You don't mention the size or type of the project area. That may have some bearing on the response. However, in general, from a water quantity standpoint, I can see no reason to detain stormwater unless it is a relatively large area where the discharge could adversely impact flooding or erosion. If you have a large drainage area discharging into a relatively small tidal stream, it could be a problem. If you are discharging into something like a bay, it probably isn't a problem.

From a water quality standpoint, your stormwater would discharge untreated into the receiving body of water. That's a problem, especially if your site is large and there is a large percentage of impervious area. You would need to treat for normal stormwater contaminants.

The question needs to be expanded.  Is the tidal body an inland lake, a stream, or a bay.  Also will the receiving
body be impacted by storm or hurricane surges.  What are
the effects of the tidal surges on your property.  

I agree with STA7240, if the receiving body is a bay, then
no detention is usually required. If it is a stream, then
usually detention is required.  But now-in-days, most
requlatory agencies are going to required a SWQ basin.

When I have a site that historically discharges into a large body ( usually a Bay or the Gulf of Mexico) I am not required to attenuate the increased discharge due to development , however I am required to treat water quality to a greater degree than an inland site. This is a State requirement however, most local municipalities have enacted detention requlations regardless of other permitting agencies.

Hi!
Tidal is Tidal!
Does the site actually outfall to the tidal waters or run through an area adjacent to the tidal waters.  This would determine if SWM is required.  Perhaps the requiring agency actually means WQM for your paticullars.  Any way there is no reason to provide SWM to tidal waters since only large body of waters and there adjacent contributary streams are tidal.  Don't confuse high water with tidal either.  All tidal areas will have a Mean High Tide elevation availble  from the ACOE since they are under there juridiction for navigable waters.  A rule of thumb is if the elevation is 10 or below, you may be tidal.

In Southwest Florida, one does not need attenuation of stormwater discharge, if the site discharges to a tidal system without flow restrictions or one with more than adequate capacity for the up to the 100 rainfall event.

However, one does need to either treat the 1st 1/2" of runoff or the runoff from the 1st inch of rainfall by percolation or wet detention (doubled detention storage)for all but the smallest sites.  In addition systems outfalling to Outstanding Florida Waters need 50% additional treatment volume.

Clifford H Laubstein
FL Certified PE #58662