Tuesday, April 21, 2020

How engineered grading and drainage works for homes in Arizona

Most houses in Arizona don't need an engineered grading and drainage plan. But some do. This post explains about that.

When is a G&D plan needed?

The decision to ask for an engineered grading and drainage plan (G&D) depends on the following factors:

  • Subdivision: If the home is on a lot of a mass-graded subdivision with all homes draining to their fronting street, it very probably does NOT need a G&D.
  • On-lot retention: If the subdivision was designed to rely on individual lot storm water retention instead of fully centralized retention basins or if the city requires on-lot retention for all homes, it DOES need a G&D
  • Unsubdivided: If the home is on an unsubdivided parcel, it very likely DOES need a G&D. This may not be true if the reviewer believes it is obvious there are no hillside (steep terrain) or off-site drainage (large watershed or noticeable washes) issues.

When is a certified/engineered pad required?

Anytime there is an engineered grading and drainage plan, there is generally also certified pad fill compaction and an inspected floor elevation.

When is a drainage report needed?

Every grading and drainage plan must be supported by a drainage report, usually including a watershed map. If possible, this report and map can be placed on the plan sheet, which helps prevent loss of the report and enables easy cross referencing of the report and plan. Sometimes this is not possible, and a separately bound technical drainage report is required. The G&D plan may be incorporated into the drainage report as an attachment for reference.

When is an erosion setback required?

For watercourses having a 100-year peak flow of 500 cfs or a watershed of 1/4 square mile (this is very rare), the Arizona State Standard 5-96 lateral migration hazard erosion setback requirements apply.

AZ State Standard 5-96 lower limit of application
In Pinal County, the Pinal County Area Drainage Master Plan Section 2.3.3 applies Standard 5-96 (above) to watercourses having a 100-year peak flow of 200 cfs (somewhat rare).

When is a scour analysis required?

If there is a required erosion setback and you cannot provide it, you must protect the home from erosion by calculating the scour depth and putting a barrier down to that elevation between the home and the watercourse. Three types of barriers (in my order of preference) are deep foundations, launchable riprap, and scour walls.

When is an elevation certificate required?

A FEMA elevation certificate prepared by a land surveyor (or civil engineer) is required (possibly at multiple stages of design and construction) when a home is in a floodplain shown on the FEMA Flood Insurance Rate Map. The civil engineer determines the Base Flood Elevation (100-yr water surface) for the home. The lowest point on the home floor (including attached garage) and the air conditioner pad etc. must be elevated possibly 2 feet above the Base Flood Elevation (sometimes 1 foot above the Floodway elevation, which can result in a little lower floor; see below). In Maricopa County, this required floor and equipment elevation is called the Regulatory Flood Elevation.

Special permitting may also be required to build a home in a floodplain.

A floodway elevation is between 0 and 1 foot above a Base Flood Elevation.

When are FEMA flood map (FIRM) revisions required?

A floodway is a special reserved corridor in the middle of some floodplains that allows development in the floodplain fringe with reduced engineering and permitting requirements.

No home can be built in a floodway without flood map revisions except possibly if it is on stilts or on a clear spanning bridge or cantilever (I have never seen this). Flood map revisions to allow building in a floodway often have unfeasible engineering and construction cost or are impossible without easements or additional property.

Before or after a home is built in a floodplain fringe (not a floodway), a flood map revision may be obtained from FEMA to reduce or eliminate flood insurance cost, and the engineering and construction cost of this is often justified by the insurance savings. To save engineering costs and schedule delays in straightforward cases, I recommend designing and constructing the home in a way that will remove it from the floodplain so that a flood map revision can be obtained after occupancy as an actual (existing condition; one step) rather than conditional (proposed condition; two steps) revision.

When is engineered grading of the yard required?

Typical building and grading inspectors focus primarily on the home, its pad compaction, and its elevation. It's rare that they focus much on yard grading per plan unless they have doubts about proper foundation drainage since every building has the same standard drainage requirements: dirt near buildings needs to drain away at least 5% for at least 10 feet.

Special engineered grading such as channels, swales, hills, riprap, and geotextile (fabric) are not usually inspected. In Maricopa County, if there is a Floodplain Use Permit, there will be a Flood Control inspector who will pay more attention to engineered grading of the yard per plan as it applies to floodplain compliance.

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