Russ,
Very good question, and also great attention to detail of the photo. Actually you are correct, the drawdown pipe inlet should be at grade or slightly above grade. Actual survey and design of the structure had the pipe at its current location in order to facilitate maximum drawdown (seemingly underground!). Unfortunately I snapped a photo just prior to the excavation of a small silt basin in front of the drawdown. Soooo the corrected picture would show the pipe in its correct orientation with silt basin installed.
In many parts of the Country, sedimentation is a real concern. If the contributing drainage has cropland that is sensitive to erosion and runoff, you are likely to have significant sedimentation. In our situation the drainage area is 100% Native Range. Soil erosion from rain events is almost non existent..
Also we do flush drawdown pipes occasionaly, leaves and other debris can be periodically flushed. Also a great pond renovating tool, just pull the plug!
Hope this helps.
Ed
