20170142 Ver 1 - MB_Buffer Calculation_MP - 6/13/2018

Wilmington District Stream Buffer Credit Calculator
(Updated 1/19/2018)
Since the implementation of the Stream Mitigation Guidelines in 2003, the Wilmington District has required that all compensatory stream mitigation projects have a minimum buffer width of 50 feet from
the Ordinary High Water Mark (OHWM) in coastal and piedmont counties and 30 feet in mountain counties. Current guidance includes an allowance of up to 5% of total stream length to be less than the
minimum width to account for areas where it is not possible to obtain the minimum wdith, such as where streams intersect property boundaries, farm crossings, or where allowable exceptions are
included with the easement. Additional stream credit may also be generated by protecting and planting buffers that are wider than the minimum stated above. The spreadsheet below is used to
calculate additional credit; however, if additional credit is sought for wider buffers, the 5% allowance does not apply and any areas that have less than the mimum width will also be considered in the
calcuation (e.g., any areas lack the minimum buffer in any direction will count as a credit deduction). This method may be used for all mitigation types, including preservation; however, this method is not
applicable to streams where credit is determined by valley length. Buffer areas may include both planted areas and areas of existing forest cover, provided they meet all vegetation and performance
standards and are protected by easement or other approved mechanism. Additionally, when seeking extra credit, some areas must be removed (clipped in GIS) from all buffer zone area calculations,
including the area of stream itself (as determined either by OHWM or average width), all easement exceptions (e.g., farm crossings, utility lines, cleared stream access points), open water resulting from
beaver ponds or other features, areas that are not meeting performance standards (e.g., areas of low stem density, low vigor, or excessive invasive species), or other areas that do not support forest
canopy species. Areas that are also generating wetland credit, state buffer or nutrient offset credit must be removed from calculations for extra credit for wider buffers but are not considered toward
credit loss where they occur inside the minimum buffer width (i.e., these areas do not have to be clipped from buffer zones that are less than 50 feet from the OHWM in piedmont and coastal counties,
or 30 feet in the mountain counties). In some cases, such as projects that were implemented after the minuimum buffers were established where easements do not meet the minimum width on more
than 5% of the total stream length, this methodology may also be used to calculate credit reductions. In these cases, areas on the project that have wider buffers may be used to offset lost credit up to
the total amount proposed in the approved Mitigation Plan, but not more.
In order to apply this methodology, GIS must be used to determine the area (in square feet) of buffer zones around all streams within a project. The applicable buffer zones and their distance from the
stream OHWM are shown in the table once the project county has been selected. Complete the table as follows:
Step 1 - Fill out the information at the top of the Buffer Calculation table, including the project county, and creditable stream length for each mitigation type. If there are stream reaches that are credited
at a ratio that is not shown, enter custom mitigation ratio multipliers, which are equal to the number of feet in the feet -to -credit mitigation ratio (e.g., for a perservation ratio of 8 feet to 1 credit, the
multiplier would be 8).
Step 2 - Clip (remove) all non -credible streams from the GIS layers prior to creating the buffer zones. This may include stream segments within road or easement crossings, streams with easements on
only one side, streams that have failed to meet performance standards, etc. Please note that some stream reaches may be removed from credit generation during monitoring, in which case additional
credit from wider buffers will be affected.
Step 3 - In GIS, use the buffer tool to create concentric buffers equal to the zones shown on the table around all creditable streams on the project (e.g., 0 to 15 feet, 15 to 20 feet, etc.). For the inner zone
(less than 15 feet), be sure to create the buffer from the outside edge of the OHWM, not the stream centerline. If OHWM is not surveyed, assign an average width to the stream centerline and create the
zone from the edge of this width. For braided channels, measurements should be taken from the OHWM of the outer-most braid on each side of the channel. Note that the outer zone stops at 150 feet
from the channel, so do not include areas that are greater than 150 feet from the channel in the outer zone.
Step 4 - Measure the square footage within each buffer zone before clipping the zones to the easement or other areas that do not count for credit. These measurements are the Ideal Buffer to be
entered in the spreadsheet, which represent the maximum potential size (in square feet) of each buffer zone measured around all creditable stream reaches. These measurements should include the
area within the arc around stream termini, regardless of whether these points are stream origins or points where streams exit/enter the easement.
Step 5 - Clip the buffer zones to eliminate all areas outside the easement and all non -credible areas within the easement (see the instructions above for more information on these areas).
Step 6 - Calculate the square footage within each zone after clipping the zones to the easement and other areas that do not count for credit. These calculations are the Actual Buffer to be entered in the
spreadsheet, which represent the creditable size (in square feet) of each buffer zone measured around all creditable stream reaches.
The following information must be submitted to the District when this methodology is used:
1. Hard copy of this spreadsheet with project information included.
2. Maps of the project identifying the credible stream reaches with different mitigation types, easement boundaries, buffer zones, and all areas that are not elegible for credit (e.g., farm crossing, open
water, areas not meeting performance standards, areas generating other credit types located outside of the minimum buffer width, etc.). Most projects will require multiple maps in order to depict the
project at a ledgible scale.
3. A digital copy of all GIS files and the excel spreadsheet.

Site Name:
USACE Action ID:
NCDWR Project Number:
Sponsor:
County:
Minimum Required Buffer Width':
Mitigation Type
Restoration (1:1)
Enhancement 1 (1.5:1)
Enhancement II (2.5:1)
Preservation (5:1)
Other (7.5:1)
Other (10:1)
Custom Ratio 1
Custom Ratio 2
Custom Ratio 3
Custom Ratio 4
Custom Ratio 5
Totals
Buffer Zones
Max Possible Buffer (square feet)4
Ideal Buffer (square feet)'
Actual Buffer (square feet)6
Zone Multiplier
Buffer Credit Equivalent
Percent of Ideal Buffer
Credit Adjustment
Total Baseline Credit
3364.33
I Meadow Brook Stream Restoration I
100024
NCDMS
Yadkin
50
Mitigation Ratio Creditable Stream
Multiplier Length
3437.00
Baseline Stream Credit
3219.00
145.33
3364.33
Wilmington District Stream Buffer Credit Calculator
Buffer Width Zone (feet from Ordinary High Water Mark)
less than 15 feet
>15 to 20 feet
>20 to 25 feet
>25 to 30 feet
>30 to 35 feet
>35 to 40 feet
>40 to 45 feet
>45 to 50 feet
>50 to 75 feet
>75 to 100 feet
>100 to 125 feet
>125 to 150 feet
103110
34370
34370
34370
34370
34370
34370
34370
171850
171850
171850
171850
102516
34512
33978
33031
31739
30865
19820
29231
141377
138130
136957
136251
00301.0712
32927.28722
32346.58514
31183.96466
29650.86564
28523.47988
27156.45374
26288.08352
81002.53273
32250.60391
10606.39381
3006.72684
50%
10%
10%
10%
5%
5%
5%
5%
7%
5%
4%
4%
1682.17
336.43
336.43
336.43
168.22
168.22
168.22
168.22
235.50
168.22
134.57
134.57
98%
95%
95%
94%
93%
92%
91%
90%
57%
23%
8%
2%
-36.34
-15.45
-16.15
-18.81
-11.07
-12.76
-15.03
-16.94
134.93
39.28
10.42
2.97
Credit Loss in Required Credit Gain for Net Change in
Total Credit
Buffer Additional Buffer Credit from Buffers
-142.55 187.60 45.05 3409.38
1Minimum standard buffer width measured from the top of bank (50 feet in piedmont and coastal plain counties or 30 feet in mountain counties)
2Use the Custom Ratio fields to enter non-standard ratios, which are equal to the number of feet in the feet -to -credit mitigation ratio (e.g., for a perservation ratio of 8 feet to 1 credit, the multiplier would be 8).
3Equal to the number of feet of stream in each Mitigation Type. If stream reaches are not creditable, they should be excluded from this measurement, even if they fall within the easement.
4This amount is the maximum buffer area possible based on the linear footage of stream length if channel were perfectly straight with full buffer width. This number is not used in calculations, but is provided as a reference.
5Maximum potential size (in square feet) of each buffer zone measured around all creditable stream reaches, calculated using GIS, including areas outside of the easement. The inner zone (0-15') should be measured from the top of the OHWM or the edge of the average stream width if OHWM is not known. Non -creditable stream
reaches within the easement should be removed prior to calculating this area wtih GIS.
6Square feet in each buffer zone, as measured by GIS, excluding non -forested areas, all other credit type (e.g., wetland, nutrient offset, buffer), easement exceptions, open water, areas failing to meet the vegetation performance standard, etc. Additional credit is given to 150 feet in buffer width, so areas within the easement that are
more than 150 feet from creditable streams should not be included in this measurement. Non -creditable stream reaches within the easement should be removed prior to calculating this area wtih GIS.