Written and reviewed by the Kuesel field operations & estimating team
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Kuesel is a family-owned site work and utility contractor based in O'Fallon, MO, building sites across St. Louis since 1963.
If any part of your parcel sits in a mapped floodplain, MSD's 2027 rules change the paperwork and a few of the numbers. The draft creates two kinds of floodplain study, a simplified one for work outside the floodway and a detailed one for work in it. It tells reviewers to use the higher of FEMA's effective or draft revised flood data. It keeps parking lots out of the 25-year floodplain (the 2018 rules said 15-year) and requires parking for residential buildings to sit above the 100-year flood level. Building elevations stay tied to the 100-year flood elevation, and strapped plumbing is the one way a basement outside the floodplain can sit below it.
This guide covers Chapter 5 of the 2027 draft and related floodplain items. Our overview of MSD's 2027 rules covers the design-storm changes. Kuesel builds the site; your civil engineer prepares the study and MSD reviews it.
Simplified or detailed study
The draft splits floodplain studies (section 5.010.02) into two types:
| Study | When MSD requires it | What it involves |
|---|---|---|
| Simplified | Building or making a site pad-ready on a parcel partly or fully in the floodplain, with no improvements or grading in the floodway; also to confirm parking is outside the 25-year floodplain | Plan sheets with the FEMA FIRMette and project extents; no hydraulic modeling required, though the municipality may ask for more |
| Detailed | Any proposed development in the floodway | A report with hydraulic modeling, usually HEC-RAS where FEMA used it, or HEC-RAS or EPA SWMM where there is no FEMA model |
Every study must be signed and sealed by a Missouri professional engineer. Detailed studies extend at least 100 feet beyond the development, or until existing and proposed profiles match, and MSD can extend the limits.
FEMA draft data can raise your flood elevation
The draft says MSD will review studies using the FEMA flow and elevation numbers that provide the most protection to the public. Effective FEMA data is normally used, but where FEMA has published draft revised data, projects must meet the higher of the effective or draft numbers. If FEMA is restudying your stream, ask your engineer which numbers apply before you set pad elevations.
If a project changes the FEMA map or base flood elevations, the CLOMR/LOMR process applies, and MSD will not approve the project until FEMA issues a CLOMR.
Elevations for buildings and basements
The 2027 draft groups these in its site plan requirements:
- The low sill of any structure in or next to the floodplain must be at least 2 feet above the 100-year flood elevation.
- The lowest floor of new or redeveloped structures must be at least 1 foot above it.
- Basements, except walkouts, must be 1 foot above it.
- A basement on a parcel outside the floodplain may sit below the 100-year flood elevation if it has a sanitary pump (strapped plumbing) with the outgoing line above that elevation. Our sister company Legacy Plumbing explains strapped plumbing under the 2027 rules.
For sewers in or affected by a known flooding area, including FEMA floodplains, the draft sets the hydraulic grade line at no less than the 100-year flood elevation. MSD explained in its comment responses that during floods, water in the gravity sanitary system can and does reach flood elevation.
Parking lots
Section 5.010.06 of the draft says no parking lots are allowed in the 25-year floodplain; the 2018 rules used the 15-year floodplain. Parking for residential facilities must be above the 100-year flood level. On a tight parcel, that can move the parking field and change the earthwork balance.
Culverts and bridges
New in the draft: culverts should be embedded at least 1 foot below the natural stream bottom unless the bottom is non-erodible, and rock sizing or erosion protection calculations must be provided. Bridges, culverts and embankments in the floodway still may not raise the 100-year water surface next to or beyond the site, and new bridges need 1 foot of freeboard above the 100-year water surface.
Levee districts
MSD's levee district provisions name five districts, including Earth City, Howard Bend, Missouri Bottoms Bridgeton and Riverport, where stormwater design follows each district's master plan. After public comment, MSD said it will add language covering "all current and future levee districts within MSD boundaries." In levee-protected and high-groundwater areas the draft also calls for buoyancy checks on below-ground structures and may require a shallower gravity sanitary system. See our Chesterfield levee and trail project for the kind of work these rules touch.
Before you set pad elevations
- Ask your engineer whether the job needs a simplified or detailed study.
- Check for FEMA draft revised data on your stream.
- Set floor, sill and basement elevations from the governing 100-year elevation.
- Locate parking outside the 25-year floodplain, and residential parking above the 100-year level.
- Price culvert embedment and outlet protection.
- Bring the site contractor in early so fill, pad and parking grades are priced from the final elevations. See our earthwork and storm sewer work, or call 636-978-3478, Monday through Friday, 7:00 AM to 4:00 PM.
Questions
- What is a simplified floodplain study under MSD's 2027 rules?
- A study for projects that build or make a pad ready on a parcel in the floodplain without improvements or grading in the floodway, or that confirm parking is outside the 25-year floodplain. It needs plan sheets with the FEMA FIRMette and project extents, but no hydraulic modeling.
- Which flood data does MSD use if FEMA has published draft revised maps?
- Under the 2027 draft, the project has to meet the higher flow and water surface elevations of FEMA's effective or draft revised data.
- Can parking lots go in the floodplain?
- The 2027 draft does not allow parking lots in the 25-year floodplain, and parking for residential facilities must be above the 100-year flood level.
- How high must a building's lowest floor be?
- At least 1 foot above the 100-year flood elevation, with the low sill at least 2 feet above it for structures in or next to the floodplain.
General information from a site contractor, not engineering or legal advice. Confirm requirements with your civil engineer, MSD and the local floodplain administrator.