Active Recall Flashcards
Manhole
4’ width
Entering pipes higher than exiting
Measure pipes to inside of wall
Pipes from inlet sizing not important
Pipes min. 8” (PVC 8-10”, conc 12”)
Max. spacing manhole 300-500 ft
Pipe Options
Reinforced Concrete (RCP)
Concrete Pipe
Corrugated metal pipe (CMP)
High Density Polyurethane (HDPE)
Plastic pipe
Polyvinyl Chloride (PVC)
Detention
Upper 1/3 of watershed to reduce flooding
Lower 1/3 will increase flooding
Excavating most efficient
2-4 above water table
3 ft above bedrock
Below frost line
100 ft from wells
10 ft from buildings
Detention Basin
Dry Detention
Empty within 72 hours
Principal spillway (10-100yr, 24 hr)
Emergency spillway (100yr+, 24 hr)
Emergency invert elev. 6” higher than principal invert
Sediment forebay (rip rap and/or leveler)
Temporary ponds wet detention trapping sediment 2 yr & 10 yr, 24 hr
1:4 slopes; Strom 1:3 slopes
4 ft depth
1:10 ledger 20’ wide
Strom Ledge 10 ft, 1 ft deep, 5:1 slope
Sediment basin longer and narrower, elongated
V shaped weir
Flow channel along length center for low drain times
SCS method or rational mass inflow method
Retention Basin
Wet Detention
Permanent pool
Forebay (riprap and/or level spreader)
Storage Basin (long and narrow, temp. Depth 4-8 ft.)
Outlet Structure
Slopes 1:4 or less; Strom 1:3
Flat shelf 1:10 or less 20 ft.
Strom Ledge 10 ft, 1 ft deep, 5:1 slope
Preferable to avoid safety fencing
V-notch weirs
2% slope basin towards outlet
Basin shape triangular with point at inlet; prevent dead zones as new water pushes old water out
Minor conveyance system
Swales, sewers
Take the frequent storms
Minor inconvenience when fails
Rational method or SCS runoff method
Major conveyance system
Can be natural or manmade
Emergency spillway, floodway
Prevent major loss and flooding
SCS runoff method
Design Storm
1.25” rainfall
2 years rainfall without erosion
Primary determinant of infiltration rates
Soil type
Urbanization impact on runoff
Simplify land cover, causing increase in runoff volumes
Greater impervious surface
Grater compaction of earth
Reducing potential for infiltration of storm water
Disconnected imperviousness
Where runoff drains from impervious surfaces as sheet flow onto pervious surfaces, runoff is minimized
Maximizing this will result in less stormwater runoff and better water quality
Connected Imperviousness
Where runoff drains directly from impervious surfaces into drainage ways, runoff maximized
Directly connected to drainage system
Rational Method
Estimate leak discharges of runoff from rainfall in urban areas
Most widely used and accepted for computing runoff
Q=KCIA
Storage and Detention
Located in upper third of watershed will reduce downstream flooding
Lower third increases flooding
Best provided at regional basis
Watershed Management System
- Characterize watershed by flooding, water quality, surface cover, land use
- Identify, assess, and prioritize problems
- Establish objectives: non-physical like land acquisition, impervious cover regulations, public education; physical like regional detention
- Implement objectives and monitor and inspect
