Recreating baseflow channels, Howards Branch, Baltimore, Maryland


[Music playing] Faith: Joe, what’s a baseflow channel? Joe:The Howards Branch project was this U- shaped incised channel, which is very common in stormwater dominated systems. It’s a result of urbanization. A really common approach to stream restoration is natural channel design, which uses that full U-shaped channel and develops a bankfull channel inside of that. And in these systems where we have very little sediment supply from the watershed, a really very disturbed hydrologic regime that’s stormwater dominated, that approach doesn’t seem to be working very well. The idea of designing this big channel that for the most of the year has very little water in it – the only time it’s full of water is when we have a stormwater runoff event – is not as purposeful from extracting some of the goods and services that ecosystems deliver as this kind of a channel, which is a baseflow channel. The idea here is you design this shallow, broad system so that when you do have those stormwater events, instead of the water getting deep, which increases sheer stresses and the ability of that channel to erode the bottom of the stream and cause channel enlargement – instead of going that route, you have this parabolic weir that when you have any increase in stage instead of having an increase in depth, you build width. This weir that we’re looking at here is 40 feet wide, but over time it’s grown in, so only this active flow path is showing. So this is bankfull, where baseflow is the only water that fits in there. Any additional water is out on the adjacent floodplain or in the riparian zone. And that serves to reduce the erosive velocities, it really maximizes the surface area of the water with the natural system – the floodplain – which results in improved sediment trapping, nutrient removal, more habitat, basically all those ecosystem services that we’re looking for. Faith: So kind of maximizing the effects that you can get from the floodplain and the connection between the floodplain and the channel. Joe: Right. We’re trying to design a channel that provides material processing rather than material conveyance. This storm-water is a water resource and the sooner we start designing those projects with the utility of that water, the sooner we’re on the right path. And that goes to the idea of the sand seepage wetlands as well. If we have a pulse of stormwater coming in here, in some cases you don’t have a big floodplain that you can get the water out onto. So then you can sort of ecologically engineer a different approach, which is to get that pulse of water and store it on the side of the floodplain in a pool that’s rimmed with this layer of sand. Over time, that water has to get back into the stream and it has to go through that sand bed to do it and it delivers all the values that we recognize with a lot of other stormwater BMPs, whether they’re bioretention facilities, sand filters, etc. So it’s another way to think about ecological engineering in a riparian system. Faith: Taking advantage of what the vegetation has to offer you. Joe: Right. [Music playing]

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