1.5 Additional Protective Measure - Infrastructure Protection

1.5.1 Thresholds

All projects that increase the amount of stormwater to the downstream stormwater conveyance system shall review this section to see if analyses or mitigation are required for the project.

1.5.2 Quantitative Analysis

Projects that impact the downstream system by increasing the amount of stormwater to the downstream private or public stormwater system may be required to complete a quantitative downstream analysis of the downstream system to ensure the system is appropriately sized before a connection to the system will be allowed. The type of analysis required shall be based upon the project impacts as described in Table 1 - 2: Quantitative Analysis Determination below. Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/Site Development Group may require different or additional analyses than those represented in Table 1 - 2: Quantitative Analysis Determination based upon project impacts such as conversions from pervious surfaces to hard surfaces, underdrained facilities, and/or lined facilities or known issues with the existing stormwater system.

Table 1 - 2: Quantitative Analysis Determination

*Increase in Surface Area and/or Increase in Surface Area Converted from Pervious to Impervious Contributing to Downstream System

Smallest Pipe Size within ¼ mile downstream of the site


Required Analysis

< 5,000 SF

Any size

None

≥ 5,000 SF and < 10,000 SF

≥ 12” φ

Pipe Analysis not Required. Inlet and Gutter Capacity Analysis required (as applicable).

≥ 5,000 SF and < 10,000 SF

< 12” φ

Single Segment Capacity Analysis and Inlet and Gutter Capacity Analyses (as applicable).

≥ 10,000 SF

≥ 12” φ

Single Segment Capacity Analysis and Inlet and Gutter Capacity Analyses (as applicable).

≥ 10,000 SF

< 12” φ

Full Backwater Analysis and Inlet and Gutter Capacity Analyses.

Any Size

Connecting to a City-owned and identified trunk main

Pipe Analyses Not Required but Inlet and Gutter Capacity Analysis Required if increase in surface area and/or increase in surface area converted from pervious to impervious contributing to downstream system is 5,000 SF or greater and a connection to the curb is made.


* Comparison shall be made between existing land cover conditions and proposed land cover conditions. See Volume 5 for additional information about the required analyses. See Volume 5 for additional information about the required analyses. Inlet and Gutter Capacity Analysis, if required based on impact, is only required for projects proposing through curb discharges.

1.5.3 Mitigation – Infrastructure Protection

Projects that discharge stormwater directly or indirectly to any of the following may have to provide mitigation for Infrastructure Protection:

To a conveyance system without capacity to convey the fully developed design event as determined through a full backwater quantitative analysis and/or Inlet and Gutter Capacity Analysis, or

To a capacity problem downstream of the project as determined by Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001, or

To any other problem, such as downstream stabilization issues, as determined by Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001.

The type of mitigation is dependent upon Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001, review of the project impacts. Mitigation may include upsizing the existing stormwater conveyance system, installing additional catch basins, onsite stormwater detention or other mitigation measures as determined appropriate by Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001.

1.5.4 Objective

To identify and evaluate offsite water quantity, erosion, slope stability, and impacts to receiving waterbodies that may be caused or aggravated by a proposed project, and to determine measures for preventing impacts and for not aggravating existing issues.