ENGINEERING TECHNOLOGY
EGTE 321 Storm-Water Management
Part IV – Design of Emergency Spillway and Top of Dam
1. Design the emergency spillway and set the elevation for the top of the dam. Recall the
following design requirements:

The crest of the emergency spillway must be no lower than the 100-yr storage elevation or
1 ft above the crest of the principal spillway (see appendix B for replacement of statement
on page 5 regarding emergency spillway crest elevation for storm-water management
ponds).

Use the tables in the reference posted on the WebCT site entitled “Design of Vegetated
Earthen Emergency Spillways” from the NRCS Engineering Field Handbook, Chapter 11
– Ponds and Reservoirs (1984) to size the emergency spillway. The downstream slope, S,
is determined by the topography at the location of the emergency spillway crest.
Maximum permissible velocity criteria for design of crest width are listed in the reference.
Use the vegetated waterway design reference from earlier in the course to specify
vegetation for the emergency spillway and downstream channel.

Pond Code 378 allows you to take storage into account for design of the emergency
spillway, but flows are relatively low in this application and there is little advantage in
terms of dam height in doing so compared to simply designing the emergency spillway
and downstream channel to carry the developed 100-yr flow rate.

Determine the elevation of the top of the settled dam by routing the 100-yr runoff event
through the pond with the principal spillway blocked. Starting water surface elevation
should be at the invert of the first opening in the principal spillway having a minimum
dimension no smaller than 0.5 ft. Alternatively, you can choose to start routing with the
water surface elevation at the riser crest or, most conservatively, simply use the rating
curve for the emergency spillway to find the depth of water over the emergency spillway
crest for a flow rate equal to the developed 100-yr peak. The top of dam must be 1 ft
above the peak water level in the pond for the 100-yr event or 2 ft above the crest of the
emergency spillway, whichever is larger.
2. Design the trapezoidal channel downstream of the emergency spillway by using the tractive
force methods outlined in HEC-15, “Design of Roadside Channels with Flexible Linings.”
Specify vegetation and its maintenance consistent with those specified for design of the
emergency spillway. Provide a drawing of the channel cross-section with dimensions
specified for a level section.
3. Design the anti-seep collars for the principal spillway barrel. See page 1, appendix B of the
Pond Code 378 handout for anti-seep collar design requirements. Another reference is
provided by the Delaware Erosion and Sediment Control Handbook (see WebCT web-page
link) in the section on Standards and Specifications for Temporary Sediment Basin. Provide
a drawing showing a profile of the dam cross-section and principal spillway barrel with antiseep collars and saturation zone depicted.
Assessment Form: Relative Contributions of Team Members1
Participant: _____________________________________ Team: ___________________________ Date: __________________
SWM Design Project Part IV
Comments
Storm-Water Management
Group Design Project Team members
(include yourself)
Individual
contributions to this
team project2
1.
2.
3.
4.
5.

1
After Fletcher, Lugowski, and Stern. 2002. Fluid Power Facilitator Guide. Sinclair Community College.
2
Allocate 100 points to your team members, including yourself. Base the allocation on your opinion of each individual’s contribution. The sum of all points
allocated must equal 100.
SWM Design Project
Part IV: Emergency Spillway Design and Top of Dam
Grading Rubric
Highly Competent
Competent
Needs Improvement
Task
(2)
(1)
(0)
Narrative, appendices, and
supporting materials as described in
assignment.
Report is complete with clear,
concise discussion of relevant
regulations, assumptions, and design
considerations. Analysis is complete
and done correctly. All relevant
supporting documentation is included
in appendices.
Report is complete; design work and
analysis are supported with adequate
documentation and some
commentary regarding major
regulatory aspects, assumptions, and
design considerations. There may be
one or two minor omissions or errors
in analysis.
Report incomplete and analysis is
inadequate or faulty with little or no
discussion of major regulatory
aspects, assumptions, and design
considerations. There may be
significant design errors or the design
may be inadequately documented.
Specific Components:


Narrative – Provide discussion of these items including all design details:
- Pond Code 378 Criteria
- Elevation of Emergency Spillway Crest determined by routing results and elevation of principal spillway crest. Include consideration of alternatives.
- Design of Emergency Spillway Crest Width. Include supporting flow data from routing and specification of vegetation.
- Design for Top of Dam with supporting hydrologic routing or hydraulics data and consideration of alternatives.
- Design for Emergency Spillway Channel by the tractive force method with supporting flow data, construction specifications, and drawings. Include
specification of vegetation and maintenance.
- Anti-Seep collar design with specifications and drawings.
Appendices
- Appropriate computer output.
- Reference figures or tables
- Appropriate calculation details for any of the design work in the Narrative.