Stream restoration for mitigation purposes has grown rapidly since the 1980s. As the science advances, some organizations (Chesapeake Bay Program, North Carolina Department of Environmental Quality) have approved or are considering providing nutrient credits for stream restoration projects. Nutrient treatment on floodplains during overbank events is one of the least understood processes that have been considered as part of the Chesapeake Bay Program’s Stream Restoration Nutrient Crediting program. This study analyzed ten years of streamflow and water quality data from five stations in the Piedmont of North Carolina to evaluate proposed procedures for estimating nitrogen removal on the floodplain during overbank flow events. The volume of floodplain flow, the volume of floodplain flow potentially treated, and the nitrogen load retained on the floodplain were calculated for each overbank event, and a sensitivity analysis was completed. On average, 9% to 15% of the total annual streamflow volume accessed the floodplain. The percentage of the average annual volume of streamflow potentially treated ranged from 1.0% to 5.1%. Annually, this equates to 0.2% to 1.0% of the total N load retained/removed on the floodplain following restoration. The relatively low nitrogen retention/removal rates were due to a majority of floodplain flow occurring during a few large events each year that exceeded the treatment capacity of the floodplain. On an annual basis, 90% of total floodplain flow occurred during half of all overbank events and 50% of total floodplain flow occurred during two to three events each year. Findings suggest that evaluating only overbank events may lead to undervaluing stream restoration because treatment is limited by hydrologic controls that restrict floodplain retention time. Treatment is further governed by floodplain and channel size.

中文翻译：

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城市河道修复工程中漫滩溢流事件期间洪水流量频率分析估计潜在的洪泛区氮处理

自 1980 年代以来，以缓解为目的的河流恢复发展迅速。随着科学的进步，一些组织（切萨皮克湾计划，北卡罗来纳州环境质量部）已经批准或正在考虑为河流恢复项目提供营养学分。漫滩事件期间对洪泛区的营养处理是人们了解最少的过程之一，它被认为是切萨皮克湾计划的河流恢复营养计入计划的一部分。本研究分析了来自北卡罗来纳州皮埃蒙特的五个站点的十年流量和水质数据，以评估在漫滩溢流事件期间估计洪泛区脱氮量的拟议程序。洪泛区流量，可能处理的洪泛区流量，并针对每个漫滩事件计算了保留在漫滩上的氮负荷，并完成了敏感性分析。平均而言，年总流量的 9% 至 15% 进入了洪泛区。可能处理的平均年流量百分比介于 1.0% 至 5.1% 之间。每年，这相当于恢复后洪泛区保留/去除的总氮负荷的 0.2% 至 1.0%。相对较低的氮保留/去除率是由于每年发生在一些大型事件中的大部分洪泛区流量超过了洪泛区的处理能力。每年，90% 的洪泛区总流量发生在一半的漫滩事件中，50% 的洪泛区总流量发生在每年两到三个事件中。研究结果表明，仅评估河堤事件可能会导致低估河流恢复，因为处理受到限制洪泛区保留时间的水文控制的限制。处理进一步受洪泛区和河道大小的控制。