Jefferson-CS 940523 THE NELSON CORPORATION
LINDA SL~/ELL PLANNING · ENGINEERING · SURVEYING BRIAN MARCUS, P.E., R.P.L.$.
s~.c..'m~s. LANDSCAPE ARCHITECTURE · CONSTRUCTION MANAGEMENT PP. sswr.~"r
5999 SUMMERSi~E DRIVE · SUITE 202 CRAIG T. CURRY
DALLAS, TEXAS 75252 on~croa
May 23, 1994 ~,~) ~0.~0~
YAX (214) ~80-2609
Mr. Kenneth M. Griffin, P.E.
Assistant City Manager/City Engineer
City of Coppell
255 Parkway Boulevard
Post Office Box 478
Coppell, Texas 75019
Subject: Storm Sewer Design Criteria
Dear Mr. Griffin:
This letter report is prepared in response to your rejection of our previously proposed
storm sewer design criteria for the project captioned above. Mr. Michael L. Clark,
P.E., of our firm requested by way of letter dated May 17, 1994 that we be permitted
to use the 10-year water surface elevation of the Elm Fork of Trinity River as the
starting hydraulic grade line for our storm sewers connected into the Elm Fork
floodplain. Your written response, dated May 19, 1994, was that this was
unacceptable; that the 100-year water surface elevation of the Elm Fork should be used
as the starting condition. The following sections provide explanations related to the
establishment of the starting hydraulic grade line condition in question.
Comparison of Watershed Areas
The usage of the Elm Fork 100-year water surface elevation as a starting hydraulic
grade line for design of the storm sewer system to convey the 100-year storm event
represents what is known as a coincident peak assumption. This implies that the peak
flow rate for the receiving waters (the Elm Fork of Trinity River) will occur at the same
time as the peak flow for the contributing storm sewered area (the Jefferson at
Riverchase property). Usual hydrology practice considers a ratio of drainage areas
between the receiving and contributing watersheds on the order of 1:1 to 2:1 to be
necessary before coincident peaks is viewed as a reasonable assumption. In other
words, the drainage areas should be relatively close to the same size.
Research of the City of Coppell Flood Insurance Study (FIS) Report, published by the
Federal Emergency Management Agency, yields a value for the drainage area of the
Elm Fork. At Sandy Lake Road, located upstream of the Jefferson at Riverchase
THE NELSON CORPORATI(~.,
Mr. Griffin, 5/23/94 Page 2 of 3
property, the unregulated watershed area is given as 104.0 square miles, or 66,560
acres. This represents the area below Lakes Grapevine and Lewisville which is not
captured by these impounding structures. The drainage area would be somewhat
greater at the location of the subject property, since it is located further down in the
Elm Fork watershed than the Sandy Lake Road crossing.
The Jefferson at Riverchase tract was filled some time ago in accordance with a
reclamation plan approved by the City of Coppell (City). This has the effect of the
property behaving as an island, such that no off-site runoff enters the property. Thus,
the only runoff to be collected by storm sewers is that generated on site. If the entire
tract were serviced by a single storm sewer system, then the contributing area would
be approximately equal to the total site area of 22 acres. For the Jefferson at Riverchase
property, the ratio of watershed areas between the receiving waters and the
contributing area thus becomes 66,560 acres to 22 acres, or about 3025:1. Clearly, this
violates usual hydrology practice such that coincident peaks is not a valid assumption
for the storm sewer design.
City of Coppell Applicable Ordinance
Storm sewer design is indirectly referenced in the City of Coppell Subdivision Ordinance
(1994), Appendix C - Section II. Subsection B, page 88, of the ordinance pertains to
Engineering Design for storm sewers and drainage. Therein, referral is made to the
City of Dallas Drainage Design Manual. The Drainage Design Manual (May 1993) by the
City of Dallas Department of Public Works addresses the starting hydraulic gradient
for storm sewers in Section 3.1.1, page 9. Item 1 of the section states:
"Starting hydraulic grade at an outfall into a creek or channel should be
the 100-year water surface unless an approved flood hydrograph is
available to provide a coincident flow evaluation for the system's peak."
The Fort Worth office of the U.S. Army Corps of Engineers (CoE) has been contacted
concerning this project. The CoE has recently updated hydrograph models available
for this segment of the Elm Fork of Trinity River. The computer models employ the
HEC-1 Flood Hydrograph Package computer program; regarded as a standard of
hydrologic engineering practice. The HEC-1 models are available for release and can
be applied to provide a coincident flow evaluation, as permitted by the City of Dallas
criteria quoted above.
In view of the drastic differences in watershed sizes as described in the previous
section, a water surface elevation for the Elm Fork to coincide temporally with the
THE NELSON CORPORATI~_.
Mr. Griffin, 5/23/94 Page 3 of 3
peaking of the 100-year storm sewer outflow from the Jefferson at Riverchase site
would in all likelihood be less than the 10-year water surface elevation for the Elm
Fork. Which is to say that a coincident flow evaluation, as permitted by ordinance, will
probably result in starting the storm sewer hydraulic grade line at an elevation lower
than the requested 10-year water surface elevation for the Elm Fork.
Establishment of Precedents by the City of Coppell
The City has encountered previous projects with very similar characteristics to the
Jefferson at Riverchase property. By virtue of the City's opinion and allowances made
for those projects, precedents have been established. The Eagle Point Subdivision and
the Fairways at Riverchase are both projects in the Riverchase area of the City, and both
of which drain into the Elm Fork of Trinity River. On each of these projects, the City
has permitted design of the related storm sewer systems by initiating the hydraulic
grade line computations at the 10-year water surface elevation for the Elm Fork.
Summary
The starting conditions for storm sewer design related to the Jefferson at Riverchase
project have been discussed herein. Three explanations have been provided to
illustrate that our original request is reasonable from the standpoint of customary
hydrologic engineering practice, and from the standpoint of procedures previously
applied by the City of Coppell. In view of this evidence offered, we once again
respectfully request that our original proposal for storm sewer design, as presented in
our May 17, 1994 letter, be honored by the City of Coppell. We trust that you will
provide us a written letter of agreement similar in form to our original submittal. Your
assistance and cooperation will be greatly appreciated.
Sincerely,
TI-IE NELSON CORPORATION
Mark D. Walter, E.T., P.E.
Chief Hydrologist
cc: Mr. Bobby Page (via fax)
Mr. Guy Brignon (via fax)