Stormwater Drainage – Sunapee NH
Vulnerability of Road Crossings: Lake Sunapee Watershed, NH
Numerous studies report that intensified precipitation resulting from anthropogenic climate change will stress civil infrastructures. Communities may have a window of opportunity to prepare, but information to support adaptation programs is sparse. For a moderately-sized watershed, the present stormwater drainage system’s capacity for conveying expected peak flow, Qp, resulting from climate change and population growth, was assessed. For a set of climate models and emissions scenarios, a modified delta method was used to downscale the design storm precipitation value to the study site. Runoff rates, current and required culvert sizes, construction costs, and Low Impact Development methods were applied using standard engineering, hydrologic, and costing methods. An outreach, education, and stakeholder participation program was applied to promote the implementation of infrastructure adaptation. 12% of culverts are already undersized for current landuse and the recent climate. 35% of culverts are estimated to be undersized for the ”most likely” estimator of a mid-21st century pessimistic climate change and population growth scenario. At the +95% confidence limit for the design storm estimated under the A1fi emissions trajectory, with population growth, 70% of culverts are projected to be undersized. The watershed-wide cost of upgrading the culvert system for the “most likely” A1fi design storm is estimated to be 12% greater than constructing culverts to the historical TP-40 design storm. Funding adaptation via a 20-year, 2% municipal bond, the average property tax bill is estimated to increase by $0.05 per $1,000 of assessed value, resulting in an average annual property tax increase of $15.00 per household, based on the recent median home price. In the context of an ongoing trend of extreme and record storms regionally proximate to the study site, the Outreach program and robust estimates of required system capacities has motivated the community to develop and implement a program of long-term adaptation.
The study found that rates of undersized culverts, and adaptation cost, are insensitive to increases in precipitation and, along with other factors, provide financial incentive to incorporating a significant safety factor into future culvert design. A long-term program to upgrade the stormwater management system, utilizing Low-Impact Development strategies and managing uncertainty and costs, may maintain historically acceptable risk levels. To enable widespread adaptation, the federal government, and civil engineering and climatological professions should promulgate a single set of TP-40-like isoplubial maps, based on best-available climate model output. Multiple climate models and ensembles should continue to be maintained for research purposes. This study makes a significant contribution to establishing the manageability of uncertainty, in support of programs to adapt civil infrastructures.