12.0 Storm Drainage System 1 Flashcards
Stormwater System
- Not intended to include any sanitary wastewater (2 isolated systems)
- important component in the City’s strategic plan to mitigate the effects of climate change and include build infrastructure into the natural water cycle
- by gravity to a natural water resource
Changes to the Natural Water Balance
- runoff volume increases in proportion to impervious area
- land uses with extensive roof and paving areas create more runoff than land uses with extensive areas of absorbent soils and forest cover
Natural Ground Cover
- 40% evapotranspiration
- 10% runoff
- 25% Shallow Infiltration
- 25% Deep Infiltration
10-20% Impervious surface
- 38% evapotranspiration
- 20% runoff
- 21% Shallow Infiltration
- 21% Deep Infiltration
35-50% Impervious Surface
- 35% evapotranspiration
- 30% runoff
- 20% Shallow Infiltration
- 15% Deep Infiltration
75-100% Impervious Surface
- 30% evapotranspiration
- 55% runoff
- 10% Shallow Infiltration
- 5% Deep Infiltration
Changes to the Natural Water Balance
- Traditional ditch and pipe systems have been designed to remove runoff from impervious surfaces as quickly as possible and deliver it to receiving waters
- resulting stormwater arrives at the receiving waters much faster and in greater volume than under natural conditions
- Changes to impact
1. property
2. ecological
3. water quality
Property Impacts
- multiple drainage culvert installations
- channel down-cutting (due to increased volume)
Ecological Impacts
- urbanization
- water quality
- aquatic habitat
Detention ponds
- accomodate existing and future upstream development
- weaken peak flows from storm water runoff
- lessen erosion downstream
Integrated Stormwater Management Plan
Components
- Land development action plan
- habitat enhancement plan
- financial and implementation program
- flood risk mitigation plan
Downsides of detention ponds
- provides a partial solution
- only treat consequences of increases impervious area (not the source)
- mitigates flooding but does not prevent the ongoing channel erosion that creates property and fisheries impacts
- do no support the sustained stream base flow (in dry months)
Integrated Stormwater Management Plan
objectives
- drainage
- stream protection
- water quality
Design storms consist the following:
- The minor system
- The major system
The minor system
- consists of gutters, catchbasins, open channels, pipes, driveway culverts, watercourses, and stormwater management facilities designed to carry flows up to the 1:5-year return period storm under free flow conditions
- minimize inconvenenience of frequent surface runoff
The major system
- consists of roadways, culverts, ditches, surface flood paths, watercourses, and stormwater management facilities designed to carry flows up to the 1:100-year return period storm
- provides safe conveyance of flows and to minimize damage to life and property
2 methods to design the storm system
- the rational method (catchment area is less or equal than 20 ha)
- the hydrograph method
Runoff coefficient
- (C) is based on ground slope, type if cover, type of ground surface, and development population density
- ranges from 0 (lots of infiltration) to 1 (heavily developed, all rainwater becomes runoff)
- must account for future development outlines in community plans and/or over a 100-year period
- generally preferable to develop a composite runoff based on the % of different types of surfaces in the drainage area
Rainfall intensity
Calculated using Intensity Duration Frequency (IDF) curves or equation
Tributary drainage area
- based on the applicable existing and proposed contours
- cumulative tributary drainage areas must be considered for catchments with multiple sub-catchments
- sub-catchment boundaries are determined by aggregating together sub-areas whose potential overland flow paths share a common direction and drain to the same collection channel
Time of concentration
sum of the inlet time and travel time in the pipe or channel to the point of interest
Manning’s Equation
Formula for open channel (unpressurized) flow
Minimum size
- 200mm diameter (residential zones)
- 250mm diameter (all other zones)
velocities and slope
- preffered min. = 0.76 m/s
- absolute min. = 0.61 m/s
- slope min. = 0.5%
- max. velocity = 4.6 m/s
- higher than 4.6 m/s, provisions shall be made for energy dissipation, erosion, and movement. Appropriate bank protection measures if discharging into open watercourses