Saturday, June 29, 2013

Will the Alki 11 Rowhouses Cause Flooding?

If you've been following these posts for a while, you are familiar with the math: for every inch of rainfall, nearly 5500 gallons of stormwater will come off the roofs and paved surfaces of the Alki 11 rowhouses and be piped into the combined sewer system. You are also familiar with how this volume of stormwater can contribute to sewage overflows into Puget Sound

Now let's look at how this project can cause flooding. The drawing below shows, in general terms, how stormwater drainage will be handled. Note that the garages will be partially underground (the entrance/exit from the parking area will be on 55th Ave SW). The only way stormwater can get out of the parking area is through the drain in the center of the autocourt. 

To get an idea of what 5500 gallons of stormwater would look like, consider this: a standard bathtub holds 50 gallons of water. So one inch of rainfall would produce the equivalent of 110 bathtubs full of stormwater. We don't often get an inch of rain in a day, but a big storm can easily drop half-an-inch on us, which would mean 55 bathtubs full. Add that to the 80 gallons of waste water per person per day (2,640 gallons or another 53 bathtubs full) that will be generated by this development and that's a lot of volume going down the drain in a single day!

With that in mind, take a look at what the the engineer who submitted the storm water drainage report for this project has to say (to read the entire drainage report, follow these directions): 
"...stormwater from roof, footings, and pavement areas will be collected in an onsite storm drainage system and discharged to the 8-inch combined sewer pipe located in Wickstrom Place SW. 
Due to the elevation of the garages, driveway, and autocourt, the discharge pipe to the combined sewer line needed to be designed at 1%. A Hold Harmless Agreement will be recorded since this pipe is under 2% as required by the Side Sewer Director's Rule SPU DR 2011-004/DPD DR 2011-04. Likewise, we were unable to design to storm outfall pipe at 1' above the crown elevation of the sewer main at the property line, so we are providing a backflow valve for this pipe as required by the Director's Rule cited above." 
I asked an attorney and a structural engineer to explain the Hold Harmless Agreement in this situation. Both said that since the sewer connection is out of compliance with city requirements, the developer is agreeing to assume financial responsibility if a problem occurs. Ominous, considering that the developer is doing business as a Limited Liability Corporation (LLC), and that buildings have a typical design life of something like 75 years. Will his company assume responsibility for damages over the life of the project? Seems doubtful. 

Developers often form separate Limited Liability Corporations for each of their projects. When a project is finished and the last unit is sold, they dissolve the corporation. When the LLC disappears, who is responsible then? The owners of the rowhouses? Or will it come back on the city, despite the hold harmless agreement, because officials were negligent for allowing this sub-standard sewer connection to be installed? These are questions that need to be answered.  

So what are the implications of this improper sewer connection? For one, drainage will be slow under the best of conditions because the stormwater drain will be almost level with the sewer line. Any amount of sediment that accumulates in the line from roof and autocourt runoff will slow drainage even more. The backflow valve will prevent sewage and stormwater from flowing back into the drain line, but if there is a big storm that sends a lot of volume into the system all at once, it's easy to see how that 8-inch sewer pipe would be overwhelmed and stormwater would back up in the garages waiting to drain. Where else can it go? 

Secondly, the combined sewer line mentioned in the engineer's report is the wild card in any discussion of how much volume can drain at any given time. Wickstrom Place SW is at the base of a steep slope. Underground streams flow off this slope year around, especially during the rainy season. This water carries sand and silt which washes into the storm drains along the street. As a result, the sewer line always has some amount of sand in it, which reduces the volume of combined sewage it can carry. The city pumps sand out of it from time to time, but during the rainy season it accumulates quickly. 

To get an idea of how much sand there can be, I talked to a neighbor on Wickstrom who had a sewer camera inspection done earlier this year. She says that the drain from her house to the main sewer line was in pretty good shape, but the technician couldn't get the camera into the main sewer line because it was so clogged with sand. 

There are only two maintenance holes on Wickstrom where the city can access the sewer line to clean it out (see sewer map here). One is at the end of the street where it dead ends at the edge of the bluff and the second is where Wickstrom bends to join 54th Ave SW. There are 5 storm drains (two on the street and three on private properties) that empty into the long stretch between maintenance holes. I doubt if there are backflow valves installed on those storm drains. So if the system is overwhelmed with waste/storm water and the sewer line is clogged, the overflow will escape through those storm drains and into garages and then into the street. Where else can it go?

Third, our climate is changing. We are beginning to experience heavier rainstorms with greater frequency. A few days ago, a storm estimated to have delivered between 0.20 and 0.50 inches of rain caused major flooding at both the Westfield Southcenter Mall and Bellvue Square Mall. It isn't far fetched to imagine that the quantity of stormwater the Alki rowhouses will dump into our sewer system will do the very same thing. 

Big storm = half an inch of rain = 55 bathtubs full of stormwater coming off the Alki 11 Rowhouses into a clogged sewer pipe = high probability of flooding. 

The Alki 11 Rowhouse project needs to go back to the drawing board. It is too large a project on too small a piece of land. Hyper-density should be accompanied by hyper-mitigation of environmental impacts. This project should be redesigned to provide space to manage all stormwater runoff on site. It should provide adequate, usable parking so residents don't have to park on the street. The number of housing units should match the carrying capacity of the waste water infrastructure. The 53rd Ave Pump Station was recently upgraded to increase the volume of waste/storm water it can handle by 2 - 3 times, but this project increases waste/storm water volume 8 times over what it has been historically. This is not sustainable development.

Related Posts:

Thursday, June 27, 2013

We Have A Meeting Date!

Meeting date: Wednesday, July 17. 2013
Time: 6:30 - 8 p.m.
Youngstown Performing Arts Center
4408 Delridge Way SW
Seattle, WA 98106

This meeting is facilitated by DPD for a land use application (3014675) that has an associated Type II MUP (Master Use Permit) component-SEPA (which refers to environmental issues).  DPD will provide a brief overview regarding the proposal; discuss the SEPA process and project milestones; and allow the public to submit verbal (limited to 2 minutes per speaker) and written comments. All comments should be directed towards the environmental aspects of the project. 

Friday, June 21, 2013

Over One MILLION Gallons of Waste Water - Where's It All Going to Go?

In my last post, I explained how the Alki 11 rowhouse project on 55th Ave SW will add more than one million gallons of combined waste and stormwater to our local sewer system every year - 8 times the volume generated with the previous housing configuration. Where is all of that polluted water going to go?

Sewer manhole cover on Wickstrom Place SW

Our Alki Beach neighborhood has a combined sewer system, meaning that storm drains tie directly into the sanitary sewer. Buildings built in the past 10 years or so also have their downspouts and parking area drains tied into the sewer. 

The drainage and waste water map of 55th Ave and Wickstrom Place shows how this works. (55th Ave is the street shown on the left. Wickstrom is the dead end street on the right.) Water lines are shown as blue-black, sewer lines are red and stormwater drain lines, turquoise. The little black rectangles are storm drain catch basins. You can see that the older buildings do not have turquoise drain lines around them - they have natural drainage for stormwater runoff from roofs - downspouts empty into yards. This keeps a lot of stormwater out of the system, which reduces the likelihood of sewage overflows into the Sound, as I will explain later. 

According to documents filed by the developer with the city, all waste and stormwater from the Alki 11 rowhouse project will empty into the sewer line on Wickstrom Ave SW. From there, it will go to the 53rd Ave Pump Station, two blocks away. Waste and stormwater is pumped from this station to the West Point treatment plant

53rd Ave Pump Station on Alki Ave SW
Considering that stormwater runoff is now the number one source of pollution in  Puget Sound, you might think that sending it all off to a sewage treatment plant is a good idea. Unfortunately, it isn't that simple. When we have heavy rains, that extra volume of water overwhelms the system. To prevent flooding and sewage backups into homes, Combined Sewer Overflows (CSOs) release combined stormwater and raw sewage into the Sound. This 53rd Ave Pump Station is one of those CSOs. Here's a map of CSO locations in Seattle. Want to know if a CSO near you is discharging sewage? Here's real-time map showing CSO status.

The 53rd Ave Pump Station was recently upgraded to add two or three times the capacity it had before. But you have to wonder: how many projects like the Alki 11 rowhouses - which adds 8 times the volume and does nothing to absorb and mitigate stormwater onsite - can the system take on without serious environmental consequences?

Sign posted next to the 53rd Ave Pump Station

Related posts:

Wednesday, June 19, 2013

How Much Sewage Will the Proposed Alki 11 Rowhouse Project Generate?

In an earlier post, I explained how to calculate the amount of stormwater runoff this project will produce. Now let's look at the amount of waste water (aka sewage) that will be going down the drain if these rowhouses are built.

According to a King County study done in 2010, per capita water usage in our area is 80 gallons per person. Nearly all of that water ends up in the sanitary sewer system by way of showers, baths, toilet flushes, handwashing, toothbrushing, dishwashing, cooking, laundry, etc.

The developer of the Alki 11 expects that this project will house 33 people.
33 people x 80 gallons of water = 2,640 gallons of waste water generated per day 
2,640 gallons per day x 365 days in a year = 963,600 gallons of waste water per year
In addition to all that wastewater, stormwater runoff from this development will flow into the sewer system, too.

As I explained previously, every inch of rain running off the buildings and autocourt of the Alki 11 will produce a combined total of 5,457 gallons.
Rainfall in Seattle averages 38 inches per year. 
38 inches x 5,457 gallons of runoff per inch = 207,366 gallons of stormwater per year
The grand total of waste and storm water from these rowhouses 
= 1,170,966 gallons per year

Compare that number with the volume that has historically been generated on this site:
Zero storm water drains into the sewer because downspouts empty into yards and off street parking is on a permeable surface
5 residents x 80 gallons per day = 400 gallons waste water per day

The grand total of waste and storm water from previous housing = 400 gallons x 365 days in a year = 146,000 gallons per year

If built, these rowhouses will introduce 8 times more waste/storm water into the sewer system than before. Even with the recently completed upgrade to the 53rd Ave Pump Station (which increased capacity by two or three times, depending on which page you look at on King County's website), this 8-fold increase is way too much. If sustainability is a goal for the city of Seattle, this is not the way to get there.  

Related posts:

The Environmental Impacts of the Alki 11 Rowhouses

How to Calculate the Amount of Stormwater Runoff the Alki 11 Rowhouses Will Produce