Stormwater Management

WHAT IS STORMWATER

Stormwater is a valuable resource as it replenishes our groundwater and surface waters and sustains our drinking water, recreational and aquatic resources. It is also essential for our natural environment, farming and landscaped areas.

However, when not properly managed, it can contribute to increased flooding and poor water quality conditions. When rainwater falls on rooftops, pavement and other impervious surfaces, the rainwater is restricted from soaking into the ground and instead runs across the land quickly. This rapid flow and excess volume can overwhelm storm drains and streams, resulting in more frequent and increased flooding. Also, as runoff flows across the land, it picks up pollutants along the way and discharges them into adjacent water bodies.

According to water quality data collected and published by the NH Department of Environmental Services (NHDES), more than 80% of the water quality impairments in our lakes, rivers and streams are due at least in part to excess stormwater runoff.

Impervious cover

What happens when land becomes more impervious?

Increased risk of flooding;
More contaminants and nutrient wash into our waterways;
Stormwater runoff has contributed to declining water quality in some of our most highly-valued water resources including Willand Pond and the Bellamy, Cochecho, and Isinglass rivers;
These rivers all drain into the Great Bay Estuary, which is not only a major recreational asset for boating and anglers but supports a significant regional economic and ecological resource for commercial fisheries and aquaculture.

Increasing Regulations

Like other New Hampshire Communities, the City is faced with having to conduct more stormwater management activities to comply with two stormwater-related permits issued by the U.S Environmental Protection Agency (EPA).

Municipal Separate Storm Sewer System (MS4) Permit - initially released in 2003 and renewed in 2017 requires the following:

A City Stormwater Management Plan and Operations and Maintenance Plan;
More frequent catch basin cleaning and street sweeping;
Inspections and sampling of all 450 outfalls;
Additional stormwater treatment measures and long-term inspection and maintenance of these measures;
Tracking inspection and maintenance of stormwater BMPs on private property;
Annual Reporting of the stormwater measures completed;

2020 Great Bay Total Nitrogen General Permit:

Contribute shared funding with other municipalities to support additional Great Bay water quality monitoring;
Additional implementation of structural and non-structural, control measures to reduce the City's nitrogen loading;
Track and report nitrogen load increases and reductions on City and private property Annual compliance costs for this Permit are estimated to be ~ $400,000.

Stormwater infrastructure

The City's storm drain infrastructure largely goes unnoticed, except perhaps during large rain events. And, yet, the City is responsible for operating and maintaining approximately 3,200 catch basins, over 100 drainage manholes, 450 stormwater outfalls, 140 culverts, 65 miles of drain pipe, and over 100 miles of swales and ditches. Some of this infrastructure is now well over 100 years old. Also, the City sweeps over 270 miles of streets each year and inspects and maintains over 60 stormwater treatment measures.

The City's annual operating stormwater budget has increased significantly in recent years largely due to increasing regulatory requirements. The budget is paid for by property taxes through the general fund. This does not include the capital costs needed for upgrades.

Flood Vulnerabilities

The flood events of May 2006 and April 2007 revealed how vulnerable various properties and City assets are to flood damage during major rainfall events. The City has identified over $5.0 million in flood resilience projects to minimize and prevent similar damage ahead of the next major flood event; however, these upgrades have been largely deferred due to competing priorities funded by the General Fund.

The City's 2018 Multi-Hazard Mitigation Plan estimated that over $200 Million in assessed property value are located in high flood hazard areas.