(A) Hydrology.
(1) Peak flow values and inflow hydrographs used for the analysis and design of detention basins shall be determined using the methodology specified within §§ 152.30 - 152.39. For purposes of detention basin analysis and design, inflow/outflow peak flow values and hydrographs shall be evaluated for the 2-, 10- and 100-year 24-hour storm return periods.
(2) When preparing plans utilizing on-site detention, the engineer should consider the impact of the retarded peak flow rates on flows from upper portions of larger drainage basins. That is, the analysis should evaluate how the detained runoff will impact the overall basin discharge rate. Cases may be encountered where on-site detention facilities would actually increase the peak discharge in the receiving stream or would have no beneficial affect. When such cases are encountered, the hydrological analysis should be included together with a request that the on-site detention be waived due to its potential detrimental effect on the receiving stream.
(B) On-line detention basins.
(1) An on-line detention basin intercepts all runoff from the upstream watershed and routes it through an outlet structure in a reduced or metered manner. This is a common type of basin that is typically used to regulate flows from on-site watersheds associated with new development. On-line basins are not recommended on natural alluvial watercourses that convey significant amounts of sediment.
(2) The outflow hydrograph and the basin volume requirements associated with an on-line detention shall be determined using the Modified Puls reservoir routing method. Commercially available software packages that perform basin routing, other than HEC-1, may also be acceptable for use in the town, if written approval is received from the Public Works Department. Figure 5-1 provides an illustration of the typical hydrograph performance of an on-line basin. Detailed, step-by-step descriptions for these procedures are presented in a number of readily available publications including:
Handbook of Applied Hydrology (1964) by V.T. Chow.
Soil Conservation Service, National Engineering Handbook, Section 4-Hydrology (Latest release).
Pima County/City of Tucson Stormwater Detention/Retention Manual (1987).
Hydrologic and Hydraulic Training Session Manual (1973) by the Arizona Department of Transportation.
(3) These methods of flood routing assume invariable discharge-storage relationships and level water-surface elevations within the storage reservoir. While these assumptions are adequate for most detention basin routing, these methods should generally not be used for most channel routing applications.
(C) Off-line detention basins.
(1) An off-line detention basin only captures a portion of runoff, allowing a certain volume to flow past the basin. After the flow event has passed, a "bleeder drain" is generally used to dispose of the stored storm water. Off-line basins generally require significantly less storage volume than on-line basins, but can be more costly to construct because of the structural appurtenances necessary to control the quantities of "capture flow" and "bypass flow".
(2) Reservoir routing is generally not required for a single purpose off-line basin. The basin size is typically determined by the volume of storm water contained within the hydrograph located above the design "bypass flow" (Figure 5.1). Reservoir routing is required where the basin drain pipe flow rate has a significant impact on downstream flow rates or when the basin has the combined function of an on-line basin for other smaller watersheds.
(D) On-line retention basins. An on-line retention basin intercepts all flow from the upstream watershed but is not provided with a gravity outlet structure. Typically the basin is designed to permit overflow at the point on the receding limb of the hydrograph that corresponds with the design outflow. The retained flow is then disposed of through infiltration or pumping. Use of this type of basin is typically limited to those situations where a conventional detention basin outlet structure will not function due to lack of grade. On-line retention basins are provided with storage volume equal to the total volume under the 100-year runoff hydrograph that corresponds to the point on the receding limb of the hydrograph that corresponds to the design outflow (Figure 5.1). For watersheds less than 160 acres in area, the Rational Method may be used. For watersheds greater than 160 acres in area, the Clark Unit Hydrograph method shall be used, in conjunction with the 100-year 24-hour storm. The minimum required storage volume is the post-development runoff storage volume from a 10-year 24-hour storm. The minimum volume is calculated using equation 5.1:
V = C*A*P10/12 (Eqn. 5.1)
Where:
P10 = 10-year 24-hour rainfall value, in inches from Table 3.1.
A = Watershed area in acres.
C = Rational Method Runoff coefficient.
V = Runoff volume, in acre-feet.
(E) Outlet structures.
(1) Because the town requires attenuation of the 2-, 10- and 100-year peak flows, multifrequency outlet structures may be necessary in the design of many on-line storm water detention facilities. There are no standardized procedures for the design of optimum multi-frequency outlet structures. The potential combinations of suitable outlets for any particular basin are numerous, and limited only by the creativity and experience of the engineer. Conceptual examples of typical multi-frequency outlet structures are shown on Figure 5.2. Additionally, the engineer is referred to a publication entitled Stormwater Detention Outlet Control Structures, by the American Society of Civil Engineers. This publication provides other information related to the design and construction of multi-frequency outlet structures.
(2) The minimum allowable pipe diameter for outlet structures is 18 inches. However, orifice plates with smaller openings may be attached at the pipe inlet to further reduce the flow capacity of a pipe. Trash racks may be required for pipe and orifice outlets that are 24 inches or less in diameter and for pipe outlets where water borne debris is expected to be present. Trash racks shall be designed to be removable, and have a surface area of at least 10 square feet. Openings in the trash rack should not exceed one-half the area of the outlet pipe for mesh screens, or one-third the diameter of the outlet for bar screens. Design of the trash rack should consider the likelihood that unclogging may be necessary when the basin is filled with water. Additionally, a concrete pad is required around the inlet in order to facilitate maintenance of the trash rack.
Figure 5.1: Hydrograph Performance for Various Basin Types
(F) Safety guidelines.
(1) Basin side-slopes and depths. The following slope/depth ratios are required for multi-use basins and basins that have unrestricted access:
(a) A maximum of 2:1 for protected side-slopes and a maximum water depth of two feet, and a maximum of 3:1 for unprotected side-slopes where depths of ponding are less than three feet.
(b) A maximum of 4:1 for side-slopes where depths of ponding are three feet or greater.
(2) Benches. A benched slope configuration is required for basins in excess of six feet deep. Benches within basins shall be proportioned so the bench width is at least three times the height of the slope above it, measured from the lowest point on the top of the slope above the bench. The minimum width of a bench shall be six feet. The maximum number of benches allowed is three. The maximum depth to the first bench, and subsequent benches, shall conform to the previous slope/depth ratios.
(3) Security barriers. Basins designed in accordance with the previously listed requirements should preclude the need for security barriers. However, detention basin design should always consider safety aspects of the facility. In the following instances security barriers are required. These barriers may consist of vegetation, masonry, wood, or chain-link. Vegetation, or a combination of vegetation and structural materials, is preferred.
(a) Security barriers shall be provided to prevent access for all basins where depths of ponding exceed three feet.
(b) Vegetative barriers must be of a width equal to or greater than overall height, with a density sufficient to restrict access. If vegetative screening is to be used, plant materials must be in place and established at the time the occupancy permit is requested.
(c) A minimum 42-inch barrier height is required.
(d) Detailed sections of proposed fences, if required, are to be shown on paving and grading plans or development plans, as appropriate.
(e) Security barriers, if required, shall not restrict the hydraulic capacity of structures.
(4) State of Arizona dam safety requirements. The Arizona Department of Water Resources (ADWR), Division of Safety of Dams, has legal jurisdiction over all dams (embankments) which exceed certain height and storage limits. A "jurisdictional dam" as defined by ADWR, is "...either 25 feet or more in height or stores more than 50 acre-feet. If it is less than six feet in height, regardless of storage capacity or does not store more than 15 acre feet regardless of height, it is not jurisdictional." The ADWR should be contacted regarding specific dam-safety requirements in conjunction with the design of any embankment that might come under their jurisdiction. Record Drawings shall be submitted in accordance with § 152.16.
(5) Emergency spillway requirements.
(a) Emergency spillways shall be provided for all stormwater storage basins. For basins with all of the design storage volume situated below existing grade (i.e. without a berm/dam), the spillway may be nothing more than grading to ensure that basin overflows will follow the downstream predevelopment drainage pattern in a safe manner.
(b) Emergency spillways shall be designed to safely convey the peak discharge from the storm listed in the table below entitled Emergency Spillway Design Capacity Requirements, exclusive of the attenuation effects of the basin.
Emergency Spillway Design Capacity Requirements
For An Embankment Berm/Dam That Is Not Regulated by ADWR | |
Berm/Dam Height*
| Spillway Design Capacity
|
H < 6 ft. | Unattenuated 100-year Inflow |
6 ft. H < 25 ft. | 1/2 Probable Maximum Flood |
* Where berm/dam height is the vertical distance from the lowest point along the downstream slope to the crest of the emergency spillway.
1. 100-year inflow is the unattenuated peak discharge from the pre- or post- development 100-year or six hour or 24 hour storm, whichever is larger.
2. Refer to § 152.62(F)(4) for information regarding dams regulated by ADWR.
(c) Emergency spillways shall be designed to convey the design peak discharge and provide erosion protection.
(d) Down-gradient properties are to be protected from flow depths and velocities in excess of pre-development conditions.
(e) A one foot minimum freeboard is required between the berm crest and the water surface elevation of the peak discharge in the emergency spillway (without attenuation from basin storage), except where the berm crest is designed to function as the emergency spillway.
(f) The finished floor elevation of adjacent structures shall be at least one foot above the peak water surface elevation of the flow passing through the emergency spillway.
(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15; Am. Ord. 938, passed 11-18-21)
Figure 5.2: Exam ples of Multip le- Frequ ency Outlet Struct ures