INFILTRATION ALTERNATIVES

1
INFILTRATION ALTERNATIVES

• Issaquah Highlands
• Prepared by the City of Issaquah Major
  Development Review Team
• June 2004

2
Issaquah Highlands Stormwater Alternatives

• 3 distinct alternatives that may be separated and
  combined into numerous variations
• Developed from the White Paper conclusions
  recommendations and Comments/Questions submitted
  as part of the process
• All alternatives meet EIS/Development Agreement
  expectations/provisions
• Alternatives for Microsoft are not included in
  Alternatives 1, 2 3 as their facilities will
  remain private (see discussion in last slide)

3
INFILTRATION ALTERNATIVES
ALTERNATIVE 1
ALTERNATIVE 2
ALTERNATIVE 3
AGRESSIVE REPAIR TO SLIDE AREA (20 CFS)
SAME AS ALT 1
NOMINAL REPAIR TO SLIDE AREA (4 CFS)
ADD EARLY WARNING SYSTEMS TO SLOPES
ADD DETENTION BASED ON 6 cfs CAPACITY IN
LOWER REID
ADD DETENTION BASED ON EXPANDED CAPACITY IN
LOWER REID
ADD 3 cfs CAPACITY IN LOWER REID (9 cfs total)
RE-TEST AND RE-VEST INFILTRATION RATES
ADD SMALL OUTLETS (DISPERSED INFILTRATION) EAST
OF BPA
SAME AS ALT 1
SAME AS ALT 1
CONTINUE WORKING W/ MICROSOFT (see Microsoft
slide)
SAME AS ALT 1
UTILIZE HILLTOP INFILTRATION SYSTEMS
PLACE HILLTOP SYSTEMS ON STANDBY (NOT MICROSOFT)
4
ALTERNATIVE 1
Additional detention (location undetermined) to
support 9 cfs L. Reid infiltration)
Install safety controls at L. Reid
Add capacity at L. Reid (9 cfs total)
Add GW monitors
Repair slide area for 4 cfs capacity (2 cfs
more than background)
Place hilltop infiltration systems on standby
5
ALTERNATIVE 2
Drill new Monitoring wells perform more
exploration
Test and re-vest infiltration rates (32 cfs
target)
Repair slide area for 20 cfs capacity (18 cfs
more than background and 12 cfs less than pipe)
Install Early Warning Devices
6
ALTERNATIVE 3
Build dispersed Infiltration East of BPA (3 cfs
target)
Additional detention (location undetermined) to
support 6 cfs _at_ L. Reid
Install safety Controls _at_ L. Reid
Place hilltop infiltration systems on standby
Repair slide area for 4 cfs capacity (2 cfs
more than background)
7
Comparison
Alternative
?1
?
?
? Probable but not guaranteed ?1 easterly
infiltration may result in slope stability issues
8
Summary
9
Microsoft

• Microsoft has expressed a preference for
  continuing the vesting process for hilltop
  infiltration. As their facilities will remain
  privately owned and maintained, the Citys
  concerns are primarily focused on environmental
  issues.
• Should it be determined that hilltop infiltration
  from the Microsoft pond at a rate necessary to
  accommodate all of their expected runoff would
  result in compromising slope stability, the
  following alternatives may be feasible and will
  be explored
• stop testing for added infiltration and build
  detention necessary for 2 cfs rate (determined to
  be stable)
• join the public system (upper Reid, flow
  splitter, etc.)
• build Valley infiltration system (similar to
  Lower Reid)
• use small scale infiltrators at each building
• reclassify the panhandle (not the DNR site) and
  make a direct stream release w/o infiltration
  (subject to LIVA recharge need)