3.18 Detention Pond Design Requirements
a. The City encourages the use of detention ponds that serve more than one development. A facility that serves more than one development will be referred to as a sub-regional facility. To maximizing the development area, reducing the overall maintenance requirement, and minimizing the overall construction cost subregional facilities are preferred.
b. The City also encourages applicants to design detention ponds to function as multipurpose facilities (i.e. parks, open space, or recreation facilities), provided that any alternative uses are compatible with the primary stormwater functions and maintenance standards.
c. The following design requirements apply to all detention pond designs:
1) Geotechnical Report.
a) Detention ponds shall have a geotechnical report that discusses the site's suitability for the type of stormwater pond being proposed and/or the engineer's recommendations as to how the site shall be improved to make the site suitable for the type of stormwater pond being proposed.
2) Pond Depth
a) The maximum active storage depth is four feet. An exception from this criterion may be approved on a case by case basis if additional safety factors can be shown to address this issue.
3) Bottom Width.
a) For ponds with an active storage depth of three feet or less, the minimum bottom width shall be 10 feet. For ponds with an active storage depth of over three feet, the minimum bottom width shall be 15 feet. An exception from this criterion may be approved on a case by case basis if required by topographical or physical boundary constraints. For the purposes of this bottom width measurement, the width shall be measured at the interior toe of slope.
4) Interior Side Slopes.
a) Interior side slopes shall be no steeper than three horizontal to one vertical (3h:1v). An exception from this criterion may be approved on a case by case basis if required by topographical or physical boundary constraints.
5) Exterior Side Slopes.
a) Exterior side slopes that have vegetated surfaces that require mowing shall be no steeper than four horizontal to one vertical (4h:1v). Exterior side slopes that have vegetated surfaces that do not require mowing shall be no steeper than two horizontal to one vertical (2h:1v}. Exterior slopes shall be landscaped so that there is no exposed soil.
6) Conveyance Outfalls.
a) Pipe outfalls from the conveyance system into the pond shall be flush with interior side slopes. Conveyance outfalls shall be designed with energy dissipation.
7) Outlet Structures.
a) Primary Outlet: Detention ponds shall have a perforated pipe underdrain system to convey water from the pond to the flow control structure.
b) Secondary Outlet: Detention ponds shall have a secondary pond outlet structure, such as a catch basin with grated lid located along an interior side slope. This secondary pond outlet will serve as a backup to convey stormwater to the flow control manhole should the primary pond outlet become clogged. The lip elevation of the secondary pond outlet should be set at approximately the ten-year design water surface.
c) Flow Control Structure: Detention ponds shall have a flow control structure with orifice and weir dimensions sized using an approved engineering method. The flow control structure shall be designed to meet the following criteria:
(1) Detention pond control structures may be either weir or orifice structures located in an enclosed manhole and meet City's standards. Locate the outlet control structure(s) outside the open water storage area.
(2) The control structure shall be designed with an internal overflow device, such as an open top riser, to pass the 25-year design storm event (or 100-year design storm for a sub-regional facility) without allowing runoff to discharge through the emergency spillway and without causing upstream or downstream flooding. The design of the internal overflow shall assume that flow control orifices are plugged during the peak design storm.
(3) Flow control manholes shall have solid locking covers. Open grates shall not be permitted in flow control manholes.
(4) Locate the flow control structure to allow maintenance access. The outlet flow control structure shall require little to no attention for normal operation.
(5) The construction drawings shall include a separate design detail for each flow control structure.
8) Emergency Overflow.
a) All ponds shall have an emergency overflow system that will safely pass runoff from a post-developed 100-year design storm through or around the detention pond and direct flows to the downstream conveyance system. The design intent of the emergency overflow system is to protect the integrity of the pond, as well as associated embankments and downstream properties, during large (rare) storm events and/or failure of the flow control structure.
b) Secondary spillway shall meet the following criteria:
(1) Locate the spillway to direct overflows safely toward the downstream conveyance system.
(2) Locate the spillway in existing soil wherever possible. Protect the spillway with riprap or an approved material that extends to, and is an appropriate distance beyond, the bottom of the berm embankment. Fill the voids of the riprap with soil and vegetate the spillway with grass or ground cover. The selection of the vegetation on the spillway shall consider the required design capacity.
(3) The invert elevation of the spillway shall be a minimum of six inches above the primary overflow elevation.
(4) The minimum spillway depth shall be nine inches from the top of the berm. The free board during the design storm event shall be a minimum of six inches.
(5) Alternate methods to accomplish the design intent of the secondary overflow system will be acceptable as long as they accomplish the same level of protection and are approved by the City Engineer.
9) Swales.
a) All vegetated swales shall use riprap or other methods to minimize and reduce erosion within the swale.
10) Inspection and Maintenance Access.
a) All manufactured systems shall include inspection and maintenance access.
11) Signage.
a) All ponds shall have signs placed so that at least one is clearly visible and legible from all adjacent streets, sidewalks, or paths. Applicants may add an indigenous, native wild bird(s) or wild animal(s) logo on the sign. Sign spacing and content shall be approved by the City Engineer prior to installation. The sign shall read:
(1) Please Do Not Disturb the Vegetation or Wildlife
(2) Silverton Stormwater Management Facility
(3) For More Information, Call Silverton Public Works at 503-873-8679.
b) The minimum sign size shall be 12-inches x 18-inches. The maximum sign size shall be 24-inches by 30-inches. The material shall be aluminum with green reflective sheeting and silk screen lettering or equal as approved by the City Engineer. The signs shall be installed on an 8-foot long by 6-inch by 4-inch treated lumber post which is set in concrete and buried 30 inches into the ground. The developer shall install these signs before the City's final acceptance of the pond.
12) Site Constraints.
a) All publicly-owned detention ponds shall be located in a separate tract dedicated to Silverton for stormwater facilities. Open ponds shall not be located in dedicated public road right-of-way areas.
1) Other Systems – Other detention facility type of systems will be considered with supporting calculations and documentation on case by case basis by the City Engineer