John,

Good point regarding the margin of safety in the TMDL development. One thing to consider is just how uncertain the flow-sediment relationship is actually. Sediment transport theory is very complex, and it is not uncommon for results to be off by an order of magnitude – this is a significant difference. But focusing specifically on the Accotink case, the two unimpaired reference watersheds used have much better draining soils (70% B soils) when compared to Accotink (70% D soils), so the inherent difference between Accotink and the unimpaired watersheds goes beyond just uncertainty, in my opinion.

Sabu,
Based upon fluvial geomorphic theory, the “channel-forming” discharge (often referred to as the bankfull or “effective” discharge) is that flow which – which looking over a long temporal horizine – carries enough erosive power yet occurs frequently enough to move the most sediment in the stream. Intuition may tell you that it is large, flooding (100-year) events that move the most sediment in a stream, but in most cases, it is a much higher-frequency event, such as the 1.5-year storm, that actually reflects the synergy between stream power and flow frequency. It should be noted that in an urban context, the channel forming flow frequency has been known to be as high as 1-year or even a 6-month event. Added to this flow dimension is the duration of flow – when flow in a channel exceeds the scouring velocity for a long period of time, the marginal amount of erosion which occurs in the channel increases over a situation where the duration of scouring is very short.

With all of this in mind, a reduction of the unit volume of discharge within a watershed for the 1-year event would indeed reduce the amount of erosion within the stream.