Some of the flood-hazard maps have flood elevation cross sections marked on the maps. These elevations can be interpolated between cross sections and are considered the property?EUR??,,????'?????<

If the map for your project does not have a 100-year cross section delineated, then a bit more cumbersome road lies ahead in determining the 100-year elevation. First, contact your state Department of Natural Resources division of water to see if their flood plain management experts can help you in determining the 100-year elevation. If this becomes a fruitless adventure, then an economic decision needs to be made to either retain the services of a professional engineering firm to perform a hydraulic study of the waterway and determine the 100-year elevation or pay the added flood hazard insurance premium required by the lender.

FEMA considers the flood protection grade (FPG) for the finished floor of the structure to be sufficient protection when it is at least 1 foot above the 100-year flood elevation. Local jurisdictions may require a higher FPG such as two feet. In the case of a walkout basement, the basement floor elevation would need to meet the minimum FPG.

Offsite Utility and Drainage Limitations

The installed sewer must maintain minimum cover requirements over the top of pipe that varies in every part of the country. The sewer utility may have minimum flood protection elevations for manhole castings that would require the manhole lids to protrude out of the ground considerably higher than the grade around the manhole. The local building code may mandate that the lowest finished floor elevation must be a minimum of one foot above the nearest sanitary sewer manhole casting. The sanitary sewer can be the governing constraint for controlling the grading in any project.

Grading is the essence of drainage design. Water flows downhill and that is precisely what is working in our favor ?EUR??,,????'?????<

In general, water is to be diverted from a high point and collected into lower areas where it can be channeled into the natural waterway of the watershed. This is accomplished in residential design by the use of swales, detention ponds, and storm pipe systems. It is good engineering practice to design the building at a high point so that water can be diverted away from the structure?EUR??,,????'?????<

The stream was relocated to an existing portion of the site, allowing for construction of the complex.

The elevation at which water leaves the site will control the grading design throughout. Allowing for storm pipe cover, pipe slope, storm structures, inlet castings, swale slopes, and overland flow will all add elevation to the grading design. Concrete storm sewer pipe typically will need at least two feet of cover over the top of pipe. A street curb inlet will require a minimum of three feet of depth from casting to invert. A typical grass drainage swale requires a one percent slope but can be as low as 0.5 percent with a pipe underdrain below the flowline. Open grass areas should have at least a one percent slope to maintain positive drainage.

Summary

The ability to grasp the vision of the homeowner, developer, or architect will lead to asking the design related questions of ?EUR??,,????'?????<

Brian Cross is an associate at Roger Ward Engineering, Inc., based in Indianapolis, Indiana.