Stream Buffers

Stream Buffer Protection Stream buffers should be established, either by public land acquisition or through reservation as permanent easements as public or private open space. These buffers should be created along all first, second and third order streams, as well as any existing or created open drainage course with a drainage area that is larger than 40 acres. Local policies and ordinances should be adopted or amended to establish protected stream buffers.

Stream buffers should be wide enough to serve the following functions:

1. Include the entirety of the regulatory 100-year (1% exceedance probability) flood plain OR where regulatory fiood plains do not exist, include areas expected to be inundated by a 24-hour, 100-year return period storm event (fiows calculated using the NRCS TR-55 method for fully developed conditions). Consider inclusion of the regulatory 500-year (0.2% exceedance probability) fiood plain within the protected buffer.
2. Allow for expected stream migration, based on recent movement patterns or historic stream channel locations.
3. Provide enough width for future streambank improvements. This plan recommends setting a line based on the existing streambank toe locations, or a line that accounts for expected future movement of the streambank toe. From that line, the buffer should include all land which falls between the stream and a projected slope line from the established toe baseline to the surface of the surrounding area. The slope line should not be steeper than a rate of 4 (horizontal) to 1 (vertical).
4. Allow width within the stream buffer for a minimum 15’ cleared maintenance path on at least one side of the stream, with a cross slope not to exceed 5%, to allow for access by trucks, tractors and other maintenance equipment. Along streams of first order or higher, these maintenance paths should be provided on both sides of the stream. These paths may be either undeveloped paths (kept clear of trees and brush by annual mowing) or paths which are surfaced with pavement or gravel.
5. Provide a minimum five-foot setback outside of the maintenance path to the edge of the reserved buffer, on the side opposite the stream from the path.
6. For engineered channels in developing areas, construct channels as bioswales where feasible to improved volume reduction and water quality treatment. Refer to the ISWMM for feasibility review and design procedures.
7. Program annual maintenance to remove invasive species and improve establish- ment of erosion resistant surface vegetation within protected buffer zones.
8. In all cases, provide a minimum 50 foot building setback from the existing top of bank for a first order stream. Provide a minimum 100 foot building setback from the existing top of bank for second and third order streams.
9. Application This plan recommends applying these standards to all new developments and where land subdivisions are planned to occur adjacent to streams subject to these provisions. Existing structures which fall within these protection zones should be identified. Past known damages to such structures may be reason to pursue opportunities to acquire and remove such structures to avoid recurrent damages.

Expected Impacts (Where Applied)

• Reduced potential for damages to buildings, property and other infrastructure during flood events
• Maximized capacity for storage and conveyance of large flood events
• Improved access for maintenance and ability to complete any necessary repairs
• Improved filtration of stormwater runoff through properly designed channels
• Opportunities for trails and other park features to be located along streams for public use

Stream buffer with trail access in Ankeny, Iowa

Stream Buffer Width Guidance Buffer widths need to vary in width from location to location. The width of the buffer to be acquired or protected by easement should include considerations for items 1 through 7 below. Recommended building setbacks may extend beyond the limit of the reserved buffer.

The orientation of these features within the buffer will vary based on local conditions. In some areas the width of the flood plain may include nearly all of these elements. In others, the projection of a stable slope, or provision for access, may extend beyond the limits of the flood plain. Ordinances can describe these features, which can then be applied to each location on a case-by-case basis.