3.20 Infiltration Systems, Drywells and French Drains
a. Infiltration systems, drywells, and french drains are not allowed as the exclusive method for draining public right-of-ways but may be used for developments on private property for buildings, paved driveways, parking and loading spaces, subject to the all of the following conditions:
1) There are no public storm drain facilities, available within a reasonable distance of the development as determined by the City Engineer. The need to acquire easements across private property to access a public storm drain facility that is within a reasonable distance shall not be grounds for allowing an infiltration system unless all other criteria are met.
2) If a design based on stormwater infiltration is proposed, soils infiltration tests shall be performed by a registered Professional Geotechnical Engineer licensed in the State of Oregon to document the permeability and infiltration capacity. The Geotechnical Engineer shall develop a recommended infiltration testing methodology using test methods and sound engineering principles appropriate to the specific site being tested Test methods proposed must demonstrate infiltration capacity of the site soils, as opposed to percolation capacity. A detailed summary of the proposed methodology and test procedures shall be submitted to the Public Works Director a minimum of 7 business days in advance of the proposed testing, for review and comment by the City Engineer.
Infiltration tests shall be conducted at the location and depth of the proposed Infiltration facility. The Geotechnical Engineer shall perform a field evaluation of the soils to demonstrate that the highest seasonal water table is not within 5 feet of the ground surface, or within 2 feet of the bottom of the proposed infiltration facility. A final infiltration report stamped by the Geotechnical Engineer shall be provided with the design drawings submitted for review by the City.
3) The system shall be engineered to ensure that adequate reserve capacity is available. Adequate reserve capacity shall include all runoff assuming the maximum amount of impervious area allowed by City Code based on zoning, and shall include an instrument recorded against the property reserving an area adequate for a replacement infiltration system equal in size to the primary system.
4) Provisions shall be made for grease and fines removal, including recording of a maintenance agreement (acceptable to the Director & City Attorney) against the property.
5) The site shall be graded so that it does not drain onto a public right-of-way without a storm drain system or neighboring property in the event that the drywell or french drain fails. The site and adjacent down gradient areas shall have no history of groundwater surfacing or being within 12-inches of the ground surface during the wet winter months, and shall not have field tile systems which may convey the infiltration water onto neighboring property.
6) The design, excluding french drains shall include pretreatment conforming with Oregon DEQ standards for groundwater injection wells, shall be acceptable to the City Engineer, and shall be approved by and registered with DEQ prior to final plan approval by the City.
b. DEO Registration Required. Where drywells, french drains or other infiltration systems are authorized & allowed, they shall be registered with the Oregon DEQ as required by OAR 340-044-005 through 340-044-055 prior to final approval by the City and construction (in addition to the standards above). Only DEQ "rule authorized" infiltration systems are acceptable, unless otherwise approved by DEQ and the Director. Under these regulations, stormwater dry wells are "rule authorized" if they meet certain highly restrictive standards (conformance with current DEQ/EPA standards must be confirmed prior to submittal for review by the City). These standards restrict the use of dry wells under the rule authorized provisions to the following.
1) No other method of storm water disposal, including construction or use of surface discharging storm sewers or surface infiltration systems, is appropriate. An appropriate method shall protect groundwater quality and may consider management of surface water quality and watershed health issues.
2) No domestic drinking water wells are present within 500 feet of the injection system.
3) The injection system does not exceed a depth of 100 feet and the bottom of the infiltration structure is a minimum of 10 feet above the highest seasonal groundwater level.
c. It should be noted that DEQ standards consider water draining from building roof areas (that has not been mixed with any other stormwater) differently, in that it can be discharged in a dry well without the same level of treatment required for other types of runoff, although it must still comply with the City and DEQ criteria above and receive DEQ approval prior to final City approval or construction.