I have personal experience with flooding. I grew up in Groves, Texas (in Jefferson County) along the Gulf Coast. My parent’s house was pier and beam construction, built on a lot at 9.5′ above sea level (before subsidence). I grew up in flat country with ~50 inches of rain per year. It was not unusual at all to see the entire school playground inundated with inches of rainwater.
Generic Rain flooding
Before and during the big rains, my father volunteered (and I was drafted) to clean the culverts downstream of our house. The construction in our area was predominantly raised crowned asphalt roads flanked by bar ditches. Most of the city sits at an elevation of 3 to 14 feet above sea level. In south Jefferson County, rainwater collected by the large drainage canals is discharged with assistance from drainage pumps — the canals are pumped down in anticipation of rain with continued pumping as needed. The pump discharge goes to the Intracoastal Canal then to Sabine Lake then to the Gulf of Mexico.
During Hurricane Audrey, a serious rain event, water was say 2′ deep in our yard, barely lapping at the finish floor elevation when vehicles left a wake while going down the road. Despite this high water, there was little property damage from flooding (at our house). In our town, Hurricane Carla brought more storm surge flooding. Landfall occurred in Port O’Connor Aransas, 185 miles ESE of Groves. Though our family evacuated well in advance of the storm, we saw the tide water rising in the shallow ditches along Highway 366. In our area, the maximum storm surge from Carla was about 7 feet. Notice that the maximum storm surge was 22 feet (!) near Port Lavaca.
Types of flooding
Above, I have described only two kinds of flooding, and both of those addressing a coastal plain. Far worse flooding is due to confined flow in or from a watershed. In river flooding, the water level rise could be due to excessive rain or snowmelt. Often there is great depth and high velocity due to variation in the terrain. Flash flooding is the rapid accumulation of flood depth. With this, I have no experience.
Compare coastal flooding to river flooding (fluvial):
In 2023, one can say all floods have a man-made component. In the coastal variety, hurricanes have been coming for centuries, and the development of towns and facilities increases the loss of life and property. Short of levee protection systems, there is not much man can do to stop or alter coastal flooding.
Rivers flood. An entirely natural river will flood and an altered river will flood. The tendency is to build along the river without leaving room for the extensive natural flood plains. The development of towns and facilities tends to hem in the river. Periodically there is loss of life and property as the river exceeds its banks.
A dramatic example is the Mississippi River through New Orleans where you are walking at street level and you see a ship traveling the river, yet the river level and the ship are ten or more feet higher in elevation. Years back, New Orleans was at a higher elevation. Pumping of underground water caused subsidence. The river was lined with levees in an effort to control flooding. Silting of the river raised the river bed. Levees were built higher. And we reach the present state where New Orleans is at far greater risk primarily due to man.
Why would anybody build a city on ground that is below sea level?
New Orleans may be a spectacular example of the evolution of a river town but the situation is unfortunately similar almost everywhere.
The common theme in most all development along rivers and coasts, is that flooding has been combatted by a rather dramatic alteration of the landscape rather than adaptation to the natural conditions. A clear and economical example of adaption is a beach house constructed on tall pilings.
Flooding is experienced well away from rivers and coasts, let’s call this generic rain flooding. Also let’s recall that hydrology is the branch of science concerned with the properties of the earth’s water, and especially its movement in relation to land. What have we learned from hydrology? The factors contributing to rain accumulation are:
- rainfall intensity
- rainfall duration
- soil conditions, and
- ground cover
The two rainfall factors are generally a function of nature. Though the other three factors might also be a function of nature, although these are most likely altered by the activities of man. Terrain with large slope, low absorption, and less ground cover tend to cause more accumulation and runoff. In hydrology, there is a formula, called the Rational Equation, that illustrates the situation.
The Rational Equation requires the following units:
Q = Peak discharge, cubic feet per second
c = Runoff coefficient
i = Rainfall intensity, inch/hour
A = Drainage area, acre
Range of runoff coefficients
Forests 0.05 - 0.25
Roofs 0.75 - 0.95
Streets 0.70 - 0.95
Another approach to quantifying runoff is given by the SCS TR-55 approach.
for Small Watersheds TR-55
As in the document below, the existing grades and proposed grades must be accounted for in controlling rainfall runoff.
In my opinion, siting is both art and science, neither of which is applied much in modern building. It is obvious to me that cities historically developed on the best land, relatively high ground albeit with near access to water. Water was needed for consumption and for shipping. As populations have grown and cities expanded, necessarily, lower quality land is used. This lower quality land might flood more easily, might be on poor soil subject to excesive settlement, etc.
Look at the sites under construction in your town. These sites were left undeveloped, rejected, for decades. Now these lower quality sites are being used. Extra care in engineering and siting might be expected: removal of incompetent soil, use of select fill, properly compacting the fill, engineered retaining walls, raising the yard elevation, raising the finish floor elevation, attention given to site slopes especially near the foundation and paving, attention to disturbed soil (plumbing, sprinkler, electrical, cable trenches). Unfortunately, I see ever more expensive houses built on ever less fit sites.
Subsidence, downward vertical movement of the Earth’s surface, is a passive contributor to flooding.