Is it true that when Hurricane Katrina hit that a county outside New Orleans blocked entry to New Orleanians?

They had tried to flee New Orleans but the roads into that county were blocked to them by local law enforcement. I believe it was a predominantly white county (or maybe it's called a parish there) and the majority of those trying to get out of New Orleans were black. Disgraceful if true. I remember seeing something about that years ago on tv news when it happened. Thanks.