Drainage calculations

need to see the survey to figure out what's going on.

We would also need a complete copy of local regulations and an understanding of local government interactions and the nuance that humans bring to the application of these regulations within the specific community. ?ÿYes, I am sober.

@dave-karoly

Is this enough information? It's the clearest picture of the gradient lines.

It tells the amount of water accumulated on your property from maximum rainfall.

The drainage numbers of your property and the surrounding area combine to compute the size of culverts and the size of underground drainage facilities and possible future flood levels of an area that all this water empties into.

In places like Houston and other metro areas, the numbers for concrete, asphalt, and other manmade surfaces like rock and slate, the numbers account for water draining from your property instead of soaking into natural ground and is used to calculate a tax you must pay.

0.02

@holdthetomatoes
"but if anything, it slopes towards lot 2."
It looks to me like the back of lot 2 would drain onto lot 1.

Even though the lowest point is on the northeast corner of lot 2?

My biggest concern is that we need to put drywells in, so we're literally sinking money into the ground. It seems extreme that we (lot 1) have double the drainage requirement of lot 2.

@a-harris
We live on Long Island, NY. We're required to put drywells in to capture rain water. So we're literally sinking money into the ground. I just want to make sure the numbers are right (or at least reasonable) before we go ahead with the installation.

Just an educated guess...the clearing on Lot 1 looks bigger than the clearing on Lot 2. The design engineer should be able to explain it to you.

Looking at the calculations, the roof drainage is about the same, but Lot 2 is about an acre while Lot 1 is about an acre and three-quarters.

Drywells certainly benefit a property that would most likely erode from too much water rushing downhill and by trapping much of that it is able to stay in the ground longer and allow grass and other plants to take root and decorate and look much better than washouts.
In some places, the houses have roofs of metal, and the gutter system is run into a series of holding tanks that filter any materials and is used around the household from washing clothes and baths and cooking and drinking because they are not able to have wells.

Lot 1 has more paved, roofed, and cleared area.?ÿ You're not allowed to increase runoff.?ÿ so, they calculate the things that will increase it from a natural forested state.?ÿ Q=CIA.?ÿ?ÿ i=a number derived from rainfall intensity in that area, the 2.5?ÿ?ÿ A=areas?ÿ?ÿ C=an empirical number derived from the surface depending on how impervious it is (a roof, paved and/or crush stone is 1, what will be lawn looks like .2).?ÿ Slopes are taken into consideration in the C value if I recall.?ÿ So, a lawn area on a slope might be .8 if it's steep.?ÿ On long island both surveyors and engineers prepare these, then a municipal engineer has to sign off on it.?ÿ Wait for the municipal engineers approval and it should be correct.

I assume this is in the Town of Islip.

see page 61 of the development standards:

https://islipny.gov/community-and-services/documents/planning-development/engineering/303-town-of-islip-subdivision-and-land-development-regulations/file

The calculation assumes a 2" rainfall, that's why it is area times 2/12 times a C value (imperviousness of the ground) equals cubic feet of storm water you are required to recharge into the ground. This is the runoff you aren't allowed to send downstream.

The linked manual has a table of C values which agree with the calculations given.

Note this is not the same as design flow for designing a storm runoff system which is given in cubic feet per second (CFS). A 12" pipe at half a percent slope will carry about 3.5cfs.

For a more general explanation see Chapter 47 of the town code:

https://islipny.gov/government/town-code

The areas are not the same as total Lot area, they are the area actually being developed.

In other words, if you build a house, you have to keep 100% of the volume of rain water which is 2ƒ? thick over the roof area and any hardscape such as concrete driveway or asphalt.

any landscaped area you have to keep 20% of the volume of 2ƒ? over that area because it is assumed 80% will soak in.

the areas for Lot 1 look larger on the map but I didnƒ??t try to measure them.

@dave-karoly
Thanks so much for getting back to me, Dave. I understand the need for the drywells, and the drainage percentages. The question I have really is that each lot is 1.5 acres (65317 sqft). Should the numbers be that much different for each lot if they are both the same size? It seems like we (lot 1) have to take care of far more than half of the drainage.

Again, I really appreciate all of your help! Thanks so much!

The total size of each lot is irrelevant.?ÿ Only the areas where something will result that is different from undisturbed portions of each lot matters in this case.?ÿ Thus, the area of impervious surfaces is relevant.?ÿ Other disturbed areas where runoff will differ from natural ground conditions are also relevant.?ÿ That is what is being shown on the data provided.

Look at it this way.?ÿ The lots in their natural state will cause a runoff of a certain quantity to occur across each.?ÿ Add a solid concrete pad 50 feet by 50 feet on each lot.?ÿ An additional amount of runoff will occur on each lot in an amount equal to the difference in permeability of the area of the concrete pad.?ÿ As they are of identical size in this simulated case, they will produce an equal amount of added runoff.?ÿ If one pad is only 25 feet by 50 feet it will produce only half as much additional runoff to be addressed.?ÿ That is why the calculations are based on the two changes in relative permeability of areas within a specific lot.?ÿ Solid surfaces will not take in moisture.?ÿ Surfaces that are less permeable than natural conditions will add some runoff but less than solid surfaces.?ÿ The undisturbed portions of each lot will continue to produce the same amount of runoff so are not of concern in these calculations.

Lot 1 has more roof but both lots have 1 dry well due to rounding up.

Lot 1 has has more than 2 times as much pavement. Don't have a plan to verify that. So that increases required dry wells.

Lot 1 has somewhat less than 2 times land clearing which looks reasonably accurate to my eye on the drawing posted here. That increased the required dry wells.

In short, lot 2 has a smaller house, less paved area and less cleared area so fewer wells are required.

Reduce your land clearing, reduced your paved area in order to reduce your required dry wells.

A fair amount of lot 2 flows into lot 1 judging by those contours, flowing down a gully towards the boundary corner. The figures presented in the calculation are indicative of the increased catchment area contributed by the rear of lot 2.

If there was a swale drain running parallel to the boundary on your neighbours side you could divert the water away and reduce your catchment area, although I don't know what they'll say, the added cost to do this may outweigh whatever savings you can make on a drywell.

@holdthetomatoes
Got thinking and seemed like really large numbers. I mean that's some serious water for a small site thousands of cubic feet per second? Units are labeled incorrectly. You have to use acres instead of sq. feet, or convert at the end if you use sq. feet, in order to get CFS.

@holdthetomatoes
It is not calculating CFS. It is a calculation of cubic feet of water assuming 2" of rainfall, the town engineer implemented the increased flow calculation this way. It uses the C value from the rational method but that's all. Calculation of storm flow for designing a storm drainage system is a separate thing; the town uses the rational method for that.