The Designed Floodplain

Area of Concern: Existing Floodplain, Marcy Street Edge

Proposed Solution: A Designed Floodplain, creating controlled, overland relief points during severe storms

The final strategy addresses the “worst-case scenario.” The central lawn is underwater. The groundwater table is inflated, filling most of the low-lying basements. A historic king tide swells the Piscataqua River up and over the inlet banks of Prescott Park, submerging the stormwater outfall several feet under. Seawater surges up the city storm drain competing for space in the pipes and structures beneath Marcy Street. The constructed wetland is full and spills outward on to the parking lot and upland central lawn. Floodwaters are rising near the elevation of the Lawrence J. Yerdon Visitor Center. Storm clouds sweep in as it begins to pour.

In this worst-case scenario, Strawbery Banke Museum can not depend upon the city storm drains and outlet infrastructure. The existing outfall will be inundated every day, twice a day, unless the closed pipe infrastructure tying Washington Street to Prescott is adapted by the city of Portsmouth. Absent this change, Strawbery Banke will need to find alternative methods to store and release floodwater during severe events.

Design Intent

The design of the future parking lot and its relationship to Hancock Street, Marcy Street, and Lawrence J. Yerdon Visitor Center will play a key role in identifying and setting critical elevations. As stormwater is directed away from the Jones House, Carpenter’s Shop, Shapley-Drisco-Pridham House, and Visitor Center, the wet swale and wetland will provide lower elevations. The conceptual design also proposes to elevate the surrounding road network of Puddle Lane and Atkinson Street to create a containment area within the central lawn.

The diagram on the previous page depicts conceptual contours at six-inch intervals, setting our designed floodplain at the 100-year elevation of 9.0’ NAVD88. The Carpenter’s Shop currently sits at elevation 9.19,’ giving this building approximately two-inches of freeboard (degree of separation) as floodwaters attempt to escape. To the east, the elevation of Marcy Street and Prescott Park are fundamental components to maintain positive drainage off and away from the campus.

Two overflow spillways are proposed within the eastern parking lot to pass floodwaters through a defined channel, across the sidewalk, and into the Marcy Street corridor. The primary spillway is disguised with plank-style pavers to extend the boardwalk feel through the parking lot. The hardscape pavers offer a stable surface for floodwaters to flow across without eroding the ground beneath.

From here, changes will need to coordinated with the City of Portsmouth to allow floodwaters to bypass Marcy Street and enter Prescott Park. As the storm subsides and high tide passes, flooded areas will gradually be released by forces of evaporation, infiltration, city storm drain conveyance, and plant absorption.

PAVED INLET FLUME

SOURCE: HORSLEY WITTEN GROUP

RAIN GARDEN

SOURCE: HORSLEY WITTEN GROUP

OVERFLOW STRUCTURE (BEEHIVE GRATE IN FOREGROUND)

SOURCE: HORSLEY WITTEN GROUP

BROAD-BASED DIP

SOURCE: ALCIAN ARAUJO

BOX CULVERT

SOURCE: HORSLEY WITTEN GROUP

EMERGENCY SPILLWAY

SOURCE: HORSLEY WITTEN GROUP