|Project Name||Steep Bank Creek Pump Station – Expansion|
|Short Project Description||Expansion of permanent pumping capacity at Steep Bank Creek Pump Station to meet Fort Bend County Drainage District criteria, taking into consideration new rainfall data released in 2018 by NOAA. Temporary Trailer Mounted Pumps and Sheds for the pumps have been purchased and are on site.|
|Additional Technical Information||
As the primary sight for the removal of water from the Steep Bank Creek Watershed, the Steep Bank Creek Pump Station is the cornerstone of our district’s storm water management system. Following Hurricane Harvey, an independent engineering review suggested that the pump station should be expanded to meet the current regulatory requirements prior to Hurricane Harvey and to meet the revised requirements including updated rainfall and river data. Note that this isn’t a “Harvey Proof” action but providing more capacity does move us in the right direction. The recommendation based on pre-Harvey data was to increase the pumping capacity by at least 40,000 gallons per minute (GPM).
The current pump station has four 20,000 GPM pumps. After reviewing several methods of expansion, the LID 19 and LID 15 Boards agreed to expand the pump station by adding three 50,000 GPM pumps.
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|Estimated Total Project Cost (including engineering and contingency)||$9,000,000|
|Estimated LID 19 Cost (if Project Costs will be shared) and Sharing Basis||The project is proposed to be shared between the District and Fort Bend County LID 15 based on relative share of acreage draining to the Steep Bank Creek watershed. Therefore, the project is proposed to be shared as follows:
LID 19: 66.82%
LID 15: 33.18%
|Estimated Schedule||Early 2022|
|Project Status||Engineering is complete and the project has bid. Expected to start construction at the end of 2020.|
|Financing Source||Project has been funded through LID 19 Bonds. The Board intends to pursue a plan of financing that is projected to not increase current LID 19 tax rates.|
To better understand what that increase does for our ability to handle water, let’s look at a few facts/numbers. First, it is important to understand that our watershed in total is approximately 2,854 acres of which we directly control 758. But make no mistake rainfall on the entire 2,854 acres ultimately exits our watershed at the outfall where the Steep Bank Creek Pump Station is located.
Next let’s talk rainfall. One inch of rain on one acre results in approximately 27000 gallons of water. Some rough math tells us that an inch of rain throughout the entire watershed will result in approximately 74,250,000 gallons that needs to be managed in one day for one inch of rain. Dividing that by 24 hours in a day and again by 60 minutes in an hour results in a needed capacity of approximately 52,000 GPM.
Knowing this, when the Brazos River causes gravity outfall to close, an additional 20,000 GPM of pumping capacity increases the maximum pumping abilities from 1.5 inches per day to 2.3 inches per day. An additional 35,000 GPM of pumping capacity increases our maximum pumping abilities to 2.69 inches per day.
You may be reading this and thinking, that doesn’t seem like a lot of rain before we are maxed out. Well, that is where detention plays a role. Detention is used to hold large amounts of water and to release that water slowly into the Steep Bank System which allows a smaller pumping station to handle the large volume of water without being overwhelmed. It simply means the pumps will be pumping long after the rainfall stops as the detention ponds slowly drain. Our watershed’s detention ponds add approximately 6 inches of rainfall capacity to our system in a given day. This means that for a given 24 hour period, if the Brazos River is at a level that we are not able to gravity outfall, our capacity will still be to handle a more than 8 inches of rainfall on the watershed.
One wrinkle to this overall capacity which the board continues to evaluate is what happens during an extended event where the detention ponds are all full over the course of extended, intense rainfall. In other words, a multi-day event like Hurricane Harvey. Well, the simple thing is that once the detention areas are all “used up”, we must calculate the needed capacity as though not only there is no detention available but add the slow release rate of the detention to the overall rainfall requirements. This would mean that to “Harvey proof” (assume a max 24 hour rainfall of 10 inches) the watershed we would need a capability to remove in excess of 520,000 GPM. That’s approximately 400,000 GPM than the expanded Steep Bank Station. With this in mind, our board continues to research and develop additional solutions.