BLOOD ON THE STEP (I)

THE ISSUE: When the crime was first discovered, Nicole was found lying on her left side on the front walk, just inside the threshold of the open gate, her buttocks against the first riser of the four steps leading up to the level of her condo (Figure 1).figure01.jpg (26147 bytes) There was a great pool of blood from her slashed throat that had flowed downhill, away from her and toward the street. It is generally believed that her throat was slashed at or very near to the position in which her body was found. There are no blood stains at all above the stairway, and since Nicole was found at the uphill end of the front walk, it is not considered that she could have bled greatly from any location east of where her body was found. The blood flow from east of her and down the walk is consistent with the source having been her throat in the position in which she was found.

However, there is also a continuation of the blood pool behind her shoulder and her back, all the way to the first riser of the steps, and this is uphill from the throat wound, and at its greatest distance is about 18” from the final position of the throat. More significant is the fact that there is a substantial pool of blood on the first step that was drained across a very wide front down the first riser. This suggests that a large amount of blood was deposited very quickly over the first step, and since the only moment in a rational scenario when a large amount of blood flows quickly occurs when the victim’s throat is slashed, it is suspected that Nicole’s throat may have been slashed over the first step, not in the position in which her body was later found.

So, the basic question considered here is: In what location, and under what conditions was Nicole’s throat slashed? It is known that when a victim suffers the kind of wounds suffered by Nicole there is a very great initial spurt of blood, and because of this, the throat slashing must have occurred somewhere over the main blood pool on the front walk, or over the first step. And if it occurred over the walk, it must have been near the step, which is the high end of the walk, to account for the continuous flow east from the bottom of the step. The issue then is reduced to slashing the throat over the first step, or over the walk near the front step.

CONSTRAINTS
: It is sometimes said that in the messy aftermath of this murder, nothing can be determined for sure, anything could have happened. But that is not completely true. The dimensions of the scene -- the width of the steps, the height of the risers, the distance to the gate, the dimensions of the front walk -- are accurately known. Nicole’s size, proportions, and the possible configurations that her body could have assumed during the attack are known. The distribution of blood -- both in its gross aspects and in its details -- have been documented, and the behavior of a fluid like blood to produce those indications can be simulated and studied. The laws of physics (liquid does not flow uphill) and physiology (a great spurt of blood occurs when the carotid arteries are severed) are known

While an analysis based on these considerations can not absolutely determine to the millimeter and the second the exact process of the murder, it can rule out certain generalities and show that others are highly likely. For example, we can be rather sure that Nicole’s throat was not slashed in the EXACT position in which her body was found because of the pattern in the blood pool ahead of her throat. The initial spurt would have projected directly forward from the throat and reached farther than the roughly 18” in that direction that the pool extends. A sideward spurt from the right carotid that would have splashed up onto the assailant is also possible, and would not be acceptable to those who think Simpson is the killer. And, the left side posture does not account for the blood on the step or behind her. So, at a minimum, it is expected that she was in the posture later seen, but face down, when the throat was slashed, and then she fell, convulsed, or was pushed, over on her left side.

HYDRODYNAMICS: To understand this situation better, it would be useful to know just how a liquid deposited on a horizontal surface, such as Nicole’s step behaves. At the outset, I scaled photographs and saw that the width of the pool on the step was about 12 inches, and the cascade down the riser at the top edge was about 18 inches. This in itself is surprising, since one usually expects the downflow to be narrower than the pool.

The common experience is that when a pool is created by a slow flow, the liquid will spread out until it encounters an edge where it can drain, it will flow down that, and further additions of liquid to the horizontal surface will be offset by draining, so that the size of the pool does not increase. In this way, the pool is wider than the trickle down the drain (the front of the riser, in this case.) But, if the liquid comes onto the horizontal surface at a very high rate (a flood), then it can accumulate and spread out before it drains, and the draining will be across a wide front down the riser. If that is true, then the wide pattern down Nicole’s first riser indicates a high flow rate onto the step. And that in turn indicates that the step was the place at which the throat was slashed, since the initial spurt from the carotid arteries is the only part of a rational scenario in which a great flow of blood occurs. That, at least is what I thought at the outset, and I undertook experiments to test this idea. I made a few other discoveries along the way.

THE EXPERIMENTS
: Since the premise is that a wide front spilling down the riser seems to imply a great flow rate on the step, I undertook to try this on my own concrete front steps, which have a 12 inch tread and an 8 inch riser. The step is flat left and right (the width) to within my ability to measure it (less than 1/8” per foot) but has a gradient font to back (the depth) of 0.64 inches per foot down toward the front. There is a slight groove (1/16” wide and 1/32” deep) front to back that could provide a slightly preferred path for very slow flow rates. In my early experiments I poured over the middle of the grove, 6 inches back from the front edge of the step.

The liquid I used was flat latex paint (yellow) diluted 1:8 with water. This may be slightly less viscous than blood, but it is noticeably thicker than water, and certainly undiluted paint would be too thick. All of the experiments described here were done in the shade, in daytime, at a temperature between 65 and 75 degrees F. Photography was done from a tripod, so that all the pictures in one series have the same perspective, and a flash was used to give consistent exposure. Fiduciary marks with a black felt pen were put on the step showing a 12 inch width on the horizontal surface of the step and showing an 18 inch width on the edge of the riser. These correspond to the maximum width of the blood pattern on Nicole’s step.

The early experiments did not work out well. Sometimes I got rushed and let an unrepresentative gush of paint spill down, one time a glob of undissolved paint distorted the picture, another time I poured from such a height that splatter obscured the pool, once I poured in the wrong part of the step. In this way, the first two rolls of film were unusable, but the experience taught me some lessons and caused me to make two changes before the third roll. 1) I made a series of pictures to explore the influence of height of pour on splatter, and 2) I moved the point of pour up from the middle of the step (the 6” point) to a place 3” from the edge.

SPLATTER: In the early experiments I discovered that the amount of splatter is controlled almost completely by the height from which the liquid is poured. It is possible to pour smoothly at a very high rate (32 ounces in 5 seconds) without producing any splatter at all, but even careful pouring from 6 inches and above at very slow rates (2 ounces in 5 seconds) will leave an indication of splatter on the step. To document this effect, I poured at the very slow rate for about 2 seconds from a height of 3, 6, 9, 12, and 18 inches taking photographs during the pour. The pictures in Figure 2 (from 6”) and Figure 3 (from 12”) are representative. figure02.jpg (18114 bytes)It is clear from these that greater height gives greater splatter, and also that the splatter pattern that results has a radial distribution away from the center of pour. Not only does the ensemble of paint marks have a radial appearance, but individual droplets also are linear, pointing away from the center of the pour.

figure03.jpg (15653 bytes)To quantify these results I undertook to estimate how far a particular pattern extended, but this is an objective task. I made two determinations: at what point there ceased to be a noticeable pattern, and the distance at which a reliably detectable pattern could be reported. I then plotted these results as a function of height of pour in Figure 4, and was surprised to find that the horizontal extent of splatter is roughlyfigure04.jpg (49601 bytes) equal to the height from which the liquid is poured, down to a height of 6 inches. Below a three inch height of pour there is not a reliable appearance of splatter.
figure05.jpg (9097 bytes)
FLOW RATE: Until these experiments I had believed that the amount of blood in the “carotid jets” was roughly 1 quart in 4 seconds. So, my original flow rate experiment (Figure 5) put 32 ounces on the step in a 5 second period, and caused a pattern that was both much wider on the step (about 20 inches) and much wider on the riser (about 30 inches) than the pattern on Nicole’s step. (In this and all other flow rate experiments, the point of pouring was 3 inches from the edge of the step for reasons that will be explained later.) Although this experiment can be explained by the possibility that some of the carotid jet went on the step, and some splashed directly onto the riser, thus preserving the “32 ounce in 4 second” concept, I was here interested only in the blood on the step.figure06.jpg (21376 bytes)

So, I tried again using 16 ounces in 5 seconds (Figure 6), and produced a pattern that was 13 inches wide on the step and 15 inches wide on the riser. Since this is close the situation at the murder scene, I considered that the flow rate onto the step had been about 16 ounces in 5 seconds. The small deviations of my observations from the crime scene could be explained by the fact that I poured the liquid vertically, whereas there was a horizontal component of velocity in the carotid jets. There are also clues in the details of the pattern on the riser.

To confirm the behavior with a very slow flow rate, I performed a third series (Figure 7) at 2 ounces per 5 seconds, and continued pouring until 16 ounces had been dropped in 40 seconds. figure07.jpg (22077 bytes)By 10 seconds, the pattern on the step was 6 inches wide, and on the riser (sum of three trails) was 3 inches wide. At the end, the pattern on the step was about 7 inches, and the riser was about 5 inches. Since a total of 16 ounces had been poured in both the fast and slow series, and only the rate was different, I considered that the experiment confirmed the concept that a wide pattern on the step, and particularly a wide pattern on the riser, indicated a high rate of flow on the step. In this case, the high rate of flow appears to be consistent with what would be expected from the carotid jets, or a large fraction of that.

figure08.jpg (45188 bytes)The quantitative results are shown in figure 8, where the pattern width is shown as a function of time for three flow rates and two surfaces (step and riser). Furthermore, I take these experiments to be a strong suggestion that Nicole’s throat was slashed over the step, and that the amount of blood that came from that wound onto the step was roughly a pint in 5 seconds.

 

 

[To be continued]

Dick Wagner • Van Nuys, CA (3/11/99) NG_527

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