20100010 Ver 1_Snowbird Creek Buffer Width Analysis_toDMS_20170508

Snowbird Creek Tributaries Mitigation Project
DMS ID #92764; Contract #000613
Contingency Report - Addendum
This document serves as an addendum to the Contingency Report that was submitted to DMS on 10/13/16
and additional information submitted to Todd Tugwell, for the IRT, on 3/27/17 for the Snowbird Creek
Mitigation Project closeout. Upon request from the IRT, a more thorough analysis of buffer width
deficiencies was conducted for noted sites on the Snowbird Creek project based on guidance provided in
the Wilmington District Stream and Wetland Compensatory Mitigation Update dated October 24, 2016
(Section XI).
The IRT requested more information regarding 11 specific areas that were either single thread channels
that lacked a full 30 -foot buffer on both sides of the channel or multi thread channels with both channels
having an impaired 30 -foot buffer on at least one bank. On the original map (Fig. 1), it was determined
that sites 1, 2, 6, 7, and 9 were instances where at least one channel had a sufficient 30 foot buffer on both
banks, so those sites were not included in this analysis as discussed and agreed upon during the IRT
meeting held on April 11, 2017. See Table 1 for more information about the sites that were considered.
Table 1
Site
Single/
Multiple
Thread
Left/
Right
Channel
Original
LF
Credit
Ratio
Original
Credits
Credit
Reduction
Factor
Credit
Reduction
Final
Modified
Credits
3
Multiple
Left
193.28
5:1
38.66
-19.39%
7.50
31.16
5
Multiple
Left
142.60
5:1
28.52
-22.60%
6.44
22.08
10
Multiple
Left
171.38
5:1
34.28
-14.95%
5.12
29.15
11
Multiple
Right
184.14
5:1
36.83
-21.12%
7.78
29.05
A
Single
-
235.79
5:1
47.16
-23.66%
11.16
36.00
B
Single
-
220.84
5:1
44.17
-12.77%
5.64
38.53
C
Single
-
188.87
5:1
37.77
-12.15%
4.59
33.18
D
Single
-
132.55
5:1
26.51
-29.17%
7.73
18.78
E
Single
-
297.59
5:1
59.52
-32.49%
19.33
40.18
F
Single
-
217.96
5:1
43.59
-36.17%
15.77
27.82
G
Single
-
184.87
5:1
36.97
-34.90%
12.90
24.07
Totals
2169.87
433.97
103.97
330.01
Methodology: Per the methodology provided in the guidance, each side of the stream was evaluated
separately using ArcMap v.10.2.2. For each site in question, the top of bank was divided into reaches that
fell within the buffer width categories shown in Table 2 of the guidance document as measured inward
from the conservation easement boundary.
For the multi thread channels, each channel was considered separately and only one was selected for
inclusion in the final crediting calculation. Buffer width measurements were made using the nearest point
method. For each bank, the credits for the total LF that fell within each category were modified based on

the established Adjustment to Stream Credit percentages from Table 2 in the October 24, 2016 guidance
document. In a few instances, a bank may have had a greater than 50 foot buffer, and this was accounted
for using the appropriate adjustment factor (+9%). The modified credit total for each bank was then
combined and then divided by two to yield the final number of credits for each site.
Using this approach, a total credit reduction of 104 credits is suggested for the 11 sites in question. Given
that 1,993 SMUs were requested in the Contingency Report (which included a 42 SMU reduction), we are
now requesting that this project be closed out recognizing an additional 104 SMU reduction and a total of
1,889 SMUs. See Table 2.
Table 2
Mitigation
Unit Summations
Stream
Riparian
Total
Nonriparian
Buffer
Restoration Level
Mitigation Units
Wetland
Wetland
Wetland (Ac)
(Ac)
(SMUs)
(Ac)
(Ac)
Restoration
467
-
-
-
-
Enhancement I
-
-
-
-
-
Enhancement II
68
-
-
-
-
Preservation
1354
-
-
-
Total
1889
-
-
-
-