Pipe Outfall Design Guidance

Pipe Outfall Design Guidance for Scour Protection

Purpose

The purpose of this design guidance is to help the designer with the process necessary to design scour protection at the pipe outfall. This design guidance is not intended as a channel lining design guide. This documentation will be limited to any pipe or culvert that is not classified as a bridge. A bridge is defined as a pipe / culvert that has greater than a 20 sf opening and a vertical clearance 4 ft or greater. Basically, any circular pipe less than or equal to a 60 in diameter. For any culverts that are non-circular, the designer will have to determine if it fits within the bridge definition and if it does, then it should be coordinated with DelDOT’s Bridge Design section for evaluation. Outfall protection will consist of one of three options: non-riprap, riprap apron, or a riprap basin / energy dissipator. The non-riprap option, which given the right situation, could be a viable substitute for riprap and would help promote enhanced vegetation and easier installation with the added benefit of reduced maintenance post construction. For the riprap outlet protection portions of this document, many references were researched and analyzed (see reference listing) and the recommendations provided are believed to be the easiest and most practical for use on DelDOT projects. Appendix A contains a brief overview of different formulas considered and the conclusion for the formulas used herein. As always, designers are encouraged to follow up with what is shown on the references sheet and complete their own research to obtain a better overall understanding. Design criteria guidance will be presented that will assist in the determination of which option to utilize; however, it is the designer’s final decision on which option best accommodates the project site conditions. In regard to the use of flared end sections on the ends of pipes, after many discussions with DelDOT personnel, it is recommended that flared end sections be used on both the entrance and exit of pipes. Outfall protection lengths where pipes discharge on a side slope and at curb openings, the scour protection should be provided to a toe of slope / where erosive forces from the runoff would not cause further degradation to the receiving channel / area. Any questions, comments, or concerns can be directed to the Stormwater section.

Non-Riprap

The recommended application of this option is in situations where the calculated design velocity of the pipe outfall is less than < 5 ft/sec and the pipe diameter is less than or equal to <= 30 in (or round equivalent) or curb openings. For a velocity or pipe size larger than these values, it is recommended to use a riprap apron as minimum outfall protection.

The main reasons or benefits for this option are an easier installation, cheaper installation cost than riprap, and an easier long-term maintenance solution. This option would also help encourage natural vegetative conditions at the outfall.

There are many prefabricated products on the market today which would fulfill this role and at the time of this writing here are some examples (in no particular order):

1. North American Green, Revetmax^TM Flexible Revetment Systems
2. Motz Enterprises, Inc, Flexamat
3. Hanes Geo Components, ScourStop® Transition Mats
4. Presto GeoSystems, Georunner®

The calculation for determining the minimum length and width of outfall protection in feet would be:
Length = 3 * Pipe Diameter (PD)
Width = 2 * PD

Keep in mind that some of the above products come in prefabricated mat sizes. It is intended that whole mat sizes be used, i.e., no partial mats. The design calculations shown below are for the minimum sizing criteria and DelDOT Construction will have to work with the Contractor in the field to place whole mat sizes. Also, most products for their installation requirements state to use their product over top of an erosion control blanket or turf reinforcement matting. When doing quantity calculations for the project, make sure to include one of these products under the outfall protection area.

Design Example (Non-Riprap):
Design a non-riprap outfall pad for the following RCP installation.
Given:
PD = 18 in (which is <= 30 in)
Q_pipe = 6 cfs
V_pipe = Q / A = 6 cfs / (hydraulic area, not just total pipe area) = 6 cfs / 1.25 sf = 4.8 ft/s
The velocity and size qualify this for a non-riprap application.

The minimum non-riprap pad dimensions would be as follows:
Length = 3 * PD = 3 * 1.5 ft = 4.5 ft, use 5 ft (Note: if answer is fractional, just round up to the next whole number)
Width = 2 * PD = 2 * 1.5 ft = 3 ft (Note: if answer is fractional, just round up to the next whole number)

Riprap Apron

This application should be used in situations where the calculated design velocity at the pipe outfall is between 5 ft/sec to 10 ft/sec or the pipe size is greater than > 30 in (round equivalent). This should also be highly considered instead of the non-riprap option if discharging into an open area, wetland, etc.

The riprap apron provides a flat armored surface at pipe/culvert outfalls to help dissipate the energy from the stormwater runoff coming out of a concentrated flow drainage system. Riprap aprons are also warranted when swales/ditches make sharp turns and at the confluence of swales/ditches. Rarely is any riprap ever needed at the inflow / upstream end of pipes, except in potentially extreme circumstances when the headwaters are causing scour conditions. This design guidance will only focus on the riprap apron at the discharge location. Riprap aprons do not dissipate significant energy except through increased roughness for a short distance, i.e. increasing the Manning’s n value. If a riprap apron is too short, it is fundamentally ineffective as the scour condition would just simply move downstream. The key design elements are the riprap size as well as the length, width, and depth of the riprap apron.

For the riprap apron length, the designer needs to ascertain what will happen at the end of the apron. If the apron is being installed at a free outfall location, evaluate whether there is enough of a downward slope beyond the apron which would potentially cause the flow to collect back into a concentrated form and erode the slope. When the apron terminates close to an active stream or ditch, evaluate the area from the end of the apron to the active flow of the receiving waterbody for erosion potential if not properly vegetated or hardened with riprap, turf reinforcement matting, etc.

One of the most common scenarios is that of a pipe discharging into a swale. For this scenario, constructability-wise, the length is not as much of a concern as the apron width. The riprap apron is going to be more confined to the swale dimensions versus being flared going away from the pipe outlet. It is suggested to maintain a constant apron width for the entire apron length versus flaring outward from the pipe outflow. The designer should evaluate the design parameters of the apron width versus that of the actual receiving swale dimensions and appraise from a constructability standpoint.

Riprap apron illustrations for the above are shown in Figure 1 and Figure 2 below.

Table 1 is taken from the DelDOT Standard Specifications. Table 2 is the consensus minimum depth needed and will need to be shown on the plans.

Table 1. Riprap Sizing Gradation Table

NSSGA* Rock Size	R-7	R-6	R-5	R-4
30"	100	---	---	---
24"	---	100	---	---
18"	15-50	---	100	---
12"	---	15-50	---	100
9"	0-15	---	15-50	---
6"	---	0-15	---	15-50
4"	---	---	0-15	---
3"	---	---	---	0-15

* National Stone, Sand, and Gravel Association

Table 2. Riprap Min. Design Depth

Riprap Size	Minimum Design Depth
R-4	18" (1.5')
R-5	24" (2')
R-6	30" (2.5')
R-7	42" (3.5')

The basic equations for riprap apron sizing are as follows:

Riprap apron stone sizing:
d₅₀ = 0.02 / Tw * (Q / PD)^1.33
d₅₀ – Median riprap size (ft)
Tw – Tailwater depth above the invert of the culvert (ft). (If Tw is unknown or < 0.4 * PD, then use 0.4 * PD.)
Q – Calculated design storm flow in pipe (cfs)
PD – Pipe diameter (ft) (Note: if elliptical pipe, use the pipe width measurement)

Riprap apron depth: (This is the same as Table 2.)

d50	Minimum Design Depth
6" (R-4)	18" (1.5')
9" (R-5)	24" (2')
12" (R-6)	30" (2.5')
18" (R-7)	42" (3.5')

Table 3. Riprap apron length:

d50	Apron Lengton (ft)*
6" (R-4)	4 * PD
9" (R-5)	5 * PD
12" (R-6)	6 * PD
18" (R-7)	7 * PD

* For PD, if elliptical, use pipe height measurement. If the end of the pipe does not include a flared end section, add 3 ft to the apron length calculated.

Riprap apron width: The minimum apron width at the pipe outlet should be 3 * PD. The width at the end of the riprap apron will be dependent upon the side flare used for the apron as based upon designer discretion. A typical application will be 3:1, but other side flares may have to be used depending on site conditions.

One of the main factors that could affect the width at the end of the riprap apron is if the apron is being placed in a swale. If that is the case, then use the minimum apron width for the length calculated above, but the width could be greater depending on designer discretion / engineering judgement.

Minimum apron width = 3 * PD (Note: if elliptical pipe, use the pipe width measurement)

Figure 1. Typical Riprap Apron

Typical Riprap Apron Perpendicular to Swale:
In this situation, the length of the riprap apron should be placed 1 ft minimum up the back slope and the width should = 3 * PD at a minimum. Again, these dimensions could be greater depending on designer discretion.

Figure 2.

Design Example (Riprap Apron):
Design a riprap apron for the following RCP installation.
Given:
Q – Pipe discharge (cfs)
PD – Pipe diameter (in)
Tw – Tailwater calculated at pipe outlet. This would be the height of the water surface elevation in the receiving channel for the storm event used in its design. This should be done to more accurately reflect the apron dimensions to use, but if Tw is unknown or < 0.4 * PD, then use 0.4 * PD.

Q = 12.0 cfs
PD = 18 in = 1.5 ft
Tw = unknown

d₅₀ = 0.02 / Tw * (Q / PD)1.33
d₅₀ = (0.02 / (0.4 * 1.5)) * (12 / 1.5)1.33
d₅₀ = (0.02 / 0.6) * (8)1.33
d₅₀ = (0.03) * (15.89)
d₅₀ = 0.52 ft = 6.3 in
The calculated d₅₀ value is slightly above R-4, but less than R-5. Since the calculated value is just barely above the R-4 range as well as the designer’s discretion / engineering judgement is that the outflow should be able to be handled with R-4 with no adverse impacts, use R-4.

Table 1. (abbreviated)

NSSGA Rock Size	R-7	R-6	R-5	R-4
9"	0-15	---	15-50	---
6"	---	0-15	---	15-50

(Note: if the d₅₀ calculated value is greater than the R-7 d₅₀ value, then consult with your supervisor.)

d50	Minimum Design Depth
6" (R-4)	18" (1.5')

Table 3. Riprap apron length:

d50	Apron Lengton (ft)*
6" (R-4)	4 * PD

(Note: if answer is fractional, just round up to the next whole number)

Apron Width at end with using 3:1 side flare = 3 * PD + (2/3) * L = (3 * 1.5) + (0.67 * 6) = 4.5 + 4.02 = 8.52ft ~ 9ft (Note: if answer is fractional, just round up to the next whole number)

Riprap Basin/Energy Dissipator

This should be used in situations where calculated design flow at the pipe outfall is > 10 ft/sec or discharging into an open area, wetland, etc.

The riprap energy dissipator is used for the same purposes as a riprap apron in that it is used to reduce the velocity and energy of the water exiting a confined conveyance system, but has the added benefit of a defined plunge pool. The plunge pool induces a hydraulic jump before the water is discharged into a receiving channel or other stable outlet. If existing downstream scour/degradation is apparent, then a dissipator would probably be a better option. If the velocity at the pipe outfall point is quite high or if discharging into waters of the US or into wetlands, a dissipator would probably be a better option to help ensure the energy component is reduced to a more acceptable level.

The current riprap energy dissipator in the DelDOT Standard Construction Details is shown here:

(Abbreviated DelDOT Standard Construction Detail E-20, Riprap Energy Dissipator)

For ease of design, up to a 30in diameter pipe, use the folowing:
D = Pipe Diameter (PD)	W1 = PD	W2 = 2 * PD	L = 3 * PD
For pipes > 30in. diameter, use the following:
D = 2.5ft	W1 = PD	W2 = 2 * PD	L = 3.5 * PD

T1 will be determined from d₅₀ calculation and minimum design depth chart.

For up to a 30in diameter pipe use the following: d₅₀ = ((0.0082*PD²)/Tw) * (Q/PD^2.5)^4/3

For pipes > 30in diameter: d₅₀ = ((0.0125 * PD²)/Tw) * (Q/PD^2.5)^4/3

For both equations, if the tailwater (Tw) is unknown or < 0.4 * PD, then use 0.4 * PD.

Design Example (Riprap Energy Dissipator):
Design riprap energy dissipator for the following RCP installation.

Given:
Q = 50 cfs
PD = 27in = 2.25ft
Tw = unknown

The riprap energy dissipator dimensions would be as follows:
(Note: For ease of constructability, just round up to the next 0.5ft increment for these calculations.)

D = 27in = 2.25ft (use 2.5ft)

W1 = 27in = 2.25ft (use 2.5ft)

W2 = 2 * PD = 2 * 27in = 54in = 4.5ft

L = 3 * PD = 3 * 27in = 81in = 6.75ft (use 7ft)

T1 determination:

d₅₀ = ((0.0082*PD²)/Tw) * (Q/PD^2.5)^4/3

d₅₀ = ((0.0082*(2.25)²)/(0.4*2.25)) * (50/(2.25)^2.5)^4/3

d₅₀ = (0.0415/0.9) * (50/7.59)^4/3

d₅₀ = 0.046 * (6.584)^4/3

d₅₀ = 0.046 * 12.263

d₅₀ = 0.564 ft = 6.77 in – The calculated d₅₀ value is above R-4, but less than R-5. For this application, it is the designer’s discretion/engineering judgment that R-5 should be used due to wanting to maximize the energy dissipation and help ensure no adverse impacts; therefore, use R-5.

T1 = 24 in = 2 ft

Table 1. Riprap Sizing Gradation Table

NSSGA* Rock Size	R-7	R-6	R-5	R-4
30"	100	---	---	---
24"	---	100	---	---
18"	15-50	---	100	---
12"	---	15-50	---	100
9"	0-15	---	15-50	---
6"	---	0-15	---	15-50
4"	---	---	0-15	---
3"	---	---	---	0-15

* National Stone, Sand, and Gravel Association

Table 2. Riprap Min. Design Depth

Riprap Size	Minimum Design Depth
R-4	18" (1.5')
R-5	24" (2')
R-6	30" (2.5')
R-7	42" (3.5')

Appendix A

The material in this appendix will delve into a little bit of the history of riprap outfall protection, as taken from Appendix D in HEC-14, as well as present the thought process for the formulas used above.  Even on the lower flow values at pipe outfalls (i.e. flat driveway culverts, etc.), it has been observed that almost always there are at least some scouring effects. After extensive research of many different states, municipalities, conservation district areas, etc. they all use a multitude of different methods and formulas for calculating riprap outfall protection.  The formulas presented on the preceding pages are what is believed to be the simpler applications to use, while still providing sound engineering judgment on the sizing of non-riprap and riprap outfalls.

As taken from HEC-14, Appendix D (Jul 2006):

“A variety of relationships for sizing riprap aprons have been developed. Six are summarized and compared in this appendix. The first is from the Urban Drainage and Flood Control District in Denver Colorado (UD&FCD, 2004). These equations consider tailwater in addition to a measure of flow intensity.

$D_{50}=0.023D\left({\frac {Q}{\alpha D^{2.5}}}\right)\left({\frac {D}{TW}}\right)^{1.2}$

(D.1a)

$D_{50}=0.014D\left({\frac {Q}{\alpha BD^{1.5}}}\right)\left({\frac {D}{TW}}\right)$

(D.1b)

where,

D₅₀ = riprap size, m (ft)

Q = design discharge, m³/s )ft³/s)

D = culvert diameter (circular) or culvert rise (rectangular), m (ft)

B = culvert span (rectangular), m (ft)

TW = tailwater depth, m (ft)

α = unit conversion constant, 1.811 (SI) and 1.0 (CU)

An equation in Berry (1948) and Peterka (1978) has been used for apron riprap sizing. It is only based on velocity.

$D_{50}=\alpha V^{2}$

(D.2)

where,

V = culvert exit velocity, m/s (ft/s)

α = unit conversion constant, 0.0413 (SI) and 0.0126 (CU)

A relationship used in the previous edition of HEC 14 from Search (1967) and also found in HEC 11 (Brown and Clyde, 1989) for sizing riprap protection for piers is based on velocity.

Equation D.3

$D_{50}={\frac {0.692}{S-1}}\left({\frac {V^{2}}{2g}}\right)$

(D.3)

where

S = riprap specific gravity

Bohan (1970) developed two relationships based on laboratory testing that considered, among other factors, whether the culvert was subjected to "minimum" tailwater (TW/D < 0.5) or "maximum" tailwater (TW/D > 0.5). The equations for minimum and maximum tailwater, respectively, are as follows:

$D_{50}=0.25DFr_{o}$

(D.4a)

$D_{50}=D(0.25Fr_{o}-0.15)$

(D.4b)

where,

Fr_o = Froude number at the outlet defined as V_o/(gD)^0.5

Fletcher and Grace (1972) used the laboratory data from Bohan, and other sources to develop a similar equation to Equation D.1.

$D_{50}=0.020D\left({\frac {Q}{\alpha D^{2.5}}}\right)^{4/3}\left({\frac {D}{TW}}\right)$

(D.5)

Finally, the USDA/SCS has a series of charts for sizing riprap for aprons. These charts appear to be based on Bohan (Equation D.4a and D.4b). Equation D.2 (Berry) and Equation D.3 (Searcy) are similar in their exclusive reliance on velocity as the predictor variable and differ only in terms of their coefficient. Equation D.1 (UD&FCD), Equation D.4 (Bohan), and Equation D.5 (Fletcher and Grace) incorporate some sort of flow intensity parameter, i.e. relative discharge or Froude number, as well as relative tailwater depth. (Bohan incorporates tailwater by having separate minimum and maximum tailwater equations.) UD&FCD and Fletcher and Grace have identical forms but differ in their coefficient and exponents. These equations and the USDA charts were compared based on a series of hypothetical situations. A total of 10 scenarios were run with HY8 to generate outlet velocity conditions for testing the equations. The 10 scenarios included the following variations:

1. Two culvert sizes, 760 and 1200 mm (30 to 48 in) metal pipe culverts
2. Discharges ranging from (1.1 to 4.2 m3/s) (40 to 150 ft3/s)
3. Slope and tailwater changes resulting in 5 inlet control and 5 outlet control cases

Figures D.1, D.2, and D.3 compare the recommended riprap size, D50, relative to the outlet velocity, V, discharge intensity, Q/D2.5, and relative tailwater depth, TW/D. The recommended D50 varies widely, but it is clear that the Berry equation (Equation D.2) results in the highest values for the range of conditions evaluated. Equations D.2 and D.3 are not recommended because they do not consider tailwater effects. Equation D.4 is not further considered because it treats tailwater only as two separate conditions, minimum and maximum. Equations D.1 and D.5 are similar in their approach and are based on laboratory data. Both would probably generate reasonable designs. For the ten hypothetical cases evaluated Equation D.1 produced the higher recommendation 3 times and the lower recommendation 7 times. Therefore, Equation D.5 is included in Chapter 10 of this manual.”

For the non-riprap aspects of this document, it has been observed that there are many pipe outfalls with lower velocity components that still exhibit scour conditions. The velocity threshold was chosen as per Figures 6-14 and 6-15 of the DelDOT Road Design Manual. This is the largest permissible velocity in an open grass mixture channel. It is also believed that riprap outfalls are not always necessary in these situations; hence, the proposal of the non-riprap option. Any of these products should be cheaper than riprap, easier to install, and easier to maintain. As always, the designer has the final judgment if they believe that riprap would be necessary even though the calculations may show otherwise.

The riprap apron portion of this document is almost the same as the HEC-14 procedures; albeit, probably slightly more conservative. For ease of use, the riprap energy dissipator calculations were mostly taken from the Practical Guidance for Design of Lined Channel Expansions at Culvert Outlets by Fletcher and Grace, Oct 1974, The Standards for Soil Erosion and Sediment Control by New Jersey Department of Agriculture, 7th Edition, January 2014, and the 2011 Maryland Standards and Specifications for Soil Erosion and Sediment Control.