I am waiting for someone to answer you. I know the area is low, but I am not aware of any place below sea level, or of any levees.
Found this about elevation in Houston:
TOPOGRAPHY: Houston lies largely in the northern portion of the Gulf coastal plain, a 40- to 50-mile-wide swath along the Texas Gulf Coast. Typically, elevation rises approximately one foot per mile inland.
Northern and eastern portions of the area are largely forested; southern and western portions are predominantly prairie grassland; coastal areas are prairie and sand.
Surface water in the Houston region consists of lakes, rivers, and an extensive system of bayous and manmade canals that are part of the rainwater runoff management system. Some 25%-30% of Harris County lies within the 100-year flood plain. Elevation ranges (a.s.l.): Brazoria 0'-146', Chambers 0'-85', Fort Bend 12'-158', Galveston 0'-43', Harris 0'-310', Liberty 0'-269', Montgomery 43'-435', Waller 80'-357'.
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Note nearly 1/3 of Harris County is in the 100 year flood plain of Buffalo Bayou and other streams in and around the city. The southern part of the city has long been flood prone.
If the storm drops enough rain and there is enough storm surge to keep it backed up, it's quite possible to have some nasty street flooding. Went through several of them in Pearland...the guy accross the street from me had to change his rugs 3 times in the five years I was there...and that was just in a hole..we weren't in the flood plain.
I don't believe there are any parts below sea level. We have lots of bayous that flood. There's a flood simulation at this link: http://www.chron.com/content/chronicle/special/05/hurricane/index.html