2018-11-07 Signed 8th Ave E 4418 BFE Letter.docx.pdfII
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Flood Plain Computations: This analysis utilized the most current version of the HEC-
RAS computer program, version 3.0.1, developed by the U.S. Army Corps of Engineers
Hydrologic Engineering Center, dated March 2001.
DRAINAGE COURSE DESCRIPTION:
The subject drainage course, as stated above, approaches the study area from the
southeast, through a broad swale running behind Alcott School. As the drainage course
enters the study area, the channel section is poorly defined in places, with rock
obstructions, and splits in the channel.
Approximately 500' east of Havana, the drainage course runs parallel to 81h Avenue as a
roadside ditch. From this point on west to Havana, the channel section is more regular,
having a bottom width ofapproximately4', a depth of approximately 5' and with 1:1 to
1: 1.5 side slopes.
300' east of Havana, the drainage course flows through a 48" concrete driveway culvert
at the entrance to the Spokane Rock Products quarry site. Presently, this culvert is
approximately Yi blocked with silt. In addition, the channel course for approximately 75'
upstream from the culvert is silted in, having a reverse slope iri places.
At the southeast comer of the intersection of 81h A venue and Havana, the drainage course
enters a 42" box culvert, and enters the City of Spokane drainage system. This culvert
extends west for 40', where a 36" storm drain joins it from the northeast, then continues
west in a 54" storm pipe. The entrance to the 42" box culvert has a bar grate mitered to
match the 2:1 slope of the end of the channel. The entrance is in good condition with no
obstructions.
ATTACHMENTS:
1. U.S.G.S. Map
2. FEMAMap
3. Copy of excerpt from Glenrose/Central Park Watershed Master Plan
4. Sub-Basin G91 runoff computations
5. HEC-RAS Flood Plain Report
6. Map of 100 Year Flood Plain Boundary
Adams & Clark, Inc.
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Attachment C
At
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8th
7th
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Hartson
Cu
b
a
19441936 1938194219461948
1934
19501952 1940
1932
1954 1956
1958
19601962
19641966
1968
197019721974 1976
1978
1
980
1982
1984
1986 198819901992
1994
1996
1998
2000
2002
2004
200 6 2008201020122014201620182020202220242026202820322034
19301928
1948
19
7
0
1
9
6
0
1962
19
7
6
196
0
2020
1
9
42
1934
19
9
2
1932
19
5
0
1974
1958
19 9 8
1958
1
9
6
2
1 9 3 2
19
5
2
1
9
6
6
197 8
1 9 6 4
1932
1
9
54
2
0
0
2
1944
1940
1
9
7
2
1
9
62
1934
1 9 6 8
1 9 3 4
1 9 7 0
1930
1 9 54
1 9 50
1
9
6
6
1942
19
3
6
1 9 5 4
1
9
7
2
1 9 86
1 9 3 2
1
9
7
6
1944
19
4
6
1 9 5 0
1952
1 9 4 8
1940
1
9
3
8
1 9 5 4
19
5
6
1936
1934
1
9
5
0
1
9
6
2
1 9 5 6
³Attachment E
Spokane Valley - 2-foot Contours at 8th and Havana