Lecture 8 - Stormwater Management Structures Flashcards
What approach has historical stormwater management design relied on?
drainage efficiency approach
What is the impact of urban stormwater?
fundamental alteration of the hydrologic cycle
What are the hydrologic watershed impacts of urban stormwater?
increased peak flows, reduced infiltration, reduced groundwater recharge, lower baseflows
What are the physical watershed impacts of urban stormwater?
channel alteration, sedimentation, disruptions to benthic habitat, elevated water temperatures
What are the water quality watershed impacts of urban stormwater?
pathogens, nutrients, inorganics (metals, salt), toxic organics
What do the watershed impact factors influence?
the structure and function of ecosystem components
What are ecological maintenance flows (EMFs)?
streamflow required to maintain acquatic habitat
What are the percentages associated with Ecological Maintenance Flows (EMFs) in the Atlantic Provinces?
25% of mean annual flow and 75% of median monthly flow
What other indices can be used to define EMFs?
Flow durations indices (Q90, Q95), 7 day average minimum flows that occur on average every x amount of years (7Q2, 7Q10, 7Q20), aqautic baseflow method (ABF)
What is the acquatic baseflow method? (ABF)
0.005 m3/(s-km2) used by USFWS for small ungauged basins in New England
Describe the relevant characteristics of the Kuhn Marsh Watershed.
28 ha urban catchment draining residential area, 27% impermeable surfaces, stormwater dishcharged to 2.2 ha wetland
Describe the study of the Kuhn Marsh Watershed.
16 month study, baseflow and stormflow loading of nutrients, bacteria, metals, organic carbon, assessment of wetland fuction
What were the water quality issues with the Kuhn Marsh watershed?
nutrient loading, elevated E. coli, heavy metals
give a general overview of stormwater management objectives
first step is to identify the design objectives for stormwater management, most jurisdictions will specify the objectives that the stormwater management systems must meet
Describe the peak flow management objective.
prevent flooding, damage to downstream infrastructure, channel erosion, most guidelines specify that post-development flows must be less than pre-development flows
Descirbe the water quantity management objective.
systems are designed to manage a specific volume of stormwater, reduces peak flows, provides water quality treatment and possibly allows for infiltration
Describe the water quality management objective.
systems are designed to remove a specific amount of contaminant load (typically sediments), could be a specified 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 24 hr)
What are the two general types of stormwater control approaches?
lot level controls (source control), end of pipe controls
What are the two different types of approaches for lot level control.
temporary storage and infiltration and long-term storage and reuse
What are two common methods of temporary storage?
parking lot storage, rooftop storage
What size watershed is lot level control appropriate for?
less than 2 ha
What are the infiltration/long-term storage methods for lot level control?
dry wells, cisterns, infiltration trenches, infiltration swales, rain gardens, pervious pavements
What is the main goal of long-term storage for most systems?
to retain water volumes from a small area and allow it to infiltrate
What are some considerations/challenges for lot level infiltration systems?
restrictive soils and geology, climate (ice, snow, frozen soils), maintenance, groundwater contamination, liability for flooding issues/
What are some end-of-pipe control strategies?
detention and retention ponds, constructed wetlands, infiltration basins, engineered filtration systems
What are detention ponds used for?
to dampen peak flow rates
What are retention ponds?
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 (2 yr, 24 hr storm), much larger than detention ponds
Give some more information about retention and detention ponds.
most widely applied stormwater management technology, can be either wet or dry ponds, also called infiltration basin
What are constructed wetlands?
same basic design principles as retention ponds, but designed to allow for emergent aquatic vegetation establishment
Why do constructed wetlands need to be quite large?
water depths need to be less than 1 m to allow for vegetation establishment, so needs to be large to achieve required retention times for treatment
What are the limitations of the load reduction approach?
does not restore/retain natural hydrologic pathways, poor removal of dissolved contaminant load, long term performance
What is the length to width ratio for detention/retention ponds?
greater than 3:1
What do wet ponds have? How deep is this?
permanent pools, typically 1-2 m
What can retention/detention ponds be designed as?
detention only, retention only, multipurpose
