Down on the Bayou (Part 2)
Recall in the last article we discussed the need for floodplain regulations. In this article the discussion will be on how to develop in the floodplain. The most important concept to understand is the elevation known as Base Flood Elevation (BFE). The base flood elevation is the elevation of the water surface to which the city or county regulates development. In our region, the BFE could be the elevation of the stormwater during the 1%-event (100-year storm) or the 0.2%-event (500-year storm), dependent on jurisdiction.
With BFE identified, the Finished Floor Elevation (FFE) can be determined. The Houston area requires FFE at 12-24 inches above BFE, dependent upon agency. The only portions of a residential structure that are permitted below the FFE are parking, building access, and storage – similar to a beach house.
The 100-year storm has the chance of occurring equal to 1% in any one year. Mathematically, that means it has approximately 26% chance of occurring once every 30 years. The 500-year storm has an annual chance of occurring of 0.2%. Mathematically, that means the chance of occurring over 30 years is 6%. When looking at flood maps, generally the darkest shaded area is the area that has at least a 1% recurrence interval. FEMA requires insurance for any federally insured properties within the 1% floodplain.In contrast, less shaded map areas that have between a 1% chance and 0.2% chance of occurring in any one year still have between a 26% chance and 6% chance of flooding over a 30-year mortgage period. FEMA does not require insurance in this area, but it may be wise to purchase. The odds of a house fire (0.25%) and the odds of a 0.2% flood event in a less shaded area are very similar, and almost all homes have fire insurance.
For residential subdivision development, an expert preemptive adaptation of the floodplain that minimizes the amount of land regulated by floodplain permitting requirements is the best policy. Through a Letter of Map Revision (LOMR), FEMA has a revision process that can accommodate a well-developed and designed alteration to the floodplain as a prelude to improvement. I would suggest contacting a well-experienced local drainage engineer to assist with this.
The best process for single lot development is to raise the finished floor elevation to meet floodplain requirements. Sensible development plans might include construction of the occupied portion of the home above the BFE, while constructing the garage at or below the BFE. This works well when the natural ground is close to the elevation of the BFE. One may also construct the unoccupied portion of the project below a raised finished floor, raising elevations 10-11 feet above the elevation of the garage. This works well when the natural ground elevation is 3 or more feet below the BFE. Remember this garage will statistically flood, so it needs to be to constructed in accordance with floodplain regulations.
Dr. Culp is the most senior hydrologist at Tetra Land Services and has three decades of civil engineering experience. His Ph.D. scholarship studied the effectiveness of structural BMP for the control of storm water pollution in Harris County while performing water quality monitoring and modeling upon selected ponds for the county. Dr. Culp also co-authored the City of Houston stormwater quality management plan. He is one of Texas’s original Certified Floodplain Managers. Recently, Dr. Culp and his staff have developed a series of drainage studies for Industrial and Oil Majors along the Texas Gulf Coast. Dr. Culp is married with two children, and lives on his farm in Southwest Houston.