"Wisdom begins in wonder" – Socrates
Category: Engineering
Potential Maximum Retention
08/16/25
“How can we quantify the maximum amount of water a watershed can hold before runoff?”
Watershed geologies can hold a certain amount of moisture before runoff occurs. We can measure this through the Potential Maximum Retention, a hydrological quantity that represents the maximum amount of water a watershed can absorb before starting runoff.
Initial Abstraction (Hydrology)
08/15/25
“How can we quantify the initial rainfall losses before runoff begins?”
Rainfall will lead to water hitting the ground. Before anything happens, some of the rainfall will get intercepted by vegetation or infiltrate into the ground and get lost from the system. Hydrologists represent this value as the Initial Abstraction. Initial abstraction is an important quantity in hydroshed and stormwater studies.
Net Positive Suction Head (NPSH)
08/14/25
“What is the minimum pressure needed to avoid cavitation in pumps?”
If pumps don’t have enough pressure, then water vapor can form in the pumps and cause cavitation in the pump blades. Pumps can prevent this from occurring by maintaining pressure above the vapor pressure. The difference between these quantities is known as the Net Positive Suction Head (NPSH). The NPSH required (NPSH-R) is the pressure at the impeller entrance needed to avoid cavitation, equal to the difference between the stagnation pressure and the vapor pressure at the inlet. The Net Positive Suction Head Available (NPSH A) is the pressure available at the pump inlet above the vapor pressure and losses due to system friction. For functional system operation, NPSH A must be greater than NPSH R.
Static Pressure
08/13/25
“What’s the pressure of a fluid’s presence?”
Fluid pressure comes in all kinds of forms. Fluids will exert a pressure from their presence alone called Static Pressure. Air in a balloon is an example of static pressure.
Stagnation Pressure
08/12/25
“What happens when a moving fluid comes to a halt?”
Many fluids move with a certain speed. But if they’re forced to abruptly come to a halt, they’ll convert all of their kinetic energy into pressure. This Stagnation Pressure is vital for understanding how static and dynamic pressure relate to each other and fluid motion.
Hydraulic Head
08/11/25
“How can we conceptualize the total potential of a fluid’s pressure?”
We know that a fluid that is higher than another will usually flow downwards if they’re connected. However, if the lower fluid has a high enough pressure or velocity, then it can overpower the higher fluid and move upwards. One way to conceptualize this is to imagine that the pressure and kinetic energy of each fluid adds extra height, and whichever fluid has the highest theoretical “height” will govern the direction of flow. This is the concept behind Hydraulic Head, as illustrated by the equation h = P/(rho_w * g) + z + v^2/(2*g), where h is the hydraulic head, P the fluid pressure, rho_w the fluid density, g the gravitational constant, z the fluid elevation, and v the fluid velocity.
Stormwater Discharge
08/10/25
“What happens when stormwater runoff enters a regulated body?”
Stormwater runoff is the water from rain that flows over the ground instead of seeping into it. Some of this rain may enter a regulated body, such as a stream or a storm drain. This water, known as Stormwater Discharge, may contain pollutants it picked up during its journey on the surface. Infrastructure operators should treat stormwater or prevent it from getting polluted before it enters water bodies. Engineers and planners must design public facilities around predicted peak stormwater discharge values and holding times.
Unit Peak Discharge
08/09/25
“How can we get a base estimate of a small watershed’s discharge potential?”
Runoff events vary by storm and watershed. However, we may want to compare the discharge potential of different watersheds without having to account for variations in different storms. So, what if we were to calculate how much peak discharge each watershed would produce for a 1-inch rain event, regardless of its area? Well, this is the idea behind Unit Peak Discharge, which is measured in flow rate per area per runoff amount. Calculating unit peak discharge is essential in estimating peak discharge using the TR-55 graphical method.
TR-55
08/08/25
“How can we use simplified procedures to estimate runoff and peak discharges in small watersheds?”
Predicting runoff and peak discharges for watersheds can be a very difficult task. There are so many factors that you have to consider without great precision. However, for small watersheds in the US, the Natural Resources Conservation Service of the U.S. Department of Agriculture has developed a procedure called TR-55 to handle these. TR-55 uses empirically derived data and hydrological modeling to determine runoff and peak discharges.
