The Eight Types of Sewer Hydraulics: Part 2 Interpreting Scattergraph Results, part a
This installment, the second of three, shows the changes induced on normal scattergraphs--in theory and points of fact--by several types of blockages
Recap and Foreword
The first installment of this three-part article introduced the concept of using depth and velocity scattergraphs from sewer-flow measurements to understand the sewer's performance, thereby to provide insight into the nature of any problems. Scattergraph results show that sewers generally fall into one of eight possible hydraulic categories. The discussion of these results is the basis for this and the last installments. Underlying all analyses are comparisons with open-channel performance.
Results
1. Normal Open-channel (Unimpeded)Flow
The first installment of this article discussed one type of normal open-channel flow--where the pipe is less than half full (see installment 1, Figure 3).
Figure 4 shows a scattergraph for a pipe with flow at depths routinely above half-pipe height and velocities at theoretical maximum. Results are for a completely full pipe in surcharged condition. Surcharged depths are measured with a pressure sensor.
2. Silt or Obstacles
Figure 5 shows a sewer profile with a hump and the flowmeter installed within the upstream pool caused by the hump.
Actually such a condition can be caused by any obstacle capable of creating a pool behind it, including cinder block, roots, or a dead animal. For this situation, the night-time flow may drop to near zero, and the flow monitor would detect a depth of a few centimeters but, perhaps, no velocity. The obstacle creating the pool produces a curve that is right offset along the depth axis (Figure 6).
A scattergraph for this hydraulic condition is shown in Figure 7. This curve offset is visible in scattergraphs despite possibly large night-time flows.
3. Bottlenecks
Any number of objects, including roots, acute turns, undersized pipes, "flat" sewers, and pump stations can cause bottlenecks in a sewer. Figure 8 is a profile of sewer that is bottlenecked by roots lodged downstream from the flowmeter.
Figure 9 plots the theoretical response to a bottleneck, showing the velocity beginning to deviate from the theoretical pipe curve above a certain depth and slowing as depth increases.
Figure 10 is a scattergraph of an actual bottleneck in a 61-cm. (24-in.) pipe. The depth-velocity relationship reasonably resembles that for open channel flow at depths less than half pipe. At depths greater than half pipe, the velocity drops until the pipe is full. The result is the classic "ski jump" shape that characterizes bottlenecks. Although the ski-jump pattern is classic, the gradual increase in velocity from full pipe (24 inches) to 183 cm. (72 inches) is unusual and shows that increased depth causes increased flow. This increase with depth suggests that whatever is causing the bottleneck behaves as an orifice or throttling structure.
4. SSO Downstream
Were the surcharge level in Figure 8 to continue to increase, the hydraulic grade line would reach the top of the manhole, causing an SSO (sanitary sewer overflow). The upstream flowmeter would detect an increase in velocity over and above that allowed by the downstream restriction. Figure 11 shows what to expect in theory--in particular, the SSO's telltale sign of radical velocity increase at a surcharged depth. Figure 11A is an actual charting.
The vertical data takeoff in Figure 11a at 145 inches on the horizontal axis results from the pressure transducer's having reached its upper measurement limit. Most open-channel flowmeters are equipped with 260-mm-Hg (5-psi) pressure transducers with an upper limit of nearly 3.6 meters (140 inches).
5. SSO Upstream
The flowmeter will not detect a change in velocity where an SSO occurs upstream of it. Whatever downstream capacity exists continues to control the velocity. The flowmeter's only indication of an upstream SSO is a relatively fixed hydraulic grade line during the overflow. The curve in Figure 12 represents the performance, in theory, of a sewer with an upstream SSO--a run of data points at a fixed surcharged elevation.
Although a sewer's behaving like this for a long period is not conclusive evidence that an upstream overflow is occurring, it is a strong symptom. Figure 13 is a scattergraph from a site with an upstream overflow. It was generated from one week's data during two major storms.
