Where does the number 640 acre-feet per square mile come from? It’s important to know that there are 640 acres in one square mile. Consider how much detention would be required if every square mile were pristine – untouched by man – and we paved over all of it while applying our existing rules for detention.
Isn’t that too strict? No, it’s forward thinking. Former Mayor Parker defined all of Houston’s incorporated area as urban, so potentially every every square mile of Houston can be covered in concrete.
For large areas, Houston requires 50% mitigation for stormwater runoff, Galveston County requires 100%, and Fort Bend requires 75%. For our hypothetical pristine square mile completely covered in concrete, Houston would have to provide 320 acre-feet of detention, Galveston County 640 acre-feet and Fort Bend County 480 acre-feet.
So why not require 320 acre-feet for Houston, why 640? It has more to do with preventing man-made flooding than an arbitrary amount of detention, and it’s logical. Here’s the logic.
As part of the coastal plains, Houston geography is primarily flat ground interrupted by natural and man-made channels designed to carry water to the Gulf. In a Houston gully-washer, these channels cannot handle the amount of water and flooding occurs. To prevent overwhelming the system, we use detention ponds to delay (detain) stormwater from reaching these channels. After the rain ends and channel capacity returns, the ponds slowly release their water and flooding is prevented.
In most Houston watersheds, a 100-year 24-hour rain event is either 13.2 or 13.9 inches per hour, so let’s assume 13.5 inches per hour. For our square mile example, that’s 720 acre-feet of water. Still think Houston’s requirement of 320 acre-feet would be enough? Even if 50% of our buildings were surrounded by manicured grass, after a couple of inches of rain, the ground is saturated and there’s 100% runoff. By our math, that brings us just a little over our estimate of 640 acre-feet of detention.
Conveyance channels should be subtracted from the required detention, but not in their entirety because they must be considered at their 100-year operating capacity. Some channels will be out of bank in such events, so will add to, rather than subtract, from the amount of detention required.
Similarly, once a baseline is established, increased or decreased fill will need to either add or subtract to required detention because water will be displaced by higher elevation fill.
Of course, green building practices can improve these numbers, as can the use of water absorbing native grasses and trees, and not every square mile of Houston will be equivalent, but we’re looking at preventing man-made flooding so we’re starting with simple assumptions that don’t require an engineering degree to understand.
We are adding fill – the elevation of properties by truckloads of dirt and concrete – to detention requirements because FEMA will soon be using relative elevations to determine our flood insurance rates. Once a baseline is established, raising elevations using fill will most likely be regulated, hence our proactive stance.
In his September 13, 2016, testimony to the Senate Committee on Banking, Housing, and Urban Affairs, Mr. Roy Wright, FEMA’s Deputy Associate Administrator for Insurance and Mitigation, says that “more than 30% of the growth in our floodplains is because of new construction … because they put concrete and asphalt down, the water is no longer absorbing and it just conveys, causing more floods.”
In his written testimony, Mr. Wright also talks about “transformative” processes, “…(including) recommendations for transitioning away from mapping the one percent (1%) annual chance flood hazard, providing structure-specific risk information, digital delivery of our products, and most of the future conditions recommendations.”
Structural-specific risk information means that flood insurance rates will soon no longer be based on whether you’re in a floodplain, but rather on the elevation of your structure. Using advanced technology like Geiger LIDAR, elevations can now be measured quickly and more accurately so that individual structures can be assigned a risk.
At the same Senate Committee meeting, Scott Edelman, PE, Senior Vice President of AECOM, who has worked on FEMA flood maps since 1981, made several comments of importance to Houston. What Mr. Edelman said is, “We also have a great deal of uncertainty within the calculations. In all actuality, the current 100-year average line shown on the flood insurance maps is perhaps closer to a safe design level of a 10-year event.”
In dept by $23B, FEMA these experts cite as an unacceptable actuarial risk that nationally 26% of flooded homes are outside any mapped flood plains. With 60% of Houston’s flooded homes outside the mapped floodplains, our region will be hit hard by the proposed new rules.
Locally, the first thing that our elected officials do is to tout that this flood was an Act of God and that there’s nothing that we can do. But there is something that can be done, and FEMA is working to make changes that will impact how Houston and the rest of the country plans for floods.
RAF has long warned that FEMA wouldn’t turn a blind eye on our sins forever. It’s time to change the discussion about our development practices.
There are a lot of reasons why Houston has flooding, but we most often point to our building practices. Early on, Houston didn’t require detention when a subdivision was built and with increasing the density (as defined in Chapter 42 of Houston’s Code of Ordinances), in-fill development is requiring less, not more detention.