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19.07.180 Watercourses.
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A. Designation and Typing. Watercourses shall be classified by the following types:

1. Type S (there are no known Type S watercourses on Mercer Island);

2. Type F;

3. Type Np;

4. Type Ns; and

5. Piped.

B. General Review Requirements.

1. Development within watercourses and/or associated buffers is prohibited unless one of the following conditions applies:

a. The proposed activity is specifically exempt pursuant to MICC 19.07.120;

b. A critical area review 1 application is reviewed and approved for one of the modifications in MICC 19.07.130; or

c. The proposed activity is permitted under subsection D of this section, Development Standards – Additional Criteria for Specific Activities.

C. Development Standards – Buffers.

1. The following minimum buffers shall be established from the ordinary high water mark or from the top of the bank if the ordinary high water mark cannot be identified:

Watercourse Type

Standard Buffer

F

120 feet

Np

60 feet

Ns

60 feet

Piped

No buffer

2. Neither lot coverage nor hardscape shall be permitted within a watercourse or watercourse buffer except as specifically provided in this chapter.

3. Any watercourse adjoined by a riparian wetland or other contiguous critical area shall have the buffer required for the stream type involved or the buffer that applies to the wetland or other critical area, whichever is greater.

4. Buffer Averaging. Buffer width averaging shall be allowed provided the following requirements are met:

a. The applicant has demonstrated how impacts will be minimized and that avoidance has been addressed consistent with MICC 19.07.100, Mitigation sequencing;

b. The applicant has demonstrated how all proposed impacts have been mitigated consistent with subsection E of this section, Mitigation Requirements, and will not result in a loss of ecological function;

c. The proposed buffer width is not less than 75 percent of the standard buffer width at any point; and

d. The total area of the buffer is equal to the area required without averaging.

Figure 1: Example of Buffer Averaging

5. Buffer Reduction. Buffer width reduction shall be allowed provided the following requirements are met:

a. The applicant has demonstrated that buffer averaging would not feasibly allow development;

b. The applicant has demonstrated how impacts will be minimized and that avoidance has been addressed consistent with MICC 19.07.100, Mitigation sequencing;

c. The applicant has demonstrated how all proposed impacts have been mitigated consistent with subsection E of this section and will not result in a loss of ecological function;

d. The proposed buffer width is not less than 75 percent of the standard buffer width at any point; and

e. The proposed buffer reduction is not proposed in conjunction with buffer averaging.

6. Piped Watercourse Setbacks.

a. The intent of applying setbacks to piped watercourses is to preserve the opportunity to daylight watercourses that were previously piped, to provide incentives to property owners to daylight and enhance previously piped watercourses, and to allow flexibility for development where daylighting piped watercourses is demonstrated to be infeasible.

b. Setbacks shall be established 45 feet from the centerline of piped watercourses.

c. Piped watercourses setback widths shall be reduced to a 15-foot buffer when the portion of the piped watercourse on the applicant’s property is daylighted and where the watercourse has been restored to an open channel, provided a restoration plan demonstrates:

i. The watercourse channel will be stable and is not expected to cause safety risks or environmental damage; and

ii. No additional impact nor encumbrance by watercourse buffer or critical area setback is added to properties neighboring the applicant(s) property.

d. Piped watercourse setback widths shall be reduced to: (i) 10 feet on lots with a lot width of 50 feet or more, and (ii) five feet on lots with a width of less than 50 feet, when daylighting is determined by qualified professional(s) to result in one or more of the following outcomes:

i. Increased risk of landslide or other potential hazard that cannot be mitigated;

ii. Increased risk of environmental damage (e.g., erosion, diminished water quality) that cannot be mitigated;

iii. The inability of a legally established existing lot to meet the vehicular access requirements of this title; or

iv. The inability of a legally established existing lot to meet the building pad standards in MICC 19.09.090.

7. Buildings and other structures shall be set back a minimum of 10 feet from the edges of a watercourse buffer. The distance may be reduced to five feet if:

a. The watercourse is Type Ns;

b. The buffer does not contain habitat for WDFW priority species;

c. A split-rail fence is installed along the perimeter of the buffer; and

d. Survey markers are installed along the perimeter of the buffer to establish its field location.

Figure 2: Example of Critical Area Setback

8. The following may be allowed in the critical area setback, provided no structures nor building overhangs may be closer than five feet from the edge of a watercourse buffer:

a. Landscaping;

b. Uncovered decks less than 30 inches above existing or finished grade, whichever is lower;

c. Building overhangs if such overhangs do not extend more than 18 inches into the setback area;

d. Hardscape and driveways; provided, that such improvements may be subject to requirements in Chapter 15.09 MICC, Storm Water Master Program;

e. Split-rail fences;

f. Trails, consistent with the requirements of this chapter; and

g. Subgrade components of foundations; provided, that any temporary impacts to building setbacks shall be restored to their previous condition or better.

D. Development Standards – Additional Criteria for Specific Activities.

1. New watercourse crossings, such as bridges and culverts, may be permitted provided the standards in WAC 220-660-190 have been demonstrated to be met.

2. The construction of trails within watercourse buffers is allowed, subject to the following:

a. Trail surfaces shall be constructed of pervious materials and may not be wider than five feet;

b. Trails shall be located to minimize the need for tree removal; and

c. Trails shall be located only in the outer 25 percent of the buffer area.

3. The trail width shall be added to the buffer width applied to the watercourse (e.g., if a trail is three feet wide, the watercourse buffer for the portion of the watercourse where the trail is located shall be expanded by three feet); except that the trail width shall not be added to the buffer width when trails are being created for public access and contained within a public access easement or right-of-way.

E. Mitigation Requirements. Mitigation measures shall achieve equivalent or greater ecological function including, but not limited to:

1. Habitat complexity, connectivity, and other biological functions;

2. Seasonal hydrological dynamics, water storage capacity and water quality; and

3. Geomorphic and habitat processes and functions. (Ord. 19C-05 § 1 (Exh. A)). (Ord. 19C-05 § 1 (Exh. A), 2019.)

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