I.8. - PROCEDURES FOR ASSESSMENT OF STRUCTURAL CONTROLS FOR RETROFITTING WATER QUALITY FEATURES

[Replaces DCM2 Section 4.4, page 4-133]

At some sites, there may be an existing structure for flood control and other water quantity control purposes. It may be possible to retrofit this structure for water quality in addition to the quantity functions. The following procedure will assist in evaluating the potential for retrofitting. In a new or major redevelopment project, new erosion and water quality control BMPs will be required, if retrofitting is not a reasonable option.

The purpose of this document is to outline the procedures for these evaluations. These procedures would then be utilized in conjunction with developing each new Drainage Basin Planning Study (DBPS) to determine the potential and feasibility for retrofitting existing structural controls (detention/retention basins).

The analysis of the structures involves three possible levels of review. The first is a qualitative review to determine if retrofitting of the structure is acceptable. The second element is quantitative to determine the pollutant removal effectiveness of the structure, both with and without water quality elements. Total Suspended Solids (TSS) will normally be the only constituent evaluated, unless other pollutants of concern are specified by the ECM Administrator, based on site-specific information such as draining to sensitive waters or high risk pollution sources. A third element of review involves developing a cost estimate for retrofitting to determine the economic feasibility.

A qualitative assessment evaluates the changes that would occur if the flood-control detention facility was modified for water quality purposes, and determines the extent to which the changes would affect these functions, and if these changes in function are acceptable. The detention pond must first be acceptable under the qualitative criteria, or the evaluation will conclude and not continue to the second level of review. A quantitative analysis involves a determination of whether the percent removal of TSS (or other specified constituent of concern) is significant. For purposes of this assessment, a significant change is defined as the percent removal of the constituent after retrofitting the detention pond is estimated to be at least 20 percent greater than the percent removal of the constituent for the detention pond without the water quality element incorporated. If a significant change is estimated, then the third element of analysis, a cost estimate of the economic feasibility, is conducted. If a significant change is not estimated, then the option to retrofit the detention pond is eliminated.

I.8.1.   Final Alternative Selection

The final alternative selection process for drainage improvement options in any new DBPS is based on the evaluation of many factors including costs, safety, environmental issues including water quality, public input, etc. If the selected alternative includes retrofitting structural controls to provide additional pollutant removal, responsibility for implementation would need to be outlined in the study. If the responsibility was determined to be a public (County) responsibility, consideration for funding any such drainage improvement project would be made by the Board of County Commissions during its annual budget approval process in conjunction with all other budget requests. If the responsibility was determined to be a private development responsibility, ECM Administration would decide when implementation would be required in conjunction with the timing of future developments.