SUB18840 HEX Exhibit 25N.L. OLSON & ASSOCIATES, INC.
4e�, Engineering, Planning & Land Surveying
2453 Bethel Avenue
Port Orchard, WA 98366
P. 360-876-2284 IF. 360-876-1487
www.niolson.com
TECHNICAL MEMORANDUM
Project: BGH, LLC, Rolling Sunrise Preliminary Plat
To: Alan Wallace — Attorney at Law, Williams Kastner
From: Norman L. Olson, P. E. — N.L. Olson & Associates
Subject: Remand Order & Hearing - Relative to Stormwater
cc: Pat Ebert
Date: August 26, 2015
N.L. Olson & Associates, Inc. (N.L. Olson) has prepared this Technical Memorandum
(TM) to provide input for the upcoming September 4`h Remand Hearing for the
referenced project. This TM provides response to Browne Wheeler's Revision 1 report,
dated May 11, 2015, which is a revision to their project preliminary drainage plan.
N.L. Olson's prior TM, dated October 8, 2014, specifically discussed Ecology's written
response that the impervious surface thresholds cannot be permissibly waived to exempt
the project from compliance with Minimum Requirements (MR's) in the manner
proposed by the applicant. Based on review of Browne Wheeler's Revision I report, they
are still proposing to exempt the project from compliance with important MR's in exactly
the same manner. On this basis, the stormwater BMP's proposed to treat roadway runoff
are unchanged from the prior design submittal and still are not allowed by Bainbridge
Island Municipal Code.
Although at the bottom of page 2 of their report they state: "...therefore Minimum
Requirements No's 1-10 apply to the project as outlined in the Stormwater Management
Manual for Western Washington (SWMMWW), 2005 edition (Figure 2.2). " They then
reference Table 2 and comment that MR's #6 & #7 don't apply or are not required
because runoff rates are reduced below thresholds given proposed infiltration. This is not
allowed as stated within the Hearing Examiner's Remand Order.
EXHIBIT
25
Rolling Sunrise Plat Project
Remand Hearing TM
Page 2 of 3
In addition, N.L. Olson's prior TM outlined the studies and data needed to ascertain the
feasibility to infiltrate stormwater to the ground at the project site: "Demonstrating such
feasibility should be accomplished based on the following:
• Geotechnical assessment and report of the subsurface conditions, native soils
and groundwater conditions.
• Preliminary civil engineering analysis and design for storm control facilities
based on computer modeling using WWIIM with site and project specific
parameters entered such as:
o Existing site conditions including existing topography, soils,
groundwater, etc.
o Historic forested conditions and parameters for modeling
o Post development site conditions including proposed clearing and
landscaping, grading, structures, septic drainfields, surfacing, etc.
It is important to note that not only do the proposed impervious surfaces need to be
recognized within the WWHM model, but all surfaces associated with the project
must be included. Surfaces such as landscaping and grass (potential surfaces over
septic drainfield areas) are not to be omitted from the model. Although relatively
pervious compared to roofs or asphalt, these surfaces must also be included in the
model and compared to the historic forested conditions (old growth forest) for
runoff contribution and compliance with MR #7. All off-site roadway
improvements that are required must also be included and recognized within the
same model. Off-site improvements cannot be separated from the project in
regards to stormwater mitigation."
In response to questions about the site's native soil suitability for infiltration, Browne
Wheeler submitted a November 6, 2014 letter, addressed to Michael Girard, which uses
soil logs prepared by the septic system designer, along with Browne Wheeler's
subsequent visual confirmation of the soil types, to justify an infiltration rate into the
native soils. Two (2) main issues are noted given this method ofjustification:
1.) They reference and attach Table 3.7 from the Ecology Manual as the basis for
setting the short term infiltration rate for the sandy loam native soil gradation.
However, they incorrectly assess a correction factor of 2 to determine a long
term rate. The Table 3.7 in the Ecology Manual clearly requires a correction
factor of 4 for the graded soil type. In-situ infiltration testing is an alternative
method allowed by the Manual to assess a different rate that requires analysis
by a professional geotechnical engineer. Browne Wheeler did not pursue that
alternative methodology.
2.) No assessment of seasonal high groundwater level is addressed in the report.
Understanding the groundwater elevation relative to the proposed infiltration
soils is also required by the Manual.
The Browne Wheeler Revision I report continues to ignore the proposed impervious
surfaces associated with development of the seven lots in conjunction with the overall
analysis and assessment of the MR's. When a project will contribute runoff to a known
"closed depression" with ` frequent ponding water on private property" no exceedance of
Rolling Sunrise Plat Project
Remand Hearing TM
Page 3 of 3
pre -developed flow magnitude or volume are allowed. The 0.1 cfs flow exceedance and
statements such as "All stormwater runoff generated from each lot will be kept on-site to
the maximum extent practicable..." are not allowed by the Ecology Manual when such a
critical drainage area is involved.