The hydrological analysis phase involves the determination of discharge rates and/or volumes of runoff that the drainage facilities will be required to convey or control. Many hydrological methods are available and most can be appropriately and effectively used under proper control and application. Particular methods recommended for highway drainage studies and circumstances for their use are listed below. When the site involves a FEMA flood study area, discharge methods and values provided in the report will take precedent over these methods for determining compliance with the regulation. The results from any hydrologic procedure should be compared to historical site information and adjustments made in the values estimated or procedure used when deemed appropriate. The designer must also consider potential future land use changes within a watershed over the life of a roadway structure and include this effect when estimating design discharges.

METHODS

Rural Watersheds - Peak Discharge

> 1 mi² The procedures and values presented in U. S. Geological Survey, Water Resources Investigations Report 01-4207 (4), shall apply.

< 1mi² The hydrological procedure and charts presented in Appendix C, N. C. Division of Highways Hydrologic Charts-1973,(C200.1 and C200.2) shall be used.

Urban Watersheds - Peak Discharge

< 10 acres If watershed is primarily composed of pavement, grassed shoulders and slopes, and/or other mixed surface type runoff, use rational formula for discharge determination. If predominately residential type development with natural drainage channels, use Highway Charts C200.1 and C200.3.

> 10 acres < 100 Use Highway drainage charts(C200.1 and C200.3). If areas have greater than 50% impervious cover and/or extensive storm drainage systems, a special procedure such as routing is recommended. The HEC-1 and NRCS, TR-20 are widely used routing procedures. Determination of specific sites for special study and selection of a design procedure must be approved by the Reviewing Engineer. This item should be addressed in the pre-design meeting.

> 100 acres Use the procedure presented in U.S. Geological Survey - Water Resource Investigation Report 96-4084(5).

Volume of Flow

The procedures presented in U.S. Geological Survey Report 96-4085(6) for developing a runoff hydrograph can be utilized to determine flow volume. For estimating purposes or minor impoundment (<1 acre-foot) a simple triangular hydrograph as described later in this chapter can be used.

"U.S.G.S. Water Resources Investigations Report 01-4207"

Two regional analysis methods are presented in this report (4). The first employs the traditional regional regression equations that are presented in Table 5, page 11. The second is the region-of-influence method that must be developed through the use of a computer software program due to the complexity of the computations. This program provides both the regional regression and region-of-influence solutions, allowing the engineer to compare and select a design value. This computer software package is available at the NC USGS home page on the internet. For gaged sites, the discharge estimate is to be determined by weighting the regional and station estimates (See Equation 7, Page 15). For sites on gaged stream and having a drainage area within 50% (0.5 to 1.5)of the gage site, the discharge estimate is to be transferred from the gage in accordance to Equation 8 and 9, Page 16.

"Highway Charts"(Appendix C)

The rural areas charts C200.1 and C200.2 are to be used within the limits previously noted. The procedure for use is as follows:

(1) From Chart C200.1 determine the hydrologic contour by location of the structure site. Interpolate to 0.5 contour interval.

(2) Determine:

Drainage area (acres or mi²) Watershed shape factor (A/L²)

• A = area
• L = length

Percent forested cover

(3) Enter chart C200.2 with drainage area and hydrological contour and read discharge.

(4) For discharges other than Q50, apply frequency adjustment factors shown on chart.

(5) Enter charts C200.4 and C200.5 to determine adjustment factors to be applied to above values for percent forested cover and watershed shape.

*NOTE: The forested cover value can be used to reduce discharge only when the watershed is mountainous, wetlands, or a designated preserve area where clearing is very unlikely. The multiple of the two adjustment factors cannot exceed the limits of 0.7 and 1.5.

The urban chart, C200.3 is to be used within the limitations previously noted. Procedure for use is as follows:

(1) From chart C200.1 determine the hydraulic contour to the nearest 0.5 interval.

(2) Determine the type and relative density of development. This should be a projection of conditions based on potential future development over the life of the structure. The development types as noted on the chart are:

• Residential-High Type; This is suburban type development with lots sizes > 0.5 acres
• Average Development; Small lots < 0.5 acres or mixture of residential and some small business
• Large Area Full Business; Area > 75 acres, no more than 50% impervious cover or extensive storm drainage systems
• Small Area Full Business; Area < 75 acres no more than 50% impervious cover or extensive storm drainage systems

(3) Enter chart C200.3 with drainage area and hydraulic contour and read discharge.

(4) Apply appropriate adjustment factor for development type.

(5) For discharges other than Q10, apply frequency adjustment factors shown on chart.

"Rational Formula"

The rational formula estimates the peak rate of runoff (Q)in ft³/s as a function of drainage area (A), in acres, runoff coefficient (C), and mean rainfall intensity (I) in in/h for a duration equal to the time of concentration (t_c), the time required for water to flow from the most hydraulically remote point of the basin to the location of analysis.

Use limitations are noted previously in the guidelines. For expanded discussion of the rational formula see "FHWA, Hydraulic Engineering Circular No. 22"(7)

Some specific criteria are:

A = 10 +/- acres maximum (When the watershed for a continuous storm drain system is greater than the suggested maximum, it is acceptable to exceed this value).

I = Use highway charts C200.7, C200.8,and C200.9.(Appendix C). Interpolate between cities for other points. The Hydrain program will provide values based on latitude and longitude location.

C = Use a weighted value = CiAi/A

Table 4-2 provides some often used values:

Time of concentration (t_c) - Use Kinematic Wave Equation for overland flow time. See page 3-8, Hydraulic Engineering Circular No.22,(6). Minimum t_c - 10 min.

USGS Report 96-4084"Estimation of Flood-Frequency Characteristics of Small Urban Streams in North Carolina"(5)

Urban regression equations are provided on page 14 of this reference. Details are provided on page 17 for use of the equations.

"Snyders Synthetic Unit Hydrograph"

This procedure can be utilized to develop a design hydrograph associated with a peak flow. It can be performed with or without precipitation and surface runoff data. It provides a graphical depiction of runoff as a function of time as well as an estimate of runoff volume. FHWA, Hydraulic Design Series No. 2(8) and No. 22(7), are reference sources for detailed direction in this procedure. The Hydrain computer program also includes this design alternate.

"Triangular Hydrograph Storage Estimate Method"

Develop inflow:

1. Determine peak discharge Qp
2. Estimate time of concentration Tc
3. Calculate time to peak Tp = Tc + 0.6Tc(hrs.)
4. Calculate total time T = Tp x 3
5. Calculate average discharge Qa = 0.33Qp
6. Calculate total runoff volume Qt = Qa/12 x T=(acre-feet)

Determine outflow:

1. Determine available storage (acre-feet) Qs
2. Calculate net runoff Qn = Qt - Qs
3. Calculate average outflow Qao = Qn x 12/T= (cfs)
4. Calculate peak outflow Qpo = 2 x Qao

The overall hydrologic analysis for a project begins with review and extrapolation of pertinent information from data sources identified during the pre-design study. Final determination of sources of watershed areas and base mapping for drainage area delineation are also made at this time. Primary resources for this information are:

• U.S.G.S. and T.V.A. quadrangle mapping
• U.S.G.S. open file report 83-211 "Drainage Areas of Selected Sites on Streams in North Carolina"
• Photogrammetric contour mapping
• Aerial photography
• Special studies (Corps, TVA, FEMA)
• Field reconnaissance (This is required for  most non-riverine drainage areas in the coastal plain as well as any small watersheds in other areas.)

The selection of a "design discharge" for a drainage feature is a risk based assessment process involving the evaluation of a range of flood magnitudes for such factors as potential damages, costs, traffic service, environmental impact, and flood plain management criteria, to determine an appropriate and acceptable structure for each site. One specific criterion on which the design is evaluated and generally referred to as the "design discharge" is the flood level and frequency which results in inundation of the travelway. Table 4-3 relates desirable minimum levels of protection from travelway inundation to roadway classification. Variation from these minimum design levels must be justified through the assessment process and appropriately documented. When roadway overtopping is not involved, the "design discharge" will be the level of flood used for establishing freeboard and/or backwater limitations.

 

TABLE 4-3
ROADWAY CLASSIFICATION	FREQUENCY
Interstate (I)	50 year
Primary (US & NC)	50 year
Secondary (Major, City thoroughfare)	50 year
Secondary	25 year

The hydrologic analysis process for a specific drainage feature is accomplished as an integral part of the hydraulic sizing and performance analysis. Specific discharge criteria and computational needs are addressed in further sections of this guideline for each particular drainage feature. Documentation of the hydrologic data is included with the hydraulic design.

The following general guidance shall be used to determine when it is appropriate to consider the overtopping flood and the limits used in defining the data. This must be applied with good judgment and considered on the particular merits of each crossing analysis.

(1) Where overtopping is not practicable and would require flood magnitude greater than state of the art capability to estimate frequency (500+ year flood), a statement similar to the following should be noted on the survey report "overtopping flood is greater than 500+ year event".

(2) An approximate frequency of occurrence must be established for the overtopping discharge. The following frequency designation will be used:

(a) If within 5% of the 200 or 500-year estimated discharge, list as 200-year +/- or 500-year +/-.

(b) If greater than 100-year flood but not within 5% of 200-year, list as 100-year +.

(c) If greater than 5% of the 200-year but not within 5% of the 500-year, list as 200-year +.

(d) If greater than 5% of the 500-year, list as 500-year +.