Another example of using infiltration to manage excess, relatively clean, stormwater during construction. As water builds up around the covered catch basin, is flows into the area to the right and infiltrates, preventing flooding.
Closeup of double plastic layers under catch basin grate and infiltration area.
This is the first project we have cut asphalt to allow construction stormwater to infiltrate. The area is paved, flat. The sweeper runs constantly but the water is still too turbid to go down the drain. The fill soil under the asphalt is sandy. If we just covered the catch basin with 30 Mil PVC, the area would just flood, then drain to another catch basin. Solution? Cut out a section of asphalt full depth to the sandy soil, fill with 3 to 5 inch rock. This way the water infiltrates but vehicles can drive over the hole. This is working really well so we will be doing this on future container dock upgrade projects.
Compost socks work really great around catch basins but need to be maintained. This doesn’t necessarily need to be replaced but the tear needs to be wrapped with plastic or netting. Safety cones or candles might keep people from backing over them.
The down side to using sandbags is that you can’t get a good seal and they leak. The idea behind this is that water ponds and some of the sediment settles out. With the leakage, little settling can occur. The catch basin insert will catch some of the sediment but since we are regulated on turbidity in Washington, neither BMP cuts turbidity low enough to meet the compliance standard. Our contract specifications require that discharges not exceed 25 Nephelometric Turbidity Units (NTUs).
The contractor had to connect a new storm system from up the hill into an existing catch basin in the foreground. After they completed the connection but before paving, they covered the pipe run with plastic and sand bags to protect from dirty water entering the catch basin. Work was completed during dry summer weather. Paving took place a couple of months later.
As of two days ago, there is new 30 mil PVC under the catch basin grate and turbid water is not draining into the catch basin. The contractor told their staff not to puncture the PVC. No word on whether they are thinking more proactively about this. The best measure is to keep the stockpile covered and the area cleaned up. We shall see what’s next.
The saga continues… the next inspection looked like the first inspection. 500 + NTU water draining into the catch basin. What happened?
As suspected, someone didn’t like the flooding, so they punctured the 30 mil PVC. I am wondering why the contractor erosion control lead is not finding this.
I inspected the open catch basin the next day, and this is what the contractor had done to keep turbid water out. I have nothing against doing this, but, since this does keep water out, flooding ensues. Flooding is okay if the catch basin is in a low spot and no one needs to work in the area.
I told the contractor that they might want to look at the storm system and see if there is a point where they can install a concrete plug temporarily and use the system for conveying the turbid water to their treatment system.
I found this open catch basin during a construction erosion inspection. The turbidity of the water draining into it from the stockpile measured at over 600 NTUs. Note that there is an insert in the catch basin; these do nothing to reduce turbidity. I notified the contractor who should have found this in their daily inspections, which I pointed out to them.
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