1. What’s New in Sedimentation and River Mechanics
Pierre Julien
Spring 2001

2. Objectives Bedforms and resistance to flow during floods.
Effects of dams on hydraulic geometry.
River response to deviations from equilibrium geometry of alluvial rivers.

3. Rhine River flood in 1998

4. Primary dune height vs discharge
Bovenrijn
0.0
0.5
1.0
1.5
2500
5000
7500
10000
Discharge (m3/s)
primary dune height (m)
Center
Left
Right
Waal
0.2
0.4
0.6
2000
4000
6000
8000

5. Roughness height vs discharge
Bovenrijn
0.0
0.2
0.4
0.6
2500
5000
7500
10000
Discharge (m3/s)
Roughness height (m)
Center
Left
Right
Waal
0.1
0.3
2000
4000
6000
8000

6. Darcy-Weisbach f vs discharge
Bovenrijn
0.02
0.03
0.04
0.05
2500
5000
7500
10000
Discharge (m3/s)
Darcy-Weisbach f
center
Left
Right
Waal
2000
4000
6000
8000

7. Manning n vs discharge Bovenrijn 0.02 0.03 0.04 2500 5000 7500 10000
Discharge (m3/s)
Manning n
Center
Left
Right
Waal
2000
4000
6000
8000

8. Downstream hydraulic geometry
The Cochiti Reach of the Middle Rio Grande is located in north-central New Mexico.
The headwaters of the Rio Grande are in southern colorado.
As a result, it is a spring snowmelt driven hydrograph with later summer monsoonal thunderstorms. Historically, the peak occurred on average in May and averaged about 120 m3/sec.
The Cochiti reach spans 45 km from Cochiti dam to the Bernalillo Highway 44 bridge, north of the city of Albuquerque.
It is located along the Rio Grande Rift, a tectonically active region prone to earthquakes and active uplift further downstream in Socorro.

9. The gaging stations used to quantify the inputs of water and sediment to the reach are shown here.
The Otowi gage located upstream of Cochiti reservoir is used as a reference for the “unregulated” conditions.
The Cochiti gage is located just downstream from the dam and represents the input to the reach.
The Bernalillo/Albuquerque gages are located at the downstream end of the reach and represent the outflow of the reach. The water and sediment discharge at these gages are also used to be representative of the downstream end of the Cochiti reach.
Three major tributaries enter the Rio Grande in the Cochiti reach. The first two are Galisteo Creek and Arroyo Tonque which are both ephemeral. The last is the Jemez River. Galisteo and the Jemez are both dammed. Jemez in 1953 and Galisteo in 1970.

10. Cochiti Dam Rio Grande Santa Fe River
The culmination of these attempts to stabilize the channel was the construction of Cochiti dam in 1973.
The dam was constructed for flood control and sediment retention.
The dam is 5 miles long and also dams the Santa Fe river, a tributary to the Rio Grande.
The agricultural land along the river is evident in this aerial photo taken Other management practices along the river were aimed at draining the high water table in the ag land, and construction of diversions for agricultural practices.
Since construction of the dam, the channel downstream has narrowed and deepened resulting in disconnection of the river from the floodplain.
Santa Fe River

11. The Rio Grande in the Cochiti reach is an alluvial river.
Historically it was an aggrading sand-bed channel that exhibited extensive lateral mobility.
The channel frequently avulsed and shifted across its floodplain, with this migration limited by valley terraces and bedrock outcroppings.
The high degree of variability in the hydrograph from year to year resulted in periods of relative stability punctuated by high mobility during extreme flows.
As is common in many rivers in semi-arid environments, it was the high magnitude, low frequency events that form the shape of the channel. During the relatively stable periods vegetation became established on the floodplain (Crawfor et al 1993).
The extreme events were necessary for maintenance of the health of the Bosque habitat. For instance, inundation of the floodplain is essential for recruitment of new cottonwood stands.
This reach is included in the critical habitat designation for the Rio Grande Silvery Minnow. Additionally, the riparian Bosque forest is important for bald eagle perches for hunting. As a result, maintenance of both the natural riparian and in-channel aquatic habitat are of interest.

12. Peak Annual Discharge
Three different statistical methods were used to identify trends in the discharge data.
The peak annual discharge prior to construction of the dam was decreasing.

13. Annual Average Daily Sediment Concentration
According to the USBR, the Middle Rio Grande historically had one of highest sediment concentrations of any major river in the world (Crawford et al. 1993).
Crawford et al (1993) states that according to Gellis (1991) there is evidence of a decrease in annual sediment concentrations throughout New Mexico, at least since the mid-1950’s.

14. 284 Cross sections 4 Reaches 1.1 1.9 2.1 2.6 3.1 4.1 3.5 26 Subreaches
4.6
3.5
4.1
3.1
2.6
26 Subreaches
284 Cross sections
4 Reaches
The numbers are the subreach #’s.

15. Cross section CO-2

16. Cross section CO-18

17. Cross section CO-24

18. Change in Mean Bed Elevation
Reach 1
Reach 2
Reach 3
Reach 4

19. Bed material size

20. Sinuosity

21. Active channel width

22. Planform geometry
1935
1972
1992
This is 10 miles d/s from the dam at the confluence with Borrego Canyon. N

23. Planform Adjustments
Braided
Meandering

24. Lateral Adjustments

25. Hydraulic Geometry Equations
Equilibrium?
Hydraulic Geometry Equations

26. Hydraulic Geometry Equations
(Julien & Wargadalam 1995)
1992
1918

27. Modeling Lateral Movement
Deviation from Equilibrium

28. Deviation from equilibrium width
Modeling Width Change
Deviation from equilibrium width

29. Exponential Model Results

30. Conclusions
Bedforms affect resistance to flow and Manning n can increase during floods.
The effects of Cochiti Dam on the Rio Grande are primarily degradation and armouring. There is relatively little effect on channel width.
The rate of change in channel width is proportional to the deviation from the equilibrium channel width.