Moultonborough holds development to a 50-year storm. The town’s Storm Water Management Ordinance says post-development peak runoff rate and volume can’t exceed what the ground put out before anyone touched it, measured against a 50-year event — a stiffer bar than plenty of towns around the lake use. Lake Kanasatka is the reason that language has teeth. A bloom there ran 121 days in the fall of 2023, the fourth straight year the lake had one.
Kanasatka is worth understanding even if you live nowhere near it, because it is the clearest lesson anybody in this town has had about where roof water and driveway water end up. The state and the watershed association traced the blooms to stormwater runoff carrying phosphorus off the watershed, plus internal loading out of the sediment once the lake had tipped. The 2022 watershed-based plan named the sources plainly: stormwater runoff, shoreline erosion, fertilizer, and septic systems that had stopped working properly. None of that is exotic. It is ordinary residential ground doing ordinary things at scale.
What it cost to fix is the part that sticks. The association ran an alum treatment in 2024 — aluminum salts that bind phosphorus and drop it to the bottom — and the bill came in over $600,000, with half a million of it a state grant out of the Cyanobacteria Mitigation Loan and Grant Fund and the rest raised locally. It worked. The lake has improved every year since and ran largely clear this past summer. But nobody in the watershed thinks that is a thing you want to do twice, which is why the residents also went after their own road ditches, culverts and drives. Keeping the phosphorus out of the water in the first place is a fraction of the cost of pulling it back out.
The ordinance itself catches subdivisions platting new roads, commercial work, and multi-family development that disturbs 20,000 square feet or more, and it wants a Storm Water Management Plan stamped by a New Hampshire licensed professional engineer, showing pre- and post-development conditions side by side. A single house on an existing lot usually won’t trip it. That’s the permitting answer, not the physical one. The water off your roof and your gravel behaves exactly the same whether or not a plan was required, and the lake is downhill of both.
On the wetland side, Article 9 sets a fifty-foot setback from jurisdictional wetlands and keeps the twenty-five feet immediately adjacent as an undisturbed, naturally vegetated buffer. Manmade ditches, swales, detention basins, and stormwater devices are exempted — which is a useful thing to know when you’re designing an outlet and someone insists the low wet spot behind the garage is untouchable.
Deal with your road ditch and your driveway before you deal with your basement. On a Moultonborough lot that is usually where the water is coming from, and it is the same fix that keeps phosphorus out of whichever lake you are uphill of.
We come look at it wet. A dry August walk over a Moultonborough lot tells you almost nothing; a walk during a March thaw, or the morning after two inches of rain, tells you everything. We want to see where the water comes onto the lot — nine times out of ten it’s the road ditch or the neighbor uphill, not the sky — and where it’s already trying to leave. The fix belongs at the top of that path. Working at the bottom is how people end up with a very expensive drain that is always full.
Curtain drains go in three to four feet deep, perforated pipe in washed stone, wrapped, run across the fall line to daylight above the house. On ground that actually takes water we’d rather infiltrate than daylight, and we’ll prove it with a hole before we design it rather than assuming. Roof water gets carried away from the building and put into the ground clean, separate from anything that has run across a driveway. Drive and yard runoff gets slowed and spread — a broad shallow swale with vegetation in it does more for phosphorus than a pipe ever will, and it costs less.
Culverts get sized for the melt and the heavy summer cell, not for the average day, and they get a stone apron at the outlet so they stop scouring a channel into whatever’s downhill. Regrading is the cheapest tool in the box and the most overlooked: a startling number of wet Moultonborough basements are wet because the yard was pitched toward the house decades ago and no one has touched it since. And whatever we build, we keep the discharge out of the twenty-five-foot vegetated strip along a wetland and off the road right-of-way, which the driveway regulations separately forbid.
Curtain and French drains in Moultonborough run $2,000–$8,000 depending on length, depth and how far it is to daylight; regrading, swales and repiping downspouts usually come in under that. Infiltration work is quoted only after a test hole proves the ground will take it, and culvert replacements after we’ve seen where the water actually goes.
Probably not as a permit matter. The ordinance reaches subdivisions platting new roads, commercial work, and multi-family development disturbing 20,000 square feet or more, and those need an engineer-stamped plan. A single house on an existing lot generally falls outside it. The water still behaves the same way, which is the part worth designing for.
Phosphorus. The watershed plan and the state pointed at stormwater runoff off the watershed, shoreline erosion, fertilizer, and failing septic systems, compounded by internal loading from the lake sediment once it had tipped over. The 2024 alum treatment addressed the sediment; the ditch and driveway work addresses what keeps arriving.
Only where the soil will take water, and only for clean roof runoff. On sandy ground near the Bay a dry well works well. On tight till or shallow syenite up the Ossipee side it simply fills and stays full. We dig a test hole and watch it drain before we will design around infiltration.
Article 9 sets a fifty-foot setback from jurisdictional wetlands with the inner twenty-five feet kept undisturbed and naturally vegetated. Manmade ditches, swales and stormwater structures are exempt from the article, but a new outlet cutting through that vegetated strip is not something to improvise. Ask first.
Because that is precisely the habit that cost the Kanasatka watershed over $600,000 to undo. A pipe delivers the phosphorus and the sediment straight to the water at speed. Spreading and infiltrating on your own ground is cheaper to build, cheaper to maintain, and it is the reason a lake stays swimmable.
Free site walks, test pits before quotes, flat pricing, and a crew that knows Lakes Region ground.
Tell us where the property is and what you’re trying to do. We’ll walk the site, talk it through, and give you a straight, flat quote.
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