I. Hydrology

A. Watershed Properties

1. Delineate Areas

a) USGS Topography Mapping

b) Aerial Mapping

c) County Auditor Mapping

2. Rational Coefficient of Runoff “C” Value

a) Use Table 1101-2 L&D, Vol. 2

b) Use “weighted” value of “C” for calculations.

A = Drainage Area in acres [hectares]

C = Coefficient of runoff

WC = Weighted coefficient of runoff

3. Time of Concentration

a) Overland Flow (1101.2.2)

(1) Equation

��

���

� −≈

−≈

3o

3o

sLC)3.26(1.1

t

sLC)1.8(1.1

t

to = Time of overland flow in minutes

C = Coefficient of runoff

L = Distance to most remote location in

drainage area in feet [meters]

s = Overland Slope (Percent)

(2) Nomograph Figure 1101-1 L&D, Vol. 2

A 1 C 1. A 2 C 2 A i C i

A 1 A 2 A iWC= WCWC

b) Shallow Concentrated Flow (1101.2.2)

(1) Equation

[ ]0.5

0.5

ksV

3.281ksV

=

=

V = Velocity in fps [m/s]

k = Intercept coefficient

(see Table 1101-1, L&D, Vol 2)

s = Overland slope (percent)

60VL

tort ds =

ts = Travel time for shallow concentrated flow

in minutes

td = Travel time for open channel or pipe flow in minutes

L = Flow length in feet [meters]

V = Velocity in fps [m/s]

c) Minimum 15 minutes to first ditch section

d) Minimum 10 minutes to first pavement catch basin

e) Use the highest time of concentration from a group of sub-areas.

4. Rainfall Intensity

a) Use local intensity curves as approved by the Hydraulic Section, Office of Structural Engineering.

b) Use Figures 1101-2 & 1101-3 from L&D, Vol. 2.

5. Rational Method of Discharge

��

���

� =

=

360CiA

Q

CiAQ

Q = Discharge in cubic feet per second [cubic

meters per second]

C = Coefficient of runoff

I = Average rainfall intensity in inches per hour [mm per hour], for a given storm frequency and for a duration equal to the time of concentration.

A = Drainage area in acres [hectares]

II. Storm Sewer Design

A. Structure Placement

1. Arbitrary catch basin locations

a) Place upstream of all intersections, bridges, pedestrian ramps, commercial drive aprons, intersection return radii, and curb termini.

b) Structures should be placed 10’ off drive aprons, intersection return radii, pedestrian ramps, or curb termini when practicable.

c) Place structure (No. 3 catch basin) in pavement sags.

d) Flank catch basin in sag on both upstream directions at 0.2 feet above the flow line of the inlet of the sag catch basin when practicable. Use No. 3-A catch basins.

2. Access to storm sewer system

a) Structures spaced at 300’ or less for diameters less than 36”.

b) Structures spaced at 500’ or less for diameters from 36” to 60” in diameter.

c) Structures spaced at 750’ to 1000’ or less for diameters greater than 60” in diameter.

B. Design Considerations

1. Use minimum pipe size of 15” for Freeways and Freeway Ramps

2. Match crown elevations at structures when practicable.

3. Minimum velocity in storm should be limited to 3.0 ft/sec.

4. Provide for adequate depth for outlet of 6” U.D. to be at least 12” above the bottom flow line of the structure.

5. Provide for sufficient depth to allow the use of precast structures (see standard drawings for details).

6. Minimum height of cover for rigid conduits is 9” as measured from the pavement subgrade to the top of the conduit.

7. Minimum height of cover for flexible conduits ranges from 12 to 24 inches (see section 1008 in L&D, Vol. 2).

8. Use Conduit, Type B under pavement and Conduit, Type C for storm not under pavement.

9. Design for “just full capacity” for a 10 year frequency and a hydraulic grade check for a 25 year frequency.

10. Verify the hydraulic grade line for a 50 year discharge if flooding of adjacent buildings is possible.

11. For highways with depressed sags that are being drained by storm sewers a 50 year hydraulic grade line check shall be used.

12. Use mannings “n” value of 0.015 for sewers 60” and smaller, thus accounting for the minor losses in the structures.

13. Use mannigs “n” value of 0.013 for sewers larger than 60” in diameter.

C. Design Procedure

1. Apply Hydrology techniques and calculations as outlined in previous section.

2. Layout initial horizontal storm sewer alignment based upon arbitrary placement of structures, access, and design considerations.

3. Size conduits based upon the “just full” capacity. The just full capacity does not use the entire diameter of the conduit. It uses 0.938xD when figuring the hydraulic radius and cross sectional area. Use design figures in the Appendix of the L&D, Vol 2 or the following equations:

a) Just full discharge

Therefore,

Q = Discharge in cubic feet per second [cubic meters per second]

D = Diameter Required in feet [meters]

S = Slope in feet per feet [meters per meters] n = Mannings n value

Q1.49

nS

1

2 A. R

2

3.

D2.00 Q. n.( )

S

3

8

Q1.00

nS

1

2 A. R

2

3.

D2.98 Q. n.( )

S

3

8

R = Hydraulic Radius in feet [meters] A = Cross Sectional Area sq. ft [sq. meters]

b) Check velocity in system using just full Mannings Equation.

Therefore,

D

=

Diameter Required in feet [meters]

S = Slope in feet per feet [meters per meters] n = Mannings n value R = Hydraulic Radius in feet [meters]

Note: If the storm sewer is extremely over sized as compared to the size required for the “just full” condition, then the actual velocity based upon the actual flow area of the conduit should be used to calculate the velocity based upon the equation Q=VA. The equations above provide a simplified approach (easier by hand) to calculate the velocity.

c) Adjust the diameter and slope of the conduits to achieve a “just full” discharge that is greater than the discharge calculated from the hydrology while maintaining the velocity between 3 & 10 fps.

d) Calculate time of flow within storm sewer based upon the velocity of flow and length of conduit. Add time to next basin. Compare the new total time to the time of concentration to the next junction (if applicable). Choose the higher of the two times to calculate the new intensity for the next rational equation discharge.

e) Add new area and weighted “C” value at the next catch basin. Total the new CA to the old CA value.

f) Multiply the new total CA value by the new intensity to get the new discharge.

g) Repeat steps a-f for the remaining storm sewer runs.

h) Calculate the hydraulic grade line elevation for the storm sewer system.

(1) Begin the hydraulic grade check at the downstream outlet with either the 25 year water surface depth or:

V1.49

nS

1

2 R

2

3.

D c D

2

V1.00

nS

1

2. R

2

3.

V0.652

nD

2

3. S

1

2. V0.438

nD

2

3. S

1

2.

Dc = Depth of Critical Flow feet [meters]

D = Diameter in feet [meters]

(2) Use the intensity of the storm sewer system at the outlet for the check on the entire system (ie the greatest time and the lowest intensity).

(3) The hydraulic grade check equation is as follows:

hf = Headloss due to friction in feet [meters] D = Diameter in feet [meters]

Q = Discharge in cubic feet per second [cubic

meters per second] n = Mannings n value

(4) Add head losses to the elevation of the downstream hydraulic grade elevation.

(5) Continue with steps 3-4 for the entire storm sewer under design. Note: The hydraulic grade line should never be below the normal depth of flow in the conduit. If it is, then use the normal depth of flow elevation as the hydraulic grade line elevation.

i) Verify that the hydraulic grade line is below the grate elevation of the structures. If not, then the conduit may have to be upsized or the slope may need to be changed.

j) For ADT greater than 2000 the hydraulic grade line shall not exceed:

(1) 12” below the edge of pavement for sections without curb.

(2) The elevation of a curb opening inlet or grate elevation of a pavement catch basin.

k) For Roadways with an ADT of 2000 consideration shall be given to a reduction in the design frequency.

l) One directional lane of a multiple lane highway or one-half of a lane on a 2-lane highway should be passable when the sewer system is discharging the 50 year storm.

hf 4.66n2 L Q2.

D

16

3

. h f 10.32 n2 L Q2.

D

16

3

.

Storm Sewer Computation Sheet Guide Column Description

1 Structure number. Assigned by the designer. Usually numbered from lowest elevation to

highest elevation. The main line trunk is numbered first and then the laterals.

2 Station of the structure as referenced from the centerline or baseline. 3 Right, Left or on the Centerline. 4 Drainage area for the referenced structure. 5 Total drainage area. This number is found by summing the ∆∆∆∆A from the current structure to

the ΣΣΣΣA directly upstream of it. 6 Time of concentration to the current structure. In some cases, this time may be calculated based

upon the length of the conduit and the velocity of the flow if there is no discharge into the next adjacent structure.

7 Total time of concentration. This number is found by summing the individual time of concentration from the current structure (column 6) to the structure directly upstream of it.

8 The rainfall intensity based upon the design year storm. At time equal to ΣΣΣΣT (column 7) 9 The rainfall intensity based upon the hydraulic grade year storm. This intensity is based upon

the greatest time of concentration to the outlet. It is used for the entire upstream storm sewer system. Do not complete until the greatest Tc is known. This intensity is used for the entire storm upstream.

10 The weighted coefficient for the watershed. 11 The multiplication of the drainage area for the structure and the weighted coefficient for the

watershed (column 4 x column 10) 12 Summation of the ∆∆∆∆CA value of the current structure added to the upstream ΣΣΣΣCA value. 13 The design discharge found by the multiplication of the ΣΣΣΣCA and the design intensity ( column

12 x column 8). 14 The design discharge found by the multiplication of the ΣΣΣΣCA and the hydraulic grade intensity (

column 12 x column 9). Do not complete until #9 is determined. 15 The diameter of the conduit. 16 The length of the conduit. 17 The slope of the conduit. 18 The invert of the incoming conduit to the current structure. 19 The invert of the outgoing conduit from the structure. 20 The velocity based upon the Manning’s “just full equation”. (see notes) 21 The discharge based upon the Manning’s “just full equation”. 22 The hydraulic friction slope. ** 23 The headloss due to friction in the conduit. hL=L*(Sf) or other equation 24 The elevation of the hydraulic grade line. Calculated by adding the head loss to the hydraulic

grade elevation of the downstream structure. At the outlet the hydraulic grade elevation is either the water surface elevation or it is calculated by the (critical depth+diameter)/2.

25 The elevation of the structure grate or cover. 26 The difference of the structure grate or cover elevation and the hydraulic grade elevation

(column 25- column 24). Notes: • A common mistake is to not use the smallest intensity (longest time of concentration) for the hydraulic grade

check. • The critical depth is calculated using the nomographs in the appendix of The Location and Design Manual,

Volume 2 or it can be approximated by using 0.8 x Diameter. **Sf= [(Q*N)/(0.465*D^(8/3))]^(2)