Redmond Rain #10 - Overlake South Mystery Cache
Hidden : 10/10/2012
Difficulty:
4.5 out of 5
Terrain:
2.5 out of 5

Size: Size: small (small)

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Geocache Description:

Redmond Rain can wash you away . . . if you are a fish! The Overlake Village neighborhood is over 300 acres of developed land, mostly pavement. When the rain lands on these hard surfaces, it rushes into pipes and ultimately dumps into Sears Creek as an overwhelming flood.  Be the Engineer, and save our salmon!


FTF: Congratulations to Samukai!

Warning!

The New Zealand Mudsnail is rapidly invading King County streams. Walking in streams will spread this scourge. If you do get your boots wet, follow decontamination procedures before stepping in a different stream. These invaders are in Kelsey Creek in Bellevue, High School Creek in Redmond, Thornton Creek in Seattle, May Creek in Renton, but have not made it to this stream, yet. (Don't be the one who brings them here.)



Redmond Rain


200 years ago, when it rained in this neighborhood, the water landed on trees in an enormous forest. What water managed to fall through all the tree branches soaked into the rich forest duff on the ground. The water would gradually seep out of the ground far away at the headwaters to a stream, cool and clean, and slow. Today, there are no trees to intercept that water. There is no rich topsoil to allow the water to soak into the ground. Instead, the water rushes off the pavement, into pipes, and quickly ends up in a stream. The fish would much prefer it if we would slow down that water and make the neighborhood act more like it did when there was a forest here.

Engineers try to use concrete boxes to mimic the natural hydrology that was once here. The Overlake South Detention vault is an enormous concrete vault that was completed in 2015, right here in the back parking lot of Sears, with the goal of making the parking lot and others like it behave more like a forest, at least in how it manages stormwater! This vault is part of a large strategy that includes multiple large vaults and distributed infiltration systems to fully retrofit the 322.7 acres of Redmond that drains into Sears Creek, in Bellevue.

Essentially, the vault has a large pipe (4 feet in diameter) going in and a small pipe (6 inches) going out. When it rains, the water rushes off the pavement and flows into the vault using the big pipe. The small pipe is too small to let the water flow out quickly, so the water backs up (is detained) within the detention vault. To find the cache, you get to play engineer and figure out how big a vault we need here to make the fish in Sears Creek think we replaced all that pavement with a forest.

We will use a simplified analysis to arrive at our answer, which will not be exactly the same answer that teams of engineers got from their much more accurate but complex math.

Using the Rational Method to Estimate Runoff

Why Estimate Runoff?

Stormwater runoff can be very damaging to stream systems. Uncontrolled runoff causes erosion of stream banks, disrupting the natural geomorphology of the stream, resulting in unchecked sediment transport, loss of places for fish spawning, loss of refuge areas for fish, loss of habitat for baby fish, and ultimately loss of fish.

There is no way to measure the amount of runoff that came from this basin when it was forested, but we can use several techniques to estimate how much water would have runoff of the ground in that forested condition, and compare it to the developed, impervious condition.

One simple way to estimate runoff is known as the "Rational Method". For the purposes of this problem, we will use the Rational Method to estimate the runoff in the forested, predeveloped condition and compare it to the runoff in the mostly paved, developed condition. (Note: This method will size a vault that will mitigate peak flows, but not manage the increased durations of flows that comes with development. That is why the City is building other vaults elsewhere.)

Objective: Calculate vault size by first estimating predeveloped runoff and developed runoff for the 100 year storm and then estimating a vault size that can mitigate for that peak flow rate.

Methodology: Use the Rational Method, as defined in the 2021 King County Surface Water Design Manual.

Assumptions:
- After each of your calculations, round the answer to two significant figures - Total Drainage Basin Area = 320 Acres
- We will divide the basin into 32 subbasins of equal area and assume their flows add up to the total basin flow
- Study Subbasin Area = 10 Acres (two significant figures)
- Predeveloped Condition is "dense forest"
- Developed Condition is 90% "pavement and roofs"
- Developed Condition is 10% "Lawns"
- Design Precipitation, 100 year 24 hour storm, P100 = 3.8 inches
- Design Storm Frequency = 100 year, 24 hour
- Predeveloped Time of Concentration, Tc = 1 hour, 39 minutes
- Developed Time of Concentration, Tc = 28 minutes

STEP 1: Use the Rational Method to solve for the 100 year peak flow for the predeveloped condition for the 10 acre subbasin.

1. Determine predeveloped runoff coefficient, Cpre. Round to the nearest hundredth.
2. Determine subbasin area, Asubbasin. (Use 10 acres).
3. Determine 100 year, 24 hour rainfall in inches from assumptions, P100.
4. From Table 3.2.1.B of King County Manual, get aR (2 significant figures) and bR(2 significant figures) for the design storm.
5. Determine time of concentration (minutes) from assumptions
6. Calculate the unit peak rainfall intensity factor, iR. Round to 2 significant digits.
7. Calculate the Peak Rainfall Intensity, IR. Round to 2 significant digits.
8. Calculate subbasin peak flow for the 10 acre subbasin in the 100 year storm, Qpre100. Round to 2 significant digits.
9. Multiply the subbasin peak flow by 32 to get the total predeveloped basin flow Qpre100total for the 320 acre watershed. (Don't forget, two significant figures!)

- BE = Qpre100total = predeveloped peak flow in cubic feet per second (CFS), rounded to the nearest whole number

STEP 2: Use the Rational Method to solve for the 100 year peak flow for the developed condition for the 10 acre subbasin.

1. Determine developed runoff coefficient, Cdev. Round to the nearest hundredth.
2. Determine subbasin area, Asubbasin. (Use 10 acres).
3. Determine 100 year, 24 hour rainfall in inches from assumptions, P100.
4. From Table 3.2.1.B of King County Manual, get aR (2 significant figures) and bR(2 significant figures) for the design storm.
5. Determine time of concentration (minutes) from assumptions
6. Calculate the unit peak rainfall intensity factor, iR. Round to 2 significant digits.
7. Calculate the Peak Rainfall Intensity, IR. Round to 2 significant digits.
8. Calculate subbasin peak flow for the 10 acre subbasin in the 100 year storm, Qdev100. Round to 2 significant digits.
9. Multiply the subbasin peak flow by 32 to get the total predeveloped basin flow Qdev100total for the 320 acre watershed.

- FGH = Qdev100total = the developed peak flow in CFS, rounded to the nearest whole number
(Quick check: B + E + F + G + H = 13)

As you look at your results, realize that a typical fire hydrant flows at only 1 to 2 CFS. Just think how many fires you could put out if you could just wait for the big rainstorm!

Estimating the Required Vault Volume

If the vault has "FGH" CFS of water flowing into it, but only "BE" CFS of water flowing out of it, it will fill up quickly. The good news is that the flows you solved for are the peak flows in your storm. The storm starts out with a much lower intensity, and gradually builds to the peak. The storm then fades until the rain ultimately stops. It can be very challenging to estimate the volume of the runoff from a storm. Typically computer models are used to evaluate how the rain falls from the sky, how it interacts with surfaces and runs down the streets and pipes. Engineers use these computer models, but sometimes use equations to create estimates to get them started in design.

For this analysis, we will use a simplified (but not particularly accurate) approach to estimate a volume that will allow the vault to capture the storm as the water rushes in off of the developed surfaces, but only releases water as if those surfaces were forested.

STEP 3: Estimate the vault volume.

To estimate the volume needed in the vault, you need the following:
- Developed Flow, Qdev100total = "FGH" CFS
- Predeveloped Flow, Qpre100total = "BE" CFS
1. Calculate the Peak Flow Difference = dQ = Qdev100total - Qpre100total = FGH - BE(CFS)
2. Calculate the number of seconds per day, S. Assume 24 hours per day. Round to 3 significant digits.
3. Calculate volume for the vault in cubic feet. V = dQ * S / 30. Round to 2 significant digits.
4. Convert volume from cubic feet to acre-feet. Round to 2 significant digits.
JK = V (Volume in Acre-Feet, rounded to the nearest whole number)

Estimating the Vault Footprint

STEP 4: Estimate the vault footprint.

Due to the shallow groundwater at this location, the vault can only be 10 feet deep on the inside. If the vault is "JK" Acre-feet in volume, and 10 feet deep, then it is "L" acres in area (L = JK / 10). (Round L to the nearest whole number.)

Locating the Vault

STEP 5: Site Visit

Now that you know just how big the vault must be, you can visit the site and determine if the vault will fit there. (No engineer worth his or her salt would design a vault without visiting the site first.) Visit each corner of the vault at the waypoint coordinates that are posted. At these reference points you will find grating panels and hatches. A hatch is a silver lid on top of the vault with hinges. A grating panel is piece of silver metal that forms a grid. You can see through a grating panel. Grating panels are heavy, so they are split into pieces small enough to be lifted by maintenance crews. You will visit each of the waypoints and count the number of grating panel pieces.

Count 1: SE Corner = 12
Count 2: SW Corner = ?
Count 3: NE Corner = 0 (closed by temporary construction fence)
Count 4: NW Corner = 12 (closed by temporary construction fence)

Add up the numbers of grating panels you counted at the corners (Count 1 + Count 2 + Count 3 + Count 4). That number is "MN".

(Quick check: J + K + L + M + N = 13)

Final Coordinates

To find the final coordinates, we just need to adjust the minutes of the posted coordinates.

Subtract from the Latitude: [ 9 * JK ] / 1000
Add to the Longitude: [ (F * 100) + ((B + L) * 10) + M + F + G ] / 1000

As you approach the hide, you will find out why the terrain rating is high. This is not recommended at night or during a storm. You don't need to get wet or enter any structures to make this find. If you are moved, there is lots of CITO opportunity. (I bring a garbage bag each time I come here, but there is a lot of work to do.) Don't go too close to the edge in your search. You are looking for a lock and lock, under a large, flat, mossy rock. See the spoiler photo.

Additional Hints (Decrypt)

[puzzle, more of a story problem] Vs lbh trg fghpx, nfx na ratvarre sbe uryc. Gur zngu jnf grfgrq bhg ba n zvqqyr fpubby fghqrag gnxvat nytroen. Ur fgehttyrq n ovg jvgu fbzr bs gur grezvabybtl ohg nsgre sbyybjvat qverpgvbaf pnzr gb gur pbeerpg nafjre. [significant figures] Jvxvcrqvn unf n avpr negvpyr. [hide] Frr gur fcbvyre cubgb.

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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