Description
Read the pdf carefully
Unformatted Attachment Preview
Part II) READING: (20 pts)
Reading 1 (10 pts): In 200 words or less, summarize the most important points from this
reading about the USGS Stream Flow measurements. https://www.usgs.gov/specialtopic/water-science-school/science/how-streamflow-measured?qtscience_center_objects=0#qt-science_center_objects
Reading 2 (10 pts): Go over the VDOT Drainage Manual Chapter 6 – Item 6.4.4.2 NRCS Methods
(Graphical Peak Discharge and Unit Hydrograph) * available here:
http://www.virginiadot.org/business/resources/LocDes/DrainageManual/chapter6.pdf
In 200 words or less, summarize the most important points from this reading.
Station Distance
A
B
C
D
E
F
G
H
[ft]
0
5
15
25
35
45
55
65
Depth
[ft]
0.0
1.0
1.2
1.5
2.5
2.5
1.5
0.0
Mean
Velocity
[ft]
0.0
1.9
2.7
3.2
4.4
4.3
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0.0
Chapter 6 – Hydrology
TABLE OF CONTENTS
CHAPTER 6 – HYDROLOGY …………………………………………………………………………………………………….. 6-1
6.1 Introduction …………………………………………………………………………………………………………… 6-1
6.1.1 Objective ………………………………………………………………………………………………. 6-1
6.1.2 Definition ………………………………………………………………………………………………. 6-1
6.1.3 Factors Affecting Floods …………………………………………………………………………. 6-1
6.1.4 Sources of Information ……………………………………………………………………………. 6-2
6.2
Design Policy…………………………………………………………………………………………………… 6-2
6.2.1 Introduction …………………………………………………………………………………………… 6-2
6.2.2 Surveys ………………………………………………………………………………………………… 6-2
6.2.3 Flood Hazards ……………………………………………………………………………………….. 6-2
6.2.4 Coordination ………………………………………………………………………………………….. 6-2
6.2.5 Documentation ………………………………………………………………………………………. 6-3
6.2.6 Evaluation of Runoff Factors……………………………………………………………………. 6-3
6.2.7 Flood History …………………………………………………………………………………………. 6-3
6.2.8 Hydrologic Methods ……………………………………………………………………………….. 6-3
6.2.9 Approved Peak Discharge Methods …………………………………………………………. 6-4
6.2.10 Design Frequency ………………………………………………………………………………….. 6-4
6.2.11 Economics…………………………………………………………………………………………….. 6-4
6.2.12 Review Frequency …………………………………………………………………………………. 6-5
6.3
Design Criteria…………………………………………………………………………………………………. 6-5
6.3.1 Design Frequency ………………………………………………………………………………….. 6-5
6.3.1.1 Factors Governing Frequency Selections …………………………………… 6-5
6.3.1.2 Minimum Criteria ……………………………………………………………………… 6-6
6.3.2 Peak Discharge Method Selection ……………………………………………………………. 6-7
6.3.3 Runoff Volume Method Selection (Hydrograph Methods) ……………………………. 6-7
6.4
Design Concepts ……………………………………………………………………………………………… 6-8
6.4.1 Travel Time Estimation …………………………………………………………………………… 6-8
6.4.1.1 Travel Time …………………………………………………………………………….. 6-8
6.4.1.2 Time of Concentration ……………………………………………………………… 6-8
6.4.1.3 Travel Time in Lakes or Reservoirs ……………………………………………. 6-9
6.4.2 Design Frequency ………………………………………………………………………………….. 6-9
6.4.2.1 Overview ………………………………………………………………………………… 6-9
6.4.2.2 Base Flow ………………………………………………………………………………. 6-9
6.4.2.3 Design Frequency………………………………………………………………….. 6-10
6.4.2.4 Review Flood ………………………………………………………………………… 6-10
6.4.2.5 Rainfall vs. Flood Frequency …………………………………………………… 6-10
6.4.2.6 Intensity-Duration-Frequency (IDF) Values ……………………………….. 6-11
6.4.2.7 Discharge Determination ………………………………………………………… 6-11
6.4.3 Peak Discharge Methods ………………………………………………………………………. 6-11
6.4.3.1 Rational Method …………………………………………………………………….. 6-11
6.4.3.1.1
Introduction …………………………………………………………. 6-11
6.4.3.1.2
Application…………………………………………………………… 6-12
6.4.3.1.3
Characteristics …………………………………………………….. 6-12
6.4.3.1.4
Equations ……………………………………………………………. 6-13
6.4.3.1.5
Infrequent Storm ………………………………………………….. 6-14
6.4.3.1.6
Time of Concentration …………………………………………… 6-14
6.4.3.1.7
Runoff Coefficients ……………………………………………….. 6-16
6.4.3.1.8
Common Errors ……………………………………………………. 6-17
6.4.3.2 Anderson Method ………………………………………………………………….. 6-17
Chapter 6-i of 3
6.4.3.2.1
Introduction …………………………………………………………. 6-17
6.4.3.2.2
Application…………………………………………………………… 6-18
6.4.3.2.3
Characteristics …………………………………………………….. 6-18
6.4.3.2.4
Equations ……………………………………………………………. 6-18
6.4.3.3 Rural Regression Method ……………………………………………………….. 6-20
6.4.3.3.1
Introduction …………………………………………………………. 6-20
6.4.3.3.2
Application…………………………………………………………… 6-20
6.4.3.3.3
Hydrologic Regions ………………………………………………. 6-21
6.4.3.3.4
Equations ……………………………………………………………. 6-21
6.4.3.3.5
Characteristics …………………………………………………….. 6-21
6.4.3.3.6
Mixed Population………………………………………………….. 6-25
6.4.3.4 Urban Regression Method ………………………………………………………. 6-25
6.4.3.4.1
Introduction …………………………………………………………. 6-25
6.4.3.4.2
Application…………………………………………………………… 6-25
6.4.3.4.3
Characteristics …………………………………………………….. 6-25
6.4.3.4.4
Equations ……………………………………………………………. 6-25
6.4.3.5 Stream Gage Data …………………………………………………………………. 6-26
6.4.3.5.1
Introduction …………………………………………………………. 6-26
6.4.3.5.2
Application…………………………………………………………… 6-26
6.4.3.5.3
Transposition of Data ……………………………………………. 6-27
6.4.4 Hydrograph Methods ……………………………………………………………………………. 6-27
6.4.4.1 Modified Rational Method ……………………………………………………….. 6-27
6.4.4.1.1
Introduction …………………………………………………………. 6-27
6.4.4.1.2
Application…………………………………………………………… 6-27
6.4.4.1.3
Characteristics …………………………………………………….. 6-28
6.4.4.1.4
Critical Storm Duration ………………………………………….. 6-28
6.4.4.1.5
Estimating the Critical Duration Storm …………………….. 6-29
*
6.4.4.2
NRCS Methods (Graphical Peak Discharge and Unit Hydrograph) .. 6-29
6.4.4.2.1
Introduction …………………………………………………………. 6-29
6.4.4.2.2
Application…………………………………………………………… 6-30
6.4.4.2.3
Characteristics …………………………………………………….. 6-30
6.4.4.2.4
Time of Concentration …………………………………………… 6-30
6.4.4.2.5
Curve Numbers ……………………………………………………. 6-30
6.4.4.2.6
Equations ……………………………………………………………. 6-31
6.5 References …………………………………………………………………………………………………………… 6-33
Chapter 6-ii of 3
List of Tables
Table 6-1. Design Storm Selection Guidelines …………………………………………………………………. 6-6
Table 6-2. Saturation Factors for Rational Formula…………………………………………………………. 6-14
Table 6-3. Anderson Time Lag Computation ………………………………………………………………….. 6-19
Table 6 4. Anderson Method Flood Frequency Ratios…………………………………………….6-20
Table 6 5. Regional Regression Equations for Estimating Peak Discharges of Streams in Virginia
…………………………………………………………………………………………………………… 6-23
Table 6 6. Transposition of Data Sample Problem…………………………………………………6-27
List of Figures
Figure 6-1. Guidelines for Peak Discharge Method Selection …………………………………………….. 6-7
Figure 6-2. Guidelines for Runoff Volume Method Selection………………………………………………. 6-7
Figure 6 3. Peak Discharge Regions for Regression Equations………………………………….6-22
List of Appendices
Appendix 6B-1 Runoff Depth for Runoff Curve Number (RCN)
Appendix 6B-2 24-hr Rainfall Depths
Appendix 6C-1 B, D, and E Factors – Application
Appendix 6C-2 B, D, and E Factors for Virginia
Appendix 6D-1 Overland Flow Time – Seelye
Appendix 6D-2 Kinematic Wave Formulation – Overland Flow
Appendix 6D-3 Overland Time of Flow
Appendix 6D-4 Overland Flow Velocity
Appendix 6D-5 Time of Concentration for Small Drainage Basins (use for channel flow) – Kirpich
Appendix 6D-6 Average Velocities for Estimating Travel Time for Shallow Concentrated Flow
Appendix 6E-1 Rational Method Runoff Coefficients
Appendix 6E-2 Rational Method Runoff Coefficients with 10-yr Cf Factor Applied
Appendix 6E-3 Rational Method Runoff Coefficients with 25-yr Cf Factor Applied
Appendix 6E-4 Rational Method Runoff Coefficients with 50-yr Cf Factor Applied
Appendix 6E-5 Rational Method Runoff Coefficients with 100-yr Cf Factor Applied
Appendix 6J-1 Major Drainage Basins
Appendix 6K-1 A and B Factors that define Intensity Duration – Frequency (IDF) Curves for use
only with the Critical Storm Duration Determination
Appendix 6K-2 Regression Constants > “a” and “b” for Virginia
Chapter 6-iii of 3
Chapter 6 – Hydrology
6.1 Introduction
6.1.1
Objective
The analysis of precipitation*, peak rate of runoff, volume of runoff, and time distribution
of flow is fundamental to the design of drainage facilities. Errors in the estimates will
result in a structure that is either undersized and causes drainage problems or
oversized and costs more than necessary. On the other hand, it must be realized that
any hydrologic analysis is only an approximation. The relationship between the amount
of precipitation on a drainage basin and the amount of runoff from the basin is complex,
and too little data are available on the factors influencing the rural and urban
rainfall-runoff relationship to expect exact solutions.
6.1.2
Definition
Hydrology is generally defined as a science dealing with water on and under the earth
and in the atmosphere. For the purpose of this manual, hydrology will deal with
estimating stormwater runoff as the result of rainfall. In design of highway drainage
structures, stormwater runoff is usually considered in terms of peak runoff or discharge
in cubic feet per second (cfs) and hydrographs as discharge versus time. For structures
which are designed to control the volume of runoff, like detention storage facilities, then
the entire inflow and outflow hydrographs will be of interest. Wetland hydrology, the
water-related driving force to create wetlands, is addressed in the AASHTO Highway
Drainage Guidelines, Chapter 10 and the AASHTO Drainage Manual, Chapter 8.
6.1.3
Factors Affecting Floods
In the hydrologic analysis for a drainage structure, it must be recognized that there are
many variable factors that affect floods. Some of the factors which need to be
recognized and considered on an individual site-by-site basis are things such as:
•
•
•
•
•
•
•
•
•
*
Rainfall amount and storm distribution
Drainage area size, shape, and orientation
Ground cover
Type of soil
Slopes of terrain and stream(s)
Antecedent moisture condition
Storage potential (overbank, ponds, wetlands, reservoirs, channels, etc.)
Watershed development potential
Type of precipitation (rain, snow, hail, or combinations thereof)
Rev. 7/19
Chapter 6-1 of 34
6.1.4
Sources of Information
The type and source of information available for hydrologic analysis will vary from site to
site and it is the responsibility of the designer to determine what information is needed
and applicable to a particular analysis.
6.2 Design Policy
6.2.1
Introduction
The following sections summarize the policies which should be followed for hydrologic
analysis for VDOT roadways. For a more detailed discussion refer to the publications
AASHTO Highway Drainage Guidelines (2007), AASHTO Drainage Manual Volumes 1
and 2 (2014), and FHWA HDS-2 Highway Hydrology (2002). *
6.2.2 Surveys
Hydrologic considerations can significantly influence the selection of a highway corridor
and the alternate routes within the corridor. Therefore, studies and investigations
should consider the environmental and ecological impact of the project. Also special
studies and investigations may be required at sensitive locations. The magnitude and
complexity of these studies should be commensurate with the importance and
magnitude of the project and problems encountered. Typical data to be included in
such surveys or studies are: topographic maps, aerial photographs, streamflow records,
historical high water elevations, flood discharges, and locations of hydraulic features
such as reservoirs, water projects, wetlands, karst topography and designated or
regulatory floodplain areas.
6.2.3
Flood Hazards
A hydrologic analysis is prerequisite to identifying flood hazard areas and determining
those locations at which construction and maintenance will be unusually expensive or
hazardous.
6.2.4
Coordination
Since many levels of government plan, design, and construct highway and water
resource projects which might have effects on each other, interagency coordination is
desirable and often necessary. In addition, agencies can share data and experiences
within project areas to assist in the completion of accurate hydrologic analyses.
*
Rev. 7/19
Chapter 6-2 of 34
6.2.5
Documentation
Experience indicates that the design of highway drainage facilities should be adequately
documented. Frequently, it is necessary to refer to plans and specifications long after
the actual construction has been completed. Thus it is necessary to fully document the
results of all hydrologic analysis.
6.2.6
Evaluation of Runoff Factors
For all hydrologic analyses, the following factors should be evaluated and included
when they will have a significant effect on the final results:
•
•
•
•
6.2.7
Drainage basin characteristics including: size, shape, slope, land use, geology,
soil type, surface infiltration, and storage
Stream channel characteristics including: geometry and configuration, slope,
hydraulic resistance, natural and artificial controls, channel modification,
aggradation, degradation, and ice and debris
Floodplain characteristics
Meteorological characteristics such as precipitation amount and type (rain, snow,
hail, or combinations thereof), rainfall intensity and pattern, areal distribution of
rainfall over the basin, and duration of the storm event
Flood History
All hydrologic analyses should consider the flood history of the area and the effects of
these historical floods on existing and proposed structures. The flood history should
include the historical floods and the flood history of any existing structures.
6.2.8
Hydrologic Methods
Many hydrologic methods are available. If possible, the selected method should be
calibrated to local conditions and verified for accuracy and reliability.
There is no single method for determining peak discharge that is applicable to all
watersheds. It is the designer’s responsibility to examine all methods that can apply to
a particular site and to make the decision as to which is the most appropriate.
Consequently, the designer must be familiar with the method sources of the various
methods and their applications and limitations. It is not the intent of this manual to
serve as a comprehensive text for the various methods of determining peak discharge.
Deleted Information *
*
Rev. 7/19
Chapter 6-3 of 34
6.2.9
Approved Peak Discharge Methods
In addition to the methods presented in this manual, the following methods are
acceptable when appropriately used:
•
•
•
•
Log Pearson III analyses of a suitable set of gage data may be used for all
routine designs provided there is at least 10 years of continuous or synthesized
flow records for 10-yr discharge estimates and 25 years for 100-yr discharge
estimates
Suitable computer programs such as the USACE’s HEC-HMS and the NRCS’
EFH-2, *WinTR-55, and WinTR-20 may be used for the hydrologic calculations.
The TR-55 method (now referred to as the NRCS Method and formerly as the SCS
Method) has been found best suited for drainage areas between 200 and 2,000
acres (ac). When using any methodology predicated on the 24-hr. rainfall event
(i.e., NRCS Method, HEC-HMS, etc.) it is necessary to use the values presented
in the NOAA Atlas 14 Point Precipitation Frequency Estimates or published in the
Chapter 11, Appendices.
Other methods may be approved where applicable upon submission to the
VDOT State Hydraulics Engineer
The 100-yr discharges specified in the FEMA flood insurance study are preferred
when the analysis includes a proposed crossing on a regulatory floodway.
However, if these discharges are deemed to be outdated or incorrect, the
discharges based on current methodology should be used.
6.2.10
Design Frequency
A design frequency should be selected commensurate with the facility cost, amount of
traffic, potential flood hazard to property, expected level of service, political
considerations, and budgetary constraints as well as the magnitude and risk associated
with damages from larger flood events. When long highway routes that have no
practical detour are subject to independent flood events, it may be necessary to
increase the design frequency at each site to avoid frequent route interruptions from
floods. In selecting a design frequency, potential upstream land uses should be
considered which could reasonably occur over the anticipated life of the drainage
facility.
6.2.11
Economics
Hydrologic analysis should include the determination of several design flood
frequencies for use in the hydraulic design. Section 6.3.1 outlines the design floods that
shall be used for different drainage facilities. These frequencies are used to size
drainage facilities for an optimum design, which considers both risk of damage and
construction cost. Consideration should also be given to the frequency flood that was
used to design other structures along a highway corridor.
*
Rev. 7/19
Chapter 6-4 of 34
6.2.12
Review Frequency
All proposed structures designed to accommodate the selected design frequency
should also be evaluated using a base flood to ensure that there are no unexpected
flood hazards.
6.3 Design Criteria
6.3.1
Design Frequency
6.3.1.1
Factors Governing Frequency Selections
The determination of design factors to be considered and the degree of documentation
required depends upon the individual structure and site characteristics. The hydraulic
design must be such that risk to traffic, potential property damage, and failure from
floods is consistent with good engineering practice and economics. Recognizing that
floods cannot be precisely predicted and that it is seldom economically feasible to
design for the very rare flood, all designs should be reviewed for the extent of probable
damage, should the design flood be exceeded. Design headwater/backwater and flood
frequency criteria should be based upon these and other considerations:
•
•
•
•
•
•
Damage to adjacent property
Damage to the structure and roadway
Traffic interruption
Hazard to human life
Damage to stream and floodplain environment
Impact to Base Flood (100 year flood) elevations
The potential damage to adjacent property or inconvenience to owners should be a
major concern in the design of all hydraulic structures.
Inundation of the traveled way indicates the level of traffic service provided by the
facility. The traveled way overtopping flood level identifies the limit of serviceability.
Table 6-1 relates desired minimum levels of protection from traveled way (edge of
shoulder) inundation to the functional classifications of roadways. The design storm
discussed here refers to roadway crossing (bridge or culvert) or roadways running
parallel to streams. Other features such as storm sewer elements, roadside ditches,
E&S, and SWM facilities will have specific design storms and rainfalls discussed in their
respective Chapters.
Chapter 6-5 of 34
6.3.1.2
Minimum Criteria
No exact criteria for flood frequency or allowable backwater/headwater values can be
set which will apply to an entire project or roadway classification. Minimum design
frequency values relative to protection of the roadway from flooding or damage have
been established. It should be emphasized that these values only apply to the level of
protection afforded to the roadway.
Table 6-1. Design Storm Selection Guidelines
(For Traveled Way Inundation)
Roadway Classification
Exceedance Probability
Return Period
Interstate, Freeways (Urban/Rural)
2%
50-year
Principal Arterial
2%
50-year
Urban Minor Arterial System
2%
50-year
Rural Minor Arterial System
4%
25 year
Rural Collector System, Major
4%
25-year
Rural Collector System, Minor
10%
10-year
Urban Collector System
10%
10-year
Local Street System
10%
10-year
Source: AASHTO Drainage Manual (First Edition), Volume One, Chapter 9, Table 9-1 *
Note: Federal law requires Interstate highways to be provided with protection from the 2%
flood. Facilities such as underpasses and depressed roadways, where no overflow relief
is available, shall also be designed for the 2% event. Where no embankment overflow
relief is available, drainage structures should be designed for at least the 1% or 100-year
event.
*
Rev. 7/19
Chapter 6-6 of 34
6.3.2
Peak Discharge Method Selection
The methods to be used are shown in Figure 6-1. For watersheds greater than 200 ac,
VDOT recommends evaluating several hydrologic methods for comparison purposes.
HYDROLOGIC METHOD
Rational Method
NRCS Graphical Peak Discharge Method
NRCS EFH-2
Anderson Method (USGS)*
USGS Regional Regression – Rural
USGS Regional Regression – Urban
Stream Gage Data
0
to
200
acres
DRAINAGE AREA SIZE
200
640
2,000
to
to
acres
640
2,000
to
2
acres
acres
20 mi
2
20 mi
to
2
20+ mi
– Range of Applicability
*For VDOT purposes, the Anderson Method is not recommended for use outside of urbanized
areas in the Northern Virginia District.
Note: The above does not indicate definite limits but does suggest a range in which the
particular method is “best suited”.
Figure 6-1. Guidelines for Peak Discharge Method Selection *
6.3.3
Runoff Volume Method Selection (Hydrograph Methods)
The hydrograph methods to be used for estimating runoff volume include the following:
HYDROLOGIC METHOD
Modified Rational Method
NRCS Unit Hydrograph Method
0
to
20 acres
DRAINAGE AREA SIZE
20
200
to
to
200 acres
640 acres
640
to
2,000 acres
– Range of Applicability
Note: The above does not indicate definite limits but does suggest a range in which the
particular method is “best suited”.
Figure 6-2. Guidelines for Runoff Volume Method Selection
For application of the technical criteria in the Virginia Stormwater Management Program
(VSMP), the Department of Environmental Quality prefers use of the NRCS Unit
Hydrograph Method for estimating runoff volume.
See Chapter 11 Stormwater
Management in this Drainage Manual for more discussion. However, the VSMP
Regulation does allow use of the Modified Rational Formula as a hydrologic method for
estimating runoff volume (see 9 VAC 25-870-72 E).
*
Rev. 7/19
Chapter 6-7 of 34
6.4 Design Concepts
6.4.1
Travel Time Estimation
Travel time (Tt) is the time it takes water to travel from one location to another in a
watershed. Tt is a component of time of concentration (tc), which is the time for runoff to
travel from the most hydraulically distant point in the watershed to a point of interest
within the watershed. The time of concentration is computed by summing all the travel
times for consecutive components of the drainage conveyance system.
The computation of travel time and time of concentration is discussed below.
6.4.1.1 Travel Time
Water moves through a watershed as sheet flow, shallow concentrated flow, open
channel flow, pipe flow, or some combination of these. The type of flow that occurs is a
function of the conveyance system and is best determined by field inspection.
Travel time is the ratio of flow length to flow velocity:
L
Where:
Tt = 3600V
Tt =
L =
V =
3600 =
(6.1)
Travel time, hour (hr)
Flow length, feet (ft)
Average velocity, feet per second (fps)
Conversion factor from seconds to hours
6.4.1.2 Time of Concentration
The time of concentration (tc) is the sum of Tt values for the various consecutive flow
segments. Separate flow segments should be computed for overland flow, shallow
concentrated flow, channelized flow, and pipe systems.
Where:
t c = Tt1 + Tt2 + ⋯ Ttm
(6.2)
tc = Time of concentration, hours (hrs)
m = Number of flow segments
Time of concentration is an important variable in most hydrologic methods. Several
methods are available for estimating tc. This chapter presents several methods for
estimating overland flow and channel flow times. Any method used should only be used
with the parameters given for the specific method. The calculated time should
represent a reasonable flow velocity.
For additional information concerning time of concentration as used in the Rational
Method, see Section 6.4.4.1.
Chapter 6-8 of 34
6.4.1.3
Travel Time in Lakes or Reservoirs
Sometimes it is necessary to compute a tc for a watershed having a relatively large body
of water in the flow path. In such cases, tc is computed to the upstream end of the lake
or reservoir, and for the body of water the travel time is computed using the equation:
Where:
VW = (gDm )0.5
(6.3) *
Vw = Wave velocity across the water, feet per second (fps)
g = Acceleration due to gravity = 32.2 ft/s2
Dm = Mean depth of lake or reservoir, feet (ft)
Generally, Vw will be high (8 – 30 fps). Note that the above equation only provides for
estimating travel time across the lake and for the inflow hydrograph to the lake’s outlet.
It does not account for the travel time involved with the passage of the inflow
hydrograph through spillway storage and the reservoir or lake outlet. This time is added
to the travel time across the lake. The travel time through lake storage and its outlet
can be determined by the storage routing procedures in Chapter 11. The wave velocity
Equation 6.3 can be used for swamps with much open water, but where the vegetation
or debris is relatively thick (less than about 25% open water), Manning’s equation is
more appropriate.
6.4.2
Design Frequency
6.4.2.1
Overview
Since it is not economically feasible to design a structure for the maximum runoff a
watershed is capable of producing, a design frequency must be established.
The frequency with which a given flood can be expected to occur is the reciprocal of the
probability, or the chance that the flood will be equaled or exceeded in a given year. If a
flood has a 20% chance of being equaled or exceeded each year, over a long period of
time, the flood will be equaled or exceeded on an average of once every five years.
This is called the return period or recurrence interval (RI). Thus the exceedance
probability (percentage) equals 100/RI.
6.4.2.2 Base Flow
Base flow (as opposed to Base Flood which is the 100 year flood event) is the typical
discharge that is found within the stream for at least 25% of the typical year. An
evaluation of the gage data throughout the state has determined that this flow in cubic
feet per second (cfs) is approximately equal to 1.1 times the drainage area in square
miles. This is typically used to aid in the design of causeways and coffer dams.
*
From Chapter 15, Part 630, Section 630.1503 of the National Engineering Handbook
Chapter 6-9 of 34
However, where stream gage data is available, the base flow could also be estimated
from the historic stream gage data instead of using the approximation method.
6.4.2.3
Design Frequency
Roadway Stream Crossings: A drainage facility should be designed to accommodate a
discharge with a given return period(s). The design should ensure that the backwater
(the headwater) caused by the structure for the design storm does not:
•
•
Increase the flood hazard significantly for property
Exceed a certain depth on the highway embankment
Based on these design criteria, a design involving roadway overtopping for floods larger
than the design event is an acceptable practice. Factors to consider when determining
whether roadway overtopping is acceptable are roadway classification, roadway use,
impacts and frequency of overtopping, structural integrity, etc. If a culvert or bridge is
designed to pass the 25-year flow, it would not be uncommon for a larger event storm to
overtop the roadway.
Storm Drains: A storm drain should be designed to accommodate a discharge with a
given return period(s). The design should be such that the storm runoff does not:
•
•
•
Increase the flood hazard significantly for property
Encroach onto the street or highway so as to cause a significant traffic hazard
Limit traffic, emerging vehicle, or pedestrian movement to an unreasonable
extent
Based on these design criteria, a design involving roadway inundation for floods larger
than the design event is an acceptable practice. Factors to consider when determining
whether roadway inundation is acceptable are roadway classification, roadway use,
impacts and frequency of inundation, structural integrity, etc.
6.4.2.4
Review Flood
After sizing a drainage facility, it will be necessary to review this proposed facility with a
higher discharge. This is done to ensure that there are no unexpected flood hazards
inherent in the proposed facilities. The review flood is usually the base flood and in
some cases, a flood event larger than the base flood is used for analysis to ensure the
safety of the drainage structure and nearby development.
6.4.2.5
Rainfall vs. Flood Frequency
Drainage structures are designed based on some flood frequency. However, certain
hydrologic procedures use rainfall and rainfall frequency as the basic input. Thus it is
commonly assumed that the 10-yr rainfall will produce the 10-yr flood.
Chapter 6-10 of 34
6.4.2.6
Intensity-Duration-Frequency (IDF) Val
