Prioritizing Roadside Ditch Retrofitting to Improve Water Quality in a Mixed-use Watershed

Abstract Roadside ditches are one of the most common landscape features in an urbanizing landscape. Poorly maintained ditches exacerbate floods and degrade water quality. Roadside ditch retrofitting offers a unique opportunity to address both water quantity and quality issues related to stormwater. However, given the extensiveness of roadside ditches, it is challenging to determine where the retrofitting efforts should take place. The study proposed a three-step procedure to prioritize roadside ditches for retrofitting based on the hydrological sensitivity and connectiveness of a landscape and applied it to the Neshanic River watershed with mixed land uses in Central New Jersey. Specifically, we delineated the hydrologically sensitive areas (HSAs) in the watershed that have high propensity to runoff generation and accumulation, selected the ditches intersecting with HSAs for consideration of retrofitting and finally ranked the selected ditches for retrofitting with high, medium and low priority based on their physical conditions. There were 853 ditches in 40.3 km mapped in the watershed, 322 ditches in 29.7 km were intersected with HSAs. Among 322 ditches considered for retrofitting, only 125 ditches were ranked for retrofitting with high priority. The study also applied the propensity score matching method to compare the conditions of the ditches that did and didn’t intersect with HSAs and found the ditches intersecting with HSAs were more likely experiencing deterioration that those not intersecting with HSAs. The proposed procedure is very useful tool to scout and prioritize roadside ditches for maintenance and improvement when personnel, time and other resources are limited.