Entrenchment

Entrenchment is the last major characteristic in the Rosgen classification system. It is expressed as a ratio, giving the new width of the river if it were to be twice as deep as normal (as might happen in a 50-100 year flood).  This, not coincidentally, helps describe the immediate area surrounding the river.

The lower the entrenchment number, the deeper the river is cut into the landscape.  The higher the number, the more it lies along the general surface, spreading wider during a flood event. 

 For example, in our sample river map we've been following for the last several posts, the river in hex 0306 is ~90 feet deep, ~1950 feet across, and is type F, which is a deeply entrenched river type.  We find the entrenchment ratio is 1.2, so during a major flood event, the river in hex 0306 will be (90x2 = 180) 180 feet deep and (1950x1.2 = 2340) 2340 feet across.  The velocity should also be recalculated...I guess I should add a flood page to the spreadsheet.

Looking closer at this info we just found, you can see that if you move directly away from the river (perpendicularly), assuming that the river spreads equally to the left and right as the floodwaters rise, the land rises 90 feet (the added flood depth) in the course of 195 feet (half the added width of the floodwaters), giving the land there a slope of (90/195 = .46) .46, a very steep grade.  Tributaries flowing into the river here could easily be type Aa+ (remember that this river type isn't randomly generated in this system), although if they run in at an oblique enough angle the steep slope might be mitigated.  Also, characters will have an extra hard time crossing the river here.  As steep as the slope is, there probably isn't a road running down to the river, so no ferries or bridges either; if they have to get across the river right here, it'll mean a climb down to the river, a long swim, and then a climb back up on the other side.

Also worth mentioning, a 50 year flood, by definition, has a 2% chance of occurring in any given year, and a 100 year flood has a 1% chance.  Therefore, if you are using an annual events table to shape your campaign, there should be a 1-2% chance of needing to use the entrenchment ratio each game year...

This is the last addition to the river generation table, which is as follows:

 

Die Roll	Slope	DA	C	E	F	D	B	G	A	Aa+
1	0.0001	x	x	x	x	x				placed as needed
2	0.0003	x	x	x	x	x
3	0.0007	x	x	x	x	x
4	0.0015	x	x	x	x	x
5	0.002	x	x	x	x	x
6	0.004	x	x	x	x	x
7	0.006		x	x	x	x
8	0.01		x	x	x	x
9	0.02					x	x	x
10	0.03					x	x	x
11	0.04								x
12	0.05								x
13	0.08								x
	Sinuosity	1	Min 1.4*	Min 1.5*	Min 1.4*	1	Min1.2*	Min 1.2*	1.0 – 1.2	1.0 – 1.1
	WDR	2d20	(d10x2)+12	d12	(d10x2)+12	(d10x4)+40	(d10x2)+12	d12	d12	d12
	Entrench.	>2.2**	>2.2**	>2.2**	(d4/10)+1	1	(d8/10)+1.4	(d4/10)+1	(d4/10)+1	(d4/10)+1
* For these river types, roll 1-6 d6, divide by 10, and add 1, for a total of 1.1 to 4.6.  If this result falls under the minimum listed, use the minimum instead.
** These river types are associated with floodplains. If obvious floodplain boundaries (hills, mountains, etc) are apparent on the map, the river will fill that area during a 50-100 year flood. If boundaries are not apparent for whatever reason, the entrenchment ratio can be simulated by rolling an “exploding” d6 (ie, if a 6 is rolled on d6, keep it and add another d6, continuing to add d6s as long as 6s are rolled), dividing by 10, and adding to 2.2 (or (d6X/10)+2.2)