Question

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

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

__The NRCS hydrologic soil group (HSG) lists soils into one of four 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. Information about the HSG classification for a soil is contained in the county soil survey.

__Initial abstractions, Ia, account 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.

__A unit hydrograph (UH) is defined as the runoff hydrograph resulting from 1-inch of rainfall occurring uniformly over a watershed in a specified time-period, i.e., D-hours.

__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.

__Detention storage is the temporary storage of stormwater runoff; detention storage is implemented mostly to regulate the peak outflow, typically at or below a target release rate defined as the pre-development runoff peak resulting from a specified return period 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 storms with rainfall depth equal to or greater than a specified amount. The NRCS CN runoff model is an equation to compute the runoff peak flow rate for a given rainfall.

__Direct storm runoff represents the immediate response of the watershed to a rainfall event and occurs mostly, if not entirely, as surface runoff. Flashy watersheds generally have runoff hydrographs that peak soon, have steep rising and recession limbs--all of which are indicative of an efficiently drained watershed. Flashy watersheds generally have a high percentage of clay soils and/or impervious cover, a well-developed internal drainage network, and creeks and other natural streams with small floodplains.

___ The Rational Method is a conceptual model that relates peak runoff to rainfall intensity, contributing area, and a runoff coefficient. The runoff coefficient is found in published tables and usually is expressed as a function of land use. These tables typically do not list single, unique values for the runoff coefficient. Instead, they cite a range for C-values for each land use. A general rule of thumb is to assign the higher C-values to the more frequent rainfall events and the lower C-values to the less frequent events.

___When using the Rational Method to estimate peak runoff rates, the design intensity is obtained from the appropriate intensity-duration-frequency (IDF) curve, which can be represented by either a plot of equation. The IDF curve is a functional relationship between intensity and time for a given return period.

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

___The NRCS travel time method for determining watershed time of concentration provides for three distinct flow path segments. Flow path segments 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 terms of land use, hydrologic soil group (soil type), watershed wetness, and surface cover conditions.

__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 no-harm release rate rule for allowable (target) peak outflow rates from stormwater detention ponds states the peak must be regulated such that no offsite (downstream) flooding problems are created or aggravated. The most common release rate rule is one that states the allowable peak following land use change (post development peak) cannot exceed the runoff peak prior to land use change (predevelopment peak) for a specified design rainfall event (single objective release rate rule). An extension to this rule states the post development peak cannot exceed the predevelopment peak for the design rainfall event and more frequent events. In other words, a pond must function such that the flood (Qp) frequency curve following development is no greater than it was prior to development for all storms with return periods greater than or equal to some specified return period, typically the return period for the single objective rule.

__When using the Rational Method, the design rainfall is obtained from an IDF curve with duration equal to time of concentration. One interpretation of time of concentration, for steady rainfall, is that it is the time required for a watershed to reach equilibrium, i.e., peak, runoff conditions. Selecting a design intensity with duration equal to time of concentration assures the runoff from the contributing area above each segment in the storm sewer system will be the peak for the specified return period event.

__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.

Answer 1

Answer (a) : True

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

Explanation

The key parameters for regional Unit Hydrograph are time to peak and peak discharge of watershed. These parameters can be used to determine the Peak Rate Factor (PRF) directly. Thus UH PRF parameterize watershed runoff potential.

Reference

"Revisit of NRCS Unit Hydrograph Procedures", by Xing Fang, Khanal Prakash, Theodore Cleveland, David Thompson and Pratistha Pradhan

Answer (b) : False

Statement: The NRCS hydrologic soil group (HSG) lists soils into one of four 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. Information about the HSG classification for a soil is contained in the county soil survey.

Explanation

Hydrologic Soils Groups (HSG) are A, B, C and D based on factors such as runoff potential, infiltration rate etc.

HSG-A is sandy type of soil having low runoff potential and high infiltration rate.

HSG-D soils are clayey with highest runoff potential and very low infiltration rates.

Reference

‘Urban Hydrology for Small Watersheds’ published by the Engineering Division of the Natural Resource Conservation Service, United States Department of Agriculture

Answer (c): True

Statement:  Initial abstractions, Ia, account 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.

Explanation

Initial abstraction is the amount of rainfall without runoff. Interception of rainfall by vegetation is often termed as initial abstraction ( in modelling). Infiltration and runoff calculations deduct the initial abstraction.

Reference

"Rainfall-Runoff Processes", by David G. Tarboton
(A workbook to accompany the Rainfall-Runoff Processes Web module).

Answer (d): True

Statement: A unit hydrograph (UH) is defined as the runoff hydrograph resulting from 1-inch of rainfall occurring uniformly over a watershed in a specified time-period, i.e., D-hours.

Explanation

Sherman (1932) proposed the unit hydrograph concept and it is defined as the direct runoff hydrograph resulting from a unit volume of excess rainfall of constant intensity (1 cm or 1 inch) and uniformly distributed over the drainage area where the effective duration labels the particular unit hydrograph.

Reference

"The relation of hydrographs of runoff to size and character of drainage‐basins", by LeRoy K. Sherman (1932)