Question

Part I. Indicate whether true or false (T or F).

____ Storm water detention ponds typically are designed to regulate the outflow peak rate at or below a single target value, such as the pre-development (pre-land use change) peak runoff rate for a specified return period event. Detention storage alters the peak but not the volume of the outflow hydrograph.

_____ Typical rating curves for weirs are concave upward. Typical rating curves for orifices are concave downward.

____ A sediment pond is designed to detain the runoff for enough time to allow a majority of the settleable solids to settle and be trapped in the pond. South Carolina Stormwater Management and Sediment Reduction Regulations state a "sediment basin shall be designed and constructed to accommodate the anticipated sediment loading from the land-disturbing activity and meet a removal efficiency of 80 percent for suspended solids or 0.5 ml/l peak settleable solids concentration, whichever is less." Consistent with this regulation, the design process involves sizing (dimensioning) the pond to provide sufficient volume, selecting the type of outlet structure and sizing (dimensioning) it, and checking the sediment trapping performance of the pond for disturbed land use conditions for a given design event, e.g., 10-year 24-hour rainfall.

____ With respect to the South Carolina regulations for sediment ponds, at least one of two criteria must be demonstrated numerically for a pond to be permitted. These criteria are 80% trapping efficiency and 0.5 ml/l settleable solids concentration in the pond effluent. The trapping efficiency criterion is a performance-based criterion, whereas, the settleable solids limit is an effluent standard.

____  The effect of turbulence in a sediment pond is increased trapping efficiency.

___Three blind mice were on a hiking trip across the United States when they encountered a watershed with relatively steep slopes, narrow floodplains, and well-defined channels. Later, they encountered another watershed, but this one had flatter slopes, broad floodplains, and meandering channels with considerable backwater effects. Having studied the South Carolina synthetic unit hydrograph method—in an earlier life, they had wanted to be civil engineers involved with stormwater management—they immediately concluded the first watershed is one with a higher unit hydrograph peak rate factor than the second. Their conclusion was correct.

___ A historic definition for time of concentration is the time required for a drop of water from the remote portion of a watershed to reach the outlet. Accordingly, time of concentration is evaluated by dividing flow length by average velocity.

___A unit hydrograph (UH) is defined as the runoff hydrograph resulting from one inch of rainfall occurring uniformly over the watershed. General application of UH methodology involves dividing a complex storm (rainfall event) into an integer number of bursts, each of duration D-hours, applying the curve number runoff model to the rainfall increment during each burst to determine the burst rainfall excess, convoluting the rainfall excess with the UH to simulate burst runoff hydrographs, and then summing the burst runoff hydrographs in real time to obtain the event runoff hydrograph.

_____ Hydrograph routing processes include translation, diffusion and dispersion. Translation is simply time shift; diffusion is spreading about a central axis (typically the peak flowrate) without a shift in volume, and dispersion is spreading resulting from a shift in volume, typically from the rising to the recession limb.

___The Muskingum-Cunge channel routing parameter K is generally interpreted as a travel time parameter, associated closely with the time of travel of the hydrograph peak. The parameter q, strictly speaking, is a weighting parameter, but it relates to the hydrograph dispersion characteristics in that for q = 0.50 the hydrograph, theoretically, does not change shape (pure translation) but for q > 0.50, the shape disperses, thereby resulting in peak attenuation and elongation of the hydrograph time base with resulting development of a “tail.”

____ The NRCS classifies soils according into one of four hydrologic soil group (HSG) categories (A, B, C and D) based on runoff potential. HSG-A soils typically are clayey soils with low infiltration rates and high runoff potential; HSG-D soils typically are sandy soils with high infiltration rates and low runoff potential.

___The NRCS travel time method for determining watershed time of concentration provides for three distinct flow path segments. These are sheet or overland flow, shallow concentrated flow, and channel flow. The travel time through each flow path segment is estimated by dividing segment length by velocity. The watershed time of concentration is 1.67 times the sum of the segment travel times.

___ Curve number is an index of watershed runoff potential and ranges from 0 (no runoff) to 100 (pure runoff). Practically, most watershed curve numbers occur between 60 and 90. Curve numbers are given in tables developed by the NRCS in terms of land use, hydrologic soil group (soil type), watershed wetness (aka, ARC or antecedent runoff conditions), and surface cover conditions.   Dry or sandy watersheds have lower CN values, while wet or clayey watersheds have higher CN values.

_____ Stormwater detention pond design involves sizing the pond and selecting the type and size of outflow structure. To confirm the proposed design works satisfactorily, the design event runoff hydrograph is routed through the pond and, typically, the peak outflow is compared with the predevelopment peak.

____ Curve Number (CN) values range between 0 (no runoff) and 100 (complete runoff). CN values are based on empirical data collected at experimental watersheds and are found in published tables as a function of soil type (HSG), land use, and surface cover conditions. CN is the percentage of rain that goes to runoff.

____ The NRCS CN runoff model is an equation to compute the runoff peak flow rate for a given rainfall.

____ Well-drained watersheds have high PRF values (up to 550) while poorly drained watersheds have low PRF values (as low as 180). PRF is a parameter used to compute the amplitude (peak) of a unit hydrograph. High PRF means the unit hydrograph has a higher peak, shorter recession limb, and more of the runoff volume occurring under the rising limb than does a unit hydrograph with a lower PRF value. The standard NRCS unit hydrograph has PRF=484 (English units).

____ Return period is the average number of years between events of magnitude equal to or greater than a specified amount.

____ Initial abstractions, Ia, accounts for rainfall lost to vegetative interception, surface depression storage, and initial high rate infiltration. It is the rainfall that occurs before measurable runoff is observed.

____ Unit hydrograph peak rate factor (PRF) parameterizes watershed runoff potential.

____ Flashy watersheds generally have runoff hydrographs that peak quickly, have steep rising and recession limbs--all of which are indicative of an efficiently drained watershed. Flashy watersheds generally have a well-developed internal drainage network and creeks and other natural streams with small floodplains.

_____ Curve number indicates the percent of rain that goes to direct storm runoff. Values are obtained from published sources as a function of land use, soil type (hydrologic soil group classification), surface cover conditions, and watershed wetness conditions.

_____ Watershed hydraulic length is measured along a path a drop of water might follow from the watershed boundary to the watershed outlet. A representative longest path is chosen for the measurement.

_____ Convolution is the mathematical process whereby one determines burst runoff hydrographs, lags them, and then sums them in real time to simulate an event runoff hydrograph.

_____ Curve number is an index of watershed drainage efficiency. Watersheds with greater drainage efficiencies have higher curve number values, meaning they have greater runoff (volumes and peaks) than watersheds with lower curve number values.

___ Routing is the mathematical procedure used to describe the translation and change in shape of a hydrograph when it moves through a stream, pond, road crossing, or pipe. During routing, the hydrograph shape usually elongates, resulting in attenuation of the peak flow rate.

___Critical parameters for the Muskingum-Cunge routing model are estimated with equations obtained by matching coefficients between the diffusion wave hydraulic routing model and the advective-diffusive form of the hydrologic routing model.

___Best management practices (BMPs) effectively treat stormwater runoff to remove pollutants by the processes (singularly or some combination) of sedimentation, filtration, vegetative uptake, and degradation.

___When using the Rational Method to estimate peak runoff rates, the design intensity is obtained from the intensity-duration-frequency data that can be found with the Precipitation Frequency Data Server. These data are the basis for an Intensity, Duration Frequency (IDF) curve, which can be represented by either a plot or equation. The IDF curve is a functional relationship between intensity and time for a given return period.

___The NRCS travel time method for determining watershed time of concentration provides for three distinct flow path segments. These are sheet or overland flow, shallow concentrated flow, and channel flow. The travel time through each flow path segment is estimated by dividing segment length by velocity. The watershed time of concentration is 1.67 times the sum of the segment travel times.

___Two assumptions inherent in the Rational Method are that Qp occurs when the entire watershed is contributing, and that the rainfall intensity is steady for duration equal to tc.

___Routing is a term for mathematical routines used to describe the movement and change in shape of a hydrograph as it passes through a pond, channel reach, or road crossing. In the case of a detention pond, the outflow hydrograph volume will equal the inflow hydrograph volume and the outflow peak will occur when the (plot of the) outflow hydrograph intersects the (plot of the) recession limb of the inflow hydrograph. At this point, the depth and storage in the pond are at their maximum values for the given event. In the case of channel routing, the outflow hydrograph does not necessarily intersect the recession limb of the inflow hydrograph.

___Generally, hydrograph dispersion is greater in a prismatic channel than in a natural channel of equal size and geometry because the flow velocity is greater in the prismatic channel, which results in greater elongation of the hydrograph shape, hence greater dispersion.

___The Muskingum-Cunge channel routing parameter K is generally interpreted as a travel time parameter, associated closely with the time of travel of the hydrograph peak. The parameter q, strictly speaking, is a weighting parameter, but it relates to the hydrograph dispersion characteristics in that for q = 0.50 the hydrograph, theoretically, does not change shape (pure translation) but for q < 0.50, the shape disperses.

_____ Unit hydrograph time to peak is estimated as watershed lag time plus burst duration, where burst duration is chosen as two-tenths of lag time (rounded, of course, to an integer multiple of a convenient time unit such as 5 minutes). The unit hydrograph time to peak also is rounded to an integer multiple of burst duration.

_____ Initial abstractions include vegetative trap (interception), depression storage and initial high rate infiltration, account for rainfall prior to the beginning of measurable runoff. The relationship between initial abstractions (Ia) and watershed retention (S), i.e., Ia=0.2S, was determined from an analysis of empirical data.

_____ Unit hydrograph peak rate factor (PRF) can be defined as an index of watershed drainage efficiency. For example, forested watersheds have dense ground cover and generally do not have an extensive number of gullies and other small channels that drain upland runoff; consequently, the PRF for forested watersheds is low, on the order of 180. On the other hand, an urban watershed typically has a high degree of imperviousness that drains to planned, constructed storm sewer systems that quickly and efficiently drain the surface runoff to the watershed outlet. The PRF at urban watersheds is high, ranging from roughly 325 for residential areas to as much as 600 at commercial sites.

_____ Historically, time of concentration has been defined as the time it takes a drop of water to travel from the watershed boundary to the outlet. In other words, it is a measure of the time it takes the entire watershed to contribute to runoff at the outlet. For steady excess rainfall input, based on hydraulic analysis of the runoff process, time of concentration is the time it takes for the watershed to reach equilibrium, or peak runoff, conditions.

_____ The primary purpose for a detention pond designed to meet the single objective release rate rule is to control the peak outflow from a site, typically following land use change that results in increased rates and volumes or runoff. To control the peak, one rule of thumb is to provide enough storage for the increase in runoff volume.

____ An example of a two-stage outflow structure is a riser with a lower-stage orifice. An example of a double, or dual, control outflow structure is a culvert that functions as a weir at low head (depth) and as an orifice at higher head.                     

Answer 1

1. True. Detention ponds are important flood control features. Detention ponds remain usually dry except during or after rain or snowmelt. Their primary purpose is to slow down water flow and hold it for a short period of time such as 24 hours to reduce damages due to the peak runoff rates.

2. False. The rating curve is concave downwards,

3. True A sediment pond is designed to detain the runoff for enough time to allow a majority of the settleable solids to settle and be trapped in the pond.

4. True. These criteria are 80% trapping efficiency and 0.5 ml/l settleable solids concentration in the pond effluent. The trapping efficiency criterion is a performance-based criterion, whereas, the settleable solids limit is an effluent standard. Trapping efficiency can be defined as the Volume of particles trapped/ Total volume of particles. It depends on the design criteria of design and overall demonstrates the performance of the feature. While 0.5 ml/l settleable solids concentration in the pond effluent is the required effluent standard. It can vary depending upon the inflow and concentration of the solids in the inflow which are variable. Efficieny is the feature of the pond, while the effluent concentration is the parameter of the pond effluent water.