HomeMy WebLinkAbout; Agua Hedionda Lagoon Tidal Effects on Flood Plain; Agua Hedionda Lagoon Tidal Effects on Flood Plain Study; 1998-03-13db Ud:£e:p Hoiuara M. unang i D i a / /oo atou
Howard H. Chang Consultants
Hydraulic, Hydrologic and Sedimentation Engineering
P.O. Box 9492
Rancho Santa Fe, CA 92067
(619)756-9050, 594-6380, FAX: (619)756-9460
March 13,1998
Ms. Sherri Howard
City of Carlsbad
Carlsbad, California
Dear Sherri:
Subject: Agua Hedionda Lagoon
Pursuant to your request, I have prepared this letter to address the tidal effects on the
floodplain study for Agua Hedionda Lagoon. I have recently completed a floodplain study for Kelly
Ranch. The study covers a portion of Agua Hedionda Lagoon.
The lagoon water surface is under the influence of the tidal variation in the ocean. However,
the computed 100-yr flood level for the lagoon is above the tidal influence as explained below.
Under the current FEMA guidelines, the starting water surface for floodplain computation at a river
mouth is the mean higher high water (MHHW) which has the elevation of about 2.8 feet NGVD. The
MHHW is exceeded by the extraordinary Spring tide which can be as high as 4.3 feet above the
mean sea level in a normal year. The mean sea level in an El Nino Year may be one foot above the
long term mean sea level. When the Spring high tide occurs in an El Nino year, the maximum tidal
level can be as high as 5.3 feet, NGVD.
The 100-yr flood level in Agua Hedionda Lagoon was determined to be 8.2 feet, NGVD.
Since this flood level is higher than the worst-case scenario for the tidal elevation, this flood level is
not under the influence of the tide.
Please let me know if you have any questions regarding this matter.
Sincerely yoi
Howard H. Chang
Ph.D, P.E.