Question 1

When designing storm sewers and open channel conveyance systems, the use of peak runoff rates is generally considered adequate. However, conveyance system designs that include flood routing components, such as retention or detention basins, which of the following is appropriate to consider?

A)Rainfall spatial variability
B)Flood hydrograph
C)Antecedent soil conditions
D)All of the above

Flood hydrograph

Accepted Answer

Conveyance system designs that include a flood routing component should consider flood hydrographs. Incorporating flood hydrographs into the design of a conveyance system can be a complex process, and is often done using a hydrology software application integrated with a computer aided drafting and design (CADD) program. The spatial variability of rainfall and antecedent moisture conditions are taken into consideration when modeling the performance of existing (built and operating) conveyance systems.

Question 2

The frequency of a storm event can be expressed in which of the following ways?&#10;A)The probability of a given intensity of rainfall being equaled or exceeded&#10;B)The recurrence interval between rainfall intensities of a given or greater amount&#10;C)Millimeters/hour (mm/h) or inches/hour (in/hr)&#10;D)A. and B.

Accepted Answer

The frequency of a storm event can be expressed as either the probability of a given intensity of rainfall being equaled or exceeded, or the recurrence interval between rainfall intensities of a given or greater amount. The intensity of a storm is the rate at which precipitation falls, and is usually stated in mm/h or in/hr.

Question 3

Erosion and sediment control best practices for construction sites include which of the following methods?&#10;A)Hay bales, silt fences and temporary sediment traps&#10;B)Slope stabilization methods&#10;C)Drainage swales and detention basins&#10;D)All of the above

Accepted Answer

Erosion and sediment control best practices for construction sites include all of the listed methods. These best practices reduce the potential impact of soil erosion when the removal of natural vegetation and topsoil make exposed areas susceptible to erosion during construction. Most communities enforce provisions of erosion and control ordinances based on U.S. EPA's guidance documents for protection local water resources from nonpoint source pollution.

Question 4

How is the concept of a 50-year design recurrence interval explained to the general public?&#10;A)2% chance storm&#10;B)May occur several times in a 50-year period&#10;C)May occur 50 times in a given year&#10;D)None of the above

Accepted Answer

The concept of a 50-year design recurrence interval is explained as a 2% chance storm event to the general public. For a 50-year recurrence interval, the probability of equaling or exceeding the resultant storm flow is 2%. A recurrence interval is not the actual interval for which a storm is expected to occur, but rather the probability that a storm event of a given magnitude is going to occur in any given year.

Question 5

Time of concentrate (T_c) is the time required for water falling on the hydraulically most remote point in a drainage area to flow to a specific point. Which of the following factors affect T_c? A)Surface roughness, channel shape and flow patterns, slope B)Surface roughness, distance, channel shape and flow patterns C)Distance, channel shape and flow patterns, slope D)None of the above

Accepted Answer

Surface roughness, channel shape and flow patterns, and slope affect the time of concentration. Hydraulically, the most remote point in a drainage area may not be the point which is the furthest away. For example, water that travels a short distance across rough, flat terrain may take a longer time to reach a given point than water traveling a longer distance across steep, smooth terrain.

Question 6

Which of the following rainfall-runoff models does not produce a storm hydrograph for a given rainfall hyetograph?&#10;A)Unit hydrographs&#10;B)Time-area method&#10;C)Rational method&#10;D)Kinematic wave model

Accepted Answer

The Rational method does not provide a storm hydrograph for a given rainfall hyetograph. The Rational method is a peak runoff model that only provides an estimate of the peak discharge from a given watershed. Unit hydrograph techniques, time-area method, and kinematic wave models provide an estimate of runoff for a given time series.

Question 7

Which of the following models is used to simulate rainfall runoff from a watershed?&#10;A)Natural Resources Conservation Service (NRCS) TR-55&#10;B)U.S. EPA Storm Water Management Model (SWMM)&#10;C)Rational method&#10;D)A. and C.

Accepted Answer

The NCRS TR-55 and Rational method are models that are used to simulate rainfall runoff from a watershed. TR-55 was originally developed in 1954 and uses the United States Department of Agriculture (USDA) Soil Conservation Service (SCS) runoff equation to predict peak runoff rates and volumes in small watersheds in the United States. The Rational method is based on the following formula that relates the potential runoff that may be produced by a watershed (C), rainfall intensity (i) and watershed drainage area (A): Q = CiA (where Q is the design discharge).

Question 8

Which of the following assumptions is not made when applying the Rational method to calculate peak runoff in a small watershed?&#10;A)Rainfall is spatially and temporally constant for the duration of the event&#10;B)The rainfall duration is shorter than the time the time of concentration of the watershed&#10;C)The runoff coefficient incorporates all losses and diffusive properties of the watershed&#10;D)None of the above

Accepted Answer

The assumption that the rainfall duration is shorter than the time of concentration of the watershed is not made when applying the Rational method to calculated peak runoff in a small watershed. The opposite assumption is made: the watershed is concentrated; therefore, the duration of the rainfall is equal to or longer than the time of concentration of the watershed. For this reason, the overall size of a watershed (also referred to as a "catchment") should be kept small and the time of concentration assumed to be less than one hour.

Question 9

Which of the following stormwater design practices is used to minimize the generation of runoff by preserving open space?&#10;A)Infiltration practices&#10;B)Conservation design&#10;C)Filtration practices&#10;D)All of the above

Accepted Answer

Conservation design is a stormwater design practice used to minimize the generation of runoff by preserving open space. The goal of conservation design is to reduce the amount of impervious surface, which increase runoff volumes, so that the predevelopment hydrology of a site is more closely replicated.
Examples of conservation design methods include the following:
• Reduced pavement widths for streets and sidewalks
• Shared driveways for multiple housing units
• Reduced structure setbacks
• Cluster development

Question 10

Low Impact Development (LID) is strategy for stormwater management that has been adopted by many cities across the U.S. in recent years. Which of the following statements best describes LID?&#10;A)LID follows a set of practices used to reduce runoff and pollutant loadings by managing runoff as close to the source as possible&#10;B)LID practices cannot be used to retrofit existing infrastructure&#10;C)Although LID practices normally increase the cost of development, the long-term environmental benefit is measureable and justifiable&#10;D)None of the above

Accepted Answer

LID follows a set of practices used to reduce runoff and pollutant loadings by managing runoff as close to the source as possible. LID techniques and methods can be used to retrofit existing stormwater systems to reduce the volume of runoff and pollutant loadings to affected receiving waters. The integrated natural systems used in LID designs can result an overall cost savings when compared to built infrastructure and the annual operation and maintenance costs of mechanical systems.

Deck 8: Hydrology