Calculations and Methodology - Semcog

Calculations and Methodology
Scott Dierks
JFNew

Calculations & Methodology
• Chapter 9 LID manual
• Describes design methods to calculate
LID techniques
– Mimicking presettlement hydrology
• Presettlement vs. Predevelopment

Implementing a Community
Stormwater Regulation
• Discuss & decide on water quality
outcomes
• Adopt design standards that achieve
desired outcomes
• Select stormwater methodologies to
meet the design standards

NPDES Phase II New Stormwater
Permit Requirements (rule issued
May 22, 2008)
• Create/enforce regulation of post-construction
runoff for new and redeveloped projects >1 acre
– Minimum treatment volume – one inch of runoff or
runoff from 90% exceedance event
– Projects must demonstrate minimum 80% TSS
removal and no more than 80 mg/L leaving site
– Post construction peak and total runoff = preconstruction
(channel protection criteria)

LID Design Criteria
• Groundwater recharge
• Channel protection
• Prevention of downstream flooding
• Water quality protection
• Evapotranspiration & the natural
hydrologic/water cycle

Channel Protection Criteria
• Bankfull condition responsible for
shaping of channel
• Match post construction runoff volume
and rate to presettlement conditions up
to bankfull

Channel Protection Criteria
• LID site design based on no increase of
presettlement runoff condition for all
storms up to the 2-year, 24-hr return
frequency
– 95% of annual rainfall volume

Rainfall Distribution by Storm
Lansing, Michigan

Waivers to Channel
Protection Criteria
• Waivers for certain site constraints
– Poor draining soils, contaminated soils,
bedrock, high water table, karst geology
• Identify alternative standards
– Detention of 1 yr, 24 hour storm and water
quality criteria
• Model ordinance

Flood Protection Criteria
• Flood control based on local needs
& flood risk
• Volume runoff is maintained at
presettlement value, peak rate will also be
maintained up to the same storm.
• Flood protection beyond presettlement
value, additional control may be needed.

Flood Protection Criteria
• Maintain presettlement runoff volume
and rate for storms up to 2 year event.
• Maintain the presettlement peak runoff
rate for all storms up to the 100 year
event or that determined by local
standard.

Additional Flood Control
• Many use 100 year storm
• Fixed release rate
• Exemptions for issues small sites or
direct discharge to certain waterbodies
• Model ordinance

Groundwater Recharge Criteria
• Essential for providing stream flows and
groundwater supplies
• Implement a volume control criteria and
maximize use of infiltration BMPs

Water Quality Criteria
• Bacteria from wildlife
• Nutrients from excessive fertilizer
• Suspended solids from eroded
streambanks, roads, & construction sites
• Hydrocarbons and trace metals
• Chlorides from road salt

Water Quality Criteria
• Quantitative goal = 80% reduction in
Total Suspended Solids based on postdevelopment
land use
• Meet channel protection volume, meet
water quality volume

Water Quality Criteria
• Options for treatment volume
– 0.5 inch of runoff
– 1 inch of runoff from impervious areas &
0.25 inches from disturbed pervious areas
– 1 inch of runoff from disturbed pervious &
impervious areas
– 90% of runoff producing storms

Other Water Quality Issues
• Soluble pollutants
• Hot spot & high risk areas
Stormwater Hot Spots
Vehicle Maintenance and Repair Facilities
Vehicle Fueling Stations
"Fast Food" Restaurants
Convenience Stores
Outdoor Chemical Mixing or Handling
Outdoor Storage of Liquids
Commercial Nursery Operations
Other Uses or Activities Designated by Appropriate Authority
Minimum Pre-Treatment Options
A, E, F, G
A, D, G
B, C, D, I, K
B, C, D, I, K
G, H
G
I, J, L
As Required

Evapotranspiration
• ET is largest component of the
hydrologic regime
• Difficult to quantify
• Minimize loss by protecting existing
vegetated areas & replacing vegetation
lost or removed.

Nonstructural BMPs
• Protected presettlement areas are subtracted
from site development for purposes of
designing LID-based treatments.
– Protect sensitive areas
– Protect riparian buffers

Stormwater Credits
• Reduces volume of runoff
• Enhance the response of a piece of land
to a storm event rather than treat the
runoff that is generated
• Based on the Curve Number method of
analysis

Stormwater Credits
• Credited BMPs
– Minimize soil compaction
– Protection of existing trees
– Soil restoration
– Native revegetation
– Riparian buffer restoration

Example Stormwater Credits
• Native revegetation/Riparian buffer
restoration
• Proposed trees and shrubs to be planted under the
requirements of these BMPs can be assigned a
Curve Number (CN) reflecting a woods in "good"
condition for an area of 200 square feet per tree or
the estimated tree canopy, whichever is greater. For
shrubs, an area of 25 square feet per shrub.

Calculating Runoff
• Runoff volume
• Peak rate
• Runoff Hydrograph

Runoff Volume Calculation
• Various methods available
– Runoff Curve Number (CN) method

Peak Rate /
Hydrograph Estimations
• NRCS (SCS) Unit Hydrograph Method
• Modified Unit Hydrograph Method for
Michigan
• The Rational Method

Computer Models for
Calculating Runoff
• HEC Hydrologic Modeling System (HEC-
HMS)
• SCS/NRCS Models: WinTR-20 & WinTR-55
• Stormwater Management Model (SWMM)
• Source Loading and Management Model
(SLAMM)
• Continuous Modeling

Calculating Peak Rate By
Utilizing Volume Control
• Volume reduction BMPs and LID
practices reduces or eliminates storage
required for peak rate
• Difficult to quantify peak rate benefits
of LID
• Milwaukee Metropolitan Sewerage District
“Quicksheets”
• Various other resources noted in manual

Precipitation Data
• Rainfall Frequency Atlas of the Midwest
– www.sws.uiuc.edu/pubdoc/B/ISWSB-
71.pdf
• United States Historical Climatology
Network
– http://cdiac.ornl.gov/epubs/ndp/ushcn/state
_MI.html

Design Calculation Process
Step 1: Provide general site information
Step 2: Map the existing features of the site
Step 3: Layout the proposed development
avoiding protected areas

Design Calculation Process
Step 4: Determining the disturbed area size
Step 5: Calculate the level of volume control
needed for channel protection

Design Calculation Process
Step 6: Select volume control BMPs
Step 7: Peak rate exemption for small sites
Step 8: Calculate peak rate control
Step 9: Determine water quality volume &
select BMPs (If needed)

Step 1:
Provide
General Site
Information

Step 2:
Map
Existing
Features

Step 3: Layout Proposed
Development
• If after development is sited, addition
sensitive areas are impacted, modify
worksheet 2.

Step 4:
Determine
Disturbed
Area Size

Step 5:
Calculate
volume
control
needed for
channel
protection

Step 6:
Select
volume
control
BMPs

Step 7:
Peak Rate
Exemption
for Small
Sites

Step 8:
Calculate
Peak Rate
Control

Step 9:
Determine
Water
Quality
Volume
(if needed)

Example for Calculating LID
BMP Peak Rate Benefits
• Milwaukee Metropolitan Sewage District LID
“Quicksheet” (See:
http://v2.mmsd.com/Assets/Documents/stormwaterweb/
PDFs/Appendix_L.pdf)
• Total site retention (sum of distributed BMP retention)
is set as uniform depth of storage across the site
• Runoff “occurs” only after runoff depth >uniform
storage depth

Calculations and Methodology
Scott Dierks
JFNew