Herrera Stormwater Review-1.docx
Stormwater Review: Sunrise of Lynnwood bld20151512
1.The area of disturbance and replaced/new impervious surfaces are inconsistent in the text of the report,
Figure 2 and the form E72 (the yellow should match yellow, green should match green). Not drastically
different, but inconsistent. Calculations were based on the areas described on P.2 and P.3 of the report.
a. P.2, Parking Lot Improvements:
i.Total site disturbance = 7,850s.f.
ii.Impervious surface = 4,825 s.f.
b.P.3, Patio Improvements:
i.Total site disturbance = 1,970s.f. (+7,850 from
parking lot = 9,820s.f.)
ii.Impervious surface = 1,390s.f. (+4,825 from
parking lot = 6,215s.f.)
c.Figure 2 Proposed Conditions:
i.Disturbance limits = 9,820s.f.
ii.PGIS (4,145s.f.) + NPGIS (2,000) = 6,145s.f.
d.Form E72-SWM
i.P.1, Line 4. Total Regulated Impervious Area =
6,145s.f.
ii.P.2, Line 3. Total area of Land Disturbing
Activity = 9,905s.f.
2.P.2 of the report states that the project is a Category 1 Small Project. The total new + replaced
impervious surface area is 6,215s.f. (or 6,145s.f.), which is over the 5.000s.f. threshold therefore
classifying it as a Category 2 Small Project. Stormwater from the 1,390s.f. patio is proposed to be fully
infiltrated, but the replaced impervious area should still be included in the threshold area when
determining the project classification.
a.As a Category 2 Small Project, the flow control standards for the 2-, 10-, and 100-year storms
must be met (for Category 1, flow control standards only apply to the 10-year storm). The table
below shows that flow control standards are only met for the 10-year storm. Flows for the 2- and
100-year storms exceed the allowable rates. This table is based on flows shown in the Site Runoff
table on page 3 of the report (modeling calculations confirmed), and allowable flowrates in Table
1 of Handout E72B.
Allowable Flow per
Flows from Existing Allowable Flow per Total Allowable Proposed Design
Event Acre Impervious
Undisturbed Site (cfs) 0.111ac (cfs) Flow (cfs) Flow (cfs)
(cfs)
2-Year 0.136 0.07 0.008 0.144 0.16
10-Year 0.226 0.25 0.028 0.254 0.233
100-Year 0.51 0.45 0.050 0.560 0.721
3.P.3, Patio Improvements states that an infiltration trench (2’wide x3’deep x50’long) will infiltrate all
stormwater from the 1,390s.f. patio at a rate of 0.25in./hr. There is no discussion of how the 0.25in./hr.
rate was determined.
4.All disturbed landscape areas shall be amended to meet BMP T5.13 (per stormwater supplement
section 5.5)
COLLEEN E. MITCHELL,
PE, LEED AP
Senior Stormwater Engineer