Eagle Point-CS 920805 NATHAN D. MAIER
CONSULTING ENGINEERS, INC.
August 5, 1992
Mr. Ken Griffin, P.E.
City of Coppell
P.O. Box 478
Coppell, Texas 75019
Riverchase; Storm Sewer Tailwater Elevation
NDM Job No. 89.10-188.F
Dear Mr. Griffin,
The purpose of this letter report is to document the analysis and results used to determine tailwater elevation
for design of storm sewers that discharge to the Swale system within the Riverchase development.
INTRODUCTION
The site drainage is impacted by the floodplain of the Elm Fork of the Trinity River and the interior drainage.
The approach is to identify a maximum tailwater elevation for storm sewer design based on either the Elm
Fork water surface elevation or the water surface elevation of the interior sumps.
i~.I.M FORK WATER SURFACE ELEVATION
An evaluation of the hydrographs and peaks on the Elm Fork was made to determine a controlling tailwater
elevation. Reco _mmended procedures allow that when a proposed storm sewer e.nters .an o~.pen channel, the
hydraulic gradient of the. proposed system should start at the 10-year water surrace elevauon of the opea
channel when the drainage area of the receiving channel to the development area is 15 or greater.
In this case, the contributing drainage area of the Elm Fork at Riverchase and below Lewisville Dam is 104.00
square miles versus a drainage area of approximately .85 square miles for Riverchase. The resulting ratio of
122 indicates it is reasonable to use the 10-year water surface elevation of the Elm Fork for the tailwater
elevations.
In addition, we compared the 100-year release from Lewisville Dam and the 100-year discharge on the Elm
Fork at the time of the 100-year peak from the Riverchase development. Attachment 1 contains a list of
hydrologic data for the Elm Fork model from'the Corps of Engineers (COE). The results for the 10-year and
100-year peak discharge from the NUDALLAS program are included along with the hydrographs for the Elm
Fork at the Carrollton gage. The 100-year peak occurs at time 20 hours and 45 minutes. This information was
obtained from the COE's hydrologic study of the Elm Fork.
The NUDALLAS program was applied to estimate the 100-year hydrograph and time of peak for the
Riverchase Development. One basin was used for the calculation with the data and results in Attachraent 2.
The peak for Riverchase occurred at 16 hours and 15 minutes into the storm. This is earlier than the pe~ak on
the main Elm Fork (20 hours and 45 minutes). The discharge of the 100-year Elm Fork hydrograph at the
peak time of the Riverchase hydrograph is 17,618 cfs from the results in Attachment 1.
The maximum average daily discharges for the high sustained flows of the 1989 and 1990 water year at the
USGS Carrollton Gage was also checked. The USGS data is included in Attachment 3.
Three NorthPark/8800 N. Central Expwy./Suite 300/Dali&s, Texas 75231/(214) 739-4741
Mr. Ken Griffin, P.E.
August 5, 1992
Page 2
The table below summarizes the discharges considered.
ELM FORK OF THE TRINITY RIVER
AT CARROLL,TON GAGE
CONDITION DISCHARGE (cfa)
100-Year Spillway Release from
Lewisville Dam 21,000
100-Year Future Elm Fork Discharge at
Time 16:15 (time of Riverchase Peak) 17,618
10-Year Future Elm Fork Peak Discharge 31,100
Max. Avg. Daily Discharge, 1989 and 1990 25,300
Thc maximum discharge on the Elm Fork of those considered for storm sewer design is 31,100 cfs, the 10-year
peak on the Elm Fork.
The COE hydraulics model from the Trinity River Reconnaissance Study was used to obtain a rating curve
and water surface elevations along the Riverchase property for this discharge. The HEC-2 data and results
from the COE analysis are included in Attachment 4.
The resulting 10-year water surface elevations are shown on the summary table. The cross section locations
are in the attached figure.
INTERIOR DRAINAGE
The 100-year water surface elevation of the interior drainage system was defined on design Sheets 2/15 and
4/15 for the Riverchase Development dated 6/85. The design sheets show rating curves for the proposed
sumps. The 100-year discharge was identified and a corresponding sump elevation can be determined.
Thc 100-year water surface elevation for the interior sumps is listed adjacent to the Elm Fork cross section
reference in the summary table. The 100-year design sump elevation is below the 10-year floodplain elevation
of thc Elm Fork.
HYDRAULIC GRADE LINE
To determine the hydraulic grade line of the interior storm sewer system that discharges to the swales for
Riverchase, the 10-year peak on the Elm Fork is used. The summary table lists recommended tailwater
elevations based on the 10-year Elm Fork water surface elevation for future discharges.
Mt. Ken Griffin, P.E.
August 5, 1992
Page 3
RIVERCHASE DEVELOPMENT
SUMMARY TABLE ELM FORK TRINITY RIVER
AND INTERIOR SUMPS
ELM FORK * 10-YR 100-YEAR
CROSS SECTION ELM FORK INTERIOR DRAINAOE
ELEVATION
ELEVATION SUMP
890+50 439.9 -
900 + O0 440.6 440_5 Sump # 11
908+80 441.6 -
920+90 442.2 439.8 Sump #13
923+00 442.3 -
936+20 443.1 -
* Recommended Tailwater Elevation for Storm Sewer Discharge to the Riverchase Swale System.
Do not hesitate to contact me if you would like to discuss this in more detail.
Sincerely,
NATHAN D. MAIER
CO NSULTINC,~NOINEERS, INC.
Mik, Daniel, P.£., R.~.L.$.
CMD/dmm