SECTION 4-03.0.0:  DESIGN STANDARDS FOR STORMWATER DETENTION AND RETENTION
Due to the size and nature of this manual, it is unable to be accommodated within the Technical Standards Manual. The manual is available online as the Pima County Regional Flood Control District Design Standards for Stormwater Detention and Retention, June 2014 version (Last Revised February 2015).
City of Tucson Amendments to the Pima County Regional Flood Control District Design Standards for Stormwater Detention and Retention, June 2014 version (Last Revised February 2015)
Section 1.1 Purpose
The following paragraph;
1.   Require first-flush retention that should be located throughout the development. The retained volume may also be used to meet all or part of the project's detention volume requirement;
Revise to read;
1.   Require first-flush retention that may be located throughout the development. The retained volume may also be used to meet part of the project's detention volume requirement;
Section 1.2 Ordinance Overview and Detention Requirements
Section 1.2.2
The following paragraph;
b.   New Development located within a Critical Basin must provide a sufficient combination of retention and detention to reduce the post-develop 2-,10-, and 100- year peak discharge rates to 90% of the pre-developed peak discharge rates. Other reductions may be specified by the Floodplain Administrator
Revise to read;
b.   New Development located within a Critical Basin must provide a sufficient combination of retention and detention to reduce the post-develop 2-,10-, and 100- year peak discharge rates to 85% of the pre-developed peak discharge rates. Other reductions may be specified by the Floodplain Administrator
Add the following paragraph;
c.   Properties within the City of Tucson that are currently in undesignated basins, shall be treated as balanced basins
Section 2.4.1 Stormwater Harvesting Basins
The following paragraph;
The maximum effective retention depth is 9 inches. If the basin is sloped, the depressed area may contain internal berms or check dams to pond water in multiple cells with planting areas on the upstream side of the berm. Each cell shall not retain greater than 9 inches of water.
Revise to read;
Without infiltration testing the maximum depth of retention in detention basins is 9 inches. If the basin is sloped, the depressed area may contain internal berms or check dams to pond water in multiple cells with planting areas on the upstream side of the berm. Retention depths greater than 9" require infiltration testing to verify that the draw down time is in accordance with section 4.5
Section 2.5 Retention within Detention Basins
The following paragraph;
Detention basins with uniform side slopes and no terraces may incorporate retention for a maximum depth of 9" below the lowest outlet elevation. The volume provided below the outlet can be counted as retention volume.
Revise to read;
Detention basins with uniform side slopes and no terraces may incorporate retention for a maximum depth of 9 inches below the lowest outlet elevation. The volume provided below the outlet can be counted as retention volume. If infiltration testing is provided, the retained depth may be increased.
The following paragraph
Terraces may be proposed within a detention basin in order to meet riparian habitat or native plant requirements. When vegetated terraces and bioretention areas are proposed, the maximum retention depth may be increase to 18 inches. Section 5.8 contains standards for retention within detention basins.
Revise to read;
Terraces may be proposed within a detention/retention basin in order to meet riparian habitat or native plant requirements Section 5.8 contains standards for retention within detention basins.
Section 3.1 Peak Discharge Rate Reduction Requirements
The following paragraph;
Within a Critical Basin, post-developed peak discharge rates shall not exceed 90% of pre-developed peak discharge rates at the project boundary, unless a different percentage is specified by the Floodplain Administrator.
Revise to read;
Within a Critical Basin, post-developed peak discharge rates shall not exceed 85% of pre- developed peak discharge rates at the project boundary, unless a different percentage is specified by the Floodplain Administrator.
Section 3.2.1 Pre- Developed Conditions
The following paragraph;
The pre-developed conditions peak discharge rates for each return period (Qpre -rp) shall be established using the modeling methods described in the District's Technical Policy, TECH-015, Acceptable Methods for Determining Peak Discharges, and Technical Policy, TECH-018, Acceptable Model Parameterization for Determining Peak Discharges, and any other technical policies as specified by the Floodplain Administrator. The District's Technical Policies are available through the Rules and Procedures page of the District's web page.
Revise to read;
The pre-developed conditions peak discharge rates for each return period (Qpre -rp) may be established using the City of Tucson STANDARDS MANUAL FOR DRAINAGE DESIGN AND FLOODPLAIN MANAGEMENT, modeling methods described in the District's Technical Policy, TECH-015, Acceptable Methods for Determining Peak Discharges, and Technical Policy, TECH-018, Acceptable Model Parameterization for Determining Peak Discharges, and any other technical policies as specified by the Floodplain Administrator. The District's Technical Policies are available through the Rules and Procedures page of the District's web page.
Section 4.9.1 Outlet Structure Standards
Add the following as paragraph 8.
8.   A bleed pipe or other outlet may be added to the basin provided that its discharge rate and invert elevation meets the requirements of section 9. Unless a waiver has been granted per Section 9, the bleed pipe shall be capped and for maintenance purposes only.
Section 4.13.2 Underground Storage Prohibitions
The following paragraph;
3.   Providing first flush retention underground is prohibited
Revise to read;
3.   Providing first flush retention underground is prohibited, without prior approval of the Floodplain Administrator
Section 5.8 Retention within a Detention Basin
The following paragraph;
3.   A maximum of 9 inches of retention is allowed, unless designed as describe below. The depth is measured from the lowest elevation on the basin floor to the lowest outlet invert elevation
a.   The retention depth of up to 18 inches may be allowed if the following conditions are met. See Figure 5.5
Revise to read;
3.   A maximum of 9 inches of retention is allowed, unless designed as described below. The depth is measured from the lowest elevation on the basin floor to the lowest outlet invert elevation
a.   A retention depth of greater than 9 inches may be allowed if the following conditions are met. See Figure 5.5
The following paragraph;
iv.   The remaining 50% of the basin area may be constructed so that the maximum retention depth is 18 inches. This area shall meet the construction requirements of part 1 of this Section.
Revise to read;
iv.   The remaining 50% of the basin shall meet the construction requirements of Part 1 of this section
Section 9 DETENTION WAIVER REQUEST AND PAYMENT OF IN-LIEU FEE
The following section Title;
Section 9 DETENTION WAIVER REQUEST AND PAYMENT OF IN-LIEU FEE
Revise to read;
Section 9 DETENTION/RETENTION WAIVER REQUEST AND PAYMENT OF IN-LIEU FEE
Add the following as the first paragraph;
A waiver for retention may be requested for situations where the volume of retention cannot be contained within the development property or where tests show that the soils have poor infiltration. The waiver may request a reduction in retention volume or the transference of design consideration from retention to detention with a maximum discharge rate of 1-cfs for sites in critical basins or 3-cfs for sites in balanced basins. These maximum discharge rates may be achieved by using a bleed pipe or weir in which the invert of the bleed pipe or weir is a maximum of 6 inches above the basin floor. The waiver may be granted by the engineering reviewer as part of the development review process. The approved waiver request will be included in the Drainage Report or Drainage Statement for the project.
Section 11.2.2 General and Permitting Notes
The following paragraphs;
3.   When a project has drainage and grading improvements that are required to mitigate off-site adverse impacts to the proposed development, an As-Built Certification or plan shall be prepared and submitted to the Floodplain Administrator. The As-Built Certification or plan requires approval prior to the issuance of any building permits, except for model home permits and any permits necessary to build drainage infrastructure.
Provide the following Permitting Note to disclose this requirement:
"Prior to issuance of any building permits an As-Built Certification (Plan) of the drainage and grading improvements that are required to mitigate off-site adverse impacts to the project shall be prepared and submitted to the Floodplain Administrator. Upon approval of the as-built plan by the Floodplain Administrator, the hold to issuance of building permits can be removed."
Revise to read;
3.   When a project has drainage and grading improvements that are required to mitigate off-site adverse impacts to the proposed development, an As-Built Certification or plan shall be prepared and submitted to the Floodplain Administrator.
Provide the following Permitting Note to disclose this requirement:
"An As-Built Certification (Plan) of the drainage and grading improvements that are required to mitigate off-site adverse impacts to the project shall be prepared and submitted to the Floodplain Administrator."
(Am. Res. 23289, 1/5/2021)