3. Urbanization Flashcards
Why does Urbanization increase peak runoff ?
1) Increased area of impervious cover
2) Decrease in time of concentration (sewers, channels, etc.)
Urban water sheds have:
1) high percentage of impervious areas
2) man-made drainage infastructure
- What are the effects of Urbanization
1) Faster response time
2) Larger runoff volumes
3) Larger peak flows
What increases peak flow in an urban area?
- percentage of impervious areas
- percentage of area served by storm sewers
When constructing a storm water detention pond, what are some things to consider while designing?
- Storage Volume
- Outlet facilities
What are the two types of storage facilities?
1) Source Control- Local disposal(infiltration, porous pavement), Inlet Control ( rooftop storage, parking), On-site detention (ditch, swales)
2) Downstream Storage- In-line detention (sewer network,..)
Off-line detention (open pond,..)
Detention at wastewater treatment plant
What are the types of Surface detention?
1) Extended detention Basins(dry detention basin) these are most widely used. Water enters the basin, impounded and the slowly discharged -Willow Grove
2) Retention Ponds (wet detention ponds) an example is a lake that can be designed to remove pollutants by settling. -Arbor Creek Canoing pond
- What is the time of concentration and what are the the factors affecting it.
Time of concentration is the time it takes the entire watershed to be contributing water to the outflow. The factors affecting it are slope, Tavel length and surface roughness.
- What are the assumptions of the Ripple and sequent-peak methods?
Ripple: 1) Demand is constant 2) year repeats itself continuously
Sequent: 1) Inflow is never greater than outflow. 2) Crit. draw down period happens within the analysis period.
How can we develop the storage-yield relationship.
Graphically on excel using the best fit line
What is flow routing?
Determining the TIME AND MAGNITUDE (flow hydrograph) of flow at a point on a watercourse from known or assumed hydrographs upstream.
What are the two general operational purposes of a reservoir?
1) Conservation (water supply, irrigation, recreation, and hydropower)
2) Flood Control (reducing the downstream flooding)
For recevoir design what is needed?
the storage-yield relationship
What is the firm yield?
the only flow guaranteed to be supplied at all time.
What mass-curve analysis? When can it be used?
A simple graphical method to determine the active storage volume. It can be used when the release is constant (the total release does not exceed the the total reservoir inflows).
What methods are used to develop a storage-yield relationship?
mass curve analysis, and sequent peak analysis.
What is the critical period?
The time period over which the flows have reached a minimum, causing the greatest drawdown of a reservoir. It starts when a reservoir is full and ends when the reservoir refills after a drought.
When is the critical drawdown period?
It starts when a reservoir is full and ends when the reservoir is empty.
What is the required active storage?
The largest difference between the firm yield and the mass-curve
What are some of the assumptions of the runoff coefficient?
1)Excess intensity is a constant fraction of the rain intensity
2)C is independent of total rainfall volume
3)C is independent of return period
4) C is independent of antecedent conditions
None of these assumptions are valid but the coefficient is still used.
When designing street and highway drainage, what are some design DECICIONS?
Spacing of inlets, and type and size of inlets
When designing street and highway drainage, what are some design ELEMENTS (criteria)?
- Spread
- Return Period
When designing street and highway drainage, what are some factors?
- Traffic Volume–Nuisance to pedestrians
- Design Speed–longitudinal slope
- Road classification–Cross slope
True or False: Design criteria are universal?
False, design criteria vary with location
