Stream Restoration Performance and its Contribution to the Chesapeake Bay TMDL: Challenges Posed by Climate Change in Urban Areas

Abstract:

In large part driven by Total Maximum Daily Load (TMDL) mandates, the stream restoration community in the Chesapeake Bay region has begun implementing novel best-management practices (BMPs), such as regenerative stream/stormwater conveyance (RSC) structures and stream-wetland complexes (SWC), in the hope that these will improve water quality. However, the nutrient and sediment reduction efficiencies of these designs are virtually unknown, and the possibility of increasing riverine flow in the Chesapeake Bay watershed associated with climate change this century necessitates an evaluation novel BMP performance in urban areas to develop and utilize those that optimize reductions in nutrient and sediment fluxes. We compare and contrast pre- and post-construction loads (TN, TP and TSS) from upland RSCs and lowland SWCs constructed at the outflows of highly developed watersheds in the Coastal Plain physiographic province of Maryland. Large RSCs performed best in relation to expected nutrient and sediment reductions because of superior water retention capability. By contrast, the performance of SWCs can be much lower. For example, by the length of river reach restored, the most effective SWC attained about 80% of its N reduction TMDL goal, but only about 20% and 3% for TP and TSS, respectively; by watershed area, the % attainment of TMDL goals are much lower. Results indicate SWCs have the potential to curtail N loading from developed catchments, but additional water quality benefits are limited and there are a number of negative impacts, such as a significant increase in water temperature. Climate change scenarios generated by the Chesapeake Bay Program’s watershed model (Phase 5.2 HSPF) generally project an increase in the frequency of larger storm sizes and stormflow runoff (10 to 20% in more developed catchments) this century in many areas of the Chesapeake Bay watershed. We use this study to exemplify the sometimes unrealistic expectation the restoration community has that all novel stream restoration techniques are effective means by which we can attain TMDL goals. Given the large-scale implementation of BMPs currently underway to accommodate TMDL goals, we recommend building resilience into many types of BMPs to help attain and maintain loads at TMDL goal levels in a wetter climate.