# Detention Basins and BMPs Under MSD's 2027 Rules: Build Details, Reports and Getting Your Deposit Back > MSD's 2027 draft basin and BMP details: outlet structures, trash racks, liners, vegetation, as-builts, the BMP hand-off meeting and deposit release. Canonical: https://kueselinc.com/guides/msd-2027-detention-bmp-construction-closeout Last updated: 2026-11-09 > Rule status, checked November 9, 2026: MSD's website still labels the 2027 Rules and Regulations and Engineering Design Requirements a draft (dated July 1, 2026). MSD posted its responses to public comment on October 15, 2026, and its proposed schedule applies the 2027 rules to plans first submitted on or after January 1, 2027; plans submitted earlier stay under the 2018 rules. Section numbers here come from the draft and could change in the final version. MSD's 2027 rules make basins and BMPs bigger, and they also change how basins are built and closed out. The draft folds the old detention review checklist into the rules themselves, adds build details for outlet structures, trash racks, embankments and vegetation, and requires two separate sealed reports. At closeout, developers who hand a project to an HOA or other end user must hold a "BMP hand-off" meeting before MSD releases the rest of the BMP deposit. The sizing changes (the 1.25-inch water quality rainfall and the larger detention storms) are in our [overview of MSD's 2027 rules](https://kueselinc.com/guides/msd-2027-rules). This guide covers what happens after sizing. Kuesel builds basins and BMPs to the approved plans; your civil engineer designs them and MSD reviews and inspects them. ## Two reports instead of one Any project with stormwater BMPs, stormwater credits or basins now needs two separate reports, each with a table of contents and sealed by a Missouri professional engineer: a Stormwater Management Facility Design and Calculations report, and a Stormwater Management Facility Operation and Maintenance Plan. The O&M plan matters to owners because it describes the maintenance they will be doing for years, including upkeep of any prairie plantings. ## Build details now written into the rules The 2018 rules carried many of these items in Exhibit 4-A, a review checklist. The draft deletes the checklist and puts its items into the rules, with some additions: - **Outlet structures:** reinforced concrete, at least 8 inches thick with one row of steel or 10 inches with two rows. No brick. Details on the plans. - **Low-flow openings:** the smallest is 1.5 inches. Openings 6 inches wide or less need a trash rack at least 12 inches wide, extending up to the next opening's sill or 24 inches, whichever is less. A concrete headwall goes around any protruding low-flow pipe. - **Wet basins:** discharge pipes at least 3 feet above the bottom, with incoming flowlines no higher than normal pool. - **Pipe:** minimum flowline slope for CMP of 0.50 percent. - **Embankments:** if the embankment contains fill, a geotechnical report may be required. Dams 35 feet or taller need Missouri DNR approval. - **Freeboard:** at least 1 foot from top of basin to maximum ponding. - **Overflow grates:** if the overflow structure has a grate top, the open area is used in the calculations with a 50 percent clogging factor. - **Subdivisions:** detention goes in common ground. - **No wetland mitigation** in BMPs or detention basins. Some channel and wetland work may need Corps 404 and DNR 401 permits. ## Liners, seepage and soils The draft says any stormwater feature must consider downhill properties and the potential for seepage, water table fluctuation or other nuisance, and impermeable liners may be required. For modeling, vegetated areas inside the post-construction disturbance limits are assumed to be hydrologic soil group D unless soil amendments are used. On a site with compacted clay, that assumption reflects what the ground actually does after grading. ## Vegetation The design engineer must specify the final vegetation method on the plans. MSD encourages prairie planting per its Landscape Guide for Stormwater BMP Design, encourages educational signage at prairie areas, and expects prairie maintenance to be covered in the O&M plan. MSD said in its comment responses that, after the water quality rainfall rose about 10 percent, maximum ponding depths in filter-only BMPs such as bioretention will rise roughly 10 percent to match. ## Closeout: as-builts, the hand-off meeting and deposits - **As-builts:** release of the stormwater quality deposit is tied to MSD-approved BMP as-built plans and an electronic points file where applicable. - **Deposit release:** MSD releases up to 75 percent of the BMP deposit with the BMP Construction Approval letter. On sites with multiple permitted BMPs, MSD will consider releasing it in proportion to BMPs built versus BMPs still to be built. - **BMP hand-off meeting:** where the developer is not the end user, for example a subdivision handed to an HOA, the developer must schedule and attend a hand-off meeting with the end user after all BMPs are built. MSD holds the balance of the deposit until that meeting has happened, the drainage area is stable and all BMPs are working. MSD said the meeting is a chance to educate, answer questions and settle any disagreements about what was approved. - **Payment:** the draft adds ACH and online credit card payment for deposits. A letter of credit must be valid for at least three years or renewable annually and covers only one deposit. Deposit refunds go to the Developer of Record. ## What this means for owners and GCs - Ask for both reports early; the O&M plan is your maintenance budget. - Make sure plans show the outlet structure, trash rack and vegetation details MSD now requires. - Plan for liners where a basin sits near basements or downhill neighbors. - Keep the drainage area stable. MSD won't release the balance of the deposit until it is. - Put the hand-off meeting on the closeout schedule if an HOA or tenant takes over. Kuesel builds [bioretention basins](https://kueselinc.com/bioretention-basins), detention and [storm sewer](https://kueselinc.com/services/storm-sewer) systems under MSD permits; see our [St. Louis County bioretention retrofit](https://kueselinc.com/projects/st-louis-county-bioretention-retrofit) and [construction BMPs guide](https://kueselinc.com/guides/construction-bmps-erosion-sediment-control). Call 636-978-3478, Monday through Friday, 7:00 AM to 4:00 PM. ## Questions ### What reports does MSD require for stormwater BMPs under the 2027 rules? Two separate reports sealed by a Missouri professional engineer: a Stormwater Management Facility Design and Calculations report and a Stormwater Management Facility Operation and Maintenance Plan. ### What is MSD's BMP hand-off meeting? Under the 2027 draft, when the developer isn't the final occupant, such as a subdivision with an HOA, the developer must schedule and attend a meeting with the end user after all BMPs are built. MSD holds the balance of the BMP deposit until the meeting has happened and the BMPs are working. ### How much of the BMP deposit does MSD release at construction approval? Up to 75 percent with the BMP Construction Approval letter, with proportional release considered on sites with multiple permitted BMPs. ### Does MSD require trash racks on basin outlets? Yes, for low-flow openings 6 inches wide or less. The rack must be at least 12 inches wide and extend up to the next opening's sill or 24 inches, whichever is less. General information from a site contractor, not engineering or legal advice. Confirm requirements with your civil engineer and MSD. ## Sources - [MSD, Rules, Regulations & Design Requirements](https://msdprojectclear.org/doing-business/development-review/rules-regulations-design-requirements/) - [2027 draft redline (sections 4.060, 4.080, Chapter 8 and 9)](https://portal.laserfiche.com/Portal/DocView.aspx?id=9591989&repo=r-a96260ce) - [MSD responses to comments (Oct 15, 2026)](https://portal.laserfiche.com/Portal/DocView.aspx?id=9676505&repo=r-a96260ce) ## Related - [Contact us](https://kueselinc.com/contact) - [MSD's 2027 Rules: What Changes for St. Louis Site Development](https://kueselinc.com/guides/msd-2027-rules) - [Redeveloping a St. Louis Site Under MSD's 2027 Rules: Existing Pavement, Cumulative Disturbance and Additions](https://kueselinc.com/guides/msd-2027-redevelopment-stormwater) - [Building Near a Floodplain Under MSD's 2027 Rules: Studies, Elevations and Parking](https://kueselinc.com/guides/msd-2027-floodplain-requirements) - [Storm Sewer and Inlet Design Under MSD's 2027 Rules: What Changes in the Field](https://kueselinc.com/guides/msd-2027-storm-sewer-inlet-design) - [MSD Work](https://kueselinc.com/msd) - [All resources](https://kueselinc.com/resources)