Stormwater
Master Plan Update
Engineering & Building
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T A B L E O F C O N T E N T S
1 Purpose
Storm Master Plan OverviewNext Steps
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E N G I N E E R I N G & B U I L D I N G
2 E N G I N E E R I N G & B U I L D I N G
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Purpose• Maintain the Current Level of Protection• Model growth per 2030 General Plan land uses• Assess capacity of the existing storm system• Identify needed improvements to support growth per 2030 General Plan
• Provide direction for incoming development• Provide costs for the update of the Storm Construction Developer Impact Fee
3 E N G I N E E R I N G & B U I L D I N G
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Storm Master Plan Redevelopment• The adopted 1994 Storm Master Plan relies heavily on discharging directly to waterways • St Johns River, Modoc Ditch,
Mill Creek, Evans Ditch, Packwood Creek, and Cameron Creek • Assumes space to enlarge
waterway channels
• The 2026 Storm Master Plan
focuses instead on regional retention basins
4 E N G I N E E R I N G & B U I L D I N G
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1994 Storm Master Plan
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Evaluation of the Existing Drainage SystemA Contemporaneous Approach 6 visalia.gov
Contemporaneous Evaluation of the Existing Drainage System • Instead of evaluating the existing system based
on the most current standards, evaluate the system based on the standards that applied at the time of installation
• Avoids moving the goalposts
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“Deficiencies” Identified Using a 10-year, 10-day “Modern” Design Storm• 17 projects identified• ~$80 Million• Increased discharges to waterways Deficiencies Identified Using a 2-year Design Storm Representative of Historical Standards• 3 projects identified• $1.4 Million• Deficiencies observed by field staff System Deficiency Projects
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E N G I N E E R I N G & B U I L D I N G
Storm Master Plan Development Approach
Geospatial analysis of existing land cover in Visalia
Correlation of land cover with General Plan land use
Forecasting anticipated impervious land coverage based on General Plan land use Development of Storm Master Plan
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Forecast Land Cover Model System During Design Storm Develop CIPs 10 E N G I N E E R I N G & B U I L D I N G
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Major Changes & Impacts1. Increased Residential Runoff 2. Detention vs Retention – Standalone Retention Basins
3. Industrial Park Exclusion
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Increase In Residential Runoff• Estimated runoff for single-family residential land use for a given rain event have increasedcompared to the 1994 plan • Geospatial analysis of Visalia satellite imagery
indicates low-density residential zoned lots are more
impervious on average than assumed in the 1994 plan
• Current development trends display increasing
density
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Residential Impervious Cover
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70 80
42 56
54 60
0 10 20 30 40 50 60 70 80 90
Residential Very Low Density
Residential Low Density
Residential Medium Density
Residential High Density
Percent Impervious
2019 Observed Average % Impervious 1994 Assumed % Impervious 13 visalia.gov 14
E N G I N E E R I N G & B U I L D I N G
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Detention vs Retention1994 Detention Approach• Detention = Discharge • Relies on waterway
capacity
• Basins designed and
sized to only temporarily hold stormwater smaller,
multipurpose
Current Retention Approach
• Retention = Recharge
• Not dependent on
waterway capacity
• Basins designed and
sized to hold stormwater until it infiltrates or evaporates larger, single-purpose
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Standalone Retention Basins
Planning-Level Design• 23’ total depth• 18’ design water depth• Perimeter fencing & landscaping
• Master plan CIP costs
include frontage improvements on 2 sides
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Understanding Visalia’s Existing Park-Basins
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Detention Park BasinsThe “Overflow Only” Design•No separate basin section•Storm drain runs under depressed park
•Only fill when the pump station
can’t keep up with the inflow
The “Part Basin” Design•Include a deeper basin portion, typically fenced
•Pumps only kick on when water
may encroach into park portion
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“Overflow Only” Detention Park Basins
Mill Creek Garden Park
Blain Park
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“Part Basin” Detention Park Basins
Seven Oaks
Riverbend
Cherry
Meadow
Kiwanis Ruiz
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Switching From Detention to Retention For New Regional Basins Detention Basins
• Temporarily detain stormwater for
flood control purposes before releasing it to waterway
Retention Basins•Retain stormwater until it
infiltrates, evaporates, or is used by plants
Detention = Discharge Retention = Recharge
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Retention Park Basins
Burke Park Basin Sunset Park Basin
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Burke Park: the Retention Park Basin
March 2023
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Sunset Park: Park With Retention Basin
•Basin area sized
to hold entire required volume
•Likelihood of
park flooding is very, very low
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“Standalone” Retention Basins
Future basins are planned as standalone facilities for sizing and impact fee purposes.The City can still plan on locating regional basins adjacent to parks. 25 visalia.gov
Basin Improvement Cost Assumptions
Local/Local Corner Lot Collector/Arterial Corner Lot
2026 Costs 26
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Unit CostElement$150,000 / acreLand Acquisition$56 / linear footPerimeter Fencing$6 / square footPerimeter Landscaping$232 / linear foot of frontage Parkway, sidewalk, curb/gutter, pavement frontage
$21,000 / basin Drive approach & corner improvements$33 / cubic yardBasin Excavation Basin Improvement Cost Assumptions
2026 Costs 27
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Industrial Park Exclusion• The Stormwater Master Plan CIPs do not address the Industrial Park
• Industrial-zoned parcels are required to retain
stormwater onsite
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Next Steps
Storm Master Plan
Report will be available
on the City Website
Return to Council 9/21
for master plan
approval and adoption
Kickoff Storm Nexus
Study & Rate Study
30 Thank you
for your time.
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