Ossipee Lake isn’t at its natural level and hasn’t been since 1919, when the dam at the head of the Ossipee River raised it something like eleven feet. Under the operating plan it’s held near 407.25 feet above sea level for summer and let down toward 404.5 for winter. In June 1998 it went to 413. Any drainage plan on the lake side of Ossipee starts with those three numbers, because where the water sits in August is not where it sits in a flood.
The lake takes in the Pine, the Beech, the Bearcamp and the Lovell, and it has exactly one way out — the Ossipee River — through channels and bays that behave like a funnel. The Ossipee Lake Alliance has been writing about it for years: the connections between the big lake and Broad, Leavitt and Berry bays are natural bottlenecks, so when a big storm comes through, water arrives far faster than it can leave. In the 1998 storm, inflow peaked at twenty-one times outflow. During Irene in 2011 the main lake stood at 409.25, two feet over summer level.
The zoning reflects this in a way people miss. On a waterfront lot in Ossipee the front setback is measured from the water, not the road — and on a dammed lake it’s measured from the normal operating level rather than a natural high-water mark. Your road side becomes your rear setback. The ordinance also asks that runoff be detained on site where practicable, with the fifty-year storm as the design period. That’s a higher bar than most homeowners expect for a house lot. It’s the right one here.
Inland the water behaves differently. The Pine River comes in from the south under Route 25 and carries a good share of the lake’s inflow, and plenty of lots back onto it or one of its brooks. On the sand, roof and driveway water will usually soak in close to where it lands if you let it. On the till up toward the Ossipee Mountains it won’t — and that’s where the wet cellars come from.
If you’re on Ossipee Lake or one of the bays, find out where your lot sits relative to 413 feet before you build anything near the water. That was the 1998 flood, and the lake still knows the way.
Over the aquifer, Ossipee’s preference is spelled out: runoff should be recharged on site by steering it toward vegetated areas to soak in at the surface, to the maximum extent possible. Dry wells come last, only where other methods aren’t feasible, and they have to be preceded by oil, grease and sediment traps. So inside the Water Resource Protection Districts we lean on swales, rain gardens and level spreaders before we bury anything. On sand those work very well. Cheaply, too.
Down by the lake, we design to the flood, not the summer pool. Anything draining toward the shore — a downspout outlet, a swale, a footing drain — needs to keep working, or at least fail harmlessly, with the lake a few feet higher than you’re used to seeing it. Outlets get stone aprons so they don’t carve a gully toward the water, and nothing gets aimed straight at the shoreline, where RSA 483-B already governs the first 250 feet.
On the heavier ground it’s the classic work. A curtain drain across the slope above the house, three to four feet down in washed stone and fabric, run out to daylight. Footing drains that actually go somewhere. Culverts under the drive sized for a real storm and the spring melt together. And the finish grade pitched away from the foundation, six inches in the first ten feet — which fixes more wet basements than any pipe we install.
Curtain and French drains in Ossipee run $2,000–$8,000 depending on length, depth and distance to daylight. Swales, regrading and surface recharge on sand often come in under that. Culverts are priced after we see what they carry, and anything inside the Water Resource Protection Districts that needs traps or a Special Use Permit drainage plan is quoted with that work included. Flat quote.
Under the state’s operating plan it’s held around 407.25 feet in summer and drawn down toward 404.5 for winter. Floods go well past that: the lake reached 413 in June 1998, and 409.25 during Irene in 2011. Shoreline drainage has to work across that whole range, not just at summer pool.
From the water. Ossipee’s ordinance makes the water side the front setback on waterfront lots, measured from the normal operating level on dammed water bodies like Ossipee Lake and from the natural high-water mark on undammed ones. The road side becomes the rear. That flips how most people sketch the lot.
Only as a last resort. Inside the Water Resource Protection Districts the town wants runoff spread toward vegetated areas to soak in at the surface first. Where a dry well is the only feasible option, it has to be preceded by oil, grease and sediment traps so contaminants don’t go straight into the groundwater.
Because the lake can only leave by the Ossipee River, and the channels between the main lake and the bays slow it down. In a big storm water arrives many times faster than it drains. Low lots near the shore and the bays feel it first. We grade and route water with the high-water mark in mind.
Different problem. Up on the till toward the Ossipee Mountains the soil won’t take water, so runoff from the slope above ends up against the foundation. A curtain drain uphill and a corrected grade usually solve it. Sand lots by the lake rarely have that particular trouble.
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