious impediment to egress and a potential drowning hazard Environmental Permitting and Other Considerations The designer must take into account environmental considerations surrounding the facility and the site during its selection, design and construction. These can include regulatory issues such as: If construction will create disturbance or otherwise modify a jurisdictional wetland, If the facility is to be located on a waterway that is regulated by the U.S. Army Corps of Engineers as a Water of the U.S., and If there are threatened and endangered species or habitat in the area. There are also non-regulatory environmental issues that should be considered. The City of Durango recommends early discussions with relevant federal, state and local regulators on these issues. Issues may include the following: Potential for encountering contaminated soils during excavation, Proper implementation of design elements to mitigate mosquito breeding (i.e., a micropool), Concern from area residents regarding the disturbance of existing riparian habitat that may be required for construction of the basin, and Colorado water rights issues related to large permanent pools or retention ponds Orifice and Weir Hydraulics The following discussion regarding weirs and orifices is adapted from Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22, Third Edition (Brown et al., 2009) Orifices Multiple orifices may be used in a detention facility, and the hydraulics of each can be superimposed to develop the outlet-rating curve. For a single orifice or a group of orifices, orifice flow can be determined using Equation 5-7. Where: 0.5 Q C ( o Ao 2gH o ) Equation 5-7 Q = the orifice flow rate through a given orifice (cfs), C o = discharge coefficient (0.60 recommended for square-edge orifices), A o = area of orifice (ft 2 ), H o = effective head on each orifice opening (ft), 5-28 City of Durango March 2017

33 g = gravitational acceleration constant (32.2 ft/sec 2 ). If the orifice discharges as a free outfall, the effective head is measured from the centroid of the orifice to the upstream water surface elevation. If the downstream jet of the orifice is submerged, then the effective head is the difference in elevation between the upstream and downstream water surfaces Weirs Flow over a horizontal spillway or drop box crest can be calculated using the following equation for a horizontal broad-crested weir. See Figure 5-12 for a graphical representation of weir flow. Horizontal Broad-Crested Weir: The equation typically used for a broad-crested weir is: Q C BCW L H 1.5 Equation 5-8 Where: Q C BCW L H = discharge (cfs), = broad-crested weir coefficient (This ranges from 2.34 to A value of 3.0 is often used in practice.) See Hydraulic Engineering Circular No. 22 for additional information, = broad-crested weir length (ft), = head above weir crest (ft). Figure Sloping Broad-Crested Weir Sloping Broad-Crested Weir: Figure 5-12 shows an example of a sloping broad-crested weir. The equation to calculate the flow over the sloping portion of the weir is as follows: Q Where: CBCW Z H Equation 5-9 Q = discharge (cfs), March 2017 City of Durango 5-29

34 Chapter 5 C BCW Z H = broad-crested weir coefficient (This ranges from 2.34 to A value of 3.0 is often used in practice.) See Hydraulic Engineering Circular No. 22 for additional information, = side slope (horizontal: vertical), = head above weir crest (ft). Note that in order to calculate the total flow over the weir depicted in Figure 5-12, the results from Equation 5-8 must be added to two times the results from Equation City of Durango March 2017

35 Figure Embankment Protection Details and Rock Sizing Chart (Adapted from: Arapahoe County) March 2017 City of Durango 5-31

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