Riverview Estates-CS 931101'1o: ~//~tayor
From:
and City Council
Kenneth M. Griffin P.E., City Engineer
Date:
Discussion of the floodplain status of a 25 acre tract of land on the
south side of Sandy Lake Road owned by Mr. Johnny Thompson.
November 1, 1993
On October 29, 1993, a memo was processed to Interim City Manager, Frank Trando, concerning my
analysis of the floodplain status of Mi'. Johnny Thompson's 25 acres on Sandy Lake Road. It is my
understanding that a copy of the memo was processed through the FYI packets to City Council.
However, another copy has been included with this agenda item. In essence, my analysis is as follows:
1 In early 1980, based on the elevation of Mr. Thompson's property and based on the elevation of the
100-year water surface, Mr. Thompson's property was most likely not in a designated floodplain.
2. The floodplain maps are based on existing conditions and they are a point in time. As property
develops, the water surface historically rises unless offsetting improvements are provided in the
adjacent streams and creeks to offset the increase in the runoff due to rooftops and concrete.
3. The current floodplain elevation adjacent to Mr. Thompson's property is approximately 2 feet higher
than in early 1980. The increase in the water surface elevation now creates a situation where Mr.
Thompson's land is lower than the 100-year water surface. Because of this, Mr. Thompson's
property is now included within a designated fioodplain.
It is important to note that each time a map is updated properties are either included or excluded
from the floodplain. This happ~s because the map is an existing condition map based on a point
in time. For Mr. Thompson to develop his property, he would have to go through the process
which developers in Coppell use to reclaim property from the floodplain. Basically, it involves a
technical flood study approved by City staff and FEMA to show that the reclamation of the property
would not have a detrimental affect on adjacent properties. During the development phase of a
subdivision, we require that the finish floor of a structure be at least 2 feet above the existing
floodplain elevation or I foot above the ultimate floodplain elevation. The ultimate elevation is based
on the Albert Halff Flood Study of 1991. The requirement for an elevated structure helps offset any
increases in the water surface due to future upstream developments.
Mr. Thompson's property is also located on the Elm Fork of the Trinity River and in the very near
future staff will be bringing forth an ordinance to officially adopt the Corridor Development
Certificate concept generated by the North Central Texas Council of Governments. The concept
of the Corridor Development Certificate for development activities was presented to and endorsed
by City Council on February 23, 1993. In essence, the requirement for development along the Elm
Fork of the Trinity River will be such that some amount of valley storage will be required on any
additional property developed along this corridor. This could affect the development of Mr.
Thompson's 25 a~res.
It is staff's opinion that all pertinent information has been reviewed and evaluated concerning this
tract of land. I have reviewed this information with an open mind to ascertain if some agreements
have fallen through the cracks on the floodplain status of Mr. Thompson's property. However, the
bottom line is that while the property may have been out of the floodplain in 1980 it is now in the
floodplain based on the current water surface elevations. Staff will be available to answer any
questions concerning this issue.
APPROXIMATE LOCATION OF A 25 ACRE TRACT
LOCATED SOUTH OF SANDY LAKE ROAD
UN£ ROAD
COPPELL CI