Stream restoration has been noted as an affordable method to accomplish total maximum daily load (TMDL) reductions and comply with MS4 requirements for sediment and nutrients in the Chesapeake Bay Watershed. However, there are effective and not so effective ways to implement stream restoration techniques for TMDL reductions in sediment, total phosphorous, and total nitrogen. Although loosely organized in conformance with the recent Chesapeake Bay TMDL expert panel recommendations (Schueler and Stack, 2013), the following reduction strategies are broad enough to be considered for other crediting programs using stream restoration:
- Remove/Reduce Bank Erosion
- Hyporheic Zone Interactions
- Hydrologic Residence Time
- Carbon Contact Time
- Step Pool Stormwater Conveyance System
Remove/Reduce Bank Erosion
Reductions of bank erosion are often addressed by using stones to protect the banks. However, there are numerous ways to use a more natural approach and fully vegetate the banks. These options include bank slope reductions and using several bioengineering techniques. Considerations of bank key-ins and tie-in locations are also important to long term stability of restoration.
Hyporheic Zone Interactions
Reductions in nutrient loads can also be separated as those applicable to baseflow (Nitrogen) and those at storm flows (nitrogen, phosphorus and sediment). By ignoring the potential benefits of baseflow nitrogen reductions, you are missing nearly 20-30% of the annual nitrogen load. By including the baseflow and hyporheic zone connection, you will be able to capture this reduction potential.
Denitrification within the hyporheic zone is a very complex process that is difficult to accurately estimate and often beyond what is done on a single stream restoration project. As a result calculating reductions are often very simplified or are completely ignored. By adding in specific restoration features that induce better hyporheic interaction and by providing the proper plantings, you are more likely to provide the conditions with which most crediting programs will acknowledge for denitrification.
Hydrologic Residence Time
Hydrologic residence time is keenly important but often ignored in restoration. There are places where this may not be possible to do (e.g. highly urban streams, significant structures in floodplain), but should be considered even if there are only small scale opportunities such as oxbows.
Carbon Contact Time
Carbon contact time is a category that helps improve the results of all previously mentioned reduction methods. It is beneficial in hyporheic zone, hydrologic residence time and stream bank erosion reductions. Studies have shown that woody debris is a sink for attracting nitrogen. There are several techniques that have been developed that can add habitat value along with helping in your TMDL reductions.
Step Pool Stormwater Conveyance System
An entirely separate category is step pool stormwater conveyance systems, which are credited by the Bay TMDL expert panel. Their benefits for erosion reduction and groundwater level increase can be substantial, but they must be placed in appropriate locations.