Landau Outfall Analysis.pdfFebruary 23, 2010
City of Edmonds
Dept. of Public Works
121 Fifth Avenue North
Edmonds, Washington 98020
Attn: Mr. Jerry Shuster
RE: 102512 T" AVENUE NORTH STORMWATER OUTFALL ANALYSIS
LANDAU
14 ASSOCIATES
Dear Jerry:
At the request of the City of Edmonds (City), Landau Associates reviewed site conditions at 1025
12t" Avenue North and on adjacent properties to evaluate the effects of relocating a stormwater outfall
pipe. The new stormwater outfall pipe is required because the existing pipe goes beneath a private
residence and is starting to fail and needs to be replaced. The new location is shifted approximately ten
feet uphill from its current location in order to avoid and minimize disturbance to multiple properties.
Our services were provided in accordance with Task Order No. 09-20 with the City.
The existing 12 -inch pipe collects stormwater from 12t1i Ave North and extends westerly
subsurface approximately 124 feet before daylighting along lot lines at the end of a private cul-de-sac east
of Olympic Avenue. Water from the outfall flows to the west in a defined flow pattern that forms a road
side ditch along the north side of the private road before entering a storm drain. The new outfall will be
shifted approximately ten feet to the north and 6.58 feet upslope of the existing outfall. The design
includes a manhole and a small rock -lined spillway to dissipate energy. Surface water flow will be
directed from the new outfall to the existing conveyance and roadside ditch.
Landau Associates visited the site on January 8, 2010 and compared the new plans to existing
conditions and evaluated anticipated flow patterns. This evaluation does not include a critical areas
(wetlands, streams, steep slopes) delineation, categorization or functional assessment.
A vacant lot just downstream of the existing outfall at 932 Olympic Avenue is characterized by
sloping topography ascending to the north and to the south. While steeper sloping areas at the base of the
slope to the north and to the south are dominated by blackberry or English Ivy, most of the lot is
dominated by reed canarygrass, horsetail, and bentgrass. These plants are hydrophyfrc and may indicate
wetland conditions. Surficial conditions on the lot were damp. Surface water on the lot is derived
primarily from seepage at the toe of the slopes. The existing stormwater outfall does not supply
hydrology to the site as this flow is channeled along a roadside conveyance along Olympic Avenue. The
new outfall has been designed in order to avoid flooding conditions on adjacent properties, such that a
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bubbler and splash guard have been incorporated into the plans in order to minimize turbulence during
intense events and to direct water to where it goes now to the existing road side conveyance ditch along
Olympic Avenue.
Based on our cursory review of site conditions, Landau Associates does not believe that there will
be any adverse affects to neighboring property as long as the water from the new outfall is diverted into
the existing channel that transports water from the existing outfall. If the water is not diverted into the
existing channel, there is a possibility that the hydrophytic vegetation and other physical conditions could
be affected due to an increase in water flows in that area.
There are two major factors to consider in this project: water quantity and water quality. Based
on the site evaluation and the information on the design plans, the quantity of water discharging from the
pipe will not change. If the water from the new outfall location is not diverted into the existing channel
for the existing outfall, hydrologic conditions would change and there could be physical changes to the
vegetation and soils on the hillside. The quality of the water could also affect the ecological and physical
conditions on the project area. It is recommended that the civil engineer include a thorough description of
the Best Management Practices (BMPs) for the activities that occur during and after the construction
phase of this project. It is important that stray sediment is prevented from transporting into the hillside
and/or downhill into the municipal stormwater system. Careful erosion and sediment control will protect
any healthy vegetation, existing slope conditions, and unsightly conditions.
Landau Associates' services have been provided for the exclusive use of the City of Edmonds for
specific purposes of evaluating the relocation of a stormwater outfall at the subject property and how it
may impact the hillside conditions. Use of this letter by others or for another purpose is at the user's sole
risk. The conclusions and recommendations of this letter have been prepared based on one site visit.
Conditions could change with time. If site conditions change or considerable time has passed, it may be
required to reevaluate the conclusions and recommendations presented in this evaluation.
Please let me know if you have any further questions or require further clarification. Once again,
thank you for this opportunity to service the City of Edmonds.
LANDAU ASSOCIATES, INC.
tjo P -�
J
W. Perry Welch, P.W.S
Senior Ecologist
WPW/rgm
2/23/10 PA074\l63\FileRm\R\C_Edmonds Olympic View outfall LTRdoc LANDAU ASSOCIATES
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