Water Quality Volume & TSS Load

Required removal

L_M = 27.2 A_N P (RG-348 Eq. 3.3), with A_N the net new impervious area in acres and P the average annual precipitation of the county (Table 3-3).

Load removed

L_R = E P (34.6 A_I + 0.54 A_P) (Eq. 3.8), with the efficiency E from Table 3-4 and the impervious and pervious areas draining to the BMP. No bypass is assumed. BMPs in series follow Eq. 3.6.

Rainfall to treat

F = L_M / L_R (Eq. 3.9). F is rounded to 0.01, as in Chapter 6 and the TCEQ worksheet, and gives the rainfall depth from Table 3-5. F above 1 means the BMP cannot remove the load, and no volume is given.

Water quality volume

WQV = depth × R_v × area (Eq. 3.10), with R_v = 1.72 IC³ − 1.97 IC² + 1.23 IC + 0.02 (Eq. 3.11).

Capture volume

WQV plus 20% for sediment, added once, as in the TCEQ worksheet. The 2009 errata puts the 20% in Eq. 3.10 and each BMP section repeats it; the two are not stacked here. The volume before the 20% is shown too.

The calculator is tested against the cases below every time its code changes.

Compared with the saved case of the TCEQ TSS worksheet, to 0.001%, and with the RG-348 Chapter 6 examples. Chapter 6 rounds R_v to 0.31 and leaves out the 20%, so its volumes are compared with the volume before the 20%, at 1.2%. Its sand filter case prints a depth of 1.46 in that Table 3-5 does not hold, so that case is not used.

• TSS only, and one BMP or one series for one drainage area. A site with several BMPs adds them by drainage area, and that total is yours. Credits from over-treatment are not computed.
• BMP dimensions are not computed: filter areas, basin shapes, irrigation areas, swale and vault sizing. Swales and wet vaults are sized by flow, at 1.1 in/hr. Proprietary BMPs in the 2017 addendum have their own rules; enter an efficiency only after reading them.
• Where RG-348 and the TCEQ worksheet differ, the page says which it uses: Eq. 3.11 throughout (the worksheet turns to a lower straight line above IC 0.844), the off-site impervious cover for off-site runoff (the worksheet; RG-348 section 3.3.3 uses the site’s), and your choice of coefficient in series (0.5 in RG-348, 0.65 in the worksheet). Table 3-5 stops at F = 0.25.
• Whether the Edwards Aquifer rules apply to a project, and the exemptions, are not addressed here. The Executive Director reviews plans; this page gives numbers.

Is this the TCEQ worksheet?

No. It follows the same order and reproduces its saved case, but it is a separate calculation aid. Use the TCEQ worksheet where your reviewer asks for it.

My project crosses two counties.

RG-348 says to use the average of the two counties’ rainfall. Choose “I will type it” and enter the average.

Why is there no volume when F is above 1?

The BMP cannot remove the required load even treating all the runoff. RG-348 says to choose a more efficient BMP; treating more impervious area also raises the load removed.

Can I attach the calculation sheet to a report?

You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews and is responsible for what goes into a report.