Will Addicks and Barker Reservoirs overtop at their flanks, erode, and fail?
Right, all of this was thoroughly covered yesterday -- ground subsidence, exploding impermeable surface, highway water barriers, packed earth construction of the 1940's dams, air gaps under the outflow culverts -- along with Monday's excellent Cat6 feature story by Dr J Masters.
https://www.wunderground.com/cat6/harvey-moves-back-over-water-historic-rainfall-will-continueHe writes that the 8000 cfs releases from the two dams, while 2x what's permitted and massive enough to exacerbate Buffalo Bayou flooding downstream, is slower than expected inflows to the two watersheds, meaning neighborhoods above the reservoirs will still flood, just somewhat less.
I'm skeptical that will persist for long because of the fairly small size of these watersheds (map below). However it takes weeks to make a dent in water levels in these reservoirs once they are filled to the brim. Here 8,000 cubic feet per second drains 15,868 acre-feet per day or 3.9% of the two full reservoirs (25 days to empty). The inflow of 30,000 cfs adds 59,504 acre-feet per day of which 22,000 cfs over culverted outflow amounts to a 43,636 acre-feet excess.
According to wikipedia, Addicks is a rolled earthen dam 11.7 miles long that's only 123' above 1929 sea level despite the distance to Galveston Bay. The maximum storage capacity of the reservoir is 201,000 acre feet but -- because releases were able to buffer inflows last time -- record storage only hit 123,100 acre-feet on 4/24/2016. The 13.8 mile long rolled earth Barker has 209,000 acre-feet of storage capacity. It's lower, at 112.5' above sea level.
We use acre-feet in the US because land was historically surveyed in acres, and this measure gives water depth conveniently. Houston has 401,280 acres, Harris County 1,137,280 acres. So failure of both reservoirs is enough to cover all of Houston by an extra foot.
However, water from failed reservoirs would all flow down into the Buffalo Bayou watershed which is 65,280 acres, for which reservoir volume pencils out to 6.3' rise in coverage if equilibrated. Buffalo Bayou does drain to the sea, but very slowly because of aforementioned shallow gradient: the Buffalo Bayou canoe trail from Addicks to Allen's Landing (at Commerce/Main downtown) is 26 miles. That gives a gradient (rise over run) of 0.0009.
The key numbers to watch this morning (at the links I provided yesterday) are 102.65' for Addicks and 95.24' for Barker because these provide flood boundaries for the April 2016 event (these depend somewhat on the event-specific rainfall accumulation rate and distribution profiles).
By Monday morning at 2:15 pm EDT, Addicks had topped the Tax Day event by a full foot and Barker was over by 2.8'.
Tuesday, Addicks began overtopping its dam an hour ago and Barker will follow shortly (108' and 104' respectively). Inflow is said to be 30,000 cfs whereas outflow is more than maxxed out at 8,000 which will leave 22,000 cfs flowing over the far ends of the dams, which they were never built to handle. The floodgates cannot handle take another 22,000 cfs, they are already over design at 8,000. Either way, this is an out of control situation.
Barker Reservoir is 34 feet above the bottom of Buffalo Bayou’s streambed and the gate outlets; Addicks is 40.5 feet above the bottom of the outlets. Those numbers give the depths at full pool.
While overflow provides an upper bound to upstream neighborhood upper flooding, it could give rise to an erosive situation like the near-miss at the Oroville dam in California. That not only destroyed the massive concrete spillway but also severely eroded earthen margins.
Holthaus is incorrect in a sense in saying upstream houses are in the 500-year floodplain; they are not (map repeated below). While Harris County indeed takes careful note of high water boundaries and maps each flooded home in each storm, the floodplain maps are not regularly redrawn from that data because they negatively impact land values for developers. (Houston has been growing by 30,000 people per year.)
Here is a neighbor's account of why the Houston Medical Center flooded last time around:
http://bigjollypolitics.com/stephen-costello-new-flood-czar/Here is a link that will open a nice satellite map of Houston and the two reservoirs:
https://www.google.com/maps/@29.8125045,-95.6550079,35976m/data=!3m1!1e3