Presenter/s: Steve Thompson
Symposium Session: 2020 - 10 Prioritizing restoration
Topics covered: beavers, floodplain, hydraulics, permits, riparian, stormwater, and urban
This talk presents the case for using stream restoration to achieve the same goals and objectives as conventional stormwater flow control BMPs (i.e. detention/infiltration/Low Impact Development) in certain locations in the watershed. This presentation begins by covering some of the background of stormwater flow control BMPs (flow control), the theory behind why we use them, and where they are most effective in the watershed.
Flow control measures are very beneficial in the upper watershed, where they can reduce peak flows on small tributary streams and help recharge the aquifer. However, these same measures are less effective lower in the watershed. They are a good preventative tool to reduce the risk of downstream channel incision, but offer only limited benefit for alluvial channels that have already scoured down to bedrock or a hard armoring. Once the damage is done the need for prevention is diminished.
Flow control requirements are governed by a one-size-fits-all policy, which strives to recover impaired rivers and streams by creating a more natural hydrologic flow regime. The limitation to flow control is that it only considers impacts from impervious surfaces, neglects all other direct human modifications to the stream channels, and assumes that channel restoration can be achieved through better stormwater infiltration.
Changes in flow regimes, such as increased peak flows resulting from urbanization, can have significant impacts, but clearing of in-stream wood and riparian vegetation, hardening of banks and loss of floodplain can have even more severe impacts. All of these alterations can lead to channel incision and can exaggerate changes in flow regime. Channel incision also leads to a loss of in-stream alluvium which in turn reduces hyporheic flow quantities which lowers water tables and worsens water quality. We must consider all of these factors if our goal is to restore natural stream functions.
This presentation offers examples of how to use stream restoration to meet the same goals and objectives as stormwater flow control requirements. For example, restoring floodplain connectivity and floodplain storage instead of building artificial ponds to store floodwater. We will discuss the regulatory framework that must be followed. We will also make a case for out-of-kind stream restoration in lieu of conventional flow control. In some locations correcting fish passage barriers, restoring riparian buffers and floodplains, and adding in-stream wood in strategic locations can offer much greater environmental benefits than building stormwater ponds or underground detention storage.