Lecture 8 Flashcards
what was the historical stormwater management approach
drainage efficiency
get it out as fast as possible
what are some impacts of development on urban stormwater graph
the post development hydrograph has a sharper peak and a lesser lag time
increased peak flows cause
channel alteration and pathogens
reduced infiltration causes
sedimentation and nutrients
reduced groundwater recharge causes
disruptions to benthic habitat and inorganics (metals,salt)
lower baseflows cause
elevated water temperatures and toxic organics
what are ecological maintenance flows (EMFs)
the streamflow required to maintain aquatic habitat
what are the methods for defining EMFS
q90/q95 flow duration indicies
7Q2,7Q10,7Q20 7 day average minimum flows that occur once on average, every 2, 10, or 20 years.
Aquatic baseflow (ABF) method
0.005m^3/(s-km^2) used by USFWS for small ungauges basins in new england
in atlantic provinces the EMF is
25% mean annual flow
75% median monthly flow
how do EMFs vary
by flow condition (severe degradation to flushing or maximum)
and time of year
(october to march) and (april to september)
fair or dgreading rec ommended EMF has
most substrate submerged
poor or minimum recommended emf has
50% of stream substrate submerged
kuhn marsh study is where
headwater catchment within morris lake watershed
kuhn marsh characteristics
28 had urban catchment draining residential area, 27% impermeable surfaces, stormwater discharged to a 2.2 ha wetland
kuhn marsh stormwater quantity and quality
16 month study
baseflow and stormflow loading of
nutrients
bactera
metals
organic carbon
assesment of wetland function
kuhn marsh water quality issues
nutrient loading
elevated E coli
heavy metals
Stormwater management objectives
Peak flow management
Water quantity management
water quality management
the first step in stormwater management is to
identify the design objectives
most jurisdictions will specify the objectives that the
stormwater management systems must meet
why manage peak flow?
to prevent:
flooding
damage to downstream infrastructure
channel erosion (most guidelines specify that post development flows must be less than pre-development flows
what does managing water quantity mean
designing a system to manage a specific volume of stormwater
(example 2yr-24hr storm to be retained for no less than 24 hours)
why manage water quantity
reduce peak flows
provide water quality treatment
possibly allow for infiltration
what does water quality management mean
systems are designed to remove a specific amount of contaminant load (typically sediments)
could be specified as a water quality metric (system must remove 50% of suspended sediment)
could be specified as a design based goal (systems must retain stormwater for at least 24hr)
Control strategies for stormwater management
- Lot level controls
(source control) (less than 2 hectares) - End-of Pipe Controls
-large scale infrastructures.
Lot level controls consist of what sub-approaches
- temporary strorage
- infiltration and long-term storage and reuse
what are the two kinds of temporary storage lot level controls common in NS
parking lot storage
rooftop storage
Infiltration/long term lot level controls include
dry wells
cisterns
infiltration trenches
infiltration swales
rain gardens
pervious pavement
the primary goal of long term level controls is
to retain water volumes from a small area and allow it to infiltrate
lot level infiltration systems challenges/considerations
restrictive soils and geology
climate (ice,snow,frozen soils)
maintenance
groundwater contamination
liability for flooding issues
end of piipe control strategies
detention and retention ponds
constructed wetlands
infiltrations basins
engineered filtration systems
detention vs retention ponds (IMPORTANT)
detention ponds are used to dampen peak flow rates
retention ponds are surface water treatment systems that are designed to retain stormwater for greater than 18 hours
-typically designed to retain runoff from a moderate rainfall event (i.e a 2yr 24hr storm)
A retention pond for 20 ha residential development in HRM would have a foodprint of
approximately 2 hectares
what is the most widely applied stormwater management tehcnology
retention and detention ponds
retention and detention ponds types
can be designed as either wet or dry ponds
can also be designed as an infiltration basin
what are constructed wetlands
they follow the same basic design principles as retention ponds, but designed to allow for emergent aquatic vegetation establishment
what depths do constructed wetlands need, why
they need a depth of <1m to allow for vegetation establishment, therefore wetlands typically need to be quite large in order to achieve required retention times for treatment
limitations of the road reduction approach
does not restore/retain natural hydrologic pathways
poor removal of dissolved contaminant load
long term performance
detention/retention pond design
can be detention, retention, or as multipurpose systems
wet or dry design
L:W >3:1
permanent pool depths in wet ponds are typically 1-2m
how would create a trial design for a detention/retention pond
there is a nifty formula that relates desired outflow hydrograph peak to desired inflow hydrograph peak
