Technology Matrix Example: Street Improvement/Retrofit

In this example, the site is residential street improvement project performed by a St. Louis County municipality and Phase II permittee, and the project’s land disturbance is 4-acres. The project consists of widening a street from 20 to 26 feet, adding curbs, and adding 4′ wide sidewalks. A project traffic study demonstrates the need for the wider street. The site is located on rolling terrain, and storm inlets exist in several locations along the street. Investigation of existing site resources indicates that existing soils are HSG D. All work is proposed within a 50′ wide existing right-of-way. 15-year 20-minute differential runoff calculations show the additional undetained runoff is <2 cfs.

Stormwater quality management is required, however, flood detention and channel protection is not required on this particular project. The city proposes to use permeable concrete for the new sidewalks, and half of all street pavement can be treated by bioretention tree boxes placed adjacent to existing storm inlets. However, utility conflicts (existing sanitary sewers), grade, and lack of other available right-of-way space for BMPs preclude the use of bioretention on the remaining half. Hydrodynamic separators (proprietary BMPs) are used in the areas where bioretention is not practical. The approach is accepted by MSD.