Projects proposing development in the 25-year and 100-year floodplains have the potential for placing our citizens and their property at risk of flooding and producing changes in floodplain elevations and plan view extent. Both City of Austin regulations and National Flood Insurance Program regulations apply. Developers and engineers that are contemplating development in floodplain areas should be especially familiar with the Land Development Code (LDC):
The following list summarizes the major issues of personal safety and property protection associated with development within floodplain areas. Each point is discussed in greater detail below. There are other issues related to environmental integrity and preservation of the natural and beneficial functions of floodplain areas that are not discussed in this document.
25-year floodplain - Encroachment of buildings and parking areas is prohibited.
100-year floodplain - Encroachment of buildings and parking areas is restricted.
No adverse impact - Proposed development must not result in additional identifiable adverse flooding on other property.
Site Plans - A site plan is required for all development proposed within the 100-year floodplain.
Variances - Variances to allow development of the floodplain are considered by the City Council. Variances must be requested in writing.
Floodplain Mapping - Approval by Federal Emergency Management Agency of changes to floodplain mapping may be required.
Discussion of Major Issues
LDC 25-7-92 (A) prohibits encroachment of buildings or parking areas within the 25-year floodplain.
LDC 25-7-92 (B) prohibits encroachment of proposed building or parking areas beyond the 25-year floodplain but within the 100-year floodplain with certain specific exceptions:
LDC 25-7-93 General Exceptions sets forth conditions and requirements to allow construction of parking areas less than 5,000 square feet, single family residential structures in a subdivision recorded before September 25, 1983, buildings within certain floodplain areas of the Colorado River, and certain boat docks.
LDC 25-7-94 Exceptions in the Central Business Area sets forth conditions and requirements that apply within the area bounded by IH-35, Riverside Drive, Barton Springs Road, Lamar Boulevard, and 15th Street.
The foundation floor slabs must be a minimum of two (2) feet above the 100-year floodplain elevation.
Pedestrian and vehicular access to the proposed buildings must be to areas above the regulatory flood datum, which is equivalent to the 100-year floodplain elevation plus a freeboard of one foot.
All development associated with the construction of buildings must compensate for any floodplain volume displaced by that construction.
The applicant must demonstrate by means of a study by a Texas registered professional engineer that the construction of the building and associated development activities improve the drainage system by exceeding the minimum requirements of Sections 25-7-2 (Obstruction of Waterways Prohibited), 25-7-3 (Duty to Maintain Unobstructed Waterways), and 25-7-4 (Standing Water Declared a Nuisance).
LDC 25-7-95 Exceptions for Parking Areas provides additional exceptions for parking areas.
LDC 25-12-3, "Local Amendments to the Building Code," Section 1612.4.1 requires that the foundation floor slabs must be a minimum of one foot above the 100-year floodplain elevation.
LDC 25-12-3, "Local Amendments to the Building Code," Section 1612.4.3 requires that normal access to proposed buildings must be to areas a minimum of one foot above the 100-year floodplain.
No Adverse Impact
LDC 25-7-61 (A) (5) requires that a final plat, subdivision construction plan, or site plan may not be approved unless the proposed development will not result in additional identifiable adverse flooding on other property and to the greatest extent feasible, preserves the natural and traditional character of the land and waterway.
LDC 25-7-151 (A) states that the owner or developer of property to be developed is responsible for the conveyance of all stormwater flowing through the property, including stormwater that is directed to the property by other developed property or that naturally flows through the property because of the topography.
Development within the floodplain may cause adverse impact to others by obstructing the existing waterway or by placing fill in floodplain storage areas.
Obstruction of the waterway reduces the cross sectional area of the channel and typically creates identifiable adverse impacts of increased erosion and scour at the obstruction and/or increased flood elevations upstream of the obstruction. A proposed site plan should not decrease the conveyance capacity of the channel and overbank areas. Hydraulic analysis is required to demonstrate that the proposed development will not create any additional identifiable adverse flooding on other property due to loss of conveyance.
Placement of fill in the floodplain reduces the natural capacity of the floodplain to store floodwaters. Loss of floodplain storage typically causes higher peak flows and additional identifiable adverse flooding on downstream properties. A proposed site should not decrease the floodplain storage volume of the channel and overbank areas. Accurate plans and cut/fill calculations must be provided for any proposed development within the floodplain to demonstrate that any proposed fill is offset by at least an equal amount of excavation within the floodplain.
Excavation and fill of the floodplain and modification of the channel are subject to requirements to preserve the natural and traditional character of the land and waterway.
The contours of the channel bottom change continually due to natural processes of deposition and scour. Temporary removal of sediments from depositional areas may have unforeseen impacts on channel erosion, and is not considered to affect calculations of cut and fill or conveyance.
The Drainage Criteria Manual, Section 1.2.6 "Flood Plain Delineations" governs the delineation of floodplain boundaries and requirements to update the effective floodplain maps when changes occur. Floodplain mapping that is outdated may create unnecessary encumbrances (such as federal requirements for flood insurance or building permit restrictions), or may allow citizens to be placed at undue risk of flooding. Both the City of Austin and the Federal Emergency Management Agency through its National Flood Insurance Program have review and approval authority for floodplain mapping within the City's jurisdiction.
Projects may change the 100-year floodplain with respect to either plan view or elevation. The applicant may submit a more accurate analysis of pre-development conditions to demonstrate that the site is less encumbered by the floodplain. Alternatively, the applicant may propose structural modifications to reduce the post-development floodplain. In either case, floodplain maps must be updated to effectively manage the floodplain.
FEMA establishes formal processes for revisions of its Flood Insurance Rate Maps (floodplain maps.) Revision of the pre-development floodplain will require a Letter of Map Revision. Changes to the post-development floodplain will require a Conditional Letter of Map Revision, i.e. the maps will be revised on the condition that proposed improvements are constructed as planned.
If the applicant proposes an encroachment on the floodplain that will result in increases in flood elevations, then FEMA must approve a Conditional Letter of Map Revision before the City of Austin will issue a development permit. For all other proposed modifications to the Flood Insurance Rate Maps, the applicant shall apply to the FEMA for a Conditional Letter of Map Revision before the City of Austin will issue a development permit. The Conditional Letter of Map Revision must be approved by FEMA before the City of Austin will issue the final letter of acceptance for a subdivision or the certificate of completion for a site plan.
The applicant is responsible to prepare the engineering analysis, to follow FEMA's administrative procedures, and to pay any fees required by FEMA. The Watershed Protection Department is the Community Representative to FEMA for the National Flood Insurance Program. Therefore, the Watershed Protection Department must review and concur with the engineering analysis prior to acceptance for review by FEMA.
The Watershed Engineering Division can assist applicants and their engineers to complete the process and to review the engineering analyses. Development assistance is available from the Development Assistance Center in One Texas Center, 505 Barton Springs Road, 1st floor, 512-974-6370. Floodplain determinations and effective hydrologic and hydraulic models (pdf) are available from the Watershed Engineering Division in One Texas Center, 505 Barton Springs Road, 12th Floor, 512-974-2843. To request a floodplain model, email us.